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°)
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
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
Random Vibration a vibration whose instantaneous amplitude is not specified at any instant of time.
Ratio the relative magnitudes of two quantities (usually expressed as a quotient)
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.
See also • acoustic impedance
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 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
see also • anti-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.
See also • anechoic
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 also came up with the formulae relating
reverberation time with sound absorption and room volume: T = 0.161 V/A
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
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. 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
RMS Value is also known as the effective value
The root-mean-square pressure, also known as the effective sound pressure is most often used to characterize a sound wave because it is directly related to the energy carried by a sound wave
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 in rectangular rooms are associated with pairs of parallel walls.
Tangential Modes involve four room surfaces
Oblique Modes all six surfaces.
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. 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. 10 log(2) = 3dB, see our sound power table for more examples.See also our sound level calculation page and the IEC decibel definition.
rpm ▫ revolutions per minute