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Published byHerbert Gilbert Modified over 2 years ago

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Intensity

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Force to Pressure The pressure wave creates a force on a slice of air. Wave amplitude s 0 Pressure amplitude p 0 The amplitudes are related by frequency and wavenumber. P + P P A xx magnitudes from harmonic waves

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Force and Intensity Intensity is the power per unit area. Pressure is force per area. Expect I = Pv Use v = s 0 Factor of ½ for average P + P P A xx

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Threshold of Hearing The minimum intensity that can be heard by humans is 10 -12 W/m 2. What is the fractional variation in air pressure? Translate intensity into pressure. Substitute values. p 0 = 10 -5 Pa Compared to P = 10 5 Pa this is one part in 10 billion.

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Intensity Ratio SoundIntensityRatio over threshold Threshold10 -12 W/m 2 10 0 Whisper10 -10 W/m 2 10 2 Mosquito10 -8 W/m 2 10 4 Talking10 -6 W/m 2 10 6 Traffic10 -4 W/m 2 10 8 Subway10 -2 W/m 2 10 10 Rock band10 0 W/m 2 10 12 Jet engine10 1 W/m 2 10 13

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Logarithmic Scale Sound intensity varies over a huge range. 13 orders of magnitude13 orders of magnitude The exponents are easier to keep track of than the values. Logarithm of the ratioLogarithm of the ratio SoundRatio exponent Threshold0 Whisper2 Mosquito4 Talking6 Traffic8 Subway10 Rock band12 Jet engine13

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Decibel The decibel is a unit that measures the intensity ratio. Symbol dBSymbol dB Exponent times 10Exponent times 10 Each 10 dB is 10 times more intensity. SounddB Threshold0 Whisper20 Mosquito40 Talking60 Traffic80 Subway100 Rock band120 Jet engine130

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Turn it Down Your younger sibling is watching TV, and its annoying you at 75 dB. You yell and she turns it down to 60 dB. By what factor has the intensity decreased? The difference in logarithms is the ratio of the values Find the difference in dB Solve for the intensity With values I 2 / I 1 = 0.032 next

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