**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.

**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 Bandwidth • Constant Percentage Bandwidths • Octave Bands • FFT- 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

**Ratio** the relative magnitudes of two quantities (usually expressed as a quotient)

**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.

**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 Particle Velocity : vo = 5 x 10

Reference Sound Energy : Wo = 10

Reference Sound Energy Density : wo = 1 pJ/m

Reference Sound Exposure : Eo = (20 μPa)

Reference Sound Intensity : Io = 1 pW/m

Reference Sound Power : Po = 1 pW = 10

Reference Sound Pressure : po = 20 x 10

Reference Sound Pressure : po = 1 x 10

Reference Vibratory Acceleration : ao = 1 μm/s

Reference Vibratory Displacement : ξo = 1pm ≡ 0 dB

Reference Vibratory Force : Fo = 10

Reference Vibratory Velocity : vo = 1 nm/s ≡ 0 dB

Reference Voltage : vo = 1 Volt ≡ 0 dB

See also : decibel scales and examples

**Reference Time** is used for calculation of
Daily Personal Noise Exposure •
Sound Exposure Level and
Time Weighed Average with reference times other than 8 hours.

Reflected Wave

Refraction

Refraction Loss

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

See also : Background Noise

See also : Acoustic Impedance

● Note : the quantity that is the measure of response should be indicated; for example, velocity resonance.

**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.

● Note : in case of possible confusion, the type of resonance must be indicated; for example, velocity resonance frequency.

Reverberant Field

Reverberant Sound Field

**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.

**Sabine's Reverberation 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

where :

V = room volume in m^{3}

A = α x S = equivalent absorption surface or area in m^{2}

α = absorbent coefficient or attenuation coefficient

T = RT60 = reverberation time in s, seconds

S = absorbing surface in m^{2}

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

See also : Early Decay Time • Schroeder • 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 / Quadratic Mean** is the square root of the arithmetic mean of the squares of the numbers in a series

**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 is also known as the Effective value**

The abbreviation RMS was formerly denoted as r.m.s. or rms, but these notations are now deprecated

**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 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 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.

Root Mean Quad : RMQ

Root Mean Square : RMS

**rpm : **revolutions per minute

**RT** : Reverberation Time

**RT60** : Reverberation Time

**RTA** : Real Time Analyser

**Rw** : Weighted Sound Reduction Index

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

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