Acoustic Terms

Acoustic Terms.

Acoustics is a multidisciplinary field that encompasses a wide range of principles, concepts and specialised terminologies.  Each branch or field has their own set of terms and concepts.   Below are some commonly encountered Acoustic Terms.

Acoustic Absorption.

This has two meanings in acoustics.  Sound may be absorbed as it passes through a bulk medium; in this case some of the energy has been converted to another form of energy, for example heat.  This cannot happen spontaneously, a process is needed, a phenomenon by which the movement of the molecules caused by a sound wave is changed to heat must be present.  Friction as in the fibres of mineral wool is an example.

In room acoustic the term is used when sound energy falls on a partition.  Sound which is not returned to the room for any reason is said to have been absorbed; but some of it may have passed through to the next room for example.

Acoustic Reflection.

This is when sound waves encounter a surface and bounce back. The angle of the surface determines the angle of the reflected sound waves.  Acoustic reflections affect sound directionality and spacial perception and be essential in certain environments like recording studios, cinemas and concert halls.


The measure of the maximum displacement of particles in a sound wave from their equilibrium position, representing the loudness of the sound.

A linear system (as e.g. an audio amplifier).

This is one in which at all times the magnitude of the output of the system is a simple multiple of the input.  An over-stressed spring or one that is coil bound would be a non-linear system.  Air with sound waves passing through is normally a linear system, although in a sonic boom, for example, the acoustic stress of the medium may be high enough for it to exhibit non-linearity.


Not uniform with direction.  Such a source would exhibit directivity.


Two sources are coherent if the sound or vibration emitted is such that the disturbance from one is a replica (allowing for a possible constant change in amplitude) of that from the other.  Two loudspeakers connected to the same channel of a stereo amplifier would produce coherent signals, as would a loudspeaker and its reflection in a hard plane.  Two cars would not be coherent.


A unit used to measure the intensity or loudness of a sound (dB), representing a logarithmic scale relative to a reference level.


A property of any wave system, which causes a disturbance to invade an area that would be forbidden to it on the basis of rectilinear propagation.  An example would be the bending of sound waves over a partial height barrier.


The scattering of sound waves in various directions by irregular surfaces, helping to distribute sound evenly in a space.


A general acoustic term describing a parameter in some system that is time varying.  We could say the angular displacement of a pendulum is a form of disturbance, as is the acceleration of a vibrating floor.

Doppler effect.

The change in frequency of a sound wave due to the relative motion between the source of the sound and the listener.


When sound waves reflect off surfaces which results in a delayed repetition of the original sound.

Free field, unobstructed propagations (as of radiation from a source).

This implies no barriers present to produce reflections.


Frequency is a measure of the number of occurrences of a disturbance in one second and is expressed as 200 cycles per second or 200 Hz (Hertz).  It is the reciprocal of the period or time for one cycle.


This terms is used when we are considering the behaviour of a system moment by moment rather than as some form of time-averaged value.


e.g. of the sound radiation from a source = uniform in all directions.


This refers to a particular form, manner or pattern of collective oscillation of a system.  Spatial averaging.  The result of averaging a parameter in some way over a region of space.

Particle velocity.

This, in contrast to the above, is the instantaneous velocity of an appropriate physical entity (a molecule in a gas) that is moving as part of a disturbance.  It could refer, e.g. to the particles of a floor that is vibrating or the particles of a gas through which a sound wave is travelling.


When two sound waves are in phase, their crests and troughs coincide.  When two sound waves are out of phase, their peaks and troughs are misaligned which reduces the sound amplitude and negatively impacts on sound quality.


The perceived highness or lowness of a sound, determined by its frequency. Higher frequencies are perceived as higher pitches.

Principle of Superposition

If a system is a linear one, as above, and a number of inputs to the system coincide in time then the total output is simply the sum of the individual outputs arising from the various inputs and the Principle of Superposition is said to hold.


The phenomenon where an object vibrates at its natural frequency in response to an external stimulus, amplifying sound waves.

Rectilinear propagation.

Propagation in straight lines as in, to all practical purposes, light waves.


A persistence sound in an enclosed space due to multiple reflections.

Shear Velocity.

This relates to a manner of stressing a material so as to cause imaginary slices of a material to slide over each other, rather in the manner of the pages of a book.  If the wind velocity changes with height above the earth, there is said to be a velocity shear.

Simple harmonic, harmonic, pure tone, sinusoidal (as of a disturbance).

These terms are synonymous and infer a sine function fluctuation with time.  The sine function is the quantity that varies with angle as encountered in trigonometry.

Sound Pressure Level.  SPL

The level of sound pressure at a specific location, often measured in decibels (dB) to quantify the loudness of a sound.

Sound Wave.

A disturbance that travels through a medium as a series of compressions and rarefactions, carrying information and energy.

Sound Wavefront.

An imaginary surface that represents points in a sound wave that are in phase at a given time.

Temporal averaging.

This is the result of averaging a parameter over a period of time.


The quality or color of a sound that distinguishes it from other sounds with the same pitch and loudness, arising from the complex combination of harmonics and overtones.


Is the acoustics/vibration context this is any device that converts disturbance into an electrical signal or vice versa.  Microphones, loudspeakers and accelerometers are examples.


A situation that is only temporary; a signal that is not repetitive.

Velocity of sound.

The velocity with which a particular state, e.g. a density peak, travels through a medium.

Wave Length

Distance between two successive points in a sound wave that are in phase, usually measured from crest to crest or trough to trough.

(Acoustics, 2009)


Acoustics, Institute of Acoustics, 2009. Notes  in Acoustics and Noise Control. United Kingdom: IOA.

📖 Dictionary of Acoustic Terms

📖 Loudness of Sound

📖 What is Reverberation? 

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