Sound masking is the addition of natural or artificial sound (such as white noise or pink noise) into an environment to cover up unwanted sound by using auditory masking. This is in contrast to the technique of active noise control. Sound masking reduces or eliminates awareness of pre-existing sounds in a given area and can make a work environment more comfortable, while creating speech privacy so workers can better concentrate and be more productive. Sound masking can also be used in the outdoors to restore a more natural ambient environment.
Sound masking can be explained by analogy with light. Imagine a dark room where someone is turning a flashlight on and off. The light is very obvious and distracting. Now imagine that the room lights are turned on. The flashlight is still being turned on and off, but is no longer noticeable because it has been "masked". Sound masking is a similar process of covering a distracting sound with a more soothing or less intrusive sound.
- 1 The Need for Sound Masking
- 2 Applications of Sound Masking
- 2.1 Commercial Facilities
- 2.2 Medical Facilities
- 2.3 Secure Facilities
- 2.3.1 Standards
- 2.3.2 Categories of Surveillance
- 2.3.3 Types of Masking Signals
- 2.3.4 Types of Masking Speakers
- 2.3.5 Locations for Protection
- 2.4 Private Applications
- 3 Status of Sound Masking
- 3.1 The Relation between Sound Masking and Acoustical Privacy
- 3.2 Three Privacy Factors
- 3.3 Degrees of Privacy
- 3.4 Types of Privacy
- 3.5 The Privacy Index
- 3.6 Advances in Sound Masking
- 3.7 Attributes of Successful Sound Masking Systems
- 3.7.1 Sound Masking Level
- 3.7.2 Sound Masking Spectrum
- 3.7.3 Spatial Uniformity
- 3.7.4 Temporal Variability
- 3.7.5 Phasing
- 3.7.6 Sound Diffusion
- 3.7.7 Broad Range of Equipment
- 3.7.8 User Acceptability
- 3.7.9 Truly Background
- 3.7.10 Centrally Controllable
- 3.7.11 Cost
- 3.7.12 Expandability
- 3.7.13 Code Compliance
- 3.7.14 Soundscaping
- 3.8 Two Design Rules
- 3.9 Two Design Objectives
- 3.10 Modeling Speech Privacy
- 3.11 Modeling Masking Speaker Arrays
- 3.12 Masking System Components
- 3.13 Recommended Sound Masking
- 4 See also
- 5 References
- 6 Further reading
- 7 External links
The Need for Sound Masking
Effects of Noise on People
A seminal work  covers the subject in some detail. Noise is defined as unwanted sound. It can have three effects depending mostly on level. At high levels, there are mechanical changes in a person, such as heating of the skin, rupture of the eardrum, or vibration of the eyeballs or internal organs. At lower levels, there are physiological (biological) changes in a person, such as elevation of blood pressure, or stress. At still lower levels, the changes are psychological (subjective) such as annoyance and complaints. Annoyance is based on factors such as the person's evaluation of the necessity of the noise, or whether it can be controlled, or whether it is normal for the environment (See Section 1.3). The levels of sound masking are sufficiently low that they have no known physical or physiological effects on people. One aim of sound masking design is to make the sound be "normal", i.e., acceptable.
Studies of Noise in the Office Environment
Since most sound masking is used in offices, a number of cognitive psychology studies have been made that relate specifically to the office environment. In one study, it was found that there was a modest stress (physiological) increase and diminished motivation caused by typical office noises, including speech. They recommended the use of sound masking under the control of the worker. In another study  they suggested that changes in level are an important factor, but that habituation to the noise can occur. In the office, habituation can be interpreted to mean “I’ve grown used to the noise and it no longer distracts me” or “Since I cannot do anything about it, I will have to live with it.” The authors of another study  point out that the specific information within the speech intrusion is not important nor is the “intensity” (level) of the sound between 48 and 76 dBA. Since the energy level of the louder sound was 1,000 times that of the least, one must assume that distraction occurred for all levels. For arithmetic tasks, both speech and non-verbal intrusive noises caused significant performance decreases. For “prose tasks” it was found that speech caused a greater performance decrease than nonverbal noises. In another study  the author added several significant observations. It was found that “during a serial recall task, the accuracy of report decreases 30 to 50%.” When the intrusive speech was increasingly filtered to a meaningless mumble, there was a monotonic increase in performance. Finally, the author states: “Perhaps the single feature that makes the irrelevant speech phenomena so fascinating is that the processing of sound is obligatory; it appears beyond the individual’s control.” Within the references cited above are further references to earlier works on this subject. There are several implications for a sound masking system. The masking must reduce the difference between the steady background level and the transient levels associated with both speech and other sounds. Motivation and productivity are improved when this is accomplished. The masking sound itself must not change rapidly and should be as meaningless as possible.
Common Opinions about Sound
Sound masking must satisfy the persons that listen to it. People ask themselves a number of questions about the acoustical environment. The following questions were deduced from employee comments about their office environment. These are questions the listeners implicitly ask themselves to determine their response to their environment. The design of a sound masking system must take these opinions into account.
- Is the sound made by me or made on my behalf?
- Is the sound "normal" for this environment?
- Is the sound necessary and can anything be done to control it?
- Does the sound have meaning?
- Is the sound frightening?
- Will the sound have an adverse effect on my health?
- What is the pitch of the sound?
- How reverberant is the room?
Complaints about Noise
Since sound masking is a shaped random sound, often erroneously called "white noise", It is important for a sound masking system to dispel that this sound is actually noise. The most important finding by Kryter is encapsulated by this statement:
- "The general finding that the performance of the more anxious personality types is more affected by noise than that of nonanxious types would attest to the existence of a stimulus-contingency factor. In terms of learning or conditioning, the task becomes disliked and is performed relatively poorly because it is related to or contingent upon the aversive noise.”
- "A possible teaching of much of the data presented in this book is that, other than as a damaging agent to the ear and as a masker of auditory information, noise will not harm the organism or interfere with mental or motor performance."
