Acoustics Glossary & Definitions

In an attempt to debunk the theory around acoustics, this is a short guide on some common acoustics language.



A frequency weighting that was calculated to account for the manner in which the human ear reacts to sound at different frequencies. The curve mirrors the 40Phon contour graph.


Acoustics is the science of sound.

Absorption Coefficient

The fraction of incident sound intensity which is either absorbed or transmitted

Airborne sound insulation test

A test involving the use of a loudspeaker generating sound (pink noise) in a source room and a measurement taken either side of a partition (wall or floor). Used as a performance indicator for Approved Document E Building Regulation Testing.


‘Without echo’ - no reflections. An anechoic chamber is lined with absorbent material for acoustic laboratory testing.


‘Apparent Sound Width’ is a measure of the lateral sound energy arriving at the listener’s position in the first 50 to 80ms after the arrival of the direct sound. It is measured using a dummy head with microphones for ears giving binaural measurements that are processed to give the ‘interaural cross-correlation coefficient’ IACC. This coefficient has been shown to be a more accurate assessment of envelopment than the Early Lateral Energy Fraction LE80


Performance space where audience members stand or sit


The clarity index measures the total sound energy arriving before an 80‐millisecond threshold, compared to the total sound energy arriving after that threshold, averaged for three mid‐frequency octave bands


A-Weighted urban traffic noise spectrum. It can be combined with DnT,W to provide a comparison between A-Weighted and Weighted Standardized. Defined in BS EN ISO 717. A greater sound insulation requirement at lower frequencies than DnT,W becuase of the A-Weighting.

Decibel (dB)

The bel is a logarithm of the ratio of two powers, and the decibel is 1/10 of a bel. The human ear responds logarithmically, and it is therefore, practical to use logarithmic units in audio systems.


The scattering of sound waves in many different directions due to an object or surface.

Diffuse Sound Field

A sound field of statistically uniform energy density where the direction of propagation of the waves are random. Also known as ‘Reverberant sound field’

Digital Signal Processing

The analysis of digital signal data. The original analogue signal is sampled at regular time intervals, and an Analogue to Digital converter converts the sampled amplitudes into a number series.

Dissipative absorber

Sound energy is dissipated as heat in the spaces in between the fibres of the absorber. Poor for low frequencies. Functions as a ‘Broadband absorber’. E.g. rubber or a porous absorber.

Early Decay Time (EDT)

Measured over the first 10 dB decay, by acousticians interested in clarity and the direct sound field. Early reflections that reach the listener within 50ms integrate with the direct sound and can improve speech clarity - see the Haas Effect.

Energy Reflection Coefficient

Linked to absorption coefficient, this is a measure of the ability of a material to reflect sound. 0 = Very good reflector. 1 = Very poor reflector. R = 1/1π.


The impression that surround sound is enhanced if the early sound includes strong lateral reflections. Often seen in concert halls prior to 1960. E.g. Vienna ‘Shoe-box’.

Free-field conditions

Sound source is unaffected by any reflective surfaces. E.g. an anechoic chamber.

Frequency Response

The changes in amplitude or sensitivity of a circuit or device with frequency.

Ground attenuation

Attenuation of sound at a distance from a receiver caused by the interference between sound waves travelling directly from source to receiver and sound arriving at receiver, after reflection at the ground.

Haas Effect

A specific area of precedence, where Helmut Haas showed that the effect (of precedence) is still present, even if the level of the delayed sound is 10dB higher

Helmholtz resonator

An acoustic tool designed to resonate at a certain frequency. For example, a milk bottle which has a small opening at the neck followed by a (resonating) cavity

Impact sound insulation test

A test conducted using a tapping machine (simulating footfall) which is placed on a floor and a sound measurement is taken in the room below. Used as a performance indicator for Approved Document E Building Regulation Testing.

Just Noticeable Difference (JND)

A concept used in psycho-acoustic measurement; the difference between two acoustic stimuli which is noticeable under a specifically defined condition. E.g. JND of the Clarity Index C50

Longitudinal Waves

Vibratory movement of the particles in the medium is parallel to the direction of travel. For example, a compressional wave in a liquid.


