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Sound waves
Sound is a wave of pressure peaks and valleys
Waves carry energy; intensity = power per unit area
$$
I = \frac{P_{\mbox{\small point source}}}{4 \pi r^2} \quad \mbox{, for a spherical wave}
$$
Decibel scale
$$
\beta = 10 \log \left( \frac{I}{I_0} \right) \quad \mbox{, in dB}
$$
Human hearing threshhold $I_0=10^{-12} \mbox{ Wm}^{-2} = 0 \mbox{ dB}$
Doppler effect ($$v = $$ speed of sound, $$f_s = $$ source frequency)
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source moving with $$v_s$$
$$
f = f_s \quad \left( {1 \mp \frac{v_s}{v}} \right)^{-1}
$$
-
observer moving with $$v_o$$
$$
f = f_s \quad \left( {1 \pm \frac{v_o}{v}} \right)
$$
-
both source and observer moving
$$
f = f_s \quad \frac{1 \pm \frac{v_o}{v}}{1 \mp \frac{v_s}{v}} \qquad
\left\{ \begin{array}{l} \mbox{ upper sign - toward} \\ \mbox{ lower sign - away from} \end{array} \right.
$$
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