In well designed sound masking systems, distraction caused by extraneous conversations (loss of privacy) far outweighs any negative response to the sound masking
The Fallacious Quest for Quiet
The biggest problem with sound masking is that when people are annoyed by the activity sounds around them (noise), they search for "quiet.” They believe that "quiet" is a desirable condition of low background sound level, but what they are really searching for is the freedom from the acoustical distractions that ultimately cause annoyance. The only way to achieve true quiet would be to maintain a low background sound level with no transient sounds; a condition that requires complete isolation from all activity sounds. A better definition of "quiet" would be the absence of distracting sounds, not the absence of all sound. This is the definition used in sound masking. Persons not familiar with this definition ask, "How can you make it quiet by adding noise?" Hopefully the above definitions help to answer this question.
Steady vs. Transient Sounds
Steady sounds are the background sounds in any environment that are reasonably continuous and long term. If a steady sound persists for a long time without change, and the level is relatively low, persons generally accept it as normal. People are seldom aware that an outdoor background sound exists. Steady sound can be tonal or random. If the sound is tonal it will create more annoyance than a random sound of the same level since the latter conveys no information to the listener. Transient sounds are conversation, paging, machine sounds as well as exterior sounds such as passing aircraft and road traffic. They are short term, can vary considerably in level, and generally distract a person's attention if the level is high relative to the steady sound level (a rise of about 10 dB is a common criterion). The distraction is further strengthened if the sound has high information content, such as conversation. At relatively low levels, the major concern is the psychological effect of distraction and annoyance. The primary use of sound masking is to reduce the distraction associated with transient sounds, and in some cases reduce the intelligibility of those transient sounds (closed offices, secure facilities).
What is sound masking?
The word “mask” means merely to cover up, or disguise, something. That something is not changed, but simply hidden. Physical masks cover the face of the wearer. Deodorants mask odors; they do not eliminate them. One-way windows mask the persons on the other side so they are not visible. Sound can mask other sounds to cover them up. In every case, the objective is to hide something that exists, it does not eliminate them. Many people have heard of noise cancellation, but incorrectly believe that it is a form of sound masking. In noise cancellation the sound is actually eliminated not covered up. So why not use it? Unfortunately, this technique works only within spatially constrained areas, such as headphones, and is not applicable to entire rooms. What is sound masking trying to mask? It covers up distracting sounds, such as conversations, by raising the background level. That level must be above the distracting sound most of the time. How is that determined? For every situation there is an optimum background sound level, just as there is an optimum light level. In the home that sound level is low, and at sporting events that level is high. The function of sound masking is to bring the background level up to the optimum; the level that provides fewer distractions without being a distraction itself. This requires persons experienced with this technique to find that optimum.
The Early History of Sound Masking
It is likely that primitive people did not want acoustical privacy, so they never camped near a rushing stream. They understood that stream noise would mask the approach of enemies or predators. The sound of fountains in Roman villas certainly served to mask the sounds of iron-rimmed chariot wheels on the cobble-stoned streets. Fountain masking has carried over to shopping malls or buildings with large atria. There are stories of a dentist, in the 1940s, applying random sound to patient’s ears through earphones to mask the terrible noise of slow speed drills. An example of a self-contained masker, made in the 1960s, is shown in the figure on the right. The application of electronics and the advent of the open office resulted in the rapid evolution of sound masking. The Quickborner Team of Germany introduced the concept of the open office to the United States in the late 1960s. Geiger-Hamme Laboratories developed a standard for open office acoustics in the 1970s. It was sponsored by the Public Building Service of the General Services Administration for use in open government offices. It included a requirement for sound masking. Many of the major furniture manufacturers, such as Herman Miller, Steelcase, and Haworth, converted much of their production to products for the open office. Herman Miller was the first to have self-contained maskers mounted pointing up on top of open office furniture panels. It did not survive primarily due to the presence of controls available to employees. Owens Corning, a manufacturer of fiberglass products, entered the open office market and introduced a centralized masking system using a speaker called the Sweeny baffle. This speaker was unusual in that the sound spectrum on axis was the same as the electrical spectrum. Unfortunately, off axis the spectrum was different; it no longer exists. Manufacturers of commercial sound systems entered the market in the 1970s; masking was an add-on to their other audio products. Soundolier (now part of Atlas Sound) sold a self contained masker that has survived until recently. The Dukane Corporation sold a masker that had two speakers contained in a heavy triangular enclosure. Companies that considered sound masking as their primary business came into existence about that time. One product, the Lahti masker, was a speaker mounted on the surface of a plastic sphere. Dynasound, Inc. introduced a masker that had a speaker mounted on the lid of two gallon paint can; the handle was used for plenum mounting. K.R. Moeller Associates sold a self-contained masker called Scamp, while the Lencore Corporation sold an equivalent unit, now called Spectra. A document was published in 1980 by the Defense Intelligence Agency. It concerned protection of secure facilities from deliberate audio surveillance; sound masking was one means of protection. Dynasound, Inc. developed a vibration device that could be attached to various surfaces such as doors, walls, and windows, to provide sound masking. In the 1980s, the Bertagni family developed a speaker that could not be distinguished from a fiberglass ceiling tile. The invisibility aspect was favorable to architects. The attempt was made to use this speaker for sound masking, but the cost and the sound radiating characteristics limited its use for that purpose. Armstrong World Industries, a manufacturer of ceiling materials, developed a similar speaker. This speaker has survived as a product of Sound Advance and is now used for applications other than sound masking. An early attitude among owners, designers, and architects was that masking was an excuse for a bad open office design. In early systems, the installers were not knowledgeable about how to provide privacy. As a result, many systems merely provided more noise and were shut off. This was countered by the rise of firms who specialized in the design, installation, and equalization, of masking systems. The evolution of sound masking since the 1980s is described in other sections.
