The Nature of Sound
What is Sound?
Sound is a pressure wave, caused by the vibration of sound. Here’s how it works: sound disturbs air molecules in it’s path. Those air molecules then pass the disturbance to adjacent air molecules. In this way the original disturbance (sound) travels through the air as a sound wave. The air molecules don’t move downstream with the wave; they simply oscillate back and forth about their individual equilibrium positions.
The following figure illustrates the transmission of a sound wave and the pressure variation (bottom half of figure) caused by the the compression and spreading (rarefaction) of air molecules (top half of figure) as the wave progresses from a rock concert to an eagerly awaiting ear.
In the figure:
- C indicates compression, where air molecules accumulate
- R represents rarefaction
- HP shows areas of higher pressure caused by compression of air molecules
- LP indicates areas of lower pressure caused by rarefaction, as downstream air molecules spread ahead of the approaching sound waves
The figure also shows the oscillating (harmonic) motion of the air molecules (white spheres) as the wave progresses toward the ear. The wave is represented by the motion of the compressed region of the pressure waves. Sound pressure is the amount of air pressure variation caused by the sound source, and we perceive this pressure as loudness. Greater compression is associated with more intense…and consequently louder sounds. If the sound is loud enough, and close enough, it can cause irreversible damage…regardless of how long the sound lasts.
Sound pressure is typically expressed in pascals (Pa) and perceptible sounds can range from .00002 Pa to 20 PA. That means there are 100,000 places between zero and one on a PA scale…and 2 million places would be required to represent the audible sound scale! Since audible sounds range over so wide a scale, it’s more productive to measure sound in decibels (dB). On a decibel scale, an increase of 10 dB corresponds to a 10 fold increase in sound intensity, and you will perceive this sound as twice as loud.
To put this in perspective, the sound of an MP3 player set at a maximum volume of 115 dB is approximately 1000 times more intense than a vacuum cleaner producing a sound volume of 85 dB…and a loud rock concert can produce sound levels (120 dB) that are 1 trillion times more intense than the softest audible sound (~0 dB)!
Side note: 1 trillion is 1 million multiplied by 1 million…or 1 followed by 12 zeros. It would take approximately 31,710 years to count to 1 trillion at 1 second per number…so don’t start this project unless you have nothing to do for the next 317 centuries!
The following videos demonstrate the nature of sound, the acoustic landscape, and how we know where sound is coming from.
This 1.5 minute video from MED-EL (www.medel.com) will help the adventurous understand the nature of sound waves. Try it…we all need a little adventure..
This video from MED-EL (www.medel.com) demonstrates the frequency components (acoustic landscape) of everyday sounds…like thunderstorms and flutes.
From MED-EL (www.medel.com), this video reveals how we determine where sounds are coming from, a process that requires two ears.
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