About Tactile Sound

"It's dark outside and the rain is pounding down hard. As condensation beads up on your landrover's windows, you sit nervously waiting for the park lights to go back on. Then, Thump...Thump. The ground beneath the landrover shakes and your head twists reflexively to the side window. An enormous reptilian eye appears in the mist and it is staring directly at you."

Remember this scene from Jurassic Park? If you found it thrilling at your local movie theater, hold onto your seats. You can experience twice the fright in your own home theater because the approach of the giant tyrannosaurus can actually make the floor vibrate and the furniture shake. All it takes is the addition of tactile transducers to your home theater system. Tactile transducers are large, electromechanical drivers designed to couple low-frequency audio energy to large physical surfaces, such as couches, seats or floors. Not only do they allow you to feel large Mesozoic-era creatures stomping about, you can also feel F-16 fighters blasting off flight decks, buildings exploding into flaming pieces and room shaking earthquakes. Whatever the event, tactile sound adds the missing component to the full sonic experience.

The Nature Of Sound. Perceptual physiologists break the perception of sound into two categories; auditory sound and tactile sound. These two types of sound are sensed by the human body in different ways.

Auditory sound is sensed by the movement of an ear drum. The actual mechanism starts with the movement of air by a sound source, let's say the beat of a bass drum. When the bass drum head vibrates, it pushes into nearby air molecules, which push other air molecules, and the resultant sound wave travels to our ear drum. When our ear drum moves, the "acoustic" energy is transferred to the Ossicles of the inner ear. The Ossicles, in turn, pushes on the Cochlea and it is in the Cochlea where the miraculous conversion of physical movement into nerve impulses occurs. The result sound!

The other mechanism of sound does not utilize the inner ear directly. It relies on the tactile (felt) movement of air molecules and other objects against our body. This is the other way that we perceive acoustic energy and it is part of the natural mechanism by which we experience louder sounds.Most people think of "sound" as the psychological perception produced by the brain as a result of air-transmitted acoustic energy reaching the ears. This perception is reinforced by the audio/video industry, because loudspeakers, the industry's standard sound reproduction device, do just that; produce air-transmitted acoustic energy.

As an example, let's wander down to Cape Canaveral and watch a shuttle launch. When the shuttle's engines ignite, there's a tremendous blast at the launch site. The enormous energy released strikes the earth and is transferred via the ground to distant observers (Path B). The subjects feel the rumble almost immediately because sound waves travel quickly through solid materials, like the earth. After several seconds, the air-transmitted acoustic energy of the event reaches our observer's ears (Path A). The total spine-tingling sensation of a shuttle launch, thus, consists of tactile and auditory senses both reinforcing each other.

Doesn't a good subwoofer deliver tactile sound? No. Subwoofers are designed to move large volumes of air to reproduce deep bass notes. This action occurs because an electrical signal drives the voice coil of the speaker and the attached speaker cone moves air molecules. Any tactile stimulus will reach our body only if the subwoofer is played loud enough so that its acoustic energy can actually move floor surfaces and chairs. Tactile transducers, in contrast, directly stimulate physical surfaces so the perceived sensation is more pleasant and natural.

Are there benefits to using tactile transducers -besides thumping? Yes, consider this: If you put tactile transducers in strategic locations around your home theater room, you can directly couple acoustic energy to room surfaces and to anyone who is contact with those surfaces. The direct benefit is this: you do not need to play the audio system at room shaking levels just to get the tactile feeling of the sound tracks.

Get Set To Rumble. Installing tactile transducers in your home theater or listening room is easy. The smaller units are often mounted directly to the wooden frames of couches and seats. Physically mounting these devices to seating surfaces isn't difficult but it may require some creativity. It may be easiest to bolt a transducer to a piece of plywood, then mount the entire assembly underneath the seating framework via drywall screws. For best effect, make a direct connection to the seating surface frame. For real earth shaking, the more powerful units, like the Clark TST 329-F or Aura Bass Shaker Pro, can be mounted directly to the floor joists underneath the listening room. The diagram shows a typical method.

Small transducers can be powered with a modest amount of power, say 25-50 watts. But driving a large inertial mass, like an entire floor surface, begs for beefy amplifiers-about 100 watts per transducer is a good starting point for a properly designed system. Actually connecting a tactile transducer to an audio system is also easy. It's like connecting an external amplifier and additional speakers. The key is to get the left and right audio signals, often from "tape monitor out" jacks, to the transducer amplifier and then the amplified signal is routed to the transducers through heavy gauge (12-16 AWG) speaker wire. Note: for best results, the tactile amplifiers need a source of full range (not subwoofer, rolled off) signal.

Transducers on the Market. The best tactile transducers are made by Clark Synthesis. This company was intimately involved in the pioneering reasearch in the tactile device market and today remain one of the leaders.