Acoustic Glossary


S : Sound and Vibration Definitions, Terms, Units, Measurements etc., ..

S/NSignal-to-Noise ratio

Sabin Absorption
Sabine Sound Absorption Coefficient
Sabine Equation

Sampling Frequency the rate at which a continuous waveform is digitised, given in Hz.

Sampling Interval how long the data is measured at each sample point.

Sampling Theorem says that ideally a signal should be sampled at a rate twice its highest frequency component.

SAWAcoustic Surface Wave

Scalar Quantity a quantity fully described by a magnitude or numerical value, for example density, mass and speed. As opposed to a vector quantity which has both magnitude and direction, for example acceleration, force and velocity

Scalar Quantities may be added, subtracted or multiplied like ordinary numbers, vector quantities can not.


Schroeder method for calculating the reverberation time from the impulse noise decay curve. The backward curve integration algorithm developed by Manfred Schroeder at Bell Labs in the nineteen-sixties.

Second (s) the second, symbol s, is the name of the SI base unit of time.

0.001 s = 1 ms (millisecond) and 0.000001 s = 1 μs (microsecond)

Seismic relating to earthquakes or other vibration in the earth

Seismic Reflection the reflection of waves at boundaries between different rock formations

Seismic Refraction the refraction of waves passing through formations of 'earth' having different seismic velocities

Seismic Velocity the velocity of wave propagation in particular ground or rock formation

Seismogram a record produced by a seismograph

Seismograph A measuring instrument for detecting and measuring the intensity and direction and duration of movements of the ground (as in ground-borne vibration) - certified seismographs

SELSound Exposure Level

Self-induced Oscillation Definition

IEC 801-24-04, a continuing oscillation that is generated in a system when non oscillatory energy is supplied, also known as self-excited oscillation

See also other oscillation terms

SENELSingle Event Noise Exposure Level

Serial Frequency Analysis the measurement of octave or third octave bands of noise where a single filter is stepped across the different bands one at a time. Superseded by real time analysis, these days.

Shear Wave


rapid transient transmission of mechanical energy.

Shock Pulse Definition

IEC 801-24-27, excitation of a system characterized by rise and fall in a time interval short in comparison with the half-period of any mode of oscillation of the system

Duration of Shock Pulse Definition

IEC 801-24-28, time required for the instantaneous value of an excitation to rise from some stated fraction of its maximum value and to decay to the same fractional value

Short Leq

SI Units the world's most widely system of units devised around the convenience of the number 10.

There are 7 base units from which other units are derived and known as SI derived units.

SI Unit prefixes, used together with a SI unit to form decimal multiples or submultiples of the unit
  Factor     Name     Symbol   Multiplying Factor    
  1012     tera     T   1,000,000,000,000
  109     giga     G   1,000,000,000
  106     mega     M   1,000,000
  103     kilo     k   1,000
  10-3     milli     m   0.001
  10-6     micro     μ   0.000.001
  10-9     nano     n
  10-12     pico     p

Example 10-6 g = 1 μg = 1 microgram or one millionth of a gram.

Sideband in frequency domain functions, pairs of frequencies with similar amplitude that appear equally spaced on either side of a centre frequency - produced by modulation.

Signal-to-Noise Ratio

the difference in dB between the measured sound level and the noise floor due to other sources present. Ideally this should be greater than 10dB - see our page on decibels and in particular the examples of adding decibels

SILSound Intensity Level
SILSpeech Interference Level.

Simple Sound Source

Sine Wave or Pure Tone characterized by it's frequency (number of cycles per second) or it's wavelength (distance it travels within a period) and the amplitude .

Single Event Noise Exposure Level (SENEL) there are two variations of this term:-

1:- the dB(A) level which if it lasted for one second would produce the same A-weighted sound energy as the actual event.

2:- similar except the start and end of the measurement is defined, usually as 10 dB below the Lmax
See also • T10

Both are similar on the Sound Exposure level (SEL) but we believe the second was developed to take account of single events like aircraft noise where the Lmax is important but the duration should also be taken factored in. An event with a higher Lmax can have a lower SEL than a longer event.

Single Number Rating (SNR), a single number rating system for hearing protectors - BS EN ISO 4869

See also • noise reduction rating used in the USA.

Single Number Rating (SNR) a rating system for duct silencers.

SLMSound Level Meter

Slow Time Weighting is also known as slow response and slow time constant.

SNR ▷ see the Single Number Rating(s) above

Sones a unit to compare the loudness of two sounds.

By definition one sone = 40 phons and also equals 40 dB on the equal loudness contours.

