# The speed of sound and Sound pressure

## The speed of sound

Velocity vector is defined as a combination of speed and direction. Wave vector is defined as a combination of wavenumber and direction.

## Not an absolute value

The speed of sound is not an absolute value. It depends on the molecules the waves are moving through, and is a fundamental property of every specific medium.

In general, the speed of sound in a matter is not constant. It is dependent on the square root of the ratio of the elastic modulus of the medium to its density.

## Environmental circumstances

Those physical parameters and the velocity of sound change with environmental circumstances. For example, the velocity of sound in gases depends on temperature. The sound amplitude has effect on the sound speed.

The outcome is the generation of harmonics and mixed tones not present in the source of sound.

## Sound pressure

Sound pressure is mathematical physical value that means the subtraction between mean ambient pressure and the pressure inside the sound wave in a specific medium.

A square of this subtraction is the basis for the calculation of the sound pressure. When the scale is a logarithmic scale, it is called decibels.

## Mathematical expressions

There are several mathematical values, based on mathematical expressions that are the basic terminology in the daily work of specialist for sound, such as engineers, audiologists and designers of sound equipment.

There are 8 common parameters: (1) Sound pressure that is called also acoustic pressure. (2) Sound pressure level (SPL), and the identical term: sound level. (3) Instantaneous sound pressure. (4) Effective sound pressure. (5) Acoustic intensity of the wave. (6) Acoustic impedance. (7) Particle displacement and the equivalent term: particle amplitude. (8) The distance law.

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