Why Use Sound Masking
Sound masking has some unique advantages over traditional methods of reducing distracting sounds.
- Sound masking is dynamic (variable) as are the sounds it is intended to block. Building elements that provide sound attenuation are static (fixed) and cannot adapt to intruding sounds that change in level. Masking can vary from location to location as well as from time to time, to adapt to changing environmental conditions.
- Sound masking is by far the least expensive tool for providing privacy.
- Sound masking systems are special audio systems that create spatially uniform sound levels. The uniformity can be used to integrate cost effective music and paging into a system.
- Sound masking works at the listener's ear and is independent of the building structure (acoustically) so concerns about how distracting sounds get from one place to another is unimportant. Structural modifications require such knowledge and are more costly.
- It helps : Overcoming noisy neighborhood noises, Snoring sounds from other family members, Soothing sleep inducing sounds for babies, Afternoon and day time naps/ sleeps are more comfortable, Noise free environment for reading/ studying, Personalized and discreet environment for confidential conversations.
There are two groups of people that should not be exposed to sound masking: those with significant hearing loss and those with very limited vision. The first group already have considerable privacy which results in masking providing too much privacy. Visually impaired persons use acoustical cues to navigate; sound masking can remove those cues.
Applications of Sound Masking
A number of studies of office acoustics have been done.
Closed offices and conference rooms often appear to provide confidentiality but actually may not. Lightweight, or movable, walls are more sound transparent and most do not extend to the ceiling deck, so speech can pass into the ceiling plenum and then to the next office. Sound masking can be used to make up for such acoustical weaknesses.
Open offices can have a background sound level that is too low. Sound level restrictions of air handling units are increasingly stricter. The conversations of others can be clearly understood. Sound masking raises the background level to make up for the absence of walls that that would otherwise block the sound.
For preliminary planning, loudspeaker spacing, S, can be found by:
S = 1.4 (2D + H - 4)
S = spacing between loudspeakers (ft) D = plenum depths (ft) H = floor-to-ceiling height (ft)
An average spacing is 15'
In a Suspended Ceiling plenum
The plenum is the space between a suspended ceiling and the structural deck above it. Since most offices have such spaces, sound masking speakers are commonly added there. For open offices, uniformity of the sound masking in the inhabited area is important and is largely determined by the spacing of the masking speakers. Speaker arrays are generally rectangular. They generally face upwards to reflect sound from the deck and broaden the distribution of the sound to create a more uniform sound field. There are other factors that determine the horizontal spacing of speakers to assist in creating a uniform sound level below. The first is the nature of the ceiling material. Mineral tile ceilings have appreciable sound loss so speaker spacing of 15 feet is common. Fiberglass ceilings, on the other hand, are highly sound absorptive but are more sound transparent, requiring speakers to be closer. The second factor is the depth of the ceiling plenum. The deeper the plenum, the further apart the speakers can be placed. The third factor, of somewhat lesser importance, is the height of the suspended ceiling itself. With higher the ceilings sound uniformity is improved, so speaker spacing can be increased. See Section 3.7.3 for a discussion of uniformity. For closed offices, uniformity of sound level is not an issue. Generally, ceiling tiles with a high transmission loss are recommended, those with CAC (Ceiling Attenuation Class) ratings of 30 or greater. This reduces the impact of sound masking in adjacent open areas creating excessive levels in those offices. Typically the ceiling plenum is used as a return air duct requiring an open grill. Sound masking in the plenum above a closed office can generate excessive levels. There are simple metal covers that are placed above the grille to reduce the impact of the hole. Available ones are sufficiently large so do not impede air flow.
In an Open Ceiling
In some offices, particularly warehouses that have been converted to office space, there is no ceiling plenum. Masking speakers are hung in a similar manner to those above suspended ceilings but the speaker spacing and height is different. Typically, the speakers are mounted higher and the spacing between them is closer. The height of the structural ceiling above is a large factor in spacing, as is the presence or absence of sound absorbing materials on the ceiling surface. Reasonably good uniformity of sound masking can be achieved. Some office designs employ the use of scattered (as opposed to continuous) ceiling tiles (clouds). Careful design is needed to achieve reasonable sound masking uniformity.
Under raised Floors
Sound masking speakers can be placed under raised floors in offices that utilize them. To carry the weight above, the floor material is structurally strong and offers high attenuation of any sound below it. Experience has shown that not only is the uniformity of the sound above exceptionally good, but occupants are hard pressed to determine where the sound source is located (good diffusion -See Section 3.7.6). The figure on the right shows masking sound levels at 48 inches above the floor; they are all within 1 dB of each other. One direction is a line between the speakers; the other direction is a line lateral to the speakers starting at the midpoint. If the depth of the cavity is sufficient, normal sound masking speakers can be used, otherwise small speakers can be used; they fit into a cavity as small as 2 inches and radiate sound horizontally. If the cavity is used as a return air duct, care must be taken to shield the opening.
Face Down in a Suspended Ceiling
The masking sound travels directly to a listener without any intervening sound attenuating materials. This arrangement, with speakers mounted in a suspended facing down, has been typical for paging systems, but is being used more recently in some sound masking applications. The speaker for this application must be specially designed to have a wider spread of sound than other speakers to make up for the lack of intervening material. They are often called "direct field" speakers to distinguish them from the other arrays that radiate sound indirectly. They can be small loudspeakers or entire ceiling tiles installed with hidden speakers. Although the speaker locations in the above sections are preferred, there are situations where use of direct field speakers is necessary. An alternative location for such speakers is wall mounting.
In Unusual Locations
In some older buildings the above locations do not exist. Vibration maskers have been applied in a number of facilities to the air supply duct, radiating masking through the air diffusers. In other cases, maskers were hidden above air ducts in open ceilings. Speakers have been placed under desks, under open office work surfaces, behind paintings, or on open office furniture panels.
