Acoustic Glossary
Noise and Vibration Terms and Parameters
S
S/N : Signal-to-noise Ratio.
Sabin : a unit of sound absorption of a surface, One square metre of 100% absorbing material for example an open window of 1 m² has a value of 1 metric sabin.
Sabin Absorption : defined by the Sabine reverberation time equation. In a reverberant room of volume V, speed of sound c, and decay rate d, the Sabine absorption is A = 0.921 Vd / c.
Sampling Frequency : the rate at which a continuous waveform is digitised.
Sampling Theorem : a signal is completely described if it is sampled at a rate twice its highest frequency component.
Scalar Quantity : a quantity only having magnitude not direction - as opposed to a vector quantity.
Schroeder : method for calculating the reverberation time from the impulse noise decay curve. Computer algorithm developed by Manfred Schroeder at Bell Labs in the 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
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.
Shock : rapid transient transmission of mechanical energy.
Short Leq : the preferred modern method of storing sound levels and displaying the true time history of a noise event. The resulting 'time histories', typically measured in 1/8 second intervals may be used to calculate the 'overall' levels for any sub-period of the overall measurement time.
In earlier times the exponential levels were averaged over time and therefore the results were dependent of the meter settings and could not be combined to get the overall result. See also exponential averaging and linear averaging and a fuller description of Leq the equivalent continuous noise level.
SI Units : is the world's most widely system of units devised around the convenience of the number 10.
SI Unit prefixes:
| Factor | Name | Symbol |
| 109 | giga | G |
| 106 | mega | M |
| 103 | kilo | K |
| 10-3 | milli | m |
| 10-6 | micro | μ |
| 10-9 | nano | n |
| 10-12 | pico | p |
Example 10-6 g = 1 µg = 1 microgram or one millionth of a gram.
However 10-6 kg = 1 mg, one milligram and is a bad mixture of prefixes.
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 .
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 Level : many people think this is the meaning of the acronym SEL - not true, but you can see how the confusion arose. SEL or Sound Exposure Level is any Leq noise level or event normalized to 1 second - see below for more details.
Single Event Noise Exposure Level : LSEL : the dBA level which, if it lasted for one second, would produce the same A-weighted sound energy as the actual event - see also sound exposure.
Single Event Noise Exposure Level : SENEL : the noise exposure level of a single event measured over the time between the initial and final points when the noise level exceeds a predetermined threshold
Single Number Rating : SNR : a single number rating system for hearing protectors, set up by the EU, BS EN ISO 4869
See also the NNR - Noise Reduction Rating used in the USA.
Single Number Rating : SNR : a rating system for duct silencers.
Single Number Rating : SNR : used in building acoustics and defined in BS ISO 717 for rating the sound insulation of building elements.
SLM : Sound Level Meter .
Slow Time Weighting : a standard time weighting applied by the sound level meter - a 1 second averaging time. In the old analogue meter days it was used to give the observer the chance to 'follow' very quick meter fluctuations. The meter response was set to 1 second, both rise and decay. Also known as Slow Response and Slow Time Constant - see also exponential averaging -
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:-
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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 340 m/s. In liquids and solids, the propagation velocity is greater - 1500 m/s in water and 5000 m/s in steel.
Sound Absorption : the product of absorption coefficient and surface area of a material. The unit is the sabin. Designates the amount of sound absorbed by a material.
Sound Absorption Coefficient : The practical unit between 0 and 1 expressing the absorbing efficiency of a material. It is determined experimentally - see sabin.
Sound Dose : D : under noise dose
Sound Energy : W : energy present in a sound field. The unit is the joule = W s. watt second.
W = Wo · 10LW/10 J .... the units are Joules = Watt seconds
Standard Reference Sound Energy Wo = 1 pJ = 10-12 J = 0 dB
Sound Energy Level : Lw = 10 log W/Wo dB re 1 pJ or 10-12 Joule.
Sound Energy Density : E or w : is the energy per unit volume.
E = I/c : where I is sound intensity in W/m2 and c is the speed of sound in m/s.
The units are J/m3 or W·s/m3
Once again because of the wide range of the human ear the absolute values of Sound Energy Density are enormous. The decibel ratio to a threshold level produces manageable values - hence the
Sound Energy Density Level : LE = 10 log (E/Eo) dB re 10-12 J/m3 or W·s/m3
* Standard Reference Sound Energy Density Level = 1 x 10-12 J/m3
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0 dB = 1 x 10-12 J/m3 20 dB = 1 x 10-10 J/m3 40 dB = 1 x 10-8 J/m3 20 dB is a factor of 100 40 dB is a factor of 10000 |
See also the table of other Standard Reference Levels
Sound Energy Flux : q : is the average rate of flow of sound energy per second through a specified area.
