Acoustic Glossary


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

S/N : Signal-to-Noise ratio the difference between the nominal or maximum operating level and the noise floor in dB

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.

SAW : Acoustic 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. 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 : 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

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, suitable mainly for steady noise signals only. Superseded, in the main, by real time analysis.

Shear Wave

Shock 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 is 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 between the nominal or maximum operating level and the noise floor in dB

SIL : Sound Intensity Level
SIL : Speech Interference Level.

Simple Sound Source

Sine Wave or pure tone is 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 - NNR used in the USA.

Single Number Rating : SNR a rating system for duct silencers.

SLM : Sound Level Meter

Slow Time Weighting - also known as Slow Response and Slow Time Constant.

SNR : see Single Number Rating 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

Sound 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

Sound Absorption
Sound Absorption Coefficient
Sound Absorption Loss

Sound Analyser under Spectrum Analyser.

Sound Dose

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 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
Sound Power Level
Sound Power Reflection Coefficient

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

Sound Pressure
Sound Pressure Level
Sound Pressure Reflection Coefficient

Sound Propagation Sound 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 Linear Exponent of Sound Propagation.

Elementary Attenuation of Propagation Definition

IEC 801-23-36, real part of the elementary exponent of sound propagation.

Elementary Dephasing of Sound Propagation Definition

IEC 801-23-38, imaginary part of the elementary exponent of sound propagation

Elementary Exponent of Sound Propagation Definition

IEC 801-23-38, with respect to a system having a periodic structure, the natural logarithm of the complex ratio of particle velocities (or pressures) measured at two successive corresponding points of the structure, when this structure is assumed to be of infinite length

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, 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, 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 also known as a 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 : 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 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

Specific Acoustic Admittance Definition

IEC 801-25-38, reciprocal of the Specific Acoustic Impedance

Specific Acoustic Impedance Definition

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

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 or rayls in MKS units.

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
Specific 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.

See also : Ambient NoiseResidual 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

Spectra is the plural of spectrum

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

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

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 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 is also known as 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 Definition

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

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 should not to be confused with Sound Particle Velocity.

Spherical Wave

SPL : Sound Pressure Level

SPP : Speech Privacy Potential

Spreading Loss Definition

IEC 801-2341, that part of the transmission loss due to the divergence, i.e. spreading, of sound waves in accordance with the configuration of the system, also known as Divergence Loss.
Note : Spreading Loss exists, for example, for spherical waves emitted by a point source.

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.

SRI : Sound Reduction Index.

Standard Atmospheric Pressure : atm atmospheric pressure is the force per unit area exerted on a surface by the weight of air above that surface. Standard Atmospheric Pressure is equal to 101.325 kPa the preferred SI units or 8760 mmHg and 1013.25 millibars.

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 stationary signal is 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 Ln of the noise.

Statistical Noise Levels
Statistical Sound Power Absorption Coefficient

Steady-state Oscillation

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 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 Wave
Surface Wavefront

SVL : Sound Velocity Level under Particle 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 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.

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

Synchronous Averaging see 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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