What is infrasound? An ongoing, never ending, quest to explain what we do….
"In the grand scheme of things we're all pretty much
blind and deaf" by Abstruse Goose
This comic from Abstruse Goose is a good place to start.
Take a second to acclimate yourself to the ranges of light and sound that we
cannot perceive. That low end of sound is the area we are going to explore.
Infrasound is technically any “sound” below 20Hz. I don’t
blame you if that makes zero sense. So let’s start with what sound is. Sound is
an oscillation of pressure. Our ears detect these vibrations and our brains
translate these signals into what we hear. Sound is a wave.
There are several different parameters we use to describe
waves. The most important (for reading this blog) is frequency. Frequency can
also be seen as the x axis on the above figure, using the unit Hz. Hertz (Hz) is
cycles per second. The following figure will help illustrate how waves of similar
amplitude look when they differ in frequency.
There are several sine waves in the above diagram with one
thing different about them. Let’s say that these waves are all 1 second long.
That means the X axis is time and the Y is amplitude. We can now talk about my
favorite word: frequency. Let’s start with the red line. Focus on the blue dots
at the trough or lowest point of the wave. Every time the wave returns to that
trough, it goes through one cycle. If we count those troughs we can see that it
goes through its oscillation 4 times. That is 4 cycles per our defined time
scale and since our time scale is one second that is 4 cycles per second. So
the red line is 4Hz. The orange, green, blue, and purple have more cycles in our
defined time scale and therefore they have a higher frequency. Let’s skip down
to the purple line and look at the frequency in Hz again. Let’s look at the
blue dots in the troughs again. There are 15 troughs in this wave, plus about
half a cycle left over, so 15.5Hz.
Now just for fun imagine if our time scale was 2 seconds
instead of 1, then the red line would be 2Hz and the purple line would be 7.75Hz.
Hopefully that made some semblance of sense, because now we
are going to use that to explore sound and infrasound. Frequency and amplitude
are two of the main variables that we use to describe a signal. We are again
going to start with the audible range.
The human ear can, in general, hear from 20Hz to 20,000Hz.
Above 20,000Hz is ultrasound (bats use it and dogs can hear it). Below 20Hz is
infrasound (whales, giraffes, and elephants use it to communicate).
Now, to put some of these numbers in context, let’s move to
the piano. In the middle of the keyboard is the “middle C”, which is blue in
the above figure. This musical note has a frequency of 261.626 Hz. In that
range is also the “A” (in yellow), that you always hear when an orchestra tunes
before a concert. This note is at 440Hz.
The lowest note on the piano is 27.5 Hz and the highest is 4186.01 Hz.
Now for some fun, here are some links so you can hear the different pitches.
First is concert A (440Hz)
In the Baroque period an A was 415 Hz
If we jump down an octave (440/2 = 220) this is what it
sounds like
… and if we go down another octave (110 Hz) this is what it
sounds like
In the sound range, we perceive amplitude as “loudness.” In
Infrasound we use Pascals (Pa) or a measure of pressure to quantify amplitude
of a signal.
Ok, enough of audible sound. That is not what this blog is
about, it is about INFRASOUND!
Infrasound, like I said, is anything below 20Hz (the value
for the base of human hearing). There are a few more variables I would like to
define before we go any farther. The speed of sound in air is one of them. In
general the work in this blog is done on signals that travel through the
atmosphere. Because the temperature and density of the atmosphere affect the
speed of sound, we are going to just use the speed of sound at sea level
throughout this blog. That value is ~340 m/sec or ~1116 ft/sec. You will
generally see us using the metric units.
Now that we have the speed and frequency of a wave, we can
talk about how long it takes to pass a single point, and how long the
wavelength is. Let’s go back to our sine wave shall we?
The
wavelength is a measure of length. In order to calculate the wavelength for a
wave you need the velocity, the frequency, and this equation: 
where 
is velocity, 
is wavelength, and 
is frequency.
where is velocity, is wavelength, and is frequency.
Let’s plug in some numbers! Let’s take our 4Hz signal from
before and 340 (m/s). If we rearrange the above equation to velocity/frequency
= wavelength we get 340(m/s) /4 (/sec) = 85 meters. The length of a sound wave that is 4Hz is 85
m. We have the velocity so we know that each second 4 waves can pass by a
single point.
Alright, now that we have all of that out of the way….
Infrasound is any sound below 20Hz. That makes more sense
now right? Good.