Sound Exposure : E : is the energy of the A-weighted sound calculated over the measurements time - units Pa²h. For a given duration, each increase of 10 dB(A) in sound pressure level corresponds to a tenfold increase in E. A sound of 85 dB(A) lasting for 8 hours (h) produces 1 Pa².h of sound exposure = 85 LAeq.
* Most modern sound level meters measure sound exposure and then calculate the required parameters.
Sound Exposure Level : SEL : symbol LAE - is an Leq normalised to 1 second. It can therefore be used to compare the energy of noise events which have different time durations. For example if a noise level of 90 dBA last for 1 second then the SEL = 90 dBA. If the same noise event lasted 10 seconds the SEL would be 100 dBA. If it lasted 20 seconds the SEL would be 103 dBA and so on.
* SEL is the Sound Exposure expressed as a logarithm and basically Leq is SEL divided by time.
* Sound Exposure Level, Sound Exposure, Leq and Dose are all mathematically related if the Time is known.
Sound Exposure Level : SEL : also sometimes used to describe the noise from an event such as an individual aircraft flyover.
Sound Exposure Meter : a small instrument designed to be worn by an individual to provide a measure of the accumulated sound exposure received by the wearer while moving about during the workday. The instrument is calibrated in Pa².h. If the meter is worn for only a representative part of the working day, the reading must be corrected appropriately. Noise dosemeter (or dosimeter) is a more general term for instruments having a similar purpose but which may be calibrated to suit differing standards around the world.
Sound Insulation : the ability of a building element or building structure to reduce the sound transmission through it. The sound insulation is measured at different frequencies, normally 100-3150 Hz.
Airborne sound insulation is expressed by a single value, Dn,c,w, Rw or R'w.
Impact sound insulation is expressed by a single value Ln,w or L' n,w
Sound Intensity : I : is the sound power per unit area and is the time averaged product of the pressure and particle velocity. Sound intensity is a vector quantity with both magnitude and direction of sound energy flow.
I = p·v ..... the SI units are W/m2 - watts per square metre.
If you know the sound power and the area you can also calculate the sound intensity.
The human ear can detect sound intensity levels as low as 1 pW and up to 20 Watts and more, a range which makes absolute sound intensity values impractical for everyday use.
A neat solution is the Sound Intensity Level descriptor.
Sound Intensity Level : LI : is a logarithmic measure of the sound power as a relation to the threshold of hearing, Io and makes the values more manageable i.e. 0 to 120+ dB and the symbol is LI.
LI = 10 log(I/Io) dB re 1 pW/m2
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140 dB = 100 W/m2 120 dB = 1 W/m2 100 dB = 0.10 W/m2 80 dB = 0.0001 W/m2 60 dB = 0.000001 W/m2 40 dB = 0.00000001 W/m2 20 dB = 0.0000000001 W/m2 0 dB = 0.000000000001 W/m2 = 10-12 W/m2 = reference level Io* |
* Standard Reference Sound Intensity I0 = 1 pW/m2 = 1 x 10-12 W/m2 = 0dB
LI uses the 10 log equation so, as a rule of thumb:
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3 dB = a factor of 2 in sound intensity 10 dB = a factor of 10 in sound intensity 20 dB = a factor of 100 in sound intensity 30 dB is a factor of 1000 40 dB is a factor of 10000 |
SIL - Sound Intensity Level is measured with a sound intensity probe, made of two closely spaced microphones. It describes as a function of frequency the direction and the amount of net flow of acoustic energy at a given position in a sound field.
See also the table of other Standard Reference Levels
other sound intensity terms include: p-I Index : Pressure Gradient : Pressure Intensity Index : Residual Intensity
Sound Intensity Probe : used to determine the sound power. Bruel & Kjaer's sound intensity probes consist of two closely spaced microphones. This enables the pressure gradient to be measured and therefore the sound velocity to be calculated. It follows therefore that the intensity is the pressure multiplied by the velocity at any given position.
Sound Level : under Sound Pressure Level.
