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 | Sound wave frequencies, periods and wavelengths and the range of human hearing
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| Interference of sound waves and the speed of sound
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 | Waves exhibit the following properties
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| Propagation - move from a source to a listener at a particular speed, wavelength, frequency and period
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| May be transverse (medium oscillates sideways to direction of propagation) or longitudinal (rarefactions and compressions - density changes - in the direction of propagation)
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| Refraction - change speed (and possibly direction) when moving from one medium to another
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| Reflection - at least partially bounce off of interface when moving from one medium to another
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| Diffraction - bending around corners and spreading out when propagating through small openings
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| Interference - adding constructively and/or destructively when added to another wave
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| Humans have a huge range of hearing, from the faintest sound at 10-12 W/m2 to the threshold of pain at 1 W/m2.
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| Sound levels are measured on a logarithmic scale and the unit of sound level is measured in tenths of a Bel (named after Alexander Graham Bell) or deciBels (dB).
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| Example of changing intensity to dB:
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| A sound intensity of normal conversation at a distance of 0.5 m is about 3 x 10-6 W/m2. This corresponds to a sound level of 65 dB when compared to the faintest sound we can hear.
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| 10*log(3x10-6/(1x10-12)) = 65 dB
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| Example of changing dB to intensity:
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| Quiet radio has a sound level of about 40 dB. This corresponds to a sound intensity of 1 x 10-8 W/m2.
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| Example of finding the sound level of the sum of two intensities:
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| If the sound level of one person speaking is 65 dB, what is the sound level of two people speaking at the same time?
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| A sound level of 65 dB corresponds to an intensity of 3 x 10-6 W/m2.
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| So, two people speaking at the same time corresponds to an intensity of 6 x 10-6 W/m2 and a sound level of
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| If there is relative motion between a source of sound and a listener, the apparent frequency heard differs from the actual frequency.
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| In the figure below the red dot represents a sound source moving to the right on the x-axis. The circular rings represent sound waves moving out from the source. Because the source moves as it emits sound, the sound waves get bunched up in the direction of motion and spread out in the opposite direction. This leads to the stationary listener (whom the sound source is approaching) hearing a higher frequency sound (because the waves are closer together) and a stationary listener (whom the sound source is leaving) hearing a lower frequency (because the waves are farther apart).
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| Apparent frequency for stationary listener when the sound source is approaching:
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 | Apparent frequency for stationary listener when the sound source is leaving:
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| The Doppler effect also occurs when the sound source is stationary and the listener is moving because the listener's speed adds to the speed of sound when approaching the sound source, and subtracts from the speed of sound when moving away from the sound source. The result is that the listener encounters sound waves more frequently when approaching the sound source and less frequently when moving away from the sound source.
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| If the listener approaches the sound source, the frequency sounds higher:
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| If the listener leaves the sound source, the frequency sounds lower:
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| Read pp. 378-400 of the text.
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