A 10dB increase, from 40 to 50 phons sounds twice as loud, so 50 phons = 2 sones and the following table applies:-
40 phon =   1 sone
50 phon =   2 sones
60 phon =   4 sones
70 phon =   8 sones
80 phon = 16 sones
90 phon = 32 sones ... and so on
Sone Definition IEC 801-29-04, unit of loudness, equal to the loudness of a pure tone presented frontally as a plane wave of frequency 1,000 Hz and a sound pressure level of 40 dB, re 20 ╬╝Pa
Note 1 : the loudness of a sound that is judged by the listener to be n times that of the 1-sone tone is n sones.


any pressure variation that the human ear can detect. Depending on the medium, sound extends and affects a greater area (propagates) at different speeds. In air, sound propagates at a speed of approximately 343 m/s. In liquids and solids, the propagation velocity is greater - 1480 m/s in water and 5120 m/s in steel, for example.

Sound Definition

IEC 801-21-01, movement of particles in an elastic medium about an equilibrium position, also known as acoustic oscillation and acoustic vibration

See also • audible soundcomplex soundinfrasoundspeed of soundultrasound sound

Sound Absorption
Sound Absorption Coefficient
Sound Absorption Loss
Sound Analyser under Spectrum Analyser.

Sound Diffraction
Sound Diffuse Field
Sound Diffusion
Sound Dose

Sound Density under Sound Energy Density

Sound Energy
Sound Energy Density
Sound Energy Flux
Sound Energy Flux Density
Sound Energy Flux Density Level

Sound Exposure
Sound Exposure Level
Sound Exposure Meter

Sound Fields
Sound Field Quantities
Inverse Square Law
Particle Displacement
Particle Velocity
Sound Pressure

Sound Flux under Sound Energy Flux

Sound Insulation

Sound Intensity

Sound Isolation Between Rooms under Sound Insulation, Level Difference

Sound Level
Sound Level Meter
Sound Level Meter Classes
See also the IEC Definition of Level

Sound Localisation

Sound Particle Acceleration.
Sound Particle Displacement.
Sound Particle Velocity.

Sound Power
Sound Power Absorption Coefficient
Sound Power Density IEC 801-211-8
Sound Power Level
Sound Power Reflection Coefficient

Sound Power through a Surface Element is also known as Sound Energy Flux

Sound Pressure
Sound Pressure Level
Sound Pressure Reflection Coefficient

Sound Propagation

sound waves can propagate through a medium such as air, water and solids as a longitudinal wave or a transverse wave

Sound Propagation Coefficient Definition

IEC 801-23-33, with respect to a uniform system, natural logarithm of the complex ratio of particle velocities (or pressures) measured at two successive points separated by unit distance, when this system is assumed to be of infinite length, also known as the Linear Exponent of Sound Propagation

See also • elementary attenuation of propagationelementary dephasing of sound propagationelementary exponent of sound propagationpropagation loss definition

Sound Rating Level (LArT), the A-weighted, Leq, sound pressure level of an industrial noise during a specified time period, adjusted for tonal character and impulsiveness

Sound Reduction Coefficient under Noise Reduction Coefficient.
Sound Reduction Index under Sound Insulation.

Sound Reference Levels

Sound Source Definition

IEC 801-29-10, a simple sound source (monopole) radiates sound equally in all directions. A complex sound source is composed of various sources, multiple frequencies and directivity patterns.

Complex Sound Source

composed of various sources, multiple frequencies and directivity patterns.

Point Sound Source Definition

IEC 801-21-34, source that radiates sound as if from a single point.

Simple Sound Source Definition

IEC 801-21-33, source that radiates sound equally in all directions in a free field and is also known as a monopole.

Strength of a Simple Sound Source Definition

IEC 801-21-33, maximum instantaneous volume velocity produced by a simple source small compared with wavelength, emitting a wave with sinusoidal variation with time and is also known as the strength of a monopole.

See also • directivitysound fieldssound waves

Sound Spectrum Definition

IEC 801-21-15, representation of the magnitudes (and sometimes of the phases) of the components of a complex sound as a function of frequency.

Sound Speed Gradient Definition

the speed of sound decreases with decreasing temperature and creates a negative sound speed gradient. An increase in temperature results in a positive sound speed gradient

Sound Transmission passage of a sound wave through a medium or series of media.

Sound Transmission Class (STC)
Sound Transmission Loss  (STL)

Sound Velocity

is usually taken to mean the speed of sound, not be confused with sound particle velocity, which is the velocity of the individual particles.

