When you hear a sound, your brain actually gets a signal from the nerve that is within your ear. This nerve fires an electric spark to the brain because the fluid within the cochlea is moving back and forth and sets some little thingies into motion which are connected to the nerve. The fluid in the cochlea is set into motion by a connected little bone called the stapes, which is connected via other small bones (the incus and malleus) to the ear drum. And it is the ear drum which starts to vibrate due to small pressure variations that entered the ear canal and it is these pressure variations, also known as sound ;-), that we will take a closer look upon.
If you would record a sound by a microphone, say for 1 second, and make a graph of the pressure changing over time, you would see the following graph:
and if you would zoom in to the first 0.1 seconds, you would see the following:
and if you would play the sound, you would hear the first second of Neil Armstrong’s famous phrase about a small step for man and a giant leap for mankind.
One thing the picture clearly shows is that the pressure which varies very quickly with time!
Sound pressure level
You may have heard about decibels or dB’s. The decibel is actually the unit for how loud a sound is.
\(SPL = 20 log_{10} \frac{p_{rms}}{p_{ref}}\)
From impedance to sound absorption coefficient
\(\alpha = \frac{2 (\zeta + \bar{\zeta})}{(\zeta + 1) (\bar{\zeta} + 1)}\)
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- A list