Dayton Audio OmniMic V2 Precision Measurement System - Manual (Page 36)

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Signal Generator
Index
With most computers, Omnimic can be used to directly generate test signals such as the sweeps for Frequency Response
or Harmonic Distortion. The signal is generated from the computer's soundcard output, which can be connected via
cabling to other amplifiers or audio systems.
When Omnimic is used in the SPL/Spectrum mode or Oscilloscope mode, it can also generate sinewave or squarewave
test tones. To use this function, select the "Config">"Generator" menu and this form will appear:
There are two tone generators included, either (or none) of which can be used simultaneously.
Tone1 can be either a sinewave or a square wave of any audio frequency and is always at a fixed level -- this level is the
same (when a sinewave) as the level at which sweep tones are ouptut from the soundcard when sound is played out in
the Frequency Response, Distortion, Reverb or Bass Decay.
This provides a handy way to provide known signal level to loudspeakers under test. With Tone1 set to about 55Hz,
sinewave, and applied to a power amplifier (which is in turn connected to an AC Voltmeter or DVM set to AC Voltage
mode), adjust the amplifier's volume control to the desired voltage level. For loudspeaker sensitivity measurements, this
is normally a voltage level of 2.83Vrms (equal to 1W into an 8 ohm load). Then change over to the Frequency Response
page, connect the speaker, and with the soundcard again providing the test tone measurements will be at the 2.83V
standard level.
Squarewave mode with Tone1 can be used to view (on the Oscilloscope) the response of speakers to a square wave. Be
aware, however, that very few loudspeakers can produce a recognizeable squarewave over any range of frequencies.
There is little if any evidence that this is actually important sonically, but the characteristic can still be interesting.
Tone2 is always a sinewave, and both its level and frequency can be adjusted as desired. Its output level (in dB) is
relative to the level of Tone1, that is, when the relative level is set to 0dB, then Tone2's level is the same as Tone1's
level. Application of the two tones simultaneously can be used to conduct single frequency-pair intermodulation tests of
loudspeakers, viewing the level of intermodulation product frequencies on the FFT Spectrum Analyzer (in the
Spectrum/SPL page). When doing this, however, be aware of complications from sound reflections in the room -- such
tests are best done with the microphone close to the speaker (if the total SPL level is less than about 110dBSPL) or
outdoors where reflections can be avoided.