
Aweighted Decibel  dB(A)  The Aweighting is a correction applied to the measured sound pressure level which is considered to approximate to the response
of the ear over the normal range of sound levels heard, and thus correlates well the subjective reaction to sound.
Ceiling Attenuation Class, CAC  A singlenumber rating of the laboratory measurement of sound attenuation between rooms sharing a common ceiling plenum.
The rating is determined in accordance with ASTM E 41387 from measurements made in accordance with ASTM 141491a over the
thirdoctave band frequency range 1254000 Hz.
Frequency  The number of times per second in Hertz ( Hz) at which sound pressure waves are generated by a source.
Hertz (Hz)  The unit of frequency measurement. One cycle per second is one Hertz.
Noise Reduction Coefficient, NRC  A singlenumber descriptor for random incidence sound absorption coefficients. Defined in ASTM 423 90a, as the arithmetical
average, to the nearest multiple of 0.05, of the measured sound absorption coefficients at the four onethird octave band
centre frequencies of 250, 500, 1000 and 2000 Hz.
Reverberation  The persistence of sound in an enclosure, due to its continued reflection from the surfaces (walls, ceiling, floor, etc...),
after the sound source has ceased. Reverberation is of significance in determining the quality and level of sound in an enclosure.
Reverberation Time  The time, in seconds, required for reverberant sound in an enclosure to decay to onemillionth (60 dB) of its original intensity
after the cessation of the sound source. Reverberation time is frequency dependent and is normally measured in thirdoctave
or octave bands.
Sound Absorption  The conversion of sound energy into heat (by friction) when passing through or striking a material or when causing a volume
of air to resonate.
Sound Attenuation  A term used in relation to the transmission of sound between rooms sharing a common ceiling plenum.
Sound Insulation  A general term describing the separation of airborne sound between one space and another provided by a barrier between.
Sound Transmission Class, STC  A singlenumber rating of the laboratory measured airborne sound transmission loss. It is calculated in accordance with ASTM
E 41387 using measured values of sound transmission loss obtained in accordance with ASTM E 9090 over the frequency range
1254000 Hz.
Weighted Sound Absorption Coefficient, alpha w  A singlenumber rating for random incidence sound absorption coefficients determined in accordance with EN ISO 11654. With
this method measured values obtained in accordance with EN ISO 20354 are converted into octave bands at 250, 500, 1000, 2000
and 4000 Hz and are plotted on a graph. A standard reference curve is then shifted towards the measured values until a "best
fit" is obtained. The derived value of alpha w will vary between 0.00 and 1.00 but is only expressed in multiples of 0.05
eg alpha w = 0.65.
Weighted Sound Reduction Index  Rw  A single number rating of laboratory measurement of airborne sound reduction index. The rating is determined in accordance
with EN ISO 7171 from measurements made in accordance with EN 201403 over the thirdoctave band frequency range 1003150
Hz.
Weighted Suspended Ceiling Normalised Level Difference, Dncw A singlenumber rating of the laboratory measurement of roomtoroom airborne sound insulation of a suspended ceiling with
a plenum above it. The rating is determined in accordance with EN ISO 7171 from measurements made in accordance with EN 201409
over the thirdoctave band frequency range 1003150 Hz.