The Congress of the United States passed the Health Insurance Portability and Accountability Act (HIPAA) into law. It mandates that individually identifiable patient health information be protected. Although written and computer files are obviously to be protected, verbal information must also be protected. “Covered entities” (those who must comply with the law) must make reasonable efforts to safeguard patient information from being overheard. The law itself gives no specific guidance on how this is to be accomplished, but a document released by the Department of Health and Human Services provides some clarification. It includes, as part of the protection, the phrase “health information whether it is on paper, in computers, or communicated orally”. The Office of Civil Rights also has published a document on this issue, stating that the law does not require retrofitting spaces, such as soundproofing of rooms, in order to comply. As a result, many medical facilities have already realized that compliance is wise and have begun retrofitting their facilities. Experience has suggested that most hospital rooms do not need sound proofing, but can benefit from sound masking.
Noise in hospitals has been a problem for at least fifty years, in part because of the need to have all surfaces hard and cleanable. A large number of measurements and reports in prestigious journals have established the problem  From the patient’s viewpoint the problem has been the distraction and annoyance caused by the noise of people, which results in less rest, poorer sleep, and possibly longer recuperation time. The increased socialization now permitted in hospitals, as well as the increased use of medical machinery, has exacerbated the problem. An extensive survey by the Public Health Service in 1963 showed that patients were frequently disturbed by speech and distress sounds in other rooms as well as staff visits during night hours. Other studies concerned the interference of sleep and recuperation by noise. One study found that the amount and rate of increase in the sound level from the constant background was the main contributor to full awakening or changes in the stage of sleep. It was determined that the magnitude of the change in level, regardless of its median value, was more significant than the level of a steady sound of the same median value. This conclusion was supported by an Environmental Protection Agency document. Suter  expanded this finding by stating “it is clear that intermittent and impulsive noise is more disturbing than continuous noise of equivalent energy, and that meaningful sounds are more likely to produce sleep disruption than sounds with neutral content.” These conclusions were the same as those found for open offices.
Sound Masking in Medical Facilities
The finding of researchers has shown that the privacy problem in medical facilities is very similar to that in open offices. As a result, sound masking has been used beneficially in a number of locations. Patient rooms, corridors, and nursing areas of hospitals are prime locations. Sound masking can be used in retirement and rehabilitation centers. It is also beneficial in medical suites or patient contact areas of medical insurance providers. Pharmacies can also provide confidential privacy at contact areas with sound masking.
Secure facilities require more care when sound masking is used. For commercial offices and even for medical facilities, the listener for confidential conversations are presumed to be casual or accidental. For secure facilities, the listener is presumed to be deliberate and may make use of sophisticated technical listening devices. Many government facilities have made use of structural solutions, i.e., rooms (room-within-a-room) that are shielded from vibration, acoustical, and electromagnetic surveillance. Unfortunately, not all secret conversations take place in such rooms. A less obvious weakness in secure rooms is that modern listening devices can be placed in locations that the building structure cannot protect against (inside wall cavities or remote detection of window vibration). Another weakness in rooms of this type is that designers may presume speech is on a controlled, but low, level. Public address systems, speaker phones, and audio/video presentations require additional protection.
There are many unclassified government documents that define how secure rooms are to be designed. One type of facility is called a SCIF (Secure Compartmented Information Facility); there are others. There are likely classified documents as well. In almost all of the documents, sound masking is one recommended audio protection method. There is similar standard that applies to financial institutions.
Categories of Surveillance
There are two categories; each must be addressed differently. Uncontrolled areas are those where the persons attempting to protect themselves have little or no control over the surrounding environment. This might be all areas outside the building in which the secure room resides, such as parking lots or other public spaces where it is possible to gain access without detection. Controlled areas are those within the building where the occupant has a measure of control.
Types of Masking Signals
Unlike sound masking in offices or hospitals, it is necessary to consider that sophisticated listeners may have technology to recover speech buried in masking sound. To inhibit such devices, it may be necessary to provide layered audio protection; several different signals are mixed together. Non-stationary random noise should be the first layer; it provides more protection than standard sound masking generators. Music may be used as the second layer; it is buried below the random noise so it is actually inaudible to room occupants. A voice babble generator or actual speech samples may be used as a third layer. The fourth layer, the actual voices to protect, should be sufficiently buried.
Types of Masking Speakers
Standard masking speakers, discussed in Section 2.1.3 must be supplemented with vibration maskers for attachment to a number of surfaces, as noted below.
Locations for Protection
Thee are a number of locations where surveillance of secure facilities can occur.
Windows generally are open to uncontrolled areas so require protection. Windows can convert interior speech to window vibration that can be detected remotely by laser or directional microphones. A vibration device attached to the window, and fed with sound masking, can be used to effectively block speech intelligibility.
Exterior walls are generally constructed of heavier material than interior walls and seldom need audio protection. Interior walls are a different story. Speech can excite a wall to vibrate and there are several locations from which conversations can be detected: remote from the wall with laser or directional microphones, on the far side of the wall with vibration detectors or direct listening, or within the wall cavity with vibration detectors or fiber optic microphones. Vibration sound masking devices can provide protection against these forms of surveillance when placed within the room to be protected and at the appropriate height and spacing, In effect, the wall becomes a masking speaker.
Doors are weak links in walls; they may be hollow or solid core, metal, or specially built for high sound attenuation. They can open to exterior uncontrolled areas or to internal controlled areas. Every door has a gap around its periphery that may have gaskets. Because carpeting is often used, there may be a significant gap at the bottom. Most surveillance of internal doors is accomplished by direct listening while other methods may be used for external doors. Because of the multiple paths of sound through and around a door, vibration sound maskers are used at appropriate locations on the door.
