343[m/s] (1230 km/h)

The strength of a sounds are measured on a logarithmic scale. We usually measure sound intensity I_1[W/m$^2$] relative to a reference level $I_0$ which is usually taken to be $I_0 = 10^{-12}$[W/m$^2$].

The decibel is ten times the logarithm base ten of the ratio of two sound intensities: \[ L_\mathrm{I} = 10\, \log_{10}\left(\frac{I_1}{I_0}\right)\ \qquad [\mathrm{dB}]. \] The decibel is a dimensionless quantity.

\[ f' = f \, \frac{v \pm v_0}{v \mp v_s} \]

$f'$ is the observed frequency, $f$ is the actual frequency, $v$ is the speed of sound ($v=336+0.6T </math>), $v_0$ is the speed of the observer, and $v_s$ is the speed of the source.

If the observer is approaching the source, use the top operator (the +) in the numerator, and if the source is approaching the observer, use the top operator (the -) in the denominator. If the observer is moving away from the source, use the bottom operator (the -) in the numerator, and if the source is moving away from the observer, use the bottom operator (the +) in the denominator.

An ambulance sounding a 400 Hz siren is moving at a speed of 30 m/s towards you. If the speed of sound is 339 m/s, what is the frequency you will hear: \[ f' = 400\,\mathrm{Hz} \left( \frac{339 + 0}{339 - 30} \right) \]

[ sound wave caugth on video with no sound ] http://www.bbc.co.uk/news/science-environment-13574197