Definitions, Terms, Units and Parameters
R
To find specific terms on a page use CTRL+FR' : Sound Reduction Index
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 velocityRandom Incidence Microphone
: also referred to as Diffuse Incidence Microphones. Designed to be omni-directional to measure sound pressure levels from multiple directions. Used for measurements in reflective room and chambers as opposed to the free-field microphone, more widely used for measurements in more open spaces. The presence of the microphone should not to effect the measurement.Because of their importance we have a full page on Measurement Microphones
See also Free-field Microphones, Pressure Microphones.
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 analysers with constant percentage bandwidths - see also white noise.Other Noise Terms.
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 PrivacyRASTI
: see Rapid Speech Transmission Index above.Rayl
: A unit of specific impedance (acoustic impedance), the ratio of the sound pressure to the particle velocity - named after Lord Rayleigh.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. See also Active Sound Field.Other Sound Fields.
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 Time 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 Window.Reference Frequencies
: under Preferred FrequenciesReference Quantities
: Standard Reference levels expressed in SI units where lg is the logarithm to the base 10.| Description | Definition, dB | Reference Quantity = 0 dB |
| Reference Acceleration Level : La | 20 lg (a/ao) | 10-6 m/s2 |
| Reference Particle Velocity Level : Lv | 10 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) | 2 x 10-5 Pa |
| 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 our decibel scales and examples
Reference Sound Pressure
: a reference pressure of 20 μPa = 20 micro pascals. This reference sound 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 Daily Personal Noise Exposure l Sound Exposure Level l Time Weighed Average : TWA 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 Noise
: this is ambient noise without the specific noise, i.e. the noise remaining when the specific noise is suppressed - LAeqT. See also Background NoiseResonance
: 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.Other Sound Fields.
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 : 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 detailsRMS
: the 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
: 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.Other averaging methods : Exponential Linear Spatial Spectrum Time Domain
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.When measuring sound insulation between rooms is common practice to Normalise the measured levels to a reference absorption A0, of 10 m²
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.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.rpm : revolutions per minute
RT60 : Reverberation Time
R'w : Weighted Sound Reduction Index
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