Listening through air ducts is a time honored source of eavesdropping since almost all modern rooms have supply ducts that connect to a multiplicity of other rooms. Ducts can be effective speaking tubes; the speech attenuation is particularly weak in unlined metal ducts. In some cases, exhaust ducts will connect to uncontrolled spaces. Surveillance can be accomplished by direct listening or with probe microphones or vibration devices within the duct where they are not visible to inspection. Vibration sound maskers, appropriately located, are attached to metallic duct walls and use the duct wall as the masking radiator. External speaker maskers are used at appropriate positions to radiate sound into fiberglass ducts.
Normally, liquid filled pipes do not carry significant speech energy nor does conduit piping filled with wires. However, empty conduit pipes are excellent speaking tubes; vibration maskers are attached to them. In some facilities, vibration maskers have been attached to support pipes and columns.
Raised floors are generally inaccessible for inspection so surveillance can be accomplished there. If the floor is part of the air handling system, probe microphones or fiber-optic microphones can be effective. If the floor is continuous, the high sound attenuation of the material makes surveillance with vibration detectors more advantageous since the stiff metal plates respond to speech. Although vibration maskers can be attached to the floor plates, the preferred protection is to use speaker maskers under the floor.
Secure rooms often have a suspended ceiling with a plenum above. Sound masking penetrates the ceiling material in open offices, so speech can go in the opposite direction into the plenum. It is likely that the perimeter walls extend to the structural ceiling. If the plenum is part of the air handling system or if there are cable run penetrations, probe microphones can be used for surveillance. If not, a small penetration can be made to insert these devices. As with commercial offices, plenum speaker maskers are installed.
Many building codes require the presence of speakers in a secure room for emergency announcements. Although speakers are intended for creating sound, the speaker cone also responds to external sound and the coil generates a minute voltage characteristic of that sound. With proper sensing, that voltage can be converted to speech. Although it is possible to place a speaker masker next to the paging speaker the recommended solution is an optical isolator. It is essentially an audio diode; sound only goes one way.
There is some evidence that the sound of key strokes can be detected to identify the characters being entered. A vibration masker placed under the keyboard will radiate sufficient masking to block surveillance without disturbing the user.
Sound masking generators have been used for personal application for many years. There are several manufacturers of devices for home, hotel, or air travel, primarily for sleeping. The figure on the right shows a masker that has been on the market for many years.
Status of Sound Masking
The Relation between Sound Masking and Acoustical Privacy
The primary goal of sound masking is privacy. In closed offices, conference rooms, or secure facilities the goal is confidentiality of speech. In open areas, the goal is sufficient privacy so occupants are free of distracting sounds, such as speech and other sources of sound. Unfortunately, sound masking is only one of three major factors that result in privacy. Thus, it is necessary to appreciate how the other factors contribute to privacy and how sound masking as a privacy tool can effectively complement those factors.
Three Privacy Factors
The factors are displayed as a diagram in the figure below.
SOUND SOURCE (LEVEL CREATION) Minus SOUND ATTENUATION (LEVEL REDUCTION) Minus BACKGROUND (LEVEL MASKING) = SPEECH INFORMATION
The source of sound is a significant factor in privacy. It creates the level that determines the effectiveness of the other factors. Most concern is about normal speech levels, but amplified speech, speaker phones, audio/video presentations must be taken into account since their levels may be considerably higher. Most sound sources are directional; level reduction can be improved by taking directionality into account, particularly in open offices. The gender of the speaker also plays a role.
En route to a listener the level of the sound will be diminished due to several factors:
- Distance. In a location with no reflecting surfaces, the sound will reduce about 6 dB per doubling of distance. While this may sound advantageous at close distances, doubling 25 feet to 50 feet entails considerable area and cost.
- Reflecting Surfaces. Sound reflecting from a surface may be added to the sound going directly to a listener, reducing privacy. Absorptive materials will reduce the level of that reflected sound.
- Blocking Structure. Any material that is in the path of sound from source to listener will reduce the level. The heavier the material, the better. A common error is to use sound absorbing materials to block sound; they are insufficient for this purpose.
In any environment, there are multiple sound paths with varying degrees of effectiveness in achieving privacy. The weakest path is the one that dominates the potential for poor privacy, which can create difficulties for sound masking.
The background sound in buildings is normally determined by the air handling system. ASHRAE has guidelines on just how loud they systems are recommended to be. The sound from well designed systems is distributed through air diffusers and of low level. It is not capable of being easily adjusted to accommodate privacy needs. The sound spectrum is weighted toward low frequencies so is not useful for improving privacy, especially speech privacy. Sound masking, being electronic, can adjust both level and spectrum at almost any place at any time to meet privacy requirements. However, care must be taken to meet acceptability requirements. See Section 3.7.8.
Degrees of Privacy
How people respond at various degrees of privacy is shown in the table.
Talker confidentiality is protected continuously at all times and from all surveillance methods.
Talker confidentiality is protected from casual listeners. Speech may be audible but not intelligible.
Listeners are protected from distracting sounds, especially speech. Speech will be audible, partially intelligible, and not distracting.
Listeners are protected from distracting sounds. Speech will be audible, mostly intelligible, and somewhat distracting.
Listeners are not protected from distracting sounds. Speech will be audible, intelligible, and completely distracting.
Types of Privacy
Listeners or talkers have privacy from all surrounding persons regardless of degree. Well designed closed offices, conference rooms, and secure facilities meet this requirement.
Listeners or talkers have privacy from some of the surrounding persons regardless of degree. Open office customer support centers are facilities that meet this requirement. The person has no privacy from those nearby, but has privacy from those further away. Because this is distance related, a term called Radius of Distraction has been employed to identify the transition distance. An example of the radius is shown in the figure on the right. It is more area related than distance related.
