Acoustic Glossary

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.

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.

Schroeder Method 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.

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

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

Shock Pulse Definition (IEC 801-24-27) excitation of a system characterised 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

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

For example the watt is the is derived unit of sound power, equivalent J/s. SI base unit kg·m²·s^-3

Examples: 10^-6 g = 1 μg = 1 microgram or one millionth of a gram.

See also T10 and SEL (sound exposure level)

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 : 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 is any air pressure variation that the human ear can detect. A sound source creates the sound power in watts and the sound energy is transmitted by sound waves, which in turn generate the sound pressure in pascals we hear.

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 Propagation, sound waves propagate through air, water and solids as longitudinal waves or transverse waves

Sound Reflection when a sound wave reaches the boundary between one medium and another, a portion of the wave undergoes reflection and a portion of the wave undergoes transmission loss across the boundary

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.

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 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 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.

The SI units for the main sound levels are :
Sound Intensity = W/m² (watt per metre-squared)
Sound Power = W (watt)
Sound Pressure = Pa (pascal)

See also sound velocity level

See also other types of averaging

Specific Acoustic Admittance Definition (IEC 801-25-38) reciprocal of the specific acoustic impedance
See also acoustic impedance and related terms

Specific Acoustic Impedance (ASA 6.35) at a point in a medium, the complex ratio of sound pressure to particle velocity in the direction of the wave propagation, unit = Pa·s/m (pascal second per metre), also known as the characteristic acoustic impedance.

Specific Acoustic Impedance Definition (IEC 801-25-35) at a point in a sound field, quotient of sound pressure by particle velocity

See acoustic impedance and related terms.

Specific Acoustic Reactance Definition (IEC 801-25-37) Imaginary part of the specific acoustic impedance

See also acoustic impedance and related terms.

Specific Acoustic Resistance Definition (IEC 801-25-36) Real part of the specific acoustic impedance

See also acoustic impedance and related terms.

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

Specific Sound Level (Ls) is the A-weighted, Leq sound level produced by a sound source during a specified period of time T.

Specific Sound Source is the sound source under investigation as defined in BS 4142, method for rating industrial noise affecting mixed residential and industrial areas.

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 acoustic impedance and related terms.

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 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 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 speed of sound (c), wavelength (λ) and frequency (f) are related by the formula c = λ·f

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

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

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 - symbol p_s.
See also atmospheric pressure • sound pressure

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.

Steady-state Oscillation Definition (IEC 801-24-24) oscillation that continues without change

See also other oscillation terms

Stimulus Definition (IEC 801-21-46) external force, or other input, applied to a system, also known as excitation

Symbol of a Quantity Definition (IEC 112-01-03) character or combination of characters denoting a quantity.
Note : 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.