Acoustic definitions

LAeq,30min         

is an average noise level over 30 minutes and is defined as the level of a notional sound which over 30 minutes would deliver the same A-weighted sound energy as the actual fluctuating sound.

DnT(Tmf,max),w

is the weighted (noise) level difference for airborne sound, in decibels, corresponding to a BB93 reference value of the reverberation time in the receiving room (Tmf,max).

L’nT(Tmf,max),w

is the weighted (noise) level difference for impact sound, in decibels, corresponding to a BB93 reference value of the reverberation time in the receiving room (Tmf,max).

T

is the room reverberation time. The reverberation time of a space is a measure of the rate at which the sound decays.

Tmf

is the mid-frequency reverberation time. The sound absorption of surfaces usually varies with frequency and therefore the reverberation time in a space also varies with frequency.

Tmf,max

is the reference reverberation time equal to the upper limit of the reverberation time, Tmf, given in Table 1.5 (of BB93) for the type of receiving room.

STI

is the Speech Transmission Index. STI is a measure of speech intelligibility in a space.

 

STI	0.1 - 0.3	0.3 - 0.45	0.45 - 0.6	0.6 - 0.75	0.75 - 1.0
Speech intelligibility	Bad	Poor	Fair	Good	Excellent

DnT,w

is the standardised weighted (noise) level difference for airborne sound, in decibels, corresponding to a reference reverberation time (T0).

Ctr

is a correction factor to account for low frequency (bass) sound that is applied to airborne sound.

Rw

is the weighted sound reduction index. It is measured in a laboratory with suppressed flanking transmission. R varies with frequency and is weighted to give a single figure number Rw.

R’w

is the apparent weighted sound reduction index. The apparent sound reduction index, of wall or floor constructions is usually lower than the laboratory measured value of R. The difference between the results is usually due to flanking transmission and a lower standard of workmanship on site.

L’nT,w

is the standardised weighted (noise) level difference for impact sound, in decibels, corresponding to a reference reverberation time (T0).

LAeq,T

is the equivalent continuous A-weighted sound pressure level. It is a value of the A-weighted sound pressure level in decibels (dB) of a continuous, steady sound, that within a specified time interval, T, has the same mean squared sound pressure as the sound under consideration that varies with time.

Dw

is the weighted level difference between rooms.

LA

is the indoor ambient noise level in a room.

NR or Noise Rating curves

are a system of quantifying frequency dependent noise levels with a single number.