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°)
Radian per second (rad/s) see angular velocity
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
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 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.
see also • anti-resonance
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
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 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 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.
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
rpm ▷ revolutions per minute