Dayton Audio OmniMic V2 Precision Measurement System - Manual (Page 41)

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comes from the speaker and reflects around the room. In other words, imagine the speaker
sent out a sudden pulse rather than the sweep this test uses. (OmniMic calculates results
equivalent to a pulse using a sweep as the sweep is better at rejecting noise and distortion).
The first spike in the ETC is the original at 0dB and 0msec time, usually a direct signal from the
speaker. The following ones are as reflected from the various surfaces around the room. You
can often hold a piece of acoustic absorber (acoustic tile or even a pillow) near surfaces near
the speaker or microphone to identifiy the sources of reflections. The controls at the top of the
form allow you to filter the ETC to include only certain frequency ranges.
Omnimic can now save and recall (as Added ETC curves) ETC measurement files,
allowing multiple curves to be shown simultaneously. Curve files can also be smoothed before
saving, to help in interpretation.
When in ETC mode and operating with full bandwidth, you can also have Omnimic calculate
the "Speech Transmission Index" (STI) or the "Rapid Speech Transmission Index" (RASTI,
a similar but less intensive version). To use this play the long sweep test signal from the
system being tested (typically a public address or theater sound system) and put the
microphone at the seating positions. It may take several sweeps for the STI or RASTI to
appear. These are very intensive calculations, so the update rate will generally be slowed
when calculating STI or RASTI. A result of "1" is perfect, "0" would be worst that could be
expressed. Good signal to noise is desirable when making the measurement, so use sufficient
test level.