Norsonic nor118, Instruction Manual

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54 Chapter 10
Reverberation time measurements
55 Norsonic Nor118
User Guide
When measuring the reverberation time, we specify
a trigger level above which a sound level occurring is
assumed to be the excitation impulse. e instrument
captures the decay, looks for the peak value and starts
the calculation 5 dB below this value. en it measures
the time it takes for sound to decay by 20 dB (by means
of the    ) and multiplies the
value found by 3. If the distance to the noise floor is more
than 10 dB and if the reverberation times are more than a
specified set of minimum values and finally, if no overload
has occurred, the values are presented as valid.
Minimum reverberation times possible
Nothing changes infinitely fast in nature. It will always
take some time to determine the level in each frequency
band with sufficient accuracy. is is related to the 
product ( ×  ) which
states that the narrower the bandwidth, the longer time
it takes to determine the level at a given standard devia-
tion.
Likewise, the filters are not cleared instantly if the input
signal is removed suddenly. Instead, they have a finite
decay, which may be considered as a virtual reverberation
time. e narrower the filter is, the longer this virtual
reverberation time will be.
e minimum reverberation times for the Nor118
are listed in the table to the right. If reverberation times
shorter than those listed in the table are encountered or
the minimum distance to the noise floor is violated, the
instrument will produce a ? in the reverberation time table
to indicate this.
On the other hand, the maximum reverberation time
possible to measure is 8 seconds in the Nor118.
Minimum reverberation times
Frequency Third-octaves Octaves
[sec] [sec]
50 Hz 1.01
63 Hz 0.83 0.43
80 Hz 0.68
100 Hz 0.56
125 Hz 0.46 0.23
160 Hz 0.38
200 Hz 0.31
250 Hz 0.26 0.13
315 Hz 0.21
400 Hz 0.17
500 Hz 0.14 0.07
630 Hz 0.12
800 Hz 0.10
1 kHz 0.08 0.04
1.25 kHz 0.06
1.6 kHz 0.05
2 kHz 0.04 0.02
2.5 kHz 0.04
3.15 kHz 0.03
4 kHz 0.02 0.02
5 kHz 0.02
6.3 kHz 0.02
8 kHz 0.02 0.02
10 kHz 0.02
A 0.09 0.09
C 0.12 0.12
Z TBD TBD