Sound Level Meter : SLM : an instrument, usually hand-held, designed to measure a frequency-weighted value of the sound pressure level in accordance with an accepted national or international standard. It consists of a microphone, amplifier, square-law rectifier, averaging circuits and indicating instrument, having a specified performance in respect of directivity, frequency response, rectification characteristic, and time-weighted averaging. The instrument is normally equipped with F, S and possibly (time-weightings as an aid to measuring fluctuating sounds. With suitable circuitry it can also perform frequency analysis, typically either with octave or one-third octave bands. See also integrating-averaging sound level meter and time weighting .
Sound Particle Acceleration : under Particle Acceleration.
Sound Particle Velocity : v : under Particle Velocity.
Sound Power : Pac : also known as Acoustic Power is the total sound energy radiated by a source per unit of time and equals the sound intensity times the area - the unit is the watt.
P = P · 10LP/10 W
The immense range of human hearing, 0.000000000001 to 10,000 watts or more, means absolute sound power values are impractical for everyday use, a neat solution is the sound power level descriptor.
Sound Power Level : LW : is a logarithmic measure of the sound power as a relation to the threshold of hearing and makes the values more manageable i.e. 0 to 160 dB and the symbol is Lw.
Lw = 10 log (W/Wo) dB re 1 pW = 10-12 W
Other terms in current use include Acoustic Power, LPac, Pac, PWL : SWL
Some typical sound power levels.
| Source | Sound Power, W | Lw re 1 picowatt * |
| Jet engine | 10,000 W | 160 dB |
| Loudspeaker rock | 100 W | 140 dB |
| Pneumatic hammer | 1 W | 120 dB |
| Outboard motor | 0.01 W | 100 dB |
| Dishwasher | 0.0001 W | 80 dB |
| Office | 0.000001 W | 60 dB |
| Fridge | 0.00000001 W | 40 dB |
| Whisper | 0.0000000001 W | 20 dB |
| Threshold | 10-12 W | 0 dB |
* Sound Power Reference Level = 1 x 10-12 W = 1 pW
LW uses the 10 log equation so, as a rule of thumb:
|
3 dB = a factor of 2 in sound power 10 dB = a factor of 10 in sound power 20 dB = a factor of 100 in sound power 30 dB is a factor of 1000 40 dB is a factor of 10000 |
See also the table of other Standard Reference Levels
Sound Pressure : p : is the difference between the pressure produced by a sound wave and the barometric pressure po at a given point in space, Sound pressure is measured in pascals - symbol Pa, units newton per square metre.
p = po · 10Lp/20 Pa where 1 Pa = 1 N/m2 = 1 J/m3 = 10-5 bar = 1 kg/(m·s2)
The immense range of human hearing means absolute sound pressure values are impractical for everyday use, a neat solution is the logarithmic sound pressure level descriptor.
Sound Pressure Level : SPL : or sound level Lp is a logarithmic measure of the rms sound pressure of a sound relative to a reference value. It is measured in decibel (dB).
Lp = 20 log (p/po) dB re 20 µPa for air - water, steel etc., are different.
Some typical sound pressure levels.
| Sources at 1 m | Sound Pressure | Lp re 20 µ Pa * |
| Rifle | 200 Pa | 140 dB |
| Threshold of pain | 20 Pa | 120 dB |
| Pneumatic hammer | 2 Pa | 100 dB |
| 6 dB = double the Pa | 1 Pa | 94 dB |
| Street traffic | 0.2 Pa | 80 dB |
| Talking | 0.02 Pa | 60 dB |
| Library | 0.002 Pa | 40 dB |
| TV Studio | 0.0002 Pa | 20 dB |
| Threshold of hearing | 20 x 10-6 Pa | 0 dB |
* Standard Reference Sound Pressure po = 2 x 10-5 Pa = 20 µPa = 0 dB
Lp uses the 20 log equation so, as a rule of thumb:
|
6 dB = a factor of 2 in sound pressure 10 dB = a factor of 3 in sound pressure 20 dB = a factor of 10 in sound pressure |
See also the table of other Standard Reference Levels
Sound Reduction Index : SRI : see LnT, L'nT, LnT,w, L'nT,w and R, R', Rw, R'w
Sound Transmission Class : STC : the single number rating of a partition's isolation value based on laboratory measurement of TL - transmission loss in one-third octave bands.
Sound Transmission Loss : STL : a measure of sound insulation provided by a structural configuration. Expressed in decibels, it is 10 times the logarithm to the base 10 of the reciprocal of the sound transmission coefficient of the configuration.
