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Intensity. 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.

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Presentation on theme: "Intensity. 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."— Presentation transcript:

1 Intensity

2 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 xx magnitudes from harmonic waves

3 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 xx

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

5 Intensity Ratio  SoundIntensityRatio over threshold  Threshold W/m  Whisper W/m  Mosquito10 -8 W/m  Talking10 -6 W/m  Traffic10 -4 W/m  Subway10 -2 W/m  Rock band10 0 W/m  Jet engine10 1 W/m

6 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

7 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

8 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 = next


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