1.1 BASIC PROPERTIES OF SOUND

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À  p

Pressure waves

an approximation for sound propagation is a sphere as a wavefront.

an ideal sound wave travels as a uniform wave in a radial direction (thus having uniform distribution over the sphere). If we watch the pressure at point `P' as a single sound wave (soliton) travels by, it will look like,

The wave equation for sound in air is given below,

If a sound is a pure tone, it will look like,

For air the speed of sound can be approximated with,

Velocities in other materials (at 21.1°C) are, [Source Irwin and Graf]

Sound powers cover such a wide range that it makes sense for the values to be expressed in a logarithmic scale, a few values are given for reference. Note: this value measures the sound source itself, not what the listener hears. [source Irwin and Graf]

The sound power level is useful when considering the source, but as we move farther away from the source the sound pressure our ears can detect will drops off. Another logarithmic scale can be used to express power levels we hear.

1.1.1 Adding/Subtracting/Averaging dB Values

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recall that log (or dB) values can't be added directly, they must be converted back to normal values first.

Obviously averaging values must observe the same restrictions as addition,

Other log properties are,

Try problems S1, S2, S3, S4