Acoustics Glossary

A-Z list of terminology commonly used within acoustics.



S (slow) time weighting: one of the standard averaging times for sound level meter displays, defined in BS EN ISO 61672-1.

Sabin: unit of sound absorption; one sabin is the amount of absorption equivalent to one square metre of perfect absorber.

Sabine’s formula: a formula for predicting reverberation times of rooms.

Sampling frequency: of a digitalised signal; the number of samples per second (see Shannon’s sampling theorem).

Sampling interval: the time interval between samples.

Scalar: a quantity that may be completely defined by its magnitude alone, i.e. it has no direction.

Semi-anechoic: a room with anechoic walls and ceiling, but with a sound reflecting floor.

Semi-reverberant: a room which is neither completely anechoic nor reverberant, but somewhere in between.

Sensation value: of a specified sound; the sound pressure level when the reference sound pressure corresponds to the threshold of hearing for the sound.

Sensitivity: of a transducer; the ratio of output to input, e.g. for a microphone the sensitivity is measured as output voltage (V)/ input pressure (Pa).

Shannon’s sampling theorem: an important result of signal processing theory that states that a minimum of two samples per hertz is needed adequately to represent the highest frequency component in a signal. Therefore a sample ate of at least 40,000 samples per second would be needed adequately to represent a signal with frequency components of up to 20,000 Hz. It is important that this condition is met otherwise false frequencies, called aliases, start to appear in the analysis.

Shear wave: a transverse wave of shear stress propagating in an elastic medium.

Shock: a sudden transient disturbance to a vibrating system caused by a rapid change in force, displacement, velocity or acceleration.

Signal-to-noise ratio: a measure of the strength of a signal, indicating its magnitude relative to the background electrical noise in the measurement system; usually expressed in decibels.

Silencer: a device for reducing noise in air and gas flow systems; silencers are either absorptive or reactive; also called attenuators or mufflers.

Simple harmonic motion: a single frequency vibration, i.e. one in which the displacement varies sinusoidally with time.

Simple source: an idealised model of an acoustic source, which radiates spherical waves, under free field conditions; see also under point source, monopole.

Sine wave: the graph of a sinusoidal function, which indicates the simplest possible repeating waveform, characterised by a single frequency and constant amplitude.

Single degree of freedom system: a vibrating system consisting of only one mass, one spring and one dashpot (damper); such a system has one natural frequency and one mode of vibration; its motion can be completely described by one variable.

Sinusoidal: relating to a sine wave.

SNR (single noise rating): a single figure method for evaluating the attenuation performance of hearing protectors.

Snubber: a device used to restrict the maximum displacement of a vibrating system, e.g. at resonance.

Sociocusis: haring loss arising from everyday activities.

Soft ground: ground such as grassland, cultivated land which is considered to be an acoustically absorbing surface, in contrast with ‘hard ground’ which is considered to be sound reflecting.

Sone: the unit of loudness; the tone scale is devised to give numbers which are approximately proportional to the loudness; it is related to the Phon scale as follows: P = 40 + 10log2S, where P represents Phons and S represents Sones.

Sound: (1) pressure fluctuations in a fluid medium within the (audible) range of amplitudes and frequencies, which excite the sensation of hearing; (2) the sensation of hearing produced by such pressure fluctuations.

Sound absorbing material: material designed and used to maximise the absorption of sound by promoting frictional processes; the most commonly used materials are porous, such as mineral fibre materials or certain types of open-cell foam polymer materials.

Sound absorption: (1) the process whereby sound energy is converted into heat, leading to a reduction in sound pressure level; (2) the property of a material, which allows it to absorb sound energy.

Sound absorption coefficient: a measure of the effectiveness of materials as sound absorbers; it is the ratio of the sound energy absorbed or transmitted (i.e. not reflected) by a surface to the total sound energy incident upon that surface; the value of the coefficient varies from 0 (for very poor absorbers and good reflectors) to 1 (for very good absorbers and poor reflectors).

Sound exposure level, SEL (LAE): a measure of A-weighted sound energy used to describe noise events such as the passing of a train or aircraft; it is the A-weighted sound pressure level which, if occurring over a period of one second, would contain the same amount of A-weighted sound energy as the event.

Sound insulating material: material designed and used as partitions in order to minimise the transmission of sound; the best materials are those which are dense and solid, such as wood, metal or brick, although lightweight panels can also be effective when in the form of double-skin constructions.

Sound insulation: the reduction or attenuation of airborne sound by a solid partition between source and receiver; this may be a building partition (e.g. a floor, wall or ceiling), a screen or barrier, or an acoustic enclosure.

Sound intensity: the sound power flowing per unit area, in a given direction, measured over an area perpendicular to the direction of flow; its units are in W/m².

Sound intensity level, LI: sound intensity measure on a decibel scale: LI = 10log10(I/I0), where I0 is the reference value of sound intensity, 1012 W/m².

Sound level: a frequency-weighted sound pressure level such as the A-weighted value.

Sound level meter: an instrument for measuring sound pressure levels.

Sound power: the sound energy radiated per unit time by a sound source, measured in watts (W).

Sound power level, LW: sound power measured on a decibel scale: LW = 10log10(W/W0), where W0 is the reference value of sound power, 1012 W.

