A speaker driver is an individual transducer that converts electrical energy to sound waves, typically as part of a loudspeaker, television, or other electronics device. Sometimes the transducer is itself referred to as a speaker, particularly when a single one is mounted in an enclosure or as surface-mounted device (as in a wall-mounted speaker, car audio speaker, and so on). There are many different types of speaker drivers. The most common ones are the woofer, mid-range and tweeter, as well as subwoofers which are becoming very common. Less common types of speaker drivers are supertweeters and rotary woofers.
Speaker drivers include a diaphragm that moves back and forth to create pressure waves in the air column in front, and depending on the application, at some angle to the sides. The diaphragm is typically in the shape of a cone for low and mid frequencies or a dome for higher frequencies, or less commonly, a ribbon, and is usually made of coated or uncoated paper or polypropylene plastic. More exotic materials are used on some drivers, such as woven fiberglass, carbon fiber, aluminum, titanium, and a very few use PEI, polyimide, PET film plastic film as the cone, dome or radiator.
All speaker drivers have a means of electrically inducing back-and-forth motion. Typically there is a tightly wound coil of insulated wire (known as a voice coil) attached to the neck of the driver's cone. In a ribbon speaker the voice coil may be printed or bonded onto a sheet of very thin paper, aluminium, fiberglass or plastic. This cone, dome or other radiator is mounted to a rigid frame which supports a permanent magnet in close proximity to the voice coil. For the sake of efficiency the relatively lightweight voice coil and cone are the moving parts of the driver, whereas the much heavier magnet remains stationary. Other typical components are a spider or damper, used as the rear suspension element, simple terminals or binding posts to connect the audio signal, and possibly a compliant gasket to seal the joint between the chassis and enclosure.
Enclosures and acoustic isolation
Drivers are almost universally mounted into a rigid enclosure of wood, plastic, or occasionally metal. This loudspeaker enclosure or speaker box isolates the acoustic energy from the front of the cone from that of the back of the cone. A horn may be employed to increase efficiency and directionality. A grille, fabric mesh, or other acoustically neutral screen is generally provided to cosmetically conceal the drivers and hardware, and to protect the driver from physical damage.
In operation, a signal is delivered to the voice coil by means of electrical wires, from the amplifier through speaker cable, then through flexible tinsel wire to the moving coil. The current creates a magnetic field that causes the diaphragm to be alternately attracted to, and repelled by, the fixed magnet as the electrical signal varies. The resulting back-and-forth motion drives the air in front of the diaphragm, resulting in pressure differentials that travel away as sound waves.
The spider and surround act as a tensioned spring centering mechanism, to both concentrically center the voice coil within the magnet assembly, and to center the front-to-back voice coil position within the magnet assembly when power is removed.
The voice coil and magnet essentially form a linear motor working against the centering spring tension of the spider and surround. If there were no restriction on travel distance imposed by the spider and surround, the voice coil could be ejected from the magnet assembly at high power levels, or travel inward deep enough to collide with the back of the magnet assembly. The majority of speaker drivers only work against the centering tension of the spider and surround, and do not actively monitor the position of the driver element or attempt to precisely position it, as occurs in servomechanisms.
Speaker drivers may be designed to operate within a broad or narrow frequency range. Small diaphragms are not well suited to moving the large volume of air that is required for satisfying low frequency response. Conversely, large drivers may have heavy voice coils and cones that limit their ability to move at very high frequencies. Drivers pressed beyond their design limits may have high distortion. In a multi-way loudspeaker system, specialized drivers are provided to produce specific frequency ranges, and the incoming signal is split by a crossover. Drivers can be sub-categorized into several types: full-range, tweeters, super tweeters, mid-range drivers, woofers, and subwoofers.
Speaker drivers are the primary means for sound reproduction. They are used among other places in audio applications such as loudspeakers, headphones, telephones, megaphones, instrument amplifiers, television and monitor speakers, public address systems, portable radios, toys, and in many electronics devices that are designed to emit sound.