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°)
Because of their importance in acoustics we have a full page on measurement microphones
Other noise descriptors • ambient noise • background noise • broadband noise • gaussian noise • narrowband noise • periodic • pink noise • pseudo random noise • residual sound • specific sound • white noise • wideband noise
See also reference time interval.
Caution • Rayls may be 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 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 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
See also • anti-resonanceResonance 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.
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
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
Reverberation Time is a significant parameter in acoustics : so we have more details
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 waveSee also • mean square
See also other types of averaging
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
This 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.Axial Modes are associated with pairs of parallel walls.
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.