R

Random incidence
A sound field in which the sound strikes a surface or an object from all angles of incidence at random. It occurs in a highly diffuse sound field.

RASTI
Rapid Speech Transmission Index. This is an objective evaluation method of quickly measuring the speech intelligibility of an enclosed space in two frequency bands of 500 Hz and 2 kHz.

Receiving Room
The room that is separated from the source room by a partition or a floor, and in which the soud transmitted from the source room is measured.

Reflection
For surfaces large compared to the wave length of impinging sound, sound is reflected much as light is reflected, with the angle of incidence equalinq the angle of reflection.

Reflection Coefficient
A measure of the sound reflective property of a surface. More specifically, the fraction of the incident sound energy reflected by a surface.

Refraction
The bending of sound waves travelling through layered media with different sound velocities.

Resilient Mounting
Any mounting or attachment system that reduces the transmission of vibrational energy from a vibrating body to an adjacent structure.

Resonance
A resonant system vibrates at maximum amplitude when tuned to its natural frequency.

Reverberant Sound
A sound field produced and dominated by reflected sound from room boundary surfaces.

Reverberation
The persistence of sound within a space after the sound source has stopped.

Reverberation Time
The time in seconds required for a sound to decay, roughly speaking, to inaudibility after the source ceases. (Strictly, the time in seconds for the sound level at specific frequency to decrease 60 dB in level after the source stops).

Room Criteria (RC) curves
A set of contours, roughly corresponding to the ear’s response to sound pressure level at various frequencies, which define the background noise level within a space, particularly that due to HVAC noise.