sound is…. (for our purposes) movement of the air that happens at human hearing frequencies: 20 -...
TRANSCRIPT
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.
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/FrequencyOrWavelength = 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.
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.
“Real World Sound”
• Many frequencies at one time• Many reflective surfaces• Much greater complexity; less predictability