**2 The Physics of Sound Waves**

**Units**

Pressure:

Pascal (Pa) |
= |
1 N/m |

Bar: 1 bar |
= |
10 |

Torr: 1 torr |
= |
133.22 Pa |

Sound wave pressures fall in the range between 0.00002 Pa or 20 μPa), corresponding approximately to the average ear’s threshold at around 3–4 kHz, to about 200 Pa, generally reckoned to be about the pain level.

**Intensity**

Watts/m^{2} (W/m^{2}), or, more practical in sound, μW/m^{2}.

**Velocity of Sound, Typical Values**

See table opposite.

A general expression for the velocity of sound in gases is given by:

*c* = (*γP/ρ*)^{−2}

where γ is the ratio of the specific heats of the gas (1.414 for air), *P* is the pressure and ρ is the density of the gas.

**Frequency ( f) and wavelength (λ)**

These are related by the expression ...

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