Acoustic Glossary

rad (radian) a derived SI Unit of angle measurement. One radian is the angle made at the centre of a circle by an arc whose length is equal to the radius of the circle.

To convert radian to degrees multiply the radian by 180/π : see also angles

Radiation Factor Definition (IEC 801-31-26) ratio of sound power radiated by a plate of a given area vibrating with a given root-mean-square velocity over the area, to that power which would be emitted as a plane wave by a plate of the same area vibrating in phase with the same vibration velocity.

Radiation Index Definition (IEC 801-31-27) in decibels, ten times the logarithm to the base ten of the radiation factor.

Because of their importance in acoustics we have a full page on measurement microphones

See also free-field microphones • pressure microphones

Random Noise Definition (IEC 801-21-09) oscillation due to the aggregate of a large number of elementary disturbances with random occurrence in time.

Other noise descriptors, ambient noise • background noise • broadband noise • gaussian noise • narrowband noise • periodic • pink noise • pseudo random noise • residual sound • sound • specific sound • white noise • wideband noise

See also reference time interval.

Caution Rayls may be published in MKS and or CGS units, which are not the same.

Real Time Analyser (RTA) an instrument which uses a number of narrow bandwidth filters connected to a display to give a visual indication of the amplitude in each frequency band simultaneously or at the same time.

Real Time Frequency Analysis measurement of octave or third octave band noise where all the filters are measured simultaneously, ensures no loss of data.

Reference Quantities expressed in derived SI units

Reference Particle Velocity (vo) = 5 x 10^-8 m/s ≡ 0 dB

Reference Sound Energy (Wo) = 1pJ = 10^-12 J ≡ 0 dB

Reference Sound Energy Density (Eo) = 1 pJ/m³ = 10^-12 J/m³ ≡ 0 dB

Reference Sound Exposure (Eo) = 20 μPa² s ≡ 0 dB

Reference Sound Intensity (Io) = 1 pW/m² = 10^-12 W/m² ≡ 0 dB

Reference Sound Power (Po) = 1 pW = 10^-12 W ≡ 0 dB

Reference Sound Pressure (po) = 20 μPa = 20 x 10^-6 Pa ≡ 0 dB in air

Reference Sound Pressure (po) = 1 x 10^-6 Pa ≡ 0 dB in liquids and solids

Reference Vibratory Acceleration (ao) = 1 μm/s² ≡ 0 dB

Reference Vibratory Displacement (ξo) = 1pm ≡ 0 dB

Reference Vibratory Force (Fo) = 10^-6 N ≡ 0 dB

Reference Vibratory Velocity (vo) = 1 nm/s ≡ 0 dB

Reference Voltage (vo) = 1 Volt ≡ 0 dB

See also our decibel reference tables

Reference Time Interval (Tr), the specified interval over which the specific sound level is determined.

Residual Sound Level (Lr) is the LAeq,T, equivalent continuous A-weighted sound pressure level of the residual sound at the assessment location over a given time interval, T

See also background noise.

Resonance Definition (IEC 801-24-05) phenomenon of a system in forced oscillation such that any change, however small, in the frequency of excitation results in a decrease in a response of the system.
Note : the quantity that is the measure of response should be indicated; for example, velocity resonance.

Resonance Frequency Definition (IEC 801-24-06) frequency at which resonance exists
Note : in case of possible confusion, the type of resonance must be indicated; for example, velocity resonance frequency.

Response Definition (IEC 801-21-47) of a device or system, the motion, or other output, that results from a stimulus (excitation) under specified conditions. The kinds of input and output being utilized must be indicated.

Reverberation is the persistence of sound as a result of multiple reflections and may continue for a second or more depending on the reverberation time

Reverberation Definition (IEC 801-21-14) sound that persists in an enclosed space, as a result of repeated reflection or scattering, after the sound source has stopped.

Reverberation Room Definition (IEC 801-31-13) room having a long reverberation time, especially designed to make the sound field therein as diffuse as possible

Reverberation Time (RT60 or RT60) is the time required for the sound pressure level to decrease by 60 dB after the sound source has stopped. However 60 dB corresponds to a decrease of a 1000 in sound pressure level and in practice the background noise often limits this measurement range. In which case the decay measured from 5 dB to 35 dB below the initial level is recorded and denoted as the RT30, or in noisy environments the RT20, values between 5 dB and 25 dB are reported.

Reverberation Time Definition (IEC 801-31-07) of an enclosure, for a sound of a given frequency or frequency band, time that would be required for the sound pressure level in the enclosure to decrease by 60 decibels, after the source has been stopped

Sabine Reverberation Time Equation in 1898 W C Sabine devised the formula relating reverberation time with sound absorption and room volume: T = 0.161 V/A, where:
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
The above equation is normalised to the speed of sound in air = 343 m/s

It follows if you know the reverberation time you can calculate the absorption coefficient and vice-versa.

Early Decay Time (EDT), 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

Reverberation Time is a significant parameter in acoustics : so we have more details

See also artificial-reverberation • early decay time • schroeder • backward curve integration

RMS (root mean square of a time-varying quantity) is obtained by squaring the amplitude at each instant, obtaining the average of the squared values over the interval of interest, and then taking the square root of this average.

RMS Value Definition (IEC 103-02-03) for a time-dependent quantity, positive square root of the mean value of the square of the quantity taken over a given time interval
Note : The root-mean-square value of a quantity may be denoted by adding one of the subscripts eff or rms to the symbol of the quantity
Note : The abbreviation RMS was formerly denoted as r.m.s. or rms, but these notations are now deprecated.

RMS Value is also known as the effective value

The root-mean-square sound pressure, also known as the effective sound pressure is most often used to characterise a sound wave because it is directly related to the sound energy carried by a sound wave

See also other types of averaging

See also aircraft noise • industrial and port noise • railway noise •

The changes are frequency dependent which makes things more complicated to predict. In large spaces air absorption can be significant at higher frequencies.

This system is considered by some to more effective than the noise criteria (NC) system.

However rooms may also have one or more modes or resonances related to the room dimensions and the wavelength of the sound. These room modes and standing waves can dramatically effect the room's acoustic performance.

Root-power Quantity was introduced in ISO 8000 Annex C and defined as the square root of a power quantity; it replaces and deprecates the term field quantity. It is essential to know which category a measurement belongs to when using decibels (dB) for comparing such quantities.

Example 1: a root power quantity (sound pressure) of 1 pascal = 94 dB SPL, add another identical sound source, the total = 2 pascals = 100 dB SPL. Two identical root power sources increase the sound pressure, a sound field quantity by 6 dB, i.e. 20 log (2) = 6dB. See our sound pressure table for more examples.

Example 2: a power quantity (sound power) of 1 Watt = 120 dB SWL, add another identical sound source, the total = 2 watts = 123 dB SWL. Two identical power sources increase the sound power output by 3 dB, i.e. 10 log (2) = 3 dB. See our sound power table for more examples.

See also our sound level calculation page and the IEC decibel definition.