User`s manual

15.4 THE CALCULATED ACOUSTICAL PARAMETERS
The acoustical parameters are calculated from a measured decay curve. A decay
curve is defined as the decay of sound pressure level as a function of time after the
sound source has ceased. The decay curves are calculated from the measured
impulse response after octave filtering has been applied; also wideband (linear or
A-weighted) decay curves are available.
Historically the most important acoustical parameter is the Reverberation Time (T
or RT) defined as the time, in seconds, that would be required for the sound
pressure to decrease by 60dB after the sound source has ceased; this is why the
most used indication you find in literature for the reverberation time is RT60. As it
is normally practically difficult to obtain a sufficient dynamic range to measure the
reverberation time directly, the norm provides for its evaluation based on a
smaller decay of 30dB; in this case the reverberation time, indicated as RT30,
should be the time evaluated considering a linear decay given by the least-square
regression of the measured curve from -5dB to -35dB. Also provided is the
possibility of evaluating RT20 and RTUser (based on user defined limits).
Sound level parameters.
Signal [dBSPL]. Measured signal level in dBSPL in the band of interest.
Noise [dBSPL]. Measured background noise level in dBSPL in the band of interest.
Balance between early and late arriving energy.
C50 [dB]. 50ms early-to-late arriving sound energy ratio (i.e. ratio between the
energy arrived in the first 50 milliseconds to the energy arrived after). C50 is
usually evaluated when results relate to speech reproduction.
C80 [dB]. Usually named “Clarity”. 80ms early-to-late arriving sound energy ratio.
C80 is usually evaluated when results relate to music reproduction.
D50 [%]. Usually named “Definition”. Directly relates to C50 with the following
equation:
dB
D
D
C )
1
log(10
50
50
50
−
=
TS [ms]. Time of centre gravity of the squared impulse response. It is another
measure of acoustic clarity; the higher Ts the poorer is clarity.
Decay time measurements.
EDT [s]. Early Decay Time i.e. time required to sound to decrease of 10dB from
the initial maximum level. EDT is directly related to the perceived reverberation
while reverberation time relates to the physical properties of the room.
RT20 [s]. Reverberation time evaluated from a 20 dB dynamic range (-5dB,-
25dB). See also below the correlation coefficient R associated with RT20.
RT30 [s]. Reverberation time evaluated from a 30 dB dynamic range (-5dB,-
35dB). See also below the correlation coefficient R associated with RT30.
15 ACOUSTICAL PARAMETERS 205