The measure of the subjective impression of the magnitude or strength of a sound, often denoted by Strength factor G


The process whereby the threshold of hearing for one sound is raised due to the presence of another, thus rendering the first sound inaudible.

Natural Frequency

The frequency of free or natural vibrations of a system.

Noise Egress

Sound Transfer through a building element, usually from an internal source to an external receiver or internal to internal. E.g. Amplified music from a venue.

Noise Ingress

Sound Transfer through a building element, usually from an external source to an internal receiver. E.g. Train or traffic noise

Noise Floor

A measure of the extraneous noise sources within a measurement system. Ideally the 'noise floor' should be 10 dB lower than the sound level to be measured.

Normal Incidence Absorption Coefficient

A measure of the effectiveness of materials as sound absorbers with sound striking at a 90-degree angle (perpendicular). 0 = Very poor absorber. 1 = Very good absorber. Only measured used an impedance tube (standing wave tube)

PA System

Public Address system, often incorporating sound reinforcement.

Panel absorber

Mass spring system (Panel = Mass. Air gap = Stiffness). Resonant absorber where each system has a natural frequency. Used for more selective sound absorption issues.

Pink Noise

Random broadband noise which contains equal power per percentage bandwidth. On a logarithmic scale, the frequency spectrum is horizontal (flat). Energy decreases at a rate of 3dB per Octave.

Plane waves

Wavefronts are plane and parallel in every direction of propagation. The energy does not diverge over increasing distance.


When sound arrives at a particular listener from several different directions (e.g. direct sound plus multiple reflections) the brain will interpret the incoming information and will identify the source of the sound as being in the direction from which the first sound arrived

Progressive waves

A wave that travels outwards from its source and is not being reflected.


The study of the relationship between the physical parameters of a sound and its human perception.

Reactive Absorber (Resonant absorber)

Determined by bulk modulus and its density. Use resonance as a mechanism for absorption. Good for low frequencies. Function as a ‘Narrowband absorber’. E.g. Helmholtz resonator or bass traps.


A wave that is reflected back and forth between two parallel surfaces.

Resonance Frequency

The frequency at which resonance occurs, for example, a piece of string subjected to continuous vibration. Forced vibration amplitude in response to a force of constant amplitude is a maximum. In an undamped system, the resonance frequency is equal to the natural frequency. In a fully damped system, the resonance frequency is slightly less than the natural frequency

Reverberation Time (RT60)

The time required for the steady sound pressure level in an enclosed space to decay by 60dB, measured from the moment the sound source is switched off. It is affected by: Room volume; Area of acoustic materials; area of any openings (doors/windows); Type of absorptive materials; frequency of source sound and room geometry.

Room Absorption

Sum of Sabine absorption, due to objects and surfaces in a room and due to dissipation of energy in the room. Involves random incidence. Testing is undertaken in a reverberation chamber.

Room Constant, Rc

A constant used in the calculation of reverberant sound pressure. 𝑅𝐶 = (𝑆.𝛼𝐴𝑉𝐺𝐸)/ (1− 𝛼𝐴𝑉𝐺𝐸)

Room Radius

The distance from the source where the direct and reverberant sound pressure levels are equal. 𝑅 = √[𝑄 𝑅𝑐/16π]

Semi-reverberant field

A sound field which is neither completely reverberant or anechoic but somewhere in between.


What humans perceive as sound is the propagation of pressure waves through a medium. The speed of sound varies from medium to medium, with denser mediums having faster wave propagation. Different sound frequencies are perceived as different tones.
An example is when an electronic signal is sent to a speaker cone, it vibrates and sends those vibrations through the air until they reach your ear. The human ear receives this sound and converts into electric impulses that are transmitted to the brain.

Sound Absorption

The process whereby sound energy is converted into heat and a reduction in sound pressure level occurs. The property of a material that allows it to absorb sound energy.

