You may not have heard of infrasound, but that's the point. Well into the sync era, movie sound consisted of dialogue, music, and mixed effects, with comparatively little emphasis on the atmospheric, immersive sound worlds that are ubiquitous in today's films. This, however, was only the result of the limitations of technology, such as the high noise of optical soundtracks and poor theatrical sound reproduction. This changed with the advent of systems such as Dolby that were able to create an environment more apprpopriate for subtle soundscapes; these were part of a general trend towards greater audio fidelity that started in the 60s, led by advances in musical recording. But what about movie sounds that audiences can't hear?
Sound and vision
Before we talk about infrasound, or any sound, it's probably useful to discuss just exactly how the sense works. According to Scientific American, sound can be defined as "pressure waves that oscillate, or alternate, between compressing and stretching the matter they travel through (in our case, air) at a certain frequency, or rate. The higher the frequency at which a sound wave oscillates, the higher the resulting sound’s pitch you hear when that wave hits your eardrums." Human perception of sound is based on the frequency (in the sense of number of repetitions) that can be sensed in one second. The more frequent, the higher the perceived pitch. For humans, audible sound falls between between 20 Hz and 20,000 kHz (with 1 kHz=1,000 Hz.)
Your brain’s perception of pitch relies on how many of these compression changes your eardrum senses per second. The wave that creates the sound of a standard whistle oscillates at a relatively high frequency, so the resulting sound is very high-pitched. The rumbling of a truck engine is produced by low-frequency oscillation, so we hear it as a very low-pitched sound. Because hearing degenerates as we age, some mischievous young people recently developed special "mosquito" ring tones to communicate with each other in class, without their totally ancient teachers catching wise (by the age of 30, most people lose the ability to hear above ~16kHz, and you can check your own relative perception here.)
Even though these waves can't be heard by us, they can be felt and have been shown to produce anxiety, extreme sorrow, and chills.
Frequencies above 20,000 kHz, audible to dogs and other animals with excellent hearing, are known as "ultrasonic," and are analogous to spectrums of ultraviolet light that are similarly invisible to humans.
The sounds in silence
On the other end of the spectrum is what's known as "infrasound," defined as, "extreme bass waves or vibrations, those with a frequency below the audibility range of the human ear (20 Hz to 22 kHz)." Despite the fact that these frequencies are inaudible to human perception in the traditional sense, they can be felt, and studies have demonstrated that infrasound is capable of producing "a range of effects...including anxiety, extreme sorrow, and chills." Scientists have even hypothesized that the infrasonic signals produced by church organ pipes can cause similar feelings in parishioners which they then attribute to metaphysical causes as the natural result of the brain's effort to contextualize environmental input.
Infrasonic frequencies might even be responsible for some ghost sightings: when British researcher Vic Tandy was working in a supposedly haunted library and, indeed, saw a "grey blob" of the corner of his eye, he was unnerved, but went looking for a scientific explanation. He found that a fan in the lab was producing an infrasonic frequency that resonated at almost the exact rate of the human eyeball, given by NASA as 18 Hz; the fan's frequency was measured by Tandy as 18.98, which was close enough to cause the optical illusions he experienced. Curiously, the same effect has been reported elsewhere, also as a result of a bent fan blade. Check out the video above to listen to an hour's worth of the frequency, if you are so inclined, which, hey, you might be (you'll need speakers or headphones with good bass, though, and we take no responsibility for any psychic torment you may experience.)
The sound design of fear
Sound design really came into its own in the '70s, with the emergence and implementation of systems like Dolby, which worked to reduce the hiss inherent to optical tracks, allowing for greater fidelity and, consequently, subtlety in film sound (the first feature film to use the Dolby system was A Clockwork Orange, in 1971.) The term "sound designer" wasn't coined until 1979, when Francis Ford Coppola used it to describe Walter Murch and his work on Apocalypse Now, and this article from Advertising Week traces the development of sound design in its modern form to 1940 and the birth of surround sound. A landmark film was 1956's Forbidden Planet, which featured the first electronic film score, by Louis and Bebe Barron. Another landmark film was Alfred Hitchcock's The Birds, and David Lynch's debut, Eraserhead is justly famous for its use of disturbing ambient sounds, sounds that are almost (but not quite, because they're distinctly audible) infrasound, and which were designed by longtime Lynch collaborator Alan Splet.
"The sound itself could be created by an instrument, but is either processed or performed in such a way as to hide the instrument." -Harry Manfredini
Infrasound has been used in many films, particularly in the horror genre, which has, historically, pioneered the use of novel techniques in scoring and sound. Speaking about non-traditional soundscapes, Harry Manfredini, composer of, among others, the score to Friday the Thirteenth, has been quoted as saying, "...the sound itself could be created by an instrument that one would normally be able to identify, but is either processed, or performed in such a way as to hide the actual instrument." And the legendary Frank Warner, working with Martin Scorsese and Thelma Schoonmaker on Raging Bull, used many sounds, including detuned drum heads and slowed down elephant calls to make the boxing scenes disturbing as possible (interestingly, elephants produce infrasonic frequencies when they communicate with each other.)
Digital technology has opened a world of possibility for sound design, while also making it easier for the low-budget filmmaker to obtain. French filmmaker Gaspar Noé has admitted to using infrasound during the first 30 minutes of his intensely disturbing and controversial Irreversible, and the effect is rumored to occur in some of the Paranormal Activity films (there are definitely audible rumbles in the film, but whether they qualify as infrasound is debatable.) Christian Stella, sound designer of the low-budget zombie film The Battery, wrote on Reddit that the filmmakers used, "...recordings of power transformers, air conditioners, etc." to get an ominous drone on the score, with the reasoning being that they wanted the sounds of objects that would be absent after a zombie apocalypse.
It goes without saying, of course, that the effectiveness of movie sound, infra or otherwise, is dependent on the playback system (and bass is notoriously more difficult to reproduce than treble), but in today's home theater world, more and more viewers have systems capable of excellent fidelity (test your system with this video.)
"You can’t hear it, but it makes you shake. In a good theater with a subwoofer, you may be more scared by the sound than by what’s happening on the screen.” -Gaspar Noé
Researcher David Blumenstein conducted experiments about the effect of "non-linear sound," irregular noises that occur in the distress signals of animals and in the music and sound design of films. After showing audiences neutral footage paired with non-linear soundtracks, he discovered that the innocuous footage took on frightening connotations, disturbing viewers for no reason they could define. Nonlinear noises, used by young animals to grab the attention of their parents, seem to also evoke an emotional response in humans. The scientist told NPR that, "Clearly, people in Hollywood know this, but it's not as though they're going out and using biologically tested algorithms."
Composer Manfredini, speaking to Atlas Obscura, related that part of his job is, "fooling the audience into believing that what we’re seeing is logical to the story we’re immersed in, and that we drew that conclusion first." Like traditional sound design, infrasound works in this way, just more sneakily: the fear felt by an audience is rationalized to fit environment and context, i.e. watching a scary movie, and anticipating fright (in this respect, it's like the pipe organs in the church study referenced above). It's the latest (and quietest) technique that filmmakers have at their disposal in order to take the audience on an emotional journey.. Of course, not everyone responds to true infrasound the same way, but enough people do, and with digital sound manipulation growing more sophisticated every day, it's almost certain to show up in more and more films as time goes by.
But you didn't hear it from me.