# Acoustics Module Glossary

This glossary contains finite element modeling terms in an acoustics context. For mathematical terms as well as geometry and CAD terms specific to the COMSOL Multiphysics software, please see the Multiphysics Glossary.

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### acoustic impedance

At a specified surface, the complex quotient of acoustic pressure by normal fluid velocity. SI unit: 1 Pa/(m/s) .

### acoustic reactance

The imaginary part of the acoustic impedance.

### acoustic resistance

The real part of the acoustic impedance.

The reciprocal of impedance.

### anisotropy

Variation of material properties with direction.

### characteristic impedance

The product of the equilibrium density and the speed of sound in a medium. SI unit: 1 Pa/(m/s).

### compliance

Reciprocal of stiffness.

### damping

Dissipation of energy with time or distance.

### decibel (dB)

Unit of level when the base of the logarithm used in defining the level is the tenth root of ten and the quantities concerned are proportional to power.

### Doppler effect

Change in the observed frequency of a wave caused by a time rate of change in the effective length of the path of travel between the source and the observation point.

### effective sound pressure

RMS instantaneous sound pressure at a point during a time interval, T, long enough that the measured value is effectively independent of small changes in T. SI unit: 1 Pa = 1 N/m2.

### eigenmode

A possible propagating mode of an acoustic wave.

### impedance

At a specified frequency, the quotient of a dynamic field quantity (such as force, sound, pressure) by a a kinematic field quantity (such as vibration velocity, particle velocity).

### instantaneous sound pressure

Total instantaneous pressure at a point in a medium minus the static pressure at the same point. SI unit: 1 Pa = 1 N/m2.

### particle velocity

In a sound field, the velocity caused by a sound wave of a given infinitesimal part of the medium relative to the medium as a whole.

### PML (perfectly matched layer)

Domain adjoined at a system boundary designed to emulate a nonreflecting boundary condition independently of the shape and frequency of the incident wave front.

### reference sound pressure

See definition in the entry for sound pressure level.

### RMS value

Root-mean-square value; for the (complex) sound pressure, p(t), over the time interval T1 < t < T2 defined as

For a harmonic pressure wave, , the time interval is taken to be a complete period, resulting in pRMS = p0 /2.

### sound energy

Total energy in a given part of a medium minus the energy that would exist at the same part in the absence of sound waves. SI unit: 1 J.

### sound-energy flux density

See sound intensity.

### sound intensity

Average rate of sound energy transmitted in a specified direction at a point through a unit area normal to this direction. SI unit: 1 W/m2.

### sound pressure

See effective sound pressure.

### sound pressure amplitude

Absolute instantaneous sound pressure in any given cycle of a sound wave at some specified time. SI unit: 1 W/m2.

### sound power density

See sound intensity.

### sound pressure level

Ten times the logarithm to the base ten of the ratio of the time-mean-square pressure of a sound, in a stated frequency band, to the square of a reference sound pressure, pref. For gases, pref = 20 μ Pa, for other media (unless otherwise specified) pref = 1 μ Pa. Unit: 1 dB (decibel).

### sound source strength

Maximum instantaneous rate of volume displacement produced by a source when emitting a harmonic sound wave. SI unit: 1 m3/s.

### specific acoustic impedance

At a point in a sound field, the quotient of sound pressure by particle velocity. SI unit: 1 Pa/(m/s).

### speed of sound

The rate of change of particle displacement with distance for a sound wave. SI unit: 1 m/s.

### static pressure

Pressure that would exist at a point in the absence of a sound wave.

### stiffness

Ratio of change of force (or torque) to the corresponding change in translational (or rotational) displacement of an elastic element.