Sound Damping

The dissipation of energy with time or distance. The collective term for the attenuation of sound due to internal structure or sound dissipative materials. Concerns vibration and solid bodies.

Sound Intensity

The sound power flowing per unit area in a given direction. Measured over an area perpendicular to the direction of flow.

Sound Pressure

The fluctuations in air pressure, from the steady atmospheric, created by sound and measured in Pascals (Pa)

Sound Pressure Level (SPL)

Sound pressure measured on a decibel scale, 𝐿𝑝=20log10𝑃/𝑃𝑜 , where Po is the reference sound pressure level 20x10-6 Pa

Sound Reduction Index (Transmission Loss)

A measure of the airborne sound insulating properties of a material, in a particular frequency band. 𝑅 = 10 log(1/𝑡)

Sound Reinforcement

A wide range of amplification, microphones and electro-acoustic equipment.

Sound Transmission Coefficient

The ratio of the sound energy transmitted by a partition or across a boundary, to the sound energy upon the partition/boundary.


Spaciousness refers to the effect where the sound appears to the listener to emanate from a source wider than the width of the actual source. This is also measured as ASW. Spaciousness is also assisted by sufficient strength of arriving energy at low frequencies; which is quantified by the low frequency Strength Factor, GLow

Speaker-Boundary Interference Response (SBIR)

Acoustic distortion occurring with coherent interference between the direct sound of a loudspeaker and the reflections in the room.

Speech Intelligibility

The ability of a listener to hear and interpret verbal messages. The quality in speech to be understood.

Sperical waves

Wavefronts are concentric spheres. Idealize model of how sound propagates in free-field conditions.

Standardized Level Difference, DnT

A measurement of Airborne Sound Insulation corrected for a Standardized Reverberation Time of 0.5 Seconds. Defined in BS EN ISO 140-4.

Standing Waves

A wave that is reflected back and forth between two parallel surfaces. Interference occurs between progressive waves. Resonance occurs when incident and reflected waves are in phase


The arithmetic average of the reverberation times in the 500Hz, 1kHz and 2kHz octave bands, or the arithmetic average of the reverberation times in the one-third octave bands from 400Hz to 2.5kHz

Transverse waves

Vibratory movement of the particles in the  medium is perpendicular (90 degrees) to the direction of travel. For example, A shear wave in a solid.


The A-weighted Voice Level - A guide to the ‘voice effort’ required for face to face communication at a distance ‘r’ (m) is VLA ≥ 1.33 (SIL + 20 log r) - 36

Weighted Apparent Sound Reduction Index, R'w

A single number rating of Airborne Sound Insulation between rooms over a range of frequencies. Includes flanking and workmanship. Field measurement.

Weighted Sound Reduction Index, Rw

A single number quantity which characterises the Airborne Sound Insulation of a building element or material over a range of frequencies. Defined in BS EN ISO 717. Used for rating partitions. Laboratory measurement.

Weighted Standardized Level Difference, DnT,w

A field measurement of Airborne Sound Transmission. Standardized to Reverberation Time of 0.5 seconds and weighted to provide a single figure value. The test includes flanking making it a total transmission calculation not just the partition.

White Noise

Random broadband noise which contains equal power per unit bandwidth. On a linear frequency scale, the frequency spectrum is horizontal (flat).


  • Department for Education (2015). BB93 Acoustic design of schools: performance standards. London: The British Standards Institution

  • Ermann, M. (2015). Architectural Acoustics Illustrated. New Jersey: John Wiley & Sons

  • Everest, F. & Pohlmann, K. (2015). Master Handbook of Acoustics, Sixth Edition. New York: McGraw-Hill Education

  • Gracey & Associates (2019). Acoustic Glossary. Huntingdon, UK: Gracey & Associates. Available from:

  • Hollins, M., Peters, R., & Smith, B. (2013). Acoustics and Noise Control, Third Edition. Oxon: Routledge

  • The Institute of Acoustics (2016). IOA Distance Learning Course - General Principles of Acoustics Module. St. Albans: IOA



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