ETC measures room decay time graphically, by showing the decay of a pulse over time. Typically, you will make an impulse sound by clapping your hands, popping a balloon, or even snapping your fingers. The graph will show the decay of this signal over time, and from this, the RT60 time can be computed.

Using external trigger mode. You can see the initial peak sound, and then the decay of the sound over time. A-weighting was used to remove low frequencies (and very high ones) from the test. We used a 400ms time window to capture the full decay of the impulse.