Dayton Audio OmniMic V2 Precision Measurement System User Manual
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Turn any equalizer or tone controls off. You can then play the 50Hz reference tone tracks and measure or set the
voltage level from the amplifier. Next, reconnect the loudspeaker, set OmniMic at the 1 meter distance from the
loudspeaker and play the desired sweep track of the test CD. Use the OmniMic software in its Frequency
Response measurement mode for Sine Sweep, and the plot should then be a reading of SPL sensitivity.
Some loudspeakers reference to a 1 watt level rather than to a voltage level. 2.83Vrms is equivalent to 1 Watt
into an 8 ohm (resistive) loudspeaker. For a 4 ohm loudspeaker at 1 Watts, the voltage to use would be
2.0Vrms, for a 16 ohm speaker use 4.0Vrms.
getting FRD files for crossover simulations
Designing a crossover is much easier if you can do it using simulations rather than by trial and error and physical
parts. To design via simulation, you need to have both magnitude and phase responses for the