Acoustic Glossary

Sampling Frequency the rate at which a continuous waveform is digitised, given in Hz.

As opposed to a vector quantity which has both magnitude and direction, for example displacement, acceleration and velocity

Scalar Quantities may be added, subtracted or multiplied like ordinary numbers, vector quantities cannot.

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

0.001 s = 1 ms (millisecond) and 0.000001 s = 1 μs (microsecond).

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

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

See also other oscillation terms

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

There are 7 base units from which other units are derived and therefore known as SI derived units.

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.

Sine Wave or Pure Tone characterised 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 and SEL (sound exposure level)

See also other hearing protector procedures

Slow Time Weighting is also known as slow response and slow time constant.

By definition one sone = 40 phons and also equals 40 dB on the equal loudness contours.

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.

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 • infrasound • speed of sound • ultrasound sound

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

See also • elementary attenuation of propagation • elementary dephasing of sound propagation • elementary exponent of sound propagation • propagation loss definition

See also reflected sound wave

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 • directivity • sound fields • sound waves

Sound Transmission passage of a sound wave through a medium or series of media.

See also other types of averaging

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 Energy the energy per unit mass, joule per kilogram = J/kg

See also • flow resistance

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 • periodic • pink noise • pseudo random noise • random noise • residual noise • white noise • wideband noise

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

See also acoustic impedance and related topics.

Spectra is the plural of spectrum

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.

Spectrum the description of a sound wave's resolution into its components of frequency and amplitude.

See also • continuous spectrum • line spectrum

See also other types of averaging

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 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 (p²/B) ÷ (po²/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 sound pressure level observed through the filter, the above relation reduces to Lps = Lp - log10 (B/Bo) dB

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

Speed of Sound Definition IEC 801-23-18, magnitude of the phase velocity of a free progressive sound wave

Spreading Loss is also known as divergence loss

Standing Wave Tube a method for measuring absorption coefficients by means of standing waves in a tube.

Static Pressure (symbol p_s)

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 L10, L90 etc., statistical levels of the noise.

See also other oscillation terms

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.

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.