16
Sep

### Acoustic Terms

#### Disturbance.

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.

#### Frequency.

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.

#### Velocity of sound.

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

#### 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.

#### Instantaneous.

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.

#### Isotropic.

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

#### Anisotropic.

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

#### 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.

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

This implies no barriers present to produce reflections.

#### 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.

#### 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.

#### Mode

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.

#### Temporal averaging.

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

#### Shear

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.

#### Transducer

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.

#### Rectilinear propagation.

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

#### Diffraction.

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.

#### Transient.

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

#### Coherent.

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.

#### 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.

(Acoustics, 2009)

Bibliography

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