According to Harry F. Olson in his book Maths, Physics and Engineering:

20 Hz is considered the normal low frequency limit of human hearing.

The lower the frequency of sound, the more difficult it us for human to hear it. [...]]]>

Sound Inspired Artwork by Andy Gilmore

Infrasound is the realm of sounds in the frequency range of 20 Hz down to 0.001 Hz.

According to Harry F. Olson in his book Maths, Physics and Engineering:

20 Hz is considered the normal low frequency limit of human hearing.

The lower the frequency of sound, the more difficult it us for human to hear it. So, in order for us to hear a sound under 20 Hz, it must be powerful.

We mostly hear infrasound through our ears, but at higher levels it is possible to feel infrasound vibrations in various parts of the body (see our post on tactile sound).

Examples of natural events that produce ultrasound include: lightning, earthquakes, volcanoes, bolides (exceptionally bright fireballs), aurorae and surf.

Human can produce ultrasound too, examples include: sonic booms, mechanical sounds from engines, subwoofers and transducers.

Perceiving Sound Under 20 Hz

We all perceive infrasound differently — an infrasound wave might be perceived as loud to one individual, but another might not perceive it at all. Wikiepedia says:

When pure sine waves are reproduced under ideal conditions and at very high volume, a human listener will be able to identify tones as low as 12 Hz.

Below 10 Hz it is possible to perceive the single cycles of the sound, along with a sensation of pressure at the eardrums.

Better quality test tones can be found on this page. You can also find tone generators available.

Paranormal Investigations

Infrasound can also cause feelings of awe or fear in humans, because it is not consciously perceived. It can be described as “shivers down your spine” or other weird sensations. An experiment in England was set up to study the effect of infrasound, Participants reported feeling uneasy and fearful when infrasound was introduced. After conducting the experiment, Professor Richard Wiseman said:

These results suggest that low frequency sound can cause people to have unusual experiences even though they cannot consciously detect infrasound. Some scientists have suggested that this level of sound may be present at some allegedly haunted sites and so cause people to have odd sensations that they attribute to a ghost—our findings support these ideas.

Some film soundtracks make use of infrasound to produce unease or disorientation in the audience. Irréversible is one such movie, as is Paranormal Activity.

Here is another infrasound investigation story from Wikipedia:

Research by Vic Tandy, a lecturer at Coventry University, suggested that an infrasonic signal of 19 Hz might be responsible for some ghost sightings. Tandy was working late one night alone in a supposedly haunted laboratory at Warwick, when he felt very anxious and could detect a grey blob out of the corner of his eye. When Tandy turned to face the grey blob, there was nothing.

The following day, Tandy was working on his fencing foil, with the handle held in a vise. Although there was nothing touching it, the blade started to vibrate wildly.

Further investigation led Tandy to discover that the extractor fan in the lab was emitting a frequency of 18.98 Hz, very close to the resonant frequency of the eye given as 18 Hz by NASA (see this PDF file). This was why Tandy had seen a ghostly figure—it was an optical illusion caused by his eyeballs resonating. The room was exactly half a wavelength in length, and the desk was in the center, thus causing a standing wave which was detected by the foil.

Tandy investigated this phenomenon further and wrote a paper entitled The Ghost in the Machine (PDF file here). Tandy carried out a number of investigations at various sites believed to be haunted, including the basement of the Tourist Information Bureau next to Coventry Cathedral and Edinburgh Castle.

So, to sum up:

Most infrasounds are caused by powerful forces.They cause things to shake, without visual cause. They are often associated with the uncanny, because they inspire fear and dread. We often only perceive them subconsciously, and it is usually in the pits of stomachs, or in our bones.

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 [...]]]>

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