The Privacy Index
Since speech privacy is the most important sound that needs evaluation, The Privacy Index has been developed; it based on the well established Articulation Index. The privacy calculation is done for each of several frequencies important for speech. Each frequency is then weighted for its importance in intelligibility and summed up. Thus it is a measureable means to ascertain the ability to understand speech and thus determine speech privacy. Numerous tests over fifty years have shown a strong relationship of that metric to the degrees of privacy. The value of PI varies from 0 (no privacy) to 100 (complete confidentiality). The curve in the figure shows that the relationship between speech privacy and Privacy Index is not a straight line, suggesting that the brain goes from nearly complete understanding of speech to virtually none with small changes in the PI. Sound masking applications make use of this fact. If the other two factors can move the Privacy Index to near 70 (the knee of the curve), addition of sound masking will have a large effect on the degree of privacy despite the fact that the PI does not change significantly.
Although people tend to think most relationships are linear (halfway in one direction yields halfway in the other), they are very familiar with non-linear school grades (50 in an exam is flunking). This analogy can be used beneficially as shown below:
- Secret PI=100+ Grade=A+
- Confidential PI-95-100 Grade=A
- Normal PI=80-95 Grade=B
- Transitional PI=60-80 Grade=C
- None PI<60 Grade=F
A newer rating is the Speech Privacy Class (ASTM E2638-08); it applies to closed rooms. PI is a measure of the speech from a talker at a specific location to a listener at a specific position. It is applicable to situations such as open offices as well useful for evaluating the efficacy of a listening device hidden in a stud wall. SPC makes use of average values so is very useful in the design and evaluation of closed rooms for speech privacy from casual listeners.
Why Calculate the Privacy Index?
In offices, it is seldom practical to make detailed measurements of the privacy provided by a sound masking system. The vast number of acoustical interactions in even small offices makes it a long and time-consuming project. As specialists gain more experience, setting up and adjusting the system to increase user acceptability will become just as important as detailed measurements. However, there are circumstances where such measurements are valuable:
- If a specification calls for the installer to take such measurements.
- If an existing office looks marginal for creating privacy. Reporting such is important.
- If facility managers need objective data to offset unreasonable complaints or justify expense by showing the improvement provided by masking.
- If the privacy achieved needs to be proven or in secure facilities at locations where listening tests cannot be performed (e.g. windows).
Privacy Index can only be calculated from a series of measurements, since it incorporates all relevant factors for speech privacy. For normal masking systems, the acoustical version is used. For secure facilities, where speech may be in vibration form, the second method must be used. To collect the necessary data, a one-third octave band Real Time Analyzer with a random incidence microphone is necessary. To create the test sound, a source with directional characteristics similar to that of the human voice is necessary. A tripod should be used to mount the sound source which is normally 48 inches high for seated talkers.
The Acoustical Privacy Index
The equations for Articulation Index and Privacy Index are:
The transmission loss spectrum (TLi) must be determined from two measurements; a total of four spectrum measurements are indicated. It is both common and acceptable to choose the normal voice spectrum (VSi) available from ASTM. The source spectrum (SSi) should be measured independently, since it can be used for multiple PI measurements. The received spectrum (RSi) and masking spectrum (MSi) must be measured at the listener location, but not at the same time. Since there are many combinations of talker/listener positions even in one workstation pair, it is best to choose a worst-case situation such as shown in the figure. The sound source (dark symbol) should face the upper workstation at seated height. Although the talker may seldom be in this position, it presents the worst privacy situation for the other parties. The listener should be at the most frequently used position; in this case, at a workstation near the talker (open chair symbols). For a second test, placing the sound source (dark symbol) at the workstation table, and facing across it, would be the worst-case for the listener in the workstation to the right.
- Step 1: The source spectrum (SSi). The sound source, mounted on a tripod, should be faced horizontally at seated height (48 inches) and pointed in the desired horizontal direction. It should be powered by a broadband random generator/amplifier. The spectrum should be as near pink (flat one-third octave band spectrum) as possible with an overall level of between 70 and 80 dBA. However, since it will be used as part of a level difference, it need not be exactly pink. It is preferable to take this measurement in an anechoic environment but such rooms are hard to find. Further, one may have to determine the spectrum in the field. In that case, the source should be at least fifteen feet from all sound reflective vertical surfaces and placed over a carpeted floor. To minimize the destructive interference of the floor reflection, measurements should be made at 2, 3, and 4 foot distances along the speaker axis. Arithmetically averaging the three spectra tends to strongly reduce the interference, which is below the frequencies important for speech anyway. Using software or Excel, the averaged spectrum can be stored and retrieved so source spectra need only be entered once.
- Step 2: The received spectrum (RSi). With the sound source on and the sound masking off (to avoid interference), the microphone should be placed at the listener’s position, 48 inches high. To avoid body reflections, the microphone may be placed on another tripod, or held at arm’s length at an angle not along the sound path.
- Step 3: The masking spectrum (MS). With the sound source off and the sound masking on, make a measurement at the same received position.
- Step 4: Calculate Privacy Index wit available software.
To minimize the tedium of these measurements, the source can be set up in one workstation, then rotated to make received measurements in all surrounding workstations. In that case, four or five measurements can be made relatively quickly.
The Vibration Privacy Index
All spectra for the Acoustical Privacy Index calculation are acoustical; not so here, both acoustical and vibration spectra must be measured. In this case the received spectrum is the vibrational response to the acoustical sound source (VRSi)and the masking spectrum is the vibrational response caused by the vibration masker (VMSi). To measure these, a vibration detector must be used. Because the equation requires only the difference between the two spectra, calibration of the detector is not necessary. It only requires sufficient gain to detect all relevant spectra. The equations are:
These equations should be used for windows and in other circumstances where surveillance vibration detectors are likely to be used. The normal speech spectrum is often used (VSi).
- Step 1: The source spectrum (SSi). The same sound source spectrum as used for the Acoustical Privacy Index may be used here. The overall level should be at least 80 dB(A).
- Step 2: The received spectrum (VRS). With the sound source on and the sound masking off, the vibration detector should be placed at the listener position on the surface. Make a measurement of the received vibration spectrum.
