Reverberation Time Calculation Glossary
Absorption
When sound waves contact a room surface such as a ceiling, wall or floor, a portion of the sound energy is reflected back into the room, and the rest is considered to be absorbed primarily by conversion into heat due to friction within a porous material, or by vibration of a solid material.
Articulation Class, AC
A measure for rating the speech privacy performance of a ceiling in an open plan environment where sound is reflected off the ceiling between two adjacent spaces divided by partial-height furniture panels. A ceiling system with AC < 150 is low performance, whereas one with AC > 200 is high performance.
Articulation Index, AI
A measure for rating speech intelligibility, where an AI < 0.05 is representative of very poor speech intelligibility, and an AI > 0.80 represents good speech intelligibility. The Speech Privacy Index PI is derived from the AI.
Background Noise
The noise level in a space, which is a composite of sound from HVAC, equipment, activity noises, etc., from both near and far, but excluding specific sources of interest such as a person talking in an adjacent space (which would be considered the signal).
Ceiling Attenuation Class, CAC
A measure for rating the performance of a ceiling system as a barrier to airborne sound transmission through a common plenum between adjacent closed spaces such as offices. A ceiling system with a CAC < 25 is consider low performance, whereas one with CAC > 35 is high performance.
Decibel, dB, dBA
A measure for rating the level of a sound, which uses a logarithmic scale. The sound level in dB is often represented as a dBA, where the "A" indicates a specific frequency weighting used to represent how we perceive loudness as a function of frequency since our hearing is non-linear with frequency. A sound level of dBA < 30 is a very low level, whereas dBA > 90 is a high level.
Frequency
The repetition rate of a sound wave measured in cycles per second, which is usually expressed in Hertz (Hz). The audible frequency range for normal hearing individual spans from 20Hz to 20 kHz. Frequency is that characteristic of a sound which is perceived by a listener as the "pitch".
Noise Reduction Coefficient, NRC
A measure for rating the overall sound absorption performance of a material when used in an enclosed architectural space such as an office, where sound is being reflected at many angles of incidence. Specifically, it is the 4 frequency averaged absorption coefficients @ 250, 500, 1000 and 2000 Hz, rounded to the nearest 0.05. A material with NRC < 0.50 is a poor absorber, and NRC > .80 is a very good absorber.
Octave Band
This is a continuous band of frequencies which span a range such that the highest frequency in the band is twice the lowest frequency. The audio frequency range is separated into octave bands as a matter of convenience, and each band is individually specified by its center frequency, e.g. 63 Hz, 125 Hz, 250 Hz, 500 Hz 1 kHz, 2 kHz, 4 kHz, 8 kHz, and 16 kHz,
Plenum
The ceiling plenum is the volume defined by the area above the back of the ceiling tile, and below the bottom of the structural slab above. Within this plenum is usually found a combination of HVAC ducts, electrical and electronic conduits, water pipes, traditional masking sound speakers, etc.
Reflector
Any material that reflects more than 50% of the sound energy that strikes it. The NRC is a measure of a sound absorber performance, so (1-NRC) will give an indication of the reflector performance.
Reverberation
The buildup of sound within an architectural space, such as a room, as a result of repeated sound reflections at the surfaces of the room. The level of the reverberant sound within a room is dependant on both the volume of the room and the amount of sound absorption installed within the room, such that small hard surfaced rooms are "louder" than large well treated rooms.
Reverberation time, RT60
A measure for rating the quality of the sound environment within an architectural space, and it appropriateness for various uses. Specifically, the reverberation time is the time it takes for reflected sound within a space to decrease by 60 dB after the sound was made, for instance, the time it takes before you cannot hear a loud clap. An RT60 < 1 sec. is beneficial for good speech intelligibility, whereas RT60 > 2.5 sec is appropriate for symphony music.
Room Criterion, RC curves
A family of noise rating curves specifying the level and frequency content of background noise from HVAC equipment which will have a varying degree of acceptability for occupied architectural spaces such as offices. For example, a background noise of between RC 25 and RC 35 is appropriate for a private office environment. The RC curves above 31 Hz are basically - 5 dB/OB slope.
Sound Paths
A trace of the path taken by specific sound waves as they move outward from the source of the sound to the receiver location. This path could be the direct sound path, a transmitted sound path, or a reverberant sound path which may include multiple reflections.
Sound Transmission Class, STC
A measure for rating the performance of a wall system as a barrier to airborne sound transmission between adjacent closed spaces, such as offices. A wall system with an STC < 35 is consider low performance, whereas one with an STC > 55 is high performance.
Transformer Tap
Refers to the power setting available on a step-down transformer (e.g. 1 watt, 2 watt, 4 watt, etc.) used in high voltage distributed sound systems (e.g. 70v, 50v, and 25v amplifier systems). The transformer is usually located on the loudspeaker.
Sound Transmission Loss, TL
A measure of the performance of a wall system as a barrier to airborne sound transmission between adjacent closed spaces, as measured in 1/3 octave bands. The TL in 1/3 OB's is used to calculate the STC which is a single number rating for the entire frequency range.