Ivie IE-45 RT60 - Reverberation Time Analysis - Manual (Page 2)

IE-35 & IE-45 RT-60 Manual
Introduction and Theory of RT60 Measurements
In theory, reverberation measurements seem easy and straight-forward to make. In actual
practice, they are not as easy as they might appear. RT60 is defined as the time, in seconds,
it takes sound in a reverberant environment to decay 60dB in level. Measurements are
usually made in narrow bands (octave or 1/3 octave), rather than broadband (20Hz to 20kHz).
ISO standard, octave bandwidths are the most common basis of measurement, with ISO
standard, 1/3 octave bandwidths being often used as well.
To make RT60 measurements, the following items are needed: A sound source to excite the
environment being measured, a sound level meter to measure the sound decay and a clock to
measure the time over which the decay occurs. If our sound source were sufficiently power-
ful, and we had good hearing protection, we could excite an environment to a level 65 or
70dB (60dB of decay plus 5 to 10dB of headroom) above the ambient sound level. We could
note the overall SPL, then shut off the sound while simultaneously starting a stop watch. We
could watch the SPL meter until the sound level decayed 60dB, and then punch our stop
watch. The time showing on the stop watch could be defined as RT60.
The accuracy of the measurement would, of course, be affected by our ability to punch the
stop watch at just the right times, and by our ability to read the SPL meter, determining
exactly when the sound had decayed 60dB. In actual practice a number of other factors
affect both the accuracy and the interpretation of the data gathered. In the first place, it is
usually not possible to generate sound levels which are 65 to 70dB above ambient - in many
environments, such a level would approach the threshold of pain. This means that decay has
to be measured over smaller windows than 60dB, with extrapolated RT60 then being
calculated, rather than actually measured. Also, the rate of decay can vary over the measure-
ment window, so, if an average of all rates of decay is desired, more calculations are involved.
Additionally, methods of exciting the environment, along with other factors, can cause
dramatic differences in the rate of decay for the first 5dB, when compared to the rest of the
decay window. Consequently, we may want to be selective as to the section of the decay
window we use to calculate decay time.
If we create a graphic plot of sound level versus time, we can observe the behavior of the
sound as it decays. Of course, the number of amplitude samples we take over our time
window will affect the resolution of decay curve we construct. Once we have this decay
curve, we can make some decisions about which section of the curve to use to calculate
RT60. After a suitable section of curve has been selected, we face the problem of determin-
ing what the average rate of decay over that section of curve is. Once we have that, we can
finish the RT60 calculation.
Perhaps the best way to handle all of these variables and calculations is through the use of a
computer. The IE-35 and IE-45 do just that. The diagram across the page helps to explain
how they work and what exactly they do.
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