Alpha w  aw (weighted sound absorption coefficient)  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.
Dncw (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.
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.
NRC (Noise Reduction Coefficient)  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 onethird octave or octave bands.
Rw (weighted sound reduction index)  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.
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 roomtoroom transmission of sound via a common ceiling plenum.
Sound Insulation  A general term describing the reduction of airborne and structure borne sound between separate spaces.
