Acoustic Glossary


R : Sound and Vibration Terms, Definitions, Units, Measurements ..

R  : Sound Reduction Index - Laboratory Measurements
R' : Apparent Sound Reduction Index - Field Measurements

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°)

To convert radians to Degrees multiply the radians by 180/π

see also : Angles

Radian per second : rad/s : see Angular Velocity

Random Incidence Microphone : also referred to 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

see also :

Free-field Microphones
Pressure Microphones

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

Random Noise IEC Definition : oscillation due to the aggregate of a large number of elementary disturbances with random occurrence in time.

Pseudo Random Noise : electronically generated noise which may appear to lack any pattern, but does consist of pulses that repeat themselves periodically. The period is determined by the generator span and the number of generator lines.

Related Terms :

Broadband Noise
Narrowband Noise
Pink Noise
White Noise

see also :

Constant BandwidthConstant Percentage BandwidthsOctave BandsFFT- Fast Fourier Transform.

Random Vibration : a vibration whose instantaneous amplitude is not specified at any instant of time.

Rapid Speech Transmission Index : RASTI


RASTI : Rapid Speech Transmission Index

Rating Level : LAR T : the A-weighted, Leq, Sound Pressure Level of an industrial noise during a specified time period, adjusted for Tonal Character and Impulsiveness.

Rayl : A unit of Specific Acoustic Impedance, the ratio of the Sound Pressure to the Particle Velocity - named after Lord Rayleigh.

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.

Raynaud's Syndrome

RC : Room Criteria

Reactance : the imaginary part of Impedance.

see also Acoustic Impedance

Reactive Intensity
Reactive Sound Field
Reactivity Index

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.

Reference Frequencies : under Preferred Frequencies

Reference Quantities : Standard Reference levels expressed in SI units where lg is the Logarithm to the base 10.

Description     Definition, dB     Reference = 0 dB
Reference Particle Velocity Level : Lv     20 lg (v/vo)   5 x 10-8 m/s
Reference Sound Energy Level : Lw   10 lg (W/Wo)   10-12 J
Reference Sound Energy Density Level : LE   10 lg (E/Eo)   10-12 J/m3
Reference Sound Intensity Level : LI   10 lg (I/Io)   10-12 W/m2
Reference Sound Power Level : LPac   10 lg (P/Po)   10-12 W
Reference Sound Pressure Level : Lp - in air     20 lg (p/po)   20 x 10-6 Pa
Reference Sound Pressure Level : - liquids and solids     20 lg (p/po)   1 x 10-6 Pa
Reference Vibration Acceleration Level : La   20 lg (a/ao)   10-6 m/s2
Reference Vibration Force Level : LF   20 lg (F/Fo)   10-6 N
Reference Vibration Velocity Level : Lv   20 lg (v/vo)   10-9 m/s
Reference Voltage Level : Lu   20 lg (v/vo)   1 Volt

see also : decibel scales and examples

Reference Sound Pressure

Reference Time : is used for calculation Daily Personal Noise Exposure l Sound Exposure Level l Time Weighed Average : TWA with a reference time other than 8 hours.

Reflected Wave

Residual Intensity

Residual Noise : the noise remaining when the specific noise is suppressed.

see also: Background Noise

Resistance : the real part of Impedance

see also Acoustic Impedance

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.

Response IEC Definition of a device or system, the motion, or other output, that results from a stimulus (excitation) under specified conditions. The kinds of input and output being utilized must be indicated

Reverberant Field

Reverberation IEC Definition : sound that persists in an enclosed space, as a result of repeated reflection or scattering, after the sound source has stopped.

Reverberation is a significant parameter in Acoustics - so we have more details

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 : RT : 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.

Reverberation Time is a significant parameter in Acoustics - so we have more details

See also Early Decay Time, Schroeder and Backward Curve Integration

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 or 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.

In acoustics the RMS value is also known as the Effective value.

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 :

Ensemble Averaging
Exponential Averaging
Linear Averaging
Spatial Averaging
Spectrum Averaging
Time Domain Averaging

Room Absorption
Room Absorption Coefficient

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 : 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 Analyser, 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.

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.

Root Mean Quad : RMQ
Root Mean Square : RMS

Rotational Wave

rpm : revolutions per minute

RT : Reverberation Time
RT60 : Reverberation Time

RTA : Real Time Analyser

RwWeighted Sound Reduction Index
R'w : Weighted Apparent Sound Reduction Index


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