Dayton Audio OmniMic V2 Precision Measurement System - Manual (Page 37)

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Harmonic Distortion
Index
To properly measure Harmonic Distortion with OmniMic, you must measure only while the sound
system is playing one of the provided "Long Sine Sweep" tracks of the OmniMic Test Track CD.
Use of other sound signals will not provide meaningful results.
 This measurement will work best when the OmniMic is positioned relatively closely to the
loudspeakers, so that sounds coming directly from the speaker are much stronger than those
coming reflected from elsewhere. Room reflections are very detrimental to measurement of
harmonic distortion.
 The microphone and speaker should be held stationary over the length of several of the test
sweeps (approximately 6 seconds each) previous to each graph update
 At very high levels (>125dB SPL), appreciable distortion may be generated from overdriving of
the OmniMic itself.
 When you position the mouse cursor over a distortion graph, a small box will appear displaying
the frequency, SPL level, and effective Harmonic Distortion percentage of the overall SPL,
relating to the position of the cursor.
 You can freeze the graph to read multiple positions by using the Pause button near the top left
of the OmniMic screen.
The graph will always display the frequency response curve (in dark black) at top, indicating
the effective SPL level sensed at the position of the OmniMic. In addition, the graph can be
configured to display
 2nd Harmonic Distortion level
 3rd Harmonic Distortion level
 4th Harmonic Distortion level
 5th Harmonic Distortion level
 2nd through 5th Harmonic Distortion levels (Total Harmonic Distortion for the first 5 products;
for loudspeaker distortion these harmonics normally are the highest)