Acoustics Glossary

A-Z list of terminology commonly used within acoustics.



R and R’: sound reduction index (R) and apparent sound reduction index (R’). Terms relating to the sound insulation performance of partitions defined in BS EN ISO 140-4, measured in octave or third octave frequency bands.

Rw and R’w: single figure frequency weighted values of R and R’ defined in BS EN ISO 717-1.

Random noise/vibration signal: a noise, vibration or signal, which has a random waveform, with no periodicity.

RASTI: rapid analysis speech transmission index; a measurement parameter for assessing the speech intelligibility in a room. IT has a value between 0 and 1: 1 representing perfect speech intelligibility and 0 representing zero speech intelligibility. RASTI is a shortened version of Speech Transmission Index, STI.

Rating level: LAr,Tr a noise index defined in BS 4142 and BS 7445; the specific noise level plus any adjustment for the characteristic features of the noise (e.g. tonal and impulsive features) during a specified time (see under specific noise level).

Ray: a straight line representing the direction in which a sound is travelling, used in situations where the size of reflecting surfaces is large compared to the sound wavelength.

Rayl: the unit of specific acoustic impedance.

Rayleigh wave: a type of elastic wave, which propagates close to the surface of a solid.

Raynaud’s disease: a disorder affecting the blood vessels, nerves, connective tissues and bones of the fingers; one of its causes is prolonged exposure to high levels of vibration.

Ray tracing: a method of modelling room acoustic performance using computer software package (also called beam tracing).

Reactance: the complex component of impedance associated with energy being stored and converted from one form to another (e.g. from potential to kinetic, or from electrostatic to electromagnetic) rather than being converted to heat.

Reactive silencer: a silencer, which reduces sound levels by using changes in impedance instead of sound absorbing materials.

Real time, in: quickly enough to observe changes in a situation as they happen.

Real-time analyser: a device, which is capable of analysing signals (usually in the frequency domain) in real time.

Real world protection: the degree of sound level attenuation provided by hearing protectors under realistic working conditions, as opposed to that from laboratory tests; an allowance to be deducted from manufacturer’s test data to allow for real world conditions.

Recruitment: an aspect of certain forms of perspective deafness; an abnormally rapid increase in the sensation of loudness with increasing sound pressure level.

Reference time interval, Tr: the specified interval over which a noise index such as equivalent continuous A-weighted sound pressure level is determined for assessment purposes e.g. in BS 7445 and BS 4142. The value of Tr is 1 hour during the day and 5 minutes at night.

Reference value: standardised values used as the basis for decibel scales of sound pressure, sound intensity, sound power, vibration acceleration, velocity and displacement.

Reflection: the redirection of waves, which occurs at a boundary between media when the size of the boundary interface is large compared with the wavelength (see also diffuse reflection and specular reflection).

Refraction: the change in direction of waves caused by changes in the wave velocity in the medium.

Repeatability: the variability of measurements when repeated under the same measurement conditions.

Reproducibility: the variability of measurements when repeated under different measurement conditions.

Residual noise: The ambient noise remaining at a given position in a given situation when the specific noise source is suppressed to a degree such that it does not contribute to the ambient noise (defined in BS 4142 and BS 7445; see also specific noise and ambient noise).

Residual noise level, LAeq,T: the equivalent continuous A-weighted sound pressure level of the residual noise.

Resonance: the situation in which the amplitude of forced vibration of a system reaches a maximum at a certain forcing frequency (called the resonance frequency).

Resonance frequency: the frequency at which resonance occurs, i.e. at which the forced vibration amplitude in response to a force of constant amplitude is a maximum; for an undamped system the resonance frequency is the same as the natural frequency of the system; for a damped system the resonance frequency is slightly reduced.

Reverberant room: a standard acoustic test environment designed to produce diffuse sound conditions throughout the space.

Reverberant sound/reverberation: the sound in an enclosed space, which results from repeated reflections at the boundaries.

Reverberant sound field: the region in an enclosed space in which the reverberant sound is the major contributor to the total sound pressure level.

Reverberation time: the time required for the steady sound pressure level in an enclosed space to decay by 60 dB, measured from the moment the sound source is switched off.

Ringing: transient free vibration of bodies caused by impact.

Robust Standard details: a method of building new dwelllings according to prescribed details which guarantees sound insulation sufficient to meet the requirements of the 2002 Building Regulations and so avoid the need for pre-completion sound insulation testing.

Room constant, Rc: a constant used in the calculation of reverberant sound pressure level in a room: Rc = S /(1 РαAVG), where S= total area of room surfaces and αAVG =average absorption of room surfaces.

Room mode: a three-dimensional standing-wave sound pressure pattern, i.e. a mode shape, associated with one of the natural frequencies of a room.

Root mean quad (RMQ): the RMQ value of a set of numbers is the fourth root of the average of the fourth powers of the numbers; for a vibration waveform the RMQ value over a given time period is the fourth root of the average value of the fourth power of the waveform over that time period.

Root mean square (RMS): the RMS value of a set of numbers is the square root of the average of their squares; for a sound or vibration waveform the RMS value over a given time period is the square root of the average of the square of the waveform over that time period.

Round window: a diaphragm or membrane at the end of the cochlea, which connects with the middle ear.