Sound pressure: the fluctuations in air pressure, from the steady atmospheric pressure, created by sound, measured in pascals (Pa).

Sound pressure level, SPL (Lp): sound pressure measured on a decibel scale: Lp = 20log10(p/p0), where p0 is the reference sound pressure, 20 x 106 Pa.

Sound propagation: the transmission or transfer of sound energy from one point to another.

Sound reduction index, R: a measure of the airborne sound insulating properties, in a particular frequency band, of a material in the form of a panel or partition, or of a building element such as a wall, window or floor; it is measured in decibels: R = 10log10(1/t), where t is the sound transmission coefficient; it is measured under laboratory conditions according to BS EN ISO 140-4; also known as transmission loss.

Soundscape: the total sound environment at a particular location, implying much more than can be described just in terms of sound level. A soundscape approach to improving the urban acoustic environment, i.e. how it may be made more pleasing to the ear, rather than simply on noise reduction.

Sound transmission: the transfer of sound energy across a boundary from one medium to another.

Sound transmission coefficient: the ratio of the sound energy transmitted by a partition, or across a boundary, to the sound energy incident upon the partition or the boundary.

Sound wave: a pressure wave in a fluid which transmits sound energy through the medium by virtue of the inertial, elastic and damping properties of the medium.

Specific acoustic impedance: at a point in a sound field; the complex ratio of sound pressure to the acoustic particle velocity. In terms of the fundamental units m, kg and s, the units of specific acoustic impedance are kgm/s2, but sometimes the quantity is expressed in terms of newtons (N) or pascals (Pa), as either NSM-3 or as Pas/m, and also as the rayl.

Specific noise: the particular component of the ambient noise which is under consideration or investigation, e.g. in connection with a planning application or noise complaint; defined in BS 4142.

Specific noise source: The noise source under investigation for assessing the likelihood of complaints (defined in BS 4142).

Specific noise level, LAeq,Tr: the equivalent continuous A-weighted sound pressure level at the assessment position produced by the specific noise source over a given reference time interval.

Spectral adaptation terms: C, Crt; terms used in connection with the measurement and assessment of airborne sound insulation and defined in BS EN ISO 7172-1.

Spectrum: a frequency spectrum is a graph showing variation of sound pressure level (or other quantity) with frequency.

Specular reflection: sound reflection which obeys the law of reflection that angle of incidence equals angle of reflection and which occurs at surfaces which are smooth on a scale comparable with the wavelength of the sound (as opposed to diffuse reflection).

Speech intelligibility: the ability of speech to be understood; the ability of a listener to hear and correctly interpret verbal messages. The concept of intelligibility is used as a criterion to determine the degree of acoustic privacy between rooms.

Speech interference level: a measure of the ambient noise level in offices which gives an indication of the degree to which speech will be intelligible; it is based on the arithmetic mean of the octave band sound pressure levels 500 Hz, 1000 Hz and 2000 Hz, which are most significant for good speech intelligibility.

Speech privacy: the degree to which speech is unintelligible between offices. Three ratings are used: confidential, normal (non-obstructive) and minimal. It is inversely related to speech intelligibility, e.g. good speech intelligibility leads to poor acoustic privacy, and vice versa.

Speech transmission index (STI): a measurement parameter for assessing the speech intelligibility in a room. It has a value between 0 and 1: 1 representing perfect speech intelligibility and 0 representing zero speech intelligibility.

Spherical waves: an idealized model of how sound propagates in free field conditions, and used as the basis of certain sound level prediction methods.

Standard deviation: a measure of the deviation or scatter of a set of values (e.g. sound pressure level measurements) from the mean value.

Standardised impact sound pressure level, L’nt: a measurement of impact sound insulation, corrected according to BS EN ISO 140-7 for room characteristics; a complete set of measurements consists of 16 values, one for each third octave frequency band from 100 Hz to 3150 Hz.

Standardised level difference, DnT: a measurement of airborne sound insulation, corrected according to BS EN ISO 140-4 for receiving room characteristics; a complete set of measurements consists of 16 values, one for each third octave frequency band from 100 Hz to 3150 Hz.

Standing waves: a wave system characterized by a stationary pattern of amplitude distribution in space arising from the interference of progressive waves; also called stationary waves.

Stapes: or stirrup; one of the three bones of the middle ear, connected to the oval window of the inner ear.

Static deflection: the deflection produced in the spring of a mass-spring system by the weight of the mass; it is related to the natural frequency of the system and is used to specify the stiffness of springs for vibration isolation.

Stationary waves: see under standing waves.

Steady noise: noise for which the fluctuations in time are small enough to permit measurement of average sound pressure level to be made satisfactorily without the need to measure LAeq using an integrating sound level meter; defined in BS 4142.

STI: see speech transmission index.

STIPA: a version of STI for public address systems.

STITEL: a version of STI for telecommunications systems.

Strain: the fractional change in shape due to an elastic deformation in a material caused by an applied stress.

Stress: force per unit area, measured in N/m2; stress applied to elastic materials causes strain.

Structure-borne sound: sound which reaches the receiver after travelling from the source via a building or machine structure; structure-borne sound travels very efficiently in buildings, and is more difficult to predict than airborne sound.

Subjective: depending upon the response of the individual.

Superposition: according to the principle of superposition, the wave disturbances in a medium caused by different sources may be combined algebraically.