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INFRASOUND - A PRIMER
Sound is, quite simply, a vibration that the human ear can detect. One note will sound higher than another if it vibrates the air at a faster rate (in other words, at a higher frequency). We’re used to talking about the visible light spectrum - the range of colours that the human eye can see. Acousticians also think of sound in spectral terms. As sound rises in pitch, from bass to treble, it moves across the audible spectrum. Just as there is infrared and ultraviolet at the cusps of the visible spectrum, there is infrasound and ultrasound at the fringes of the spectrum of audible sound.
Infrasound lies at the extreme bass end of our hearing range. It’s usually defined as a vibration that occurs fewer than 20 times a second. Humans (unlike some other animals) don’t communicate with infrasound and are not very good at detecting it. But infrasound isn’t always inaudible. To understand why, it’s worth knowing more about human sensitivity to sound.
Physicists measure frequency in units called hertz (Hz) and call a thousand hertz one ‘kilohertz’ (kHz). Most physics textbooks say we can hear airborne vibrations that occur between 20 and 20,000 times a second (20 - 20kHz). But in truth, this is a gross simplification. Hearing varies from person to person, with countless factors influencing the range of frequencies that any one of us can detect. Your age and genetic makeup play a part — so do many other variables, such as the time you’ve punished your ears in foundries or heavy metal concerts and the amount of wax in your ears.
Rather than cutting off sharply at 20Hz and 20kHz, our hearing ability fades gradually as we approach these frequency limits. A piano’s bottom note C, for example, vibrates at roughly 33Hz, a frequency near the edge of our hearing range. Top C on the piano vibrates at around 4190Hz, a mid-range frequency where human hearing is extremely acute. To seem as loud as top C, bottom C needs to make a sound that is roughly a thousand times more powerful (in acoustic terms, 30dB louder). In general, extreme bass and treble sounds need more power than mid-range sounds, in order to cross the ‘threshold of hearing’ – the minimum loudness that can be heard. With enough volume, even sounds that lie outside the often quoted ‘20 to 20k’ frequency range can be heard. This is true of infrasound.
Infrasound clearly lies on the cusp of our perception, rather than outside it. But our experience of infrasound is still a mysterious issue. When we sense these vibrations, what do we actually hear? Researchers at University of Salford asked this when they tested our ability to hear low frequencies in 1967. Subjects described the sensation of infrasound as ‘rough’, a ‘popping effect’. Infrasound below 5Hz was described as a ‘chugging or ‘whooshing’, a sensation they could ‘feel’. (Yeowart, Bryan and Tempest, 1967) The chance to hear infrasound in a large auditorium seems very enticing. But the hypothesis that infrasound can affect people’s mood intrigues us even more. The existence of infrasound, in sacred music and reputedly haunted sites, makes an exploration of infrasound and mood all the more fascinating.
THE INFRASONIC ZOO
Far from being an exotic phenomenon, infrasound is with us all the time. We continually bathe in a sea of barely perceptible, ambient infrasonic noise. Sometimes described as the ‘infrasonic zoo’, most of this is generated by natural processes and events: thunderstorms, earth tremors, ocean waves, volcano eruptions and curious phenomena such as meteor impacts, aurora and ‘sprites’ (sudden electrical discharges in the upper atmosphere).
Human activity also contributes to background infrasound. Deep below the rumble of city traffic, there is a cacophony of very-low-frequency noise from factories, lorry engines, fireworks, passing aircraft, distant quarrying and many other human sources. In 1957, the French physicist Vladimir Gavreau highlighted this overlooked noise pollution, citing it as a possible cause of city dwellers’ stress. (Gavreau, Condat and Saul, 1966)