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

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.

Random Noise : 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.

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.

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.

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.

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.

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.

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.