Rapid Speech Transmission Index RASTI
Rapid Speech Transmission Index RASTI is a measure for the speech intelligibility like STI. The measurement method for RASTI uses a limited frequency range compared with the STI method.

RC (Room Criteria) curves
Noise rating curves used mainly in the USA to evaluate background noise from e.g. HVAC components.

Reflection
Reflection occurs when a sound wave hits a surface which has a low absorption coefficient. The sound wave is reflected by the surface and returns into the room.

Resonance
The condition that results when a system is acted upon by a periodic driving force the frequency of which coincides with one of the natural frequencies of the system. The steady-state amplitude of the system, for fixed amplitude of the driving force, is a local maximum at a resonance frequency.

Resonance frequency
The frequency at which any system vibrates naturally when excited by a stimulus. A tuning fork, for example, resonates at a specific frequency when struck. In building acoustics the resonance of the mass-spring-mass system for a double construction reduces the sound insulation of this construction at this frequency. See also Resonance.

Reverberant sound field (diffuse sound field)
A sound field made of reflected sounds in which the time average of the mean square sound pressure is everywhere the same and the flow of energy in all directions is equally probable. This requires a room with hard surfaces with very low absorption coefficients like a reverberation room for acoustic measurements.

Reverberation
Reverberation is an expression of the fact that a sound continues after the sound source has stopped. See reverberation time.

Reverberation time
Reverberation time is a measurement used in acoustic design. It is defined as the length of time taken for a sound to decay 60 dB after the source has stopped.
Some times, the reverberation time is called T20 – indicating that the 60 dB decay time is extrapolated from the decay -5 dB to -25 dB or T30, extrapolated from the decay -5 dB to -35 dB.

Rw. R’w: weighted sound reduction index, dB
A single-number rating of the sound reduction through a wall or other building element. Since the sound reduction may be different at different frequencies, test measurements are subjected to a standard procedure which yields a single number that is about equal to the average sound reduction in the middle
of the human hearing range. The weighting procedure is standardized in ISO 717-1 for the sound reduction of airborne noise.
R’w (dB) is the weighted apparent sound reduction index in a building, including flanking sound transmission. Rw is the laboratory value for the weighted sound reduction without flanking transmission.

Room acoustics
Room acoustics describes how sound behaves in an enclosed space. Deals with the sound propagation inside a room (sound absorption and reflection on surfaces, absorption in air). Important parameters are reverberation time, echoes and speech intelligibility.