Acoustic Glossary


R : Sound and Vibration Terms and Definitions ...

R : sound reduction index - laboratory measurement
R' : apparent sound reduction index - field measurement

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.

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

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

rad/s : radian per second, under angular velocity
rad/s2 : radian per second-squared, under angular acceleration

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.

Railway Noise (UK Government Environmental Noise Regulations)
Indicators LAeqLdayLevening.

See also aircraft noiseindustrial and port noiseroad traffic noise

Random Incidence Microphone also known as diffuse field 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 Incidence Sound Wave

Random Noise, 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 soundsoundspecific soundwhite noisewideband noise

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

Rapid Speech Transmission Index


RASTI : rapid speech transmission index

Rate of Fluid Flow under volumetric flow rate

Rating Level (LAr,Tr), the specific sound level plus any adjustment for the characteristic feature of the sound, BS 4142 refers

See also reference time interval.

Ratio the relative magnitudes of two quantities (usually expressed as a quotient) , for example sound pressure and the reference sound pressure when calculating sound pressure levels in decibels (dB).

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 published in MKS and or CGS units, which are not the same.

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

Rayleigh Wave
Raynaud's Syndrome

RC : room criteria

Reactance the imaginary part of impedance.

See also acoustic impedance

Reactive Sound Intensity
Reactive Sound Field
Reactivity 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 - UK Government Health and Safety Executive 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 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 Sound Source - Pistonphone

Reference Time is used for calculation of daily noise exposuresound exposure level and time weighed average with reference times other than 8 hours.

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

Reflected Sound Wave
Refraction Loss

Relaxation Time under time constant

Residual Sound the ambient sound remaining when the specific sound is suppressed.

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.

Residual Sound Intensity

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. Some devices may keep oscillating, for a time after the source is removed. If something tends to have resonance it is said to be resonant.
See also antiresonance

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 under reverberant sound field

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.

See also anechoic

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 Definition (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 (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.

Reference Reverberation Time of 0.5 seconds is often cited as the typical value for a medium sized, carpeted and furnished living room and does not require detailed and accurate knowledge of the dimensions of the test rooms. The reference reverberation time is used in sound insulation measurements

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-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 (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 (Quadratic Mean) is the square root of the arithmetic mean of the squares of the numbers in a series

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 mean square

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

Road Traffic Noise (UK Government Environmental Noise Regulations)
Indicators LA10,18hLAeq,16hLdayLevening.

See also aircraft noiseindustrial and port noiserailway noise

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 more complicated to predict. In large spaces air absorption can be significant at higher frequencies.

See also room modes

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 : N for Neutral.

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

Room Modes the sound entering a room 'decays' in time due to the sound waves interacting with the room surfaces and objects in the room, see sound absorption for more details. The time taken for the sound to decay is known as the reverberation time.

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.

Axial Modes are associated with pairs of parallel walls.
Tangential Modes involve four room surfaces
Oblique Modes all six surfaces.

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 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.

Rotational Wave

RT : reverberation time
RT60 : reverberation time

RTA : real time analyser

Rw : weighted sound reduction index
R'w : weighted apparent sound reduction index

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