Acoustic Glossary


S : Sound and Vibration Terms, Definitions, 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 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 Superseded, in the main, by Real Time Analysis.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.

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:
  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 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 Exposure
Sound Exposure Level
Sound Exposure Meter

Sound Fields

Sound Field Quantities : that follow

The Inverse Square Law
Sound Pressure
Sound or Particle Velocity
Particle Displacement or Displacement Amplitude

Sound Flux

Sound Insulation

Sound Intensity
Sound Intensity Level

Sound Level
Sound Level Meter
Sound Level Meter Classes

Sound Localisation

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

Sound Power
Sound Power Density
Sound Power Level

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

Sound Pressure : p
Sound Pressure Level : SPL

Sound Reduction Coefficient : under Noise Reduction Coefficient.
Sound Reduction Index

Sound Reference Levels

Sound Source IEC Definition : 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 IEC Definition : source that radiates sound as if from a single point.

see also :

Sound Fields
Sound Waves

Sound Spectrum IEC Definition : 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, Should 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 :

Ensemble Averaging
Exponential Averaging
Linear Averaging
RMS Averaging
Spectrum Averaging
Time Domain Averaging

Specific Acoustic Impedance

Specific Energy

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 Noise and Residual Noise.

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

Spectra : is the plural of spectrum

Spectral Density : IEC Definition 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).

see also : Energy Spectral Density.

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

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 Spectrum.
Line 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 :

Ensemble Averaging
Exponential Averaging
Linear Averaging
RMS Averaging
Spatial Averaging
Time Domain Averaging

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

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 : 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 : Divergence 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

Standing Wave

Standing Wave Tube : a method for measuring Absorption Coefficients by means of Standing Waves in a tube.

Static Pressure IEC Definition : 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.

Steady State Oscillation : Oscillation that continues without change.

STC : Sound Transmission Class.

STI : Speech Transmission Index.
STIPA : Speech Transmission Index for Public Address systems.

Stimulus : IEC Definition external force, or other input, applied to a system

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 and Simple Sound Source : IEC Definition maximum instantaneous Volume Velocity produced by a simple source small compared with Wavelength, emitting a wave with sinusoidal variation with time

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.

Surface Area
Surface Wave
Surface Wavefront

SVL : Sound Velocity Level : see Particle Velocity Level

SWL : Sound Power Level

Synchronous Averaging : see Time Domain Averaging.


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