Definitions, Terms, Units and Parameters
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S/N : Signal-to-Noise ratio: the difference between the nominal or maximum operating level and the noise floor in dB
Sabin: a unit of sound absorption of a surface, A square metre of 100% absorbing material has a value of 1 metric sabin. An example of this would be a 1 m² open window.
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 Sabin Absorption is A = 0.921 Vd / c.
Note the unit is the sabin, not the sabine.
Sabine: Wallace Clement : American pioneer in architectural acoustics. He derived an expression for the duration T of the residual sound to decay below the audible intensity. The associated absorption unit was named after him - sabin without the e.
Sabine Equation: in 1898 W C Sabine also came up with the formulae relating Reverberation Time with Sound Absorption and Room Volume.
T = 0.161 V/A
V = room volume in m3
A = α x S = equivalent absorption surface or area in m2
α = Absorbent coefficient or attenuation coefficient
T = RT60 = reverberation time in s, seconds
S = absorbing surface in m2
It follows if you know the Reverberation Time you can calculate the A, Absorption Coefficient and vice-versa.
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.
Scalar Quantity: a quantity having only magnitude and not direction, 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 can 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' 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
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 Superseded, in the main, by Real Time Analysis.
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:
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
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.
Both are based on the Sound Exposure Level : SEL definition 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, 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.
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 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 fraction of sound energy absorbed by a material. It is expressed as a value between 1.0, perfect absorption and no reflection and 0, total reflection and zero absorption. The value varies with frequency and angle of incidence.
It is determined experimentally - see sabin.
Sound Fields: there are various
Active Sound Field
Anechoic Sound Field
Diffuse Sound Field
Random Incidence Sound Field
Reactive Sound Field
Sound Field Quantities: that follow the Inverse Square Law
Sound or Particle Velocity
Particle Displacement or Displacement Amplitude
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 Leq - equivalent sound level and time weighting .
We also have a full page dedicated to Sound Level Meters and Leq Basics.
Sound Level Meter Classes: Sound Level Meters are divided into classes, originally known as types. Class 1 are Precision instruments with a wider frequency range and tolerances than the Class 2 instruments. Class 0 is a requirement for Laboratory measurements
Sound Transmission: passage of a sound wave through a medium or series of media.
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
Spatial Averaging: taking measurements at various positions and averaging the results. Mandatory in sound insulation measurements and recommended anywhere multiple reflections are present.
Other averaging methods : Exponential Linear RMS Spectrum Time Domain
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 metre 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 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.
Other averaging methods : Exponential Linear RMS Spatial Time Domain
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 Privacy Potential : SPP: combines the partition sound insulation performance, installed Dw : Weighted Difference Level with the measured background noise level in the receiving room (dBA). The higher the resulting SPP, the higher the level of privacy between the rooms.
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 : c: at a temperature of 20 °C, the speed of sound in air at sea level is approximately 344 m/s - not to be confused with sound particle velocity.
The frequency f, wavelength λ and wave velocity v are related by the formulae λ = v/f
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.
For example 2 × 2 = 4 , so the square root of 4 = 2.
The symbol is √
Another example: √ 36 = 6 (because 6 × 6 = 36)
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.
Standardised: measurement in accordance with a Standard or 'Norm'.
Standardised Impact Sound Pressure Level : LnT
Standardised Impact Sound Pressure Level : L'nT
Standardised Level Difference : DnT :
Standardised Impact Sound Pressure Level : L'nT
Standardised Level Difference : DnT :
Standing Wave: a phenomenon when a sound is reflected back and forth between two parallel surfaces, such as two side walls in a room. Combination of two waves each having the same amplitude and frequency.
Standing Wave Tube: a method for measuring acoustic absorption coefficients by means of standing waves in a tube.
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.
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.
Synchronous Averaging: see time domain averaging.
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