Larson Davis System 824, Руководство по обучению

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3-2 824 Training Manual 3/22/01
the level by some dB for some time and extrapolated to
obtain a time for a 60 dB drop
How does my 824 measure an
RT60?
RT60 measurements are usually made with real-time analyz-
ers with 1/3 octave filters. The 824 uses the high speed RTA
option to gather spectra at a rate of up to 400/second. These
spectra are stored in what is known as an autostore ByTime
record. The ByTime autostore records show each time slice
(2.5mS) as the sound is decaying in the room. The
“ByTime” display of the 824 shows each of the 1/3 octave
frequencies over time. The following diagram shows the
level versus time of a single frequency.
The curve above could be any 1/3 octave frequency from
12.5 Hz to 20 kHz. For each of these frequencies a ByTime
curve is generated. The slope of the decay part of the curve
determines the time it takes the sound to decay.
The slope is determined by using the least squares method
for a dB down setting chosen by the user.
Time
Level
Slope of decay
ByTime Graph
∆
dB
60 dB
∆
T
RT60
5 dB
RT60
=
∆
∆
Τ
dB