*R •* sound reduction index (laboratory measurements)

*R' •* apparent sound reduction index (field measurements)

**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

Indicators • LAeq,6h • LAeq,16h • LAeq,18h • Lday • Levening.

Random Incidence

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

See also • free-field microphones • pressure microphones

**Random Noise** is a signal whose instantaneous value varies randomly with time.

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

Rapid Speech Transmission Index

Rarefaction

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

Rayleigh Wave

Raynaud's Syndrome

Reactive Sound Intensity

Reactive Sound Field

Reactivity Sound Index

**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 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/m^{3} = 10^{-12} J/m^{3} ≡ 0 dB

*Reference Sound Exposure (Eo)* = (20 μPa)^{2} s ≡ 0 dB

*Reference Sound Intensity (Io)* = 1 pW/m^{2} = 10^{-12} W/m^{2} ≡ 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/s^{2} ≡ 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

Reflected Sound Wave

Reflection

Refraction

Refraction Loss

See also • background noise

Residual Sound Intensity

**Resistance** the real part of impedance

See also • acoustic impedance

See also • anti-resonance

● Note : the quantity that is the measure of response should be indicated; for example, velocity resonance.

● Note : in case of possible confusion, the type of resonance must be indicated; for example, velocity resonance frequency.

Reverberant Field

See also • anechoic

● Note : Reverberation rooms are used in particular for the measurement of absorption coefficients of materials and of the sound power of sound sources

V = room volume in m

A = α x S = equivalent absorption surface or area in m

α = absorbent coefficient or attenuation coefficient

T = RT60 = reverberation time in s, seconds

S = absorbing surface in m

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-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 • mean squareSee also other types of averaging

Indicators • LA10,18h • LAeq,16h • Lday • Levening.

Room Absorption

Room Absorption Coefficient

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

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

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 Mean Quad

Root Mean Square

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

*Rw •* weighted sound reduction index

*R'w •* weighted apparent sound reduction index