Acoustic Glossary


R : Sound and Vibration Definitions, Terms, Units, Measurements etc., ..

Rsound reduction index - laboratory measurements
R'apparent sound reduction index - field measurements

Radian (rad) 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.

Since the circumference of a circle = 2·π·r, then one radian equals 360°/(2·π) ≈ 57.3° and π/2 radians equals a right angle (90°)

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

Radian per second (rad/s) see angular velocity

Radiation Factor

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

IEC 801-31-27, in decibels, ten times the logarithm to the base ten of the radiation factor.

Random Incidence

Random Incidence Microphone also known as diffuse incidence microphones.
Used for measurements in reflective room and chambers as opposed to the Free-field Microphones, used for measurements in more open spaces. The presence of the microphone should not to effect the measurement.

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

See also • free-field microphonespressure microphones

Random Noise

is a signal whose instantaneous value varies randomly with time.

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 noisebackground noisebroadband noisegaussian noisenarrowband noiseperiodicpink noisepseudo random noiseresidual noisespecific noisewhite noisewideband noise

Random Vibration a vibration whose instantaneous amplitude is not specified at any instant of time.

Rapid Speech Transmission Index


RASTIrapid speech transmission index

Rate of Fluid Flow under volumetric flow rate

Ratio the relative magnitudes of two quantities (usually expressed as a quotient)

Rayl a unit of specific acoustic impedance, the ratio of the sound pressure to the particle velocity - named after Lord Rayleigh.

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

Rayleigh Disk a disk on a torsion suspension designed to measure the sound particle velocity in a fluid.

Rayleigh Wave

Raynaud's Syndrome

RCRoom Criteria

Reactance the imaginary part of impedance.

See also • acoustic impedance

Reactive Sound Intensity
Reactive Sound Field
Reactivity Sound Index

Real (of a number or quantity) having no imaginary part.

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.

Real World +4 dB in the real world there are factors that can reduce the effectiveness of hearing protectors: imperfect fitting and the condition of the protectors are two examples. To allow for this the HSE recommends the addition of 4 dB to the calculated level at the ear.

Rectangular Window a time window that has a zero value outside the specified time record and unity within the record length. In the FFT analyser, the rectangular window is actually no window at all. It is also called rectangular weighting, or uniform weighting, and is used when the signal to be analysed is a transient rather than a continuous signal : see also windowing.

Reference Frequencies under preferred frequencies

Reference Quantities expressed in SI units

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

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

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

Reference Sound Exposure (Eo) = (20 μPa)2 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 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 is used for calculation of daily personal noise exposuresound exposure level and time weighed average with reference times other than 8 hours.

Reflected Wave
Refraction Loss

Residual Intensity

Residual Noise the noise remaining when the specific noise is suppressed.
See also • background noise

Resistance the real part of impedance
See also • acoustic impedance


resonance is the tendency of a mechanical or electrical system to vibrate or oscillate at a certain frequency when excited by an external source, and to keep oscillating after the source is removed. If something tends to have resonance it is said to be resonant.

see also • anti-resonance

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.

Reverberant Field

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 Chamber

a specially constructed test room in which all the surfaces are hard and reflective so that none of the noise produced will be lost by absorption, often used for the measurement of sound power levels of noisy machines.

Reverberation Room

IEC 801-31-13, room having a long reverberation time, especially designed to make the sound field therein as diffuse as possible
Note : Reverberation rooms are used in particular for the measurement of absorption coefficients of materials and of the sound power of sound sources

Reverberation Time (RT)

RT60 or T60 the time it takes for the sound pressure level to fall by 60 dB after the sound has been turned off. 60 dB corresponds to a decrease in sound pressure by a factor of 1000.

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's Reverberation-time 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
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 normalized 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.

Measuring reverberation times also enables the calculation of the total sound absorption of a room. The reverberation time varies with frequency.

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

See also • artificial-reverberationearly decay timeschroederbackward curve integration

RMQ (Root Mean Quad) is used in Vibration Dose VDV measurements to take account of the impulsive nature of these measurements. The procedure is similar to the more commonly used RMS method below except the 4th power average is calculated before taking the ∜ - quad root or 4th root.

RMS (Quadratic Mean)

is the square root of the arithmetic mean of the squares of the numbers in a series

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. For a sine wave, if you multiply the RMS value by the square root of 2 (1.414), you get the peak value of the wave.

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. Also known as the effective value

RMS Averaging also called power spectrum averaging, calculates the weighted average of the sum of the squared levels. The weighting is either linear or exponential. RMS averaging reduces random fluctuations in the levels but does not reduce the noise floor.

See also other types of averaging

Room Absorption
Room Absorption Coefficient

Room Acoustics sound waves are reflected by the walls, ceiling, floor and any object they come into contact with. The reflected wave is modified in various ways. sound absorption is a major contributor, hard reflecting surfaces common in reverberant rooms leave the sound largely unchanged whereas soft absorbent surfaces found in the home produce significant changes.

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

Room Criteria (RC) a single-number for rating room noise. Based on the preferred speech interference level values. Suffixes are added, R for Rumble, RV for Vibration and Rattle, H for Hiss and N for Neutral.

This system is considered by some to more effective than the Noise Criteria (NC) system.

The B&K 2250 sound analyser, measures RC values.

Room Modes when sound is generated in a room, by a loudspeaker for example, the sound 'decays' in time due to the sound interacting with the room surfaces and objects in the room see sound absorption. The time taken for the sound to decay is known as the reverberation time.

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

Root Mean Quad
Root Mean Square

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 sound source of 1 pascal = 94 db SPL, add another identical sound source, total = 2 pascals = 100 dB SPL. Two identical source increase the sound pressure, a sound field quantity by 6 dB. 20 log(2) = 6dB

Example 2: a sound source of 1 Watt = 120 db SWL, add another identical sound source, total = 2 Watts = 123 dB SWL. Two identical sources increase the sound power output by 3 dB. 10 log(2) = 3dB

10 dB is a 10x change in power quantities and 20x change in field quantities

Rotational Wave

rpm ▷ revolutions per minute

RTreverberation time
RT60reverberation time

RTAreal time analyser

Rwweighted sound reduction index
R'wweighted apparent sound reduction index

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