Sound Velocity Level under Particle Velocity Level

Sound Waves

Sound Wavelengths

Sound Wave Velocity

Spatial Averaging taking measurements at various positions and averaging the results. Mandatory in sound insulation measurements and recommended anywhere multiple reflections are present.

See also other types of averaging

Spatial Frequency is a measure of how often sinusoidal components of the structure repeat per unit of distance. The SI unit of spatial frequency is cycles per meter.

Specific Acoustic Admittance Definition

IEC 801-25-38, reciprocal of the specific acoustic impedance

Specific Acoustic Impedance (z)

the ratio of the effective sound pressure at a point in an acoustic medium to the effective particle velocity at that point. z = p/v the SI units are Pa·s/m3.

Specific Acoustic Impedance Definition

IEC 801-25-35, at a point in a sound field, quotient of sound pressure by particle velocity

See also acoustic impedance and related topics

Specific Acoustic Reactance Definition

IEC 801-25-37, Imaginary part of the specific acoustic impedance

Specific Acoustic Resistance Definition

IEC 801-25-36, Real part of the specific acoustic impedance

Specific Energy the energy per unit mass, joule per kilogram = J/kg

Specific Flow Resistance Definition

IEC 801-31-34, quotient of the difference in air pressure between the two faces of a sheet of porous material by the particle velocity of airflow through the sheet

See also • flow resistance

Specific Noise

noise from the source under investigation as defined in BS 4142 method for rating industrial noise affecting mixed residential and industrial areas. The specific noise is compared to the background noise to assess the likelihood of complaints.

Specific Noise Level

is the A-weighted, Leq level produced by a noise source during a specified period of time.

Other noise descriptors • ambient noisebackground noisebroadband noisegaussian noisenarrowband noiseperiodicpink noisepseudo random noiserandom noiseresidual noisewhite noisewideband noise

Specific Volume (v) the number of cubic metres occupied by one kilogram of the substance : m3/kg.

Specific Wall Admittance Definition

IEC 801-31-25, quotient of the particle velocity normal to a wall, by the sound pressure acting on the wall

Specific Wall Impedance Definition

IEC 801-31-24, quotient of the sound pressure acting on a wall, or a wall covering, by the particle velocity normal to the wall

See also acoustic impedance and related topics

Spectra is the plural of spectrum

The Spectral Density

of a wave, when multiplied by an appropriate factor, will give the power carried by the wave, per unit frequency, known as the power spectral density of the wave and is commonly expressed in watts per hertz (W/Hz).

Spectral Density Definition

IEC 801-21-43, limit as the bandwidth approaches zero, of the mean square value of a field quantity divided by bandwidth. The kind of field quantity must be specified, such as sound pressure, particle velocity, particle acceleration. Also known as spectrum density

Spectral Density Limit as the bandwidth approaches zero, of the mean square value of a field quantity divided by bandwidth.

See also • energy spectral density.

Spectral Leakage

Spectral Lines the number of constant bandwidth lines used in the measurement of spectra.

Spectrum the description of a sound wave's resolution into its components of frequency and amplitude.

See also • continuous spectrumline spectrum

Spectrum Adaption Term (C and Ctr)

Spectrum Analyser an instrument to analyse a sound or vibration wave into it's frequency components. A spectrum analyser converts a signal from the time domain into the frequency domain,. The FFT, Octave and 1/3-octave analysers are the most common type today, but there are many other types.

Spectrum Averaging a short term spectrum analysis may include information due to external sources, for example background noise. Repeating the measurements over a longer period and averaging the spectra will cause any random signals to be 'discarded' and your confidence in the measurement will improve.

See also other types of averaging

Spectrum Density Definition

IEC 801-21-43, limit as the bandwidth approaches zero, of the mean square value of a field quantity divided by bandwidth. The kind of field quantity must be specified, such as sound pressure, particle velocity, particle acceleration.

Spectrum Density is also known as spectral density

Spectrum Density Level Definition

IEC 801-22-13, level of the limit, as the width of the band approaches zero, of the ratio of a specified quantity distributed within a frequency band to the width of the band.
Note 1 : the kind of quantity must be specified, such as by (squared) sound pressure spectrum level.
Note 2 : in view of the fact that filters have finite bandwidths, practically the sound pressure spectrum level Lps is obtained for the centre frequency of the band by the formula: Lps = 10 log10 (p2/B) ÷ (po2/Bo) dB,

where p and po are respectively the given field quantity and the reference quantity; B and Bo are respectively the effective bandwidth of the filter and the reference bandwidth of 1 Hz.

When Lp is the band sound pressure level observed through the filter, the above relation reduces to Lps = Lp - Log10 (B/Bo) dB

Spectrum Level under Spectrum Density Level

Specular Reflection

Speech - Articulation Index (AI)
Speech - Articulation Intelligibility
Speech - Intelligibility
Speech Interference Level (SIL)
Speech Interference Level (SIL3)
Speech Power
Speech Privacy
Speech Transmission Index (STI and STIPA)

Speed of Sound (c)

≈ 331.5 + 0.60 T(°C), at 20 °C, the speed of sound in air is approximately 343 m/s and the decrease of speed with temperature is referred to as a negative sound speed gradient. The speed of sound is also dependent, to a minor extent, on atmospheric pressure and relative humidity.

Sound travels faster in liquids and solids. For example the speed of sound in water is 1,480 m/s and for iron 5,120 m/s, these values are also temperature dependent, also giving rise to Sound Speed Gradients.

The frequency f, the wavelength λ and the wave velocity v are related by the formulae λ = v/f

Speed of Sound Definition

IEC 801-23-18, magnitude of the phase velocity of a free progressive sound wave

The Speed of Sound should not to be confused with the Sound Particle Velocity.


Spherical Wave

SPLSound Pressure Level

SPPSpeech Privacy Potential

Spreading Loss Definition

IEC 801-23-41, that part of the transmission loss due to the divergence, i.e. spreading, of sound waves in accordance with the configuration of the system
Note : Spreading Loss exists, for example, for spherical waves emitted by a point source, also known as the strength of a simple sound source

Square when you multiply a whole number times itself, the resulting product is called a square number, or a perfect square or simply a square.

square metre (m2) Area

square root (√), the square root of a number is a smaller number that, when multiplied by itself equals the original number.

SRISound Reduction Index.

Standard Atmospheric Pressure

Standardized measurement in accordance with a Standard or 'Norm'.

Standardized Impact Sound Pressure Level (LnT)
Standardized Impact Sound Pressure Level (L'nT)
Standardized Level Difference (DnT)

Standard Reference Levels

Standing Wave

Standing Wave Tube a method for measuring absorption coefficients by means of standing waves in a tube.

Static Pressure Definition IEC 801-21-18, at a point in a medium the pressure that would exist at that point in the absence of sound waves.

Stationary Signal a signal whose average statistical properties over a time interval of interest are constant. In general, the vibration signatures of rotating machines are stationary.

Stationary signals are either deterministic or random.

Statistical Analysis a calculation performed by a sound level meter on the noise levels measured during the measurement period to describe the statistical levels of the noise.

Statistical Noise Levels
Statistical Sound Power Absorption Coefficient

Steady-state Oscillation Definition

IEC 801-24-24, oscillation that continues without change

See also other oscillation terms

STCSound Transmission Class

STISpeech Transmission Index.
STIPASpeech Transmission Index for Public Address systems

Stimulus Definition

IEC 801-21-46, external force, or other input, applied to a system, also known as excitation

STLSound Transmission Loss

Stochastic the details of individual events may be unpredictable but the overall character of the sound is. For example rain falling, sound of insects, birds, etc.

Strength of a Monopole
Strength of a Simple Sound Source

Structure-borne Noise

a significant portion of the transmission path from source to receiver takes place in a solid structure rather than through the air.

Subharmonic Response

Superposition Principle under Sound Waves.

Surface Area
Surface Normal
Surface Wave
Surface Wavefront

SVLsound velocity level, under Particle Velocity Level

SWLSound Power Level

Symbols, the names, symbols and definitions for quantities and units of acoustics are given in BS EN ISO 80000-8 - BSI copyright precludes us publishing any standard. However we can state 'well-known' general facts

Symbol of a Quantity Definition IEC 112-01-03, character or combination of characters denoting a quantity.
Note 1 : a simple quantity symbol is preferably one, or in some cases two, letters of the Latin or Greek alphabets and may include subscripts, superscripts, or other modifying signs. The letters are in italic (sloping) type, using preferably a font with serifs. The subscripts and superscripts are printed either in roman (upright) type, or, when they denote quantities, variables, or running numbers, in italic (sloping) type. also known as Quantity Symbol.

See • ISO 80000-1 and IEC 60027-1 for more details and for the combination of symbols.

Synchronous Averaging under Time Domain Averaging.

System of Units set of base units and derived units, together with their multiples and submultiples, defined in accordance with given rules, for a given system of quantities - see the SI units above


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