SOUND IS…. (for our purposes) Movement of the air that happens at human hearing frequencies: 20 - 20,000 cycles per second.

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SOUND IS…

(for our purposes) Movement of the air that happens at human hearing frequencies: 20 - 20,000 cycles per second

The easiest case to imagine is a “monopole” sound source. Imagine a ball suspended in the air that expands and contracts, pushing and pulling on the air around it.

Here, the compressed air is represented in blue, and the rarified air is represented in pink.

One “cycle” would consist of one expansion and one contraction. The expansion would form a region of compressed air. The contraction would form a region of rarified air.

The compressed/rarified regions form a “wave” that moves away from the source at the speed of sound.

The compressed air portion of the wave pushes on our eardrums, then the rarified portion pulls on the eardrum. This is how humans experience sound.

The distance from the beginning of the compression to the end of the rarifaction is the “wavelength”. The amount of time it takes to go from compression, through rarifaction is the “frequency”.

Since sound travels only just so fast through the air, if we increase the frequency, we decrease the wavelength.

If we double the frequency, we halve the wavelength.

The formula is: Wavelength= Speed of Sound/Frequency Or Wavelength = 1128/Frequency

The speed of sound at sea level at 68 degrees Fahrenheit is 1128 feet per second. If we know the frequency is, say, 440 cycles per second, then we can calculate the wavelength at 2.56 feet. (Wavelength = 1128/Frequency)

If this wavelength is 2.56 feet, then its frequency must be 440 Cycles per second

If this wavelength is 1/2 of the one below, then it’s frequency must be 880 Cycles per second.

440 HZ 2.56’ Wavelength 880 HZ 1.28’ Wavelength Hertz or HZ is the same as Cycles per second

Knowing the wavelength is important for being able to predict the behavior of a sound.

When a sound wave encounters an object that is smaller than its wavelength, it flows around the object, unchanged.

The sound “refracts” or bends around the object. If you stood behind the object, you would hear the sound.

When a sound wave encounters an object that is larger than its wavelength, it will be absorbed, or reflected by that object, causing a “shadow” behind it.

If you stood behind this object you would not hear the sound.

The sound will tend to be reflected if the material it encounters is rigid.

The sound will tend to be absorbed if the material of the object is less rigid.

“Make Believe Sound” One frequency at a time No reflective surfaces Simple and predictable

“Real World Sound” Many frequencies at one time Many reflective surfaces Much greater complexity; less predictability

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