Sound Velocity is usually taken to mean the speed of sound and should not be confused with particle velocity, which is the velocity of the individual particles.
Sound Velocity Level : SVL under Particle Velocity Level
Specific Acoustic Impedance : Acoustic Impedance.
Specific Energy : joule per kilogram - J/kg
Specific Noise : noise from the source under investigation. Specific noise is a component of ambient noise and can be identified and associated with the specific source. See also residual noise and background noise.
Specific Volume : v : cubic meter per kilogram - m3/kg
Spectral Density : 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. Power spectral density is commonly expressed in watts per hertz (W/Hz) - see also energy spectral density.
Spectral Leakage : a phenomenon whereby the measured spectral energy appears to leak from one frequency into other frequencies. Spectral leakage occurs when a sampled waveform does not contain an integral number of cycles over the time period during which it was sampled. The technique used to reduce spectral leakage is to multiply the time-domain waveform by a window function. See also windowing.
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.
Spectrum Adaption Term : C and Ctr : these spectrum adaptation terms express how much the airborne sound insulation afforded by the wall or floor being tested would vary from the DnT,w figure if the noise source was A-weighted pink noise (C) or urban road traffic noise (Ctr)
Spectrum Analyzer : an instrument for measuring, and usually recording, the spectrum of a signal. A spectrum analyzer converts a signal from the time domain into the frequency domain, and the FFT analyzer is the most common type today, but there are many other types.
Speech Intelligibility : is directly dependent on the level of background noise, reverberation time and the shape of the room. Different methods are used to evaluate speech intelligibility, the most common is the Speech Transmission Index - see below.
Speech Interference Level : SIL : degree to which background noise interferes with speech. The arithmetic average of sound pressure levels at the 500 Hz, 1 kHz, 2 kHz and 4 kHz in octave band levels.
Speech Interference Level : SIL3 : based on the highest three octaves is the arithmetic average of the 1 kHz, 2 kHz and 4 kHz octave bands.
Note: although SIL, SIL3 and the PSIL Preferred Speech Interference Level are defined for octave band levels they are also calculated from the 1/3-octave bands within each octave before doing the average.
Speech Privacy : : The acoustic dissatisfaction in offices is frequently related to speech privacy - overhearing unwanted conversations, or feeling that one is overheard. See Speech Transmission Index below or our separate download file on Speech Privacy
Speech Transmission Index : STI : a number between 0 and 1 which qualifies speech intelligibility, often implemented in a simplified version known as RASTI - RApid Speech Transmission Index. This method uses a transmitter to broadcast a special modulated noise test signal from a loudspeaker at the talker's location. A receiver with a microphone gives a direct read out of the RASTI value at the receiver position. The full version is derived from a family of Modulation Transfer Function (MTF) curves
Speed of Sound : at a temperature of 21 degrees Celsius, the speed of sound in air at sea level is approximately 344 m/s - not to be confused with sound velocity.
The frequency f, wavelength λ and wave velocity v are related by the formulae λ = v/f
SPL : Sound Pressure Level.
square metre : m2 : area.
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 Impact Sound Pressure Level : LnT : the impact sound pressure level in a stated frequency band, corrected for the standardized reverberation time of 0.5 seconds. Laboratory measurement.
Standardized Impact Sound Pressure Level : L'nT : the impact sound pressure level in a stated frequency band, corrected for the standardized reverberation time of 0.5 seconds. Field measurement.
Standardized Level Difference : DnT : for airborne sound transmission. Similar to the Dn, but this index corrects the measured difference to a standardized reverberation time of 0.5 seconds. This RT value is often cited as approximately average for a medium sized, carpeted and furnished living room. It does not require detailed and accurate knowledge of the dimensions of the test rooms.
Standing Wave : a phenomenon when a sound is reflected back and forth between two parallel surfaces, such as two side walls in a room.
Stationary Signal : a stationary signal is a signal whose average statistical properties over a time interval of interest are constant, and it may be deterministic or not. 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 - more details.
STC : Sound Transmission Class .
STI : Speech Transmission Index .
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.
Structure-borne Noise : noise for which a significant portion of the transmission path from source to receiver takes place in a solid structure rather than through the air.
Structure-borne Sound : sound for which a significant portion of the transmission path from source to receiver takes place in a solid structure rather than through the air.
SVL under Particle Velocity Level
SWL : Sound Power.
Synchronous Averaging : time domain averaging.
Other Glossary Terms
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