- Step 3: The masking spectrum (VMS). With the sound source off and the sound masking on, make a vibration measurement of the vibration masking spectrum at the same position on the surface as for the received spectrum.
- Step 4: Calculate Privacy Index with appropriate software.
Advances in Sound Masking
The advent of sound masking made use of various components typical of other types of sound systems. Since that time, use of masking has grown so that manufacturers have added a number of beneficial functions to their systems. Several are listed below. Chanaud has given a more detailed discussion.
Initial Ramp Up Function
On the initiation of a sound masking system, it is important not raise the background level experienced by occupants from a low existing level to a higher masking level. This function permits the level to be raised slowly and automatically over periods as long as 30 days.
Fast Ramp Up Function
Buildings will have power failures shutting the masking system down. This function prevents the level from jumping up when power is restored. Typical recovery times are in minutes.
Programmed Level Control
The need for privacy varies throughout the day. Persons desire privacy during busy times, but do not need as much when occupancy is low in the evening or on weekends. Security guards do not want privacy as they patrol an office at night. This function permits the level to change smoothly from one hour to the next for each day of the week. See the figure on the right.
The required masking levels are based on the activity levels in a particular environment. In the 2000s, Soft dB was the pioneer of adaptive masking control. This function automatically adjusts the masking level based on distractions and ambient sound instead of only environnemental sound. See the figure on the right.
Most sound systems require visits for adjustment. Newer masking systems now have the capability to be adjusted remotely, either locally or over the internet. Because of this added capability, some systems now permit software to adjust each masking speaker independently. This function does not require the use of monitor panels.
Attributes of Successful Sound Masking Systems
There are a number of factors that contribute to the success of a sound masking system. Good design takes into account each of these factors; a system with more of them will survive longer.
Sound Masking Level
The system should be able to generate sound that masks the intelligibility of speech for the various degrees of privacy. It should also be able to mask the sound of aircraft, the sound of vehicles, the barks of dogs, the music of neighbors, and other sources of annoyance should the need arise. To do this, the equipment must have a broad range of levels and an adequate number of zones.
Sound Masking Spectrum
The system should be able to apply different masking spectra at different locations. To do this, the system must have an adequate number of channels to set the needed spectra. Speakers in open offices, closed offices, and vibration maskers all require different spectra.
In open offices, a high uniformity of the sound masking level over a given area is advantageous. Persons moving from one area to another should not detect any changes in the acoustical environment. This is accomplished by proper placement of the masking speakers during installation and inspection and adjustment of the equipment prior to occupancy.
The system should be able to vary the masking level with time to conform to the changing need for privacy during the day. To do this, the system must have a clock. It may have a programmed level control function or it may have an environmental sensor that sends activity sound level information to the system so the masking can be adjusted automatically. An ON-OFF clock-controlled switch has been found to be disastrous and is not recommended.
When two adjacent speakers radiate the identical masking sound a listener passing by the midpoint between them may detect a "swishing" sound. It relates to phase relationships at various frequencies. This negative effect is most noticeable when speakers are placed in a plenum above a fiberglass ceiling tile, or when the speakers are placed in an open ceiling. A wiring arrangement where every other speaker is wired to one masking channel and the other the speakers are wired to a separate masking channel will strongly reduce that effect. It is called a checkerboard wiring array.
When sound is coming from many directions, little attention is paid to it; it is considered "background" and is more acceptable. The sound field is called "diffuse". When the direction of the sound can be identified or the source located, the sound is less acceptable. Masking speakers should be positioned so they cannot be located. Under floor systems create excellent diffusion.
Broad Range of Equipment
The system manufacturer should have a sufficient number of types, sizes, and shapes of speakers available so it can accommodate a wide variety of masking applications. To do this, the system must be able to handle indoor and outdoor applications, large or small plenum ceilings, and large or small access floor cavities. Both loudspeakers and vibration devices should be available. For visible masker locations, speaker shape and color must be acceptable to the owner. The system should be able to incorporate paging and music; it increases system utility and provides an economic advantage to the user. The system should be able to equalize each signal independently and be able to set relative levels between the signals.
The sound masking must be acceptable to the listener. To accomplish this, established design rules must be met. The person equalizing the system must know how to take into account the existing acoustical environment and the potential response of occupants to masking. The advanced functions, noted in Section 3.6, all improve user acceptability.
The system should be truly background; it does not call attention to itself. This includes general inaccessibility to controls, lack of visibility of speakers, spatial uniformity, diffuseness of the sound field, inability for listeners to locate the maskers, and slow level changes.
The system should be centrally controllable either manually or remotely. All controls must be available to the installer; a limited number of controls should be available to the owner, and none to employees. The controls available to the owner must only be those to change level. They must be stepped (not analog) and have a small step and a limited level range so the correct functioning of the system is retained. Manual controls are acceptable if the cabinet or room is lockable. A computer software interface with the controls is preferable since passwords can be used.
Masking system equipment costs should be comparable to those of other sound systems. Equipment should be designed to minimize the cost of installation.
The system should be readily expandable. Centralized systems should have excess cabinet space, spare zone controls, and adequate power capacity. Distributed systems require only correctly located power outlets for expansion.
The system must comply with local building codes as well as national and international standards.
In most environments, the level of sound experienced changes gradually as one moves from place to place. Gradual change is more acceptable than rapid change. The same is true of sound masking. Persons moving from masked areas to unmasked areas will notice the change in level as they walk. This typically occurs when moving from an open office area to a corridor or to an unmasked support area such as for printers or copiers. This event violates the design rule that sound masking should not call attention to itself. This event can be avoided by changing the sound level more gradually. It is done by adding a string of speakers, each of which is successively reduced in level to the unmasked background level.
Two Design Rules
- The masking should be placed by the listener. Inexperienced persons will often want to place the masking by the talkers. This will require them to speak louder or closer. It will not mask listeners very effectively.
