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Sound Part 2

Sound Part 2. Pressure Waves. a.k.a . longitudinal waves Pressure is easier to measure than displacement Compression- pressure is higher since molecules are closer together Rarefaction- pressure is less than normal since molecules are farther apart than normal

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Sound Part 2

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  1. Sound Part 2

  2. Pressure Waves • a.k.a. longitudinal waves • Pressure is easier to measure than displacement • Compression- pressure is higher since molecules are closer together • Rarefaction- pressure is less than normal since molecules are farther apart than normal • note that the displacement wave is 1/4 λout of phase with the pressure wave

  3. Pressure Waves When pressure is a max. or min., displacement from equilibrium position is zero When pressure variation is zero, displacement is at a max. or min.

  4. Intensity of Sound: Decibels • Loudness- relates to intensity or energy transported per unit time acrossunit area. • I ∝ A2 and E/t = Power • Therefore I is measured in W/m2 • Human ear- 10-12W/m2 to 1 W/m2 • More than the one W/m2 and you feel pain!

  5. Intensity of Sound: Decibels • Producing a sound twice as loud requires a sound wave with 10x the intensity • 10-2W/m2 sounds twice as loud as 10-3 W/m2 and four times as loud as 10-4 W/m2 • Alogarithmic scale is needed to compare loudness easily

  6. Intensity of Sound: Decibels • Bel - the unit of the logarithmic scale used to compare loudness • Named after Alexander Graham Bell • Decibel - 1/10th of a Bel(10 dB = 1 Bel) • Intensity:

  7. Intensity of Sound: Decibels • Io= 1x 10-12W/m2, the threshold for human hearing • So, for a sound with an intensity = 1 x 10-10W/m2

  8. Intensity of Sound: Decibels • An increase in intensity by a factor of • 10 means a level increase of 10 dB • 100 means a level increase of 20 dB • A50 dB sound is 100 times more intense than a 30 dB sound

  9. Intensity of Sound: Decibels

  10. Intensity of Sound: Decibels • A sound level difference of 3 dB (twice the intensity) corresponds to a very small change in the subjective sensation of apparent loudness. • Loudness decreases as you get farther from the source • I ∝ (1/r2)

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