The reverberation time (RT) is one of the most fundamental room measurements.

It gives a broad brush stroke description of the general acoustical behavior of a space, paving the way for the use of other metrics to determine clarity and direct-to-reverberant ratios.

Wallace Clement Sabine was the first to formalize the RT into an acoustic metric. His method of measurement involved a program source, a stopwatch and a quiet room.

Sabine determined that 60 dB of room decay was audible in a quiet space, so he formalized the RT60 to describe it. Sabine’s work provided the statistical RT formulas that are still in use today.

Further refinements came with the ability to measure the Room Impulse Response (RIR ) (Figure 1). ISO3382 is an international Standard for “scoring” the RIR .

First, the squared RIR is backward integrated (energy summed from right-to-left, a.k.a. the Schroeder curve). The slope of the Schroeder curve is used to determine various decay times.

Two of the ISO 3382 decay metrics that are commonly used by sound system designers include:

Figure 1 – The Room Impulse Response or RIR. This is the amplitude vs. time response of the room to an impulsive source.

Reverberation Time T30 – The reverberation time for 60 dB of room decay, based on a straight line curve fit between the -5 dB to -35 dB points on the Schroeder curve (Figure 2).

This time is doubled to be made consistent with the traditional reverberation time for 60 dB of decay (RT60). The T30 is a measure of the “persistence” of sound in the space.

Early-Decay Time EDT – Based on a straight-line curve fit between the 0 dB to -10 dB points of the Schroeder curve, extrapolated to 60 dB of decay (Figure 3). A strong direct field will produce an EDT that is less than (steeper slope) than the T30.

Figure 2 – The cursor placements for determination of T30 according to ISO3382.

This is a desirable attribute for increased speech intelligibility or music clarity. Strong early reflections have a similar effect, which is why acousticians often specify band shells and acoustic clouds to enhance the stage sound in large spaces.

The T30 and EDT provide important information about the way that sound behaves in a room. Both are most applicable when a homogeneous reverberant field exists (i.e. medium to large rooms with low absorption).

Figure 3 – The cursor placements for determination of EDT according to ISO3382.

Their use in small spaces (i.e. control rooms, home theaters, and other rooms with high absorption relative to room volume) is questionable.

Caveats
The determination of T30 and EDT according to ISO3382 seems straightforward, so one would expect to get the same result from any measurement program. But, this is not the case.

So much for Standards!

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