- Every attempt should be made to make the system truly background, conveying no information those exposed to it.
Two Design Objectives
- The masking sound should random and incoherent; meaningful sound should not be used. The sounds from adjacent speakers should not have any correlation with each other. This is best accomplished in two ways: (1) by having intervening materials (e.g. ceilings) between the speakers and listeners; or (2) adjacent speakers are fed a signal from a different masking source.
- The sound field should be as diffuse as possible to reduce awareness. Intervening materials help in this direction.
Modeling Speech Privacy
Since sound masking is only one factor for creating speech privacy, several companies have developed programs that permit the sound masking designer to determine how successful masking will be prior to installation of the system. The program takes into account the other factors in office design such as furniture panels in the open office or walls and doors in closed offices. It also accounts for existing background sound levels and the sound absorbing and sound attenuating characteristics of ceiling materials. The first screen for one such program is shown in the figure on the right. In that screen, the design of an open office workstation is shown. Many of the important factors such as panel height, ceiling material and height, carpet type, workstation size, and occupant position are user variable. Once the design is fixed, the program then analyzes all the applicable sound paths from the person talking to the listener and determines how much speech is attenuated when it arrives at the listeners ear. The program then permits the user to input the desired degree of privacy from which the desired masking spectrum and level is calculated. If the sound attenuation is insufficient, requiring high levels of sound masking; the installation can be cancelled. If the masking level is acceptable, the program can be used to demonstrate the cost effectiveness of sound masking to the prospective owner. For open offices with low panels, the program can be used to calculate the Radius of Distraction (See Section 3.4.2)
Modeling Masking Speaker Arrays
Offices requiring sound masking, particularly open offices, can have shapes other than rectangular. if the office is large, creating the array manually can be tedious. Several companies have developed programs that can automatically create speaker arrays given the dimensions and shape of the room as well as the desired the vertical location of the speakers (See Section 2.1.3). The program can be used to help develop a materials list and provide a printout for an installer to locate each speaker. An example printout is shown in the figure on the right for an office with a central core of elevators. The vertical and horizontal location of each speaker is shown in the printout as well as the channel, zone, and speaker tap setting. In cases where tap settings vary, the speaker shape is changed.
Masking System Components
All masking systems are composed of several basic components. The primary component is the source of one or more random electrical signals. Typically the source may create either pink or white noise. These particular spectra are seldom acceptable to listeners when converted to masking sound, so a spectrum equalizer is needed to create the proper sound spectrum a listener hears. Most professionals recommend the equalizer cover at least the speech frequency range from 160 Hz to 8000 Hz in 1/3 octave bands; most products cover a broader range. Auxiliary signals, such a paging and music, can be added from an outside source. Those electrical signals are seldom proper when converted to sound, so a more limited spectrum equalizer should be added. All signals are added in a mixer to set their relative levels and then sent to one or more power amplifiers. From the amplifiers, the mixed signal is sent to devices that control the overall level in various areas called zones and thence to either loudspeakers or vibration devices to create sound. Many newer masking systems enclose all components up to the amplifier in one reasonably small cabinet, which can be rack, shelf, or desk mounted. There may be more than one of each component in any system and than one system in a large facility. Most recent system designs have incorporated the ability to control many of the components by software, either locally or remotely. See the dashed lines in the figure. The advantage of such controls lies in the simplicity with which the settings of the various components can be altered. The disadvantage is the added cost to add these controls, especially if system changes are infrequently required.
Recommended Sound Masking
CHANNELS: The number of places where the masking frequency spectrum can be set. For commercial facilities, one channel is required for each of the different speaker locations noted in Section 2.1.3. For secure facilities, one channel is required for each of the different speaker locations noted in Section 2.3.5.
ZONES: The number of places where the overall masking level can be set (generally in terms of dB(A)). Levels can be set along with the spectrum in some generators. It can also be set at each power amplifier attached to a number of masking speakers. The speakers can be further subdivided into zones with the use of multiple attenuators attached to each amplifier. Finally, each masking speaker should be capable of individual level control. In a sense, each speaker can be made into a separate zone. The primary purpose of such detailed control is to offset the variations of level caused by local geometric influences, such as air ducts. In some advanced systems, zone control can be done centrally.
Recommended open office levels
- Panel Height: less than 150 centimetres (59 in) Level: 48 dB(A)
- Panel Height: near 150 centimetres (59 in) Level: 47 dB(A)
- Panel Height: near 170 centimetres (67 in) Level: 46 dB(A)
- Panel Height: near 180 centimetres (71 in) Level: 45 dB(A)
- Panel Height: near 200 centimetres (79 in) Level: 44 dB(A)
Recommended open office spectrum
The sound masking spectrum shown in the table on the right is one whose overall level is 47 dB(A). This is a default level and can be adjusted based the office design. For higher or lower levels, add or subtract 1 dB at each frequency. See recommendations above. The chosen spectrum contour is based on numerous measurements and is a mean value based on spectra used by numerous consultants. See the upper figure on the left for examples of the range of spectra used. The importance of spectrum contour is shown in the lower figure on the left where a wide variety of masking spectra produce various levels of privacy even at the same overall level.
Recommended closed office spectrum
Closed offices can be constructed such that Confidential Privacy can be achieved with closed doors and without sound masking. However, the expense to do this is considerable in both materials and in installation labor. More common are STC45 walls that end at a suspended mineral tile ceiling, return air grilles and a continuous ceiling plenum between rooms. It that case, Confidential Privacy can be achieved with sound masking. The table on the right shows a sound masking spectrum that accomplishes that objective under the conditions listed above. The overall level for that spectrum is 44 dB(A). Variations in installation quality have shown that overall levels from 42 to 45 dB(A) may be acceptable.
- Architectural acoustics
- Noise health effects
- Noise mitigation
- Noise Reduction Coefficient
- Noise regulation
- White noise machine
- Tinnitus masker
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