Acoustic Glossary
Noise and Vibration Terms and Parameters
R
Radian : rad : a derived SI unit of angle measurement. One radian is the angle at the centre of a circle that cuts off an arc of length equal to the radius. Since the circumference equals 2 π times the radius, then one radian equals 360°/(2 π) or 180°/π, approximately 57.3°
To convert radians to degrees multiply the radians by 180/π
Radian per second : rad/s : see angular velocity
Random Noise : random noise is a signal whose instantaneous value varies randomly with time. In the context of assessing hearing damage, any audible sound should be regarded as noise. Pink noise is random noise whose spectrum falls at 3 dB per octave: it is useful for use with sound analyzers with constant percentage bandwidths - see also white noise.
Random Vibration : a vibration whose instantaneous amplitude is not specified at any instant of time.
Rapid Speech Transmission Index : RASTI : is an objective way of measuring speech intelligibility. It is measured by placing a loudspeaker, which transmits sound from the location of the person speaking, and a microphone where the listeners are situated - see also speech transmission index and the B&K 3361. See also our separate download file on RASTI Measurements and Speech Privacy
RASTI : under Rapid Speech Transmission Index.
Rayl : is the SI derived unit of acoustic impedance. 1 MKS rayl = 1 pascal-second per metre, Pa·s/m - 1 newton-second per cubic metre, N·s/m³ - named after Lord Rayleigh.
Raynaud's Syndrome : under Hand Arm Vibration Syndrome
Reactive Intensity : the part of the sound field that does not contribute to the net flow of energy - see sound intensity.
RC : see Room Criteria
Reactive Sound Field : a sound field in which the particle velocity is 90° out of phase with the pressure, a standing wave is an example of this type of field.
Real Time Analyzer : RTA : a device 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 Time Window : a time window that has a zero value outside the specified time record and unity within the record length. In the FFT analyzer, 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 Hanning Window.
Reference Quantities : Standard Reference levels expressed in SI units where log = log10.
| Description | Definition, dB | Reference Quantity = 0 dB |
| Acceleration Level : La | 20 log (a/ao) | 10-5 m/s2 |
| Particle Velocity Level : Lv | 10 log (v/vo) | 5 x 10-8 m/s |
| Sound Energy Level : Lw | 10 log (W/Wo) | 10-12 J |
| Sound Energy Density Level : LE | 10 log (E/Eo) | 10-12 J/m3 |
| Sound Intensity Level : LI | 10 log (I/Io) | 10-12 W/m2 |
| Sound Power Level : LPac | 10 log (P/Po) | 10-12 W |
| Sound Pressure Level : Lp - in air | 20 log (p/po) | 2 x 10-5 Pa |
| Vibration Velocity Level : Lv | 20 log (v/vo) | 10-9 m/s |
| Voltage Level : Lu | 20 log (v/vo) | 1 Volt |
see also our decibel scales and examples
Reference Sound Pressure : a reference pressure of 20 μPa = 20 micro pascals. This reference pressure was chosen conventionally to correspond to the quietest sound at 1000 Hz that the human ear can detect.
Reference Time : is used for calculation of Lep,d,v Sound Exposure Level and TWAv Time Weighed Average with a reference time other than 8 hours.
Refraction : the bending of a sound wave from its original path, either because it is passing from one medium to another with different velocities or by changes in the physical properties of the medium, for example, a temperature or wind gradient in the air.
Residual Intensity : LI,R : the sound intensity level measured when the same signal is fed to both channels of a sound intensity measuring system and is due to phase mismatch between channels - see the B&K 4297 Sound Intensity Calibrator.
Residual Intensity Level : subtract the pressure residual intensity index spectrum from the mean sound pressure spectrum.
Residual Noise : this is ambient noise without the specific noise. It is the noise remaining at a point under certain conditions when the noise from the specific noise is suppressed. See also background noise
Residual p-i Index : the residual pressure-intensity Index describes the phase mismatch between the two channels used for sound intensity measurements. It is equal to the mean pressure minus the intensity with the probe in a sound field where the pressure is uniform over the volume containing the microphones - for example, using the B&K 4297 Sound Intensity Calibrator.
Resonance : resonance is the tendency of a mechanical or electrical system to vibrate or oscillate at a certain frequency when excited by an external source, and to keep oscillating after the source is removed. If something tends to have resonance it is said to be resonant.
Resonant Frequency : the frequency at which resonance occurs.
Reverberant Field: the region in a room where the reflected sound dominates, as opposed to the region close to the noise source where the direct sound dominates. The same as diffuse field - see also far field : near field
Reverberation: sound that persists in an enclosed space, as a result of repeated reflection or scattering, after the sound source has stopped - more detail.
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 Time : RT60 or T60 - the time it takes for the sound pressure level to fall by 60 dB after the sound has been turned off. 60 dB corresponds to a decrease in sound pressure by a factor of 1000. Measuring reverberation times also enables the calculation of the total sound absorption of a room. The reverberation time varies with frequency - more detail
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. The RMS value, also called the effective value of the sound pressure, is the best measure of ordinary continuous sound, but the peak value is necessary for assessment of impulsive noises. Also, used to describe the mathematical process of determining the average value of a complex signal.
RMS averaging : averaging technique used to average the energy, or power, of a signal. RMS averaging reduces signal fluctuations but not the noise floor. RMS averaging for dual-channel measurements is defined in such a way to preserve important phase information.
Room Absorption : sum of Sabin absorption due to objects and surfaces in a room, and due to dissipation of energy in the medium within the room.
Room Absorption Coefficient : ratio of Sabine absorption of a surface to the area of the surface.
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.
See also reverberation above
Room Criteria : RC : A single-number for rating room noise. Based on the PSIL 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 analyzer, measures the 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 - see above. 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.
See also reverberation above
Root-Mean-Square : under RMS
rpm = revolutions per minute. :
RT : under Reverberation Time.
RTA : see Real Time Analyzer.
RT60 : under Reverberation Time.
R : Sound Reduction Index : the measured quantity which characterises the sound insulating properties of a material or building element in a stated frequency band - laboratory measurement.
R' : Sound Reduction Index : the measured quantity which characterises the sound insulating properties of a material or building element in a stated frequency band - field measurement.
Rw : Weighted Sound Reduction Index : a single-number quantity which characterises the airborne sound insulation of a material or building element over a range of frequencies - based on the laboratory measurement of R.
R'w : Weighted Sound Reduction Index : a single-number quantity which characterises the airborne sound insulation of a material or building element over a range of frequencies - based on the field measurement of R'.
Other Glossary Terms
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