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

Sabin

Sabin Absorption

Sabine

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.

**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

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 Superseded, in the main, by real time analysis.

**Shock** rapid transient transmission of mechanical energy.

**Shock Pulse** IEC Definition, 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

**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**.

Factor | Name | Symbol | Multiplying Factor |
---|---|---|---|

10^{12} | tera | T | 1,000,000,000,000 |

10^{9} | giga | G | 1,000,000,000 |

10^{6} | mega | M | 1,000,000 |

10^{3} | 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.

**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 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.

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

50 phon = 2 sones

60 phon = 4 sones

70 phon = 8 sones

80 phon = 16 sones

90 phon = 32 sones ... and so on

**Sound** IEC Definition, movement of particles in an elastic medium about an equilibrium position

**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 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 Intensity

Sound Level

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

Sound Power Absorption Coefficient

Sound Power Density

Sound Power Level

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

Sound Pressure

Sound Pressure Level

**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** IEC Definition, 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**

**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

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 Directivity • 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 other types of Averaging

**Specific Acoustic Admittance** IEC Definition, reciprocal of the **Specific Acoustic Impedance**

**Specific Acoustic Impedance** IEC Definition, 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/m^{3} or rayls in MKS units.

**Specific Acoustic Reactance**IEC Definition, Imaginary part of the **Specific Acoustic Impedance**

**Specific Acoustic Resistance**IEC Definition, Real part of the **Specific Acoustic Impedance**

Specific Energy

**Specific Flow Resistance** IEC Definition, 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

**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 • Residual Noise.

**Specific Volume : v** the number of cubic metres occupied by one kilogram of the substance : m^{3}/kg.

**Specific Wall Admittance** IEC Definition, quotient of the particle velocity normal to a wall, by the sound pressure acting on the wall

**Specific Wall Impedance** IEC Definition, 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** 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).

**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 • 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 other types of Averaging

**Spectrum Density Level** IEC Definition, 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 (p^{2}/B) ÷ (po^{2}/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 pressure level observed through the filter, the above relation reduces to
Lps = Lp - Log10 (B/Bo) dB

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** IEC Definition, 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

**The Speed of Sound should not to be confused with the** Sound Particle Velocity.

Spelling

Spherical Wave

SPP : Speech Privacy Potential

**Spreading Loss** IEC Definition, 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 : m ^{2}** : Area

**square root : √** the square root of a number is a smaller number that, when multiplied by itself equals the original number.

**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 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

Statistical Sound Power Absorption Coefficient

**Steady-state Oscillation** IEC Definition, 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

**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.

**Subharmonic Response** IEC Definition, periodic response of a system at a frequency that is a submultiple of the excitation frequency

**Superposition principle** under Sound Waves.

Surface Area

Surface Wave

Surface Wavefront

**SVL : Sound Velocity Level** under Particle Velocity Level

**Quality Symbols** IEC Definition, character or combination of characters denoting a quantity. 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