Acoustics Glossary & Definitions

​​BSI - British Standards Institute

dB - Decibel

IANL - Indoor Ambient Noise Level

HMO - House in Multiple Occupation
IOA - Institute of Acoustics
RT - Reverberation Time

SITMA - Sound Insulation Testing & Measurement Association
SLM - Sound Level Meter
SPL - Sound Pressure Level

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Absorption Coefficient - The fraction of incident sound intensity which is either absorbed or transmitted.

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

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ASW - ‘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 (IOA, 2017).

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Auditoria - Performance space where audience members stand or sit.

C80 - 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 (Ermann, 2015, p.67).

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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 (Everest & Pohlmann, 2015).

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Diffusion - The scattering of sound waves in many different directions due to an object or surface (Hollins et al, 2013).

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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’ (Hollins et al, 2013).

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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 (Gracey & Associates, 2019).

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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.

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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 (Gracey & Associates, 2019).

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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.

Envelopment - 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.

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Frequency Response - The changes in amplitude or sensitivity of a circuit or device with frequency (Everest & Pohlmann, 2015).

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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 (Hollins et al, 2013).

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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 (IOA, 2016).

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 (Hollins et al, 2013).

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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 (Hollins et al, 2013).

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Loudness - The measure of the subjective impression of the magnitude or strength of a sound, often denoted by Strength factor G (Hollins et al, 2013).

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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 (Gracey & Associates, 2019).

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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) (IOA, 2016)

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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.

Psychoacoustic - The study of the relationship between the physical parameters of a sound and its human perception (Hollins et al, 2013).

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Precedence - 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 (IOA, 2016).

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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.

Resonance - A wave that is reflected back and forth between two parallel surfaces (Hollins et al, 2013).

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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 (Hollins et al, 2013).

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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.

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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 (Hollins et al, 2013).

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Room Constant, Rc - A constant used in the calculation of reverberant sound pressure.                                                                                    

Room Radius - The distance from the source where the direct and reverberant sound pressure levels are equal.

 

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

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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.

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Sound Pressure - The fluctuations in air pressure, from the steady atmospheric, created by sound and measured in Pascals (Pa) (Hollins et al, 2013).

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Sound Pressure Level (SPL) - Sound pressure measured on a decibel scale,  , where Po is the reference sound pressure level 20x10-6 Pa (Hollins et al, 2013).

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Sound Reduction Index (Transmission Loss) - A measure of the airborne sound insulating properties of a material, in a particular frequency band.  

   

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

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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 - 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 (IOA, 2016).

 

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

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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 (Hollins et al, 2013).

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Tmf - 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 (DfE, 2015)

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VLA - 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 (IOA, 2016)