One of the main investigative areas at Reactual Labs is tactile sound. So what is tactile sound exactly? It is the sensation of sound transmitted directly to the human body by contact, rather than by sound waves through the ears (tympanic sensation).

More specifically, it is sound perceived through bone conduction, through muscles and deep tissue, and via skin sensation. We perceive the vibrations through our haptic senses which include both kinesthetic (muscle) and touch (cutaneous) senses. We feel both inaudible (infrasonic frequencies) and audible frequencies. Thus, tactile sound is not just brute rumbles and shock waves. Tactile sound can be perceived in high fidelity up to the mid-range frequencies. According to NASA research we perceive 0 to 1 kHz through the sense of touch, 0 to 3 kHz through kinesthesia (the sense of the relative position of neighboring parts of the body), and 0.20 kHz to 20 kHz through our ears. Note there’s an overlap — we can hear the range of 0.20 kHz to 3 kHz both with our ears and through other parts of our body. Also, some hearing-impaired individual can hear sounds up to 5 kHz through their skin.

Tactile sound is a sensation that many seek out. People often buy expensive subwoofers to achieve deep bass effects. In this pursuit, subwoofers are often pushed past recommend levels, resulting in damaged equipment, muddied sound fields, and disturbed neighbors. Realistically, subwoofers not produce tactile sound very well. They are designed to transmit low frequency sound through the air. Note, however, that tactile transducers don’t usually produce bass below 25 – 30Hz. Correctly installed and enclosed subwoofers can producer lower and louder deep bass, but they will not produce the same level of tactile vibrations.

So what kind of device produces tactile sound? Tactile transducers, or electro-mechanical devices that are built specifically to vibrate objects. They are very similar to a loudspeaker woofer driver minus the cone.

There are two basic flavors of transducer: inertial shakers and linear actuators. Let’s take a look at the differences between the two:

Inertial Shakers

Inertial Shakers (Image via HomeToys.Com)

Inertial Shakers operate by shaking a mass in an enclosed housing. The resulting vibrations are transferred to an attached object, like the wooden crossbeam of a couch, for example. Usually, shakers are bolted directed to an object.

Companies that make Inertial Shakers include: Clark Synthesis, Aura Sound, Earthquake Audio, Buttkicker and Sonic Immersion Technologies. These devices vary widely in price — some produce a simple shaking effect, some produce high-fidelity sounds that compliment to a stereo system or home theater (such as those offered by Clark Synthesis, my personal favorites).

A good tactile transducer to start with is $150, entry-level Clark Synthesis Silver Edition (TST239) — this will be more than enough bass for our home system. It will produce solid definition to drum hits, or shake your fillings loose if you so desire. The transducer can be installed in less than an hour. It took me about 45 minutes to bolt it to the bottom of my couch, using the parts provided with the transducer. In my test installation, I removed the fabric from the button of a couch, and bolted the transducer to a crossbeam underneath. Once installed, the entire couch functions as a bass speaker.

But wait, we need to power the tactile transducer! Clark Synthesis sells a $350 customized tactile amplifier to do this, but you can actually use any amplifier that outputs at least 100 watts (check the Clark Synthesis documentation for more info). Well, Cambridge Audio makes good amplifiers, but their entry-level models start at $330. We can instead use an AudioSource amplifier. This is a 2-channel 100-watt (50 watts x 2) power amplifier, which can be purchased for under $100. This amplifier is well-reviewed —  111 reviewers on Amazon gave it an average of 4 out of 5 stars. You attach your amplifier to the tactile transducer using standard speaker cables. You then connect the amplifier to the Sub Out connector on your receiver.

Linear Actuators

Linear Actuators. (Image via HomeToys.Com)

Linear Actuators are transducers that push against a hard surface. They apply a force directly to an object, in contrast to Inertial Shakers which vibrate themselves, and then transmit that vibration into another object. Commercially available Linear Actuators usually replace the feet of a chair. They operate by pushing against the floor, and directly lifting and dropping a chair. They tend to be more expensive that Inertial Shakers, and they can be only installed under chairs — they can’t be installed under a floor or in a swimming pool like the shakers above.

Companies that offer Linear Actuator include: Crowson Technology and d-Box Technologies.

See also: The Tactile Transducer FAQ