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Reverb/ETC
Index

Reverberation is a measure of how quickly sound reflections die down in a room, and will depend on the frequency range of the sound, the size and shape of the room, what is in the room, and how the room surfaces are constructed.  Determination of RT60 can be made using  this tool.

ETC stands for "Energy-Time Curve", which is a similar display, but which shows individual reflection spikes more clearly to help in locating where they are coming from.  This display is also able to calculate "Speech Transmission Index" (STI) for auditorium and hall acoustics evaluations.

A third display option "Log IR" shows the dB level of the Impulse responses (like the one at the bottom of the Frequency Response tab page, but in a decibel format), a very similar result to the ETC in most cases.

These tests must be done while the sound system is playing the "long sine sweep" test signals  (as downloaded from the Dayton Audio site, or off of the OmniMic Test Track CD or DVD, a generated WAV file, or played out from the soundcard of your computer).  Use of other sound signals will not provide meaningful results!


Reverb (RT60)

OmniMic provides a graph of the reverberation decay curve.  A properly done reverberation decay curve will drop 40 decibels (dB) or more, relatively smoothly, from left to right.  Using the mouse cursor on the graph and the mouse button,  you can obtain a measure of the RT60 value -- the time needed for a reverberant field falling at that rate to die down by 60 decibels.
  • Set the value of the "Integration time" to a value approximately equal to the expected RT60 of the room (around 500 milliseconds typically).  The optimum value is the one that makes the decay line the straightest and longest.
  • Set the lower and upper frequency limits to define each frequency range over which you wish to measure.  The width of the range will be displayed in Hz and in Octaves.  If you wish to force the controls to hold current number or octaves of range, click the small "lock" icon and the controls will track accordingly.  Click again to unlock.  
  • Click on the graph to define a line parallel to the decay curve.  The slope of this curve defines the value of RT60, which will be displayed on the screen until the next graph update.  This may be easier to do if you first freeze the graph with the Pause button at top left of the OmniMic screen.


    ETC (Energy-Time Curve)

    The curve shown is the energy relative to the level of the energy in the first peak as an impulse 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 identify 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 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 can be expressed.  Good signal to noise is desirable when making the measurement, so play the test signal loudly enough.