Tactile sound is the sensation of sound transmitted directly to the human body by contact, rather than by sound waves through the ears. For example, when you stand on a train platform you can feel the train approaching as well as hearing it. Explosions, crashes, sonic booms, and thunder are all normally felt in addition to being heard. Tactile sound is also present in the vibratory signature of musical instruments, your automobile, in aircraft, someone walking across the floor or slamming a door, even the vibration from the compressor in your refrigerator. When we talk we experience our voice in a tactile format, if we are in intimate contact with a talking individual, we can feel their voice. Besides air pressure, tactile sound can be conducted through ground motion. Tactile sound can also be transmitted through water, for example in a swimming pool or hot tub.
In addition to being produced naturally, tactile sound can be produced by a transducer in the same way that sound can be produced through a loudspeaker. Tactile sound has been used by the military for flight and tank simulators, for rides in amusement parks, medical research, musical tactile massage, home cinema, computer games, car audio, dance floors, water beds, patio decks and for musical performance as tactile feedback for drummers and other musicians. It has even been used recently to promote weight loss,improve muscle tone and improve blood circulation by a resistance training company.
Various designs for tactile transducers have been presented since the 1960s, most of which fall under the "shaker" category. Shakers create a vigorous vibration by moving a mass (usually a magnet) which is bolted to a final mass (like a chair or couch). A simple example of this is the vibration available on a common cellphone. Another way of producing tactile sound uses "linear actuators", which move furniture (usually up and down), rather than shaking it. The main advantage of linear actuators is that they deliver actual motion (ground excursion), not just vibration.
Recently, tactile sound transducers have evolved to include higher frequencies and produce higher fidelity. The human tactile frequency range is from 1 Hz, very low frequency such as earthquakes, up to 5 kHz in some hearing impaired individuals. For most individuals 2 to 3 kHz is the upper threshold for tactile reception.
These newer devices must have higher resolution than previous "shakers" to produce these frequencies. Most humans have tactile resolution to 2 Hz which is the smallest change in frequency that can be perceived. The primary use for this extended bandwidth is to reproduce the vibratory signature for musical instruments such as violins, guitars, the human voice or sound effects in movies (for example, the Speeders in Star Wars). Also higher frequencies may be used to augment hearing through bone conduction, a consideration for people who have compromised their hearing from exposure to loud music.
Tactile sound is often used to increase the realism of an artificial environment. For example, mounting a tactile sound transducer in a chair or couch in a home cinema setup can give more of a sense of "being there". For such use, the transducer is often connected to the LFE channel of an A/V receiver. Tactile sound is often used in combination with a subwoofer so that low frequencies can be both felt and heard.
To facilitate broadband tactile sound, all channels are summed to provide a full range signal to the transducer amplifier. Graphic equalizers can also be used to further modify the effect.
For musical performance, drummers will often use a tactile sound transducer mounted on their drum stool so they can "feel" themselves playing, rather than using a more conventional stage monitor. The size and power of a stage monitor required to adequately reproduce low frequency drum sounds would be expensive and hard to transport, while a tactile sound transducer can be rather small and require much less power to get the job done.