Help Topics - R

Random noise

An oscillation whose instantaneous magnitude is not specified for any given instant of time. It can be described statistically by the probability distribution function giving the fraction of the total time that the magnitude of the noise lies within a specified range.

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Real time analysis

The process of analyzing the frequency components of a complex sound into octave or third octave bands when all of the band levels are obtained at the same time. This is sometimes referred to as simultaneous frequency analysis and is a feature usually found on the more sophisticated models in a manufacturer’s range of instruments due to the complexity of the signal processing involved. This type of analysis is required for the correct determination of transient sounds such as aircraft flyovers or impulsive signals from blasts. Since all of the bands are calculated simultaneously no energy is lost or missed when performing this type of analysis.

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Reflection

The return of a sound wave from a hard surface. Also the scattering of a light wave by a dust particle in the air from its original straight path through the medium.

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Refraction

The bending of a sound wave or light wave from its original path, either because it is passing from one medium to another or by changes in the physical properties of the medium. Examples of this are temperature or wind gradients in the atmosphere.

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Relative humidity

The amount of water vapor in the air at any given time is usually less than that required to fully saturate the air. The relative humidity is the percentage of the saturation humidity, generally calculated in relation to saturated vapor density and expressed as a percentage figure. When air temperature and dew point temperatures are very close, the air is said to have a high relative humidity. This unit is usually abbreviated to RH%.

Resonance

The relatively large amplitude of sound (or vibration) when the frequency of some source of sound matches the natural frequency of some object or component of the system.

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Resonator

A device that responds or resounds in sympathy with a source of sound or vibration.

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Respirable

The mass fraction of particles with a mean diameter of less than 4 microns.

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Reverberant field

The region in an enclosed space or room where the reflected sound dominates, as opposed to the region close to the noise source where the direct sound dominates.

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Reverberation

The persistence of a sound in an enclosed space as a result of multiple reflections after the source of the noise has stopped.

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Reverberant room

A specially designed room having a long reverberation time, usually used to make the sound field inside it as diffuse as possible. The walls are usually constructed from hard material such as smoothly plastered concrete blocks and the opposing walls are made non-parallel to prevent the buildup of standing waves with the space.

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Reverberation Time (RT60)

The reverberation time of a room is the time taken for the sound to decay by 60 dB from its steady state value when the source of the sound energy is suddenly stopped. It is a measure of the persistence of an impulsive sound in a room as well as the amount of acoustical absorption present inside the room. Rooms with long reverberation times are said to be “live” rooms while rooms with short RT’s are said to be “dead” rooms. If a room has an RT that is too long, speech will be difficult to follow and absorption material may have to be placed in the room to reduce the RT value to acceptable limits.

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RMS. Sound pressure

The root mean square (rms) value of a time varying signal is obtained by squaring the function at each instant, obtaining the average of the squared values over the interval of interest, and then taking the square root of the average value. The purpose of this is to convert an a.c. waveform into its equivalent d.c. value such that the variations can be read on a sound-measuring instrument. The rms value represents the effective energy value and is the best measure of steady continuous sounds. The period of interest, or averaging time, in sound measuring instruments is sometimes called the time weighting and is standardized in international regulations to specific values called the Slow, Fast and Impulse responses.

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RMS. level

The rms level is the slowly varying sound level in decibels read from the display of a sound level meter (or other equivalent device) that shows the instantaneous sound pressure level with a selected time weighting applied to the signal. This is usually referred to as the Slow or Fast sound pressure level depending upon the selection of the time weighting in the instrument.

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Rotameter

Same as a flow meter for verifying the flow rate of a sampling pump.

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Run

The complete measurement process from the start time to the stop time encompassing all of the noise during that interval. A run can have just a single value attributed to the total noise level or it can be comprised of a number of regular shorter intervals that show how the noise level changed over time.

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