Acoustic Glossary


S : Sound and Vibration Terms and Definitions ...

S/N : signal-to-noise ratio

Sabine Absorption
Sabine Absorption Coefficient
Sabine Reverberation Time 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.

SAW (surface acoustic wave)

Scalar Quantity is a physical quantity fully described by a magnitude or single number only, for example length, area, volume, pressure and power

As opposed to a vector quantity which has both magnitude and direction, for example displacement, acceleration and velocity

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


Schroeder Method for calculating the reverberation time from the impulse noise decay curve. The backward curve integration algorithm was 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

SEL : sound 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

SENEL : single 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 Superseded by real time analysis, these days.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

Shock rapid transient transmission of mechanical energy.

Shock Pulse Definition (IEC 801-24-27) excitation of a system characterised 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 are the world's most widely used system of measurement units devised around the convenience of the number 10.

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

For example the watt is the is derived unit of sound power, equivalent J/s. SI base unit kg·m2·s-3

SI Unit prefixes are 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

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

SI units of sound
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 is 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.

SIL : sound intensity level
SIL : speech interference level

Simple Sound Source

Sine Wave or Pure Tone characterised 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 filter 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 and SEL (sound exposure level)

Single Number Rating (SNR) a single number rating system for hearing protectors. This method requires a C-weighted average sound level measurement for each 'noise risk' area and the manufacturers SNR figure for the specific protector..

See also other hearing protector procedures

SLM : sound level meter

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

SNR : single number rating

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 μPa
Note : the loudness of a sound that is judged by the listener to be n times that of the 1-sone tone is n sones.

See also loudness and related terms

Sound is any air pressure variation that the human ear can detect. A sound source creates the sound power in watts and the sound energy is transmitted by sound waves, which in turn generate the sound pressure in pascals we hear.

Sound Definition (IEC 801-21-01) movement of particles in an elastic medium about an equilibrium position. Sound is also known as acoustic oscillation and acoustic vibration.

See also audible soundcomplex soundinfrasoundspeed of soundultrasound

Sound Absorption
Sound Absorption Coefficient
Sound Absorption Loss
Sound Analyser under spectrum analyser.

Sound Damping
Sound Density
Sound Diffraction
Sound Diffuse Field
Sound Diffusion
Sound Dissipation
Sound Dispersion
Sound Dose

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

Sound Exposure
Sound Exposure Level
Sound Exposure Meter

Sound Fields
Sound Field Quantitiesinverse square lawparticle displacementparticle velocitysound pressure

Sound Flanking
Sound Flux

Sound Insulation

Sound Intensity

Sound Isolation between Rooms

Sound Level
Sound Level Difference under sound insulation level difference
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
Sound Power Level
Sound Power Quantity
Sound Power Reflection Coefficient
Sound Power Through a Surface Element

Sound Pressure
Sound Pressure Level
Sound Pressure Reflection Coefficient

Sound Propagation, sound waves propagate through air, water and solids as longitudinal waves or transverse waves

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 acoustic phase coefficientelementary attenuation of propagationelementary dephasing of sound propagationelementary exponent of sound propagationpropagation loss definition

Sound Radiation
Sound Radiation Directivity

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

Sound Reduction Coefficient
Sound Reduction Index

Sound Reference Levels

Sound Reflection when a sound wave reaches the boundary between one medium and another, a portion of the wave undergoes reflection and a portion of the wave undergoes transmission loss across the boundary

See also reflected sound wave

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.

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 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
Sound Transmission Loss

Sound Units, the common 'unit' across the range of sound levels is the dB (decibel), to compress the immense range we hear into manageable numbers. Strictly speaking decibels are unitless as they are the ratio of two numbers which have the same units.

The SI units for the main sound levels are :
Sound Intensity = W/m2 (watt per metre-squared)
Sound Power = W (watt)
Sound Pressure = Pa (pascal)

Sound Velocity is usually taken to mean the speed of sound and should not be confused with sound particle velocity, which is the velocity of the individual particles in the media.

See also sound velocity level

Sound Volume Velocity
Sound Waves
Sound Wavelength
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. Also known as space and time averaging.

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
See also acoustic impedance and related terms

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/m.

Specific Acoustic Impedance (ASA 6.35) at a point in a medium, the complex ratio of sound pressure to particle velocity in the direction of the wave propagation, unit = Pa·s/m (pascal second per metre), also known as the characteristic acoustic impedance.

Specific Acoustic Impedance Definition (IEC 801-25-35) at a point in a sound field, quotient of sound pressure by particle velocity

See acoustic impedance and related terms.

Specific Acoustic Reactance Definition (IEC 801-25-37) Imaginary part of the specific acoustic impedance

See also acoustic impedance and related terms.

Specific Acoustic Resistance Definition (IEC 801-25-36) Real part of the specific acoustic impedance

See also acoustic impedance and related terms.

Specific Energy the energy per unit mass. SI derived unit J/kg (joule per kilogram).

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 Sound Level (Ls) is the A-weighted, Leq sound level produced by a sound source during a specified period of time T.

Specific Sound Source is the sound source under investigation as defined in BS 4142, method for rating industrial noise affecting mixed residential and industrial areas.

Other noise descriptors, ambient soundbackground noisebroadband noisegaussian noisenarrowband noiseperiodicpink noisepseudo random noiserandom noiseresidual soundwhite 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 acoustic impedance and related terms.

Specular Reflection
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

Specular Reflection

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

Speed, in physics speed and velocity are different, they both relate to the distance travelled in time, but velocity is a vector quantity and includes information on the direction of travel

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 speed of sound (c), wavelength (λ) and frequency (f) are related by the formula c = λ·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

SPL : sound pressure level

SPP : speech 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.

Spreading Loss is also known as divergence loss

square metre (m2) under metre-squared

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

SRI : sound reduction index

Standard Atmospheric Pressure

Standardised measurements are carried out in accordance with a Standard or 'Norm' for example,
Standardised Impact Sound Pressure Level (LnT)
Standardised Impact Sound Pressure Level (L'nT)
Standardised Level Difference (DnT)

Standard Reference Levels

Standing Wave
Standing Wave 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 - symbol ps.
See also atmospheric pressuresound pressure

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 L10, L90 etc., statistical levels of the noise.

Statistical Noise LevelsL10, L90, Ln etc.,
Statistical Noise Levelsthe full version
Statistical Sound Power Absorption Coefficient

Steady-state Oscillation Definition (IEC 801-24-24) oscillation that continues without change

See also other oscillation terms

STC : sound transmission class

STI : speech transmission index
STIPA : speech 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

STL : sound 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

Stress applied to a material is the force per unit area, measured in N/m2 (newtons per metre-squared) or pascals (Pa)

Structure-borne Noise, noise caused by the vibration of elements of a structure, the source of which is within a building or structure with common elements, for example poor impact sound insulation, building services plant, manufacturing machinery and local construction or demolition work. 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.

Surface Acoustic Wave

Surface Acoustic Wave
Surface Area
Surface Normal
Surface Wavefront

SVL : sound velocity level

SWL : sound 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 : 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.

Symmetry, exact reflection of form on opposite sides of a dividing line, plane or waveform.

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