S : Sound and Vibration • definitions • terms • units • measurements ...
▫ signal-to-noise ratio
Sabine Absorption Coefficient
Sabine Reverberation Time Equation
Sampling Frequency the rate at which a continuous waveform is digitised, given in Hz.
how long the data is measured at each sample point.
says that ideally a signal should be sampled at a rate twice its highest frequency component.
SAW - surface acoustic wave
, under rayleigh waves
a quantity fully described by a magnitude or numerical value, for example density
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.
, 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.
EDT : Early Decay Time based on the impulse decay curve, 0 dB to -10 dB
T10 decay time based on the impulse decay curve, -5 dB to -15 dB
T20 decay time based on the impulse decay curve, -5 dB to -25 dB
T30 decay time based on the impulse decay curve, -5 dB to -35 dB
The Schroeder Method
is also known as Backward Curve Integration
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)
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
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
▫ 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
▫ 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 Superseded by real time analysis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.
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
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 ||0.000.000.001|
| 10-12 || pico || p ||0.000.000.000.001|
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.
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 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
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 SEL (sound exposure level) 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. 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
Single Number Rating (SNR) a rating system for duct silencers.
▫ sound level meter
Slow Time Weighting is also known as slow response and slow time constant.
SNR, see the • Single Number Rating(s) above
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 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
Sound is also known as Acoustic Oscillation and Acoustic Vibration
See also •
audible sound •
complex sound •
speed of sound •
Sound Diffuse Field
under Sound Energy Density
Sound Energy Density
Sound Energy Flux
Sound Energy Flux Density
Sound Energy Flux Density Level
Sound Exposure Level
Sound Exposure Meter
Sound Field Quantities
Inverse Square Law
under sound energy flux
Sound Isolation Between Rooms
under sound insulation, level difference
Sound Level Meter
Sound Level Meter Classes
See also the IEC Definition of Level
Sound Particle Acceleration.
Sound Particle Displacement.
Sound Particle Velocity.
Sound Power Absorption Coefficient
Sound Power Density
Sound Power Level
Sound Power Reflection Coefficient
Sound Power Through A Surface Element Definition IEC 801-21-37, time average
product of the in-phase components of the instantaneous sound pressure
and the volume velocity
through the surface element considered, and is also known as the sound energy flux
Sound Pressure Level
Sound Pressure Reflection Coefficient
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
• elementary attenuation of propagation
• elementary dephasing of sound propagation
• elementary exponent of sound propagation
• propagation loss definition
Sound Rating Level (LArT)
, the A-weighted
, sound pressure level
of an industrial noise during a specified time period, adjusted for tonal character
Sound Reduction Coefficient under noise reduction coefficient
Sound Reduction Index under sound insulation
Sound Reference Levels
, when a sound wave reaches the boundary between one medium
and another medium, a portion of the wave undergoes reflection and a portion of the wave undergoes transmission 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.
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 •
sound fields •
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)
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 Volume Velocity
under volume velocity
Sound Wave Velocity
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
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
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 noise •
background noise •
broadband noise •
gaussian noise •
narrowband noise •
pink noise •
pseudo random noise •
random noise •
residual noise •
white 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
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 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 •
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.
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
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
/B) ÷ (po2
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
under spectrum density level
Speech - Articulation Index (AI)
Speech - Articulation Intelligibility
Speech - Clarity
Speech - Intelligibility
Speech Interference Level (SIL)
Speech Interference Level (SIL3)
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.
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
▫ sound pressure level
▫ 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
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.
▫ sound reduction index
Standard Atmospheric Pressure
measurement in accordance with a Standard or 'Norm'.
Standard Reference Levels
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
Static Pressure (symbol ps)
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
▫ 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
▫ 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
a significant portion of the transmission path from source to receiver takes place in a solid structure rather than through the air.
under Sound Waves
Surface Acoustic Wave (SAW)
, under rayleigh waves
▫ sound velocity level
, under particle velocity level
▫ sound power level
, 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.
, exact reflection of form on opposite sides of a dividing line, plane or waveform
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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