Svantek SV 971A - E.2 Definitions and Calculation of the RT 60 Reverberation Time

SV 971A User Manual – Appendixes
292
(RT 60) is obtained using the slope coefficient of the decay curve. The type of the definition from which slope
coefficient is calculated (EDT, RT 20, RT 30 or user defined) depends on the difference between levels L
1
and L
2
(the difference between background noise level and sound source level) of the decay curve and it
depends on significantly from the acoustic source ability. If the level difference is larger than 45 dB, the
RT 60 parameter can be calculated using three definitions: EDT, RT 20 and RT 30.
The real measurement results are not as smooth as the curves presented on graphs in Figure 1. In order to
point out the interesting decay curve region (the position of the markers t
1
and t
2
) some measurement data
processing (in general signal smoothing by averaging) need to be applied.
E.2 DEFINITIONS AND CALCULATION OF THE RT 60 REVERBERATION TIME
➢ EDT (early decay time):
The EDT decay curve region is pointed out by markers t
1
and t
3
(cf. Fig. 2). It is checked whether the selected
decay curve region has proper dynamics for the EDT calculation:
L
1
– L
2
>= 10 dB
L
2
– L
3
>= noise margin
It is recommended by the ISO-3382 standard to set 10 dB value for noise margin.
In case of the impulse method, the sound pressure level values between points t
1
(with L
1
level) and t
2
(with L
2
) are approximated with the straight line (y = a · x + b) by the linear regression. Before approximation
the EDT value is calculated using the slope coefficient ‘a’ according to the formula:
EDT = – 60.0 / a
In case of the decay method, the EDT value is calculated according to the formula:
EDT= 6 · (t
2
– t
1
)
Fig 2. The EDT evaluation
➢ RT 20 (reverberation time calculated with 20 dB dynamics):
The RT 20 decay curve region is pointed out by markers t
1
and t
4
(cf. Fig. 3). It is checked whether the
selected decay curve region has proper dynamics for the RT 20 calculation:
L
1
– L
4
> 5 dB + 20 dB + noise margin
It is recommended by the ISO-3382 standard to set 10 dB value for noise margin.
t [s]
L
[dB]
t
3
t
1
L
1
L
3
noise
margin
linear approximation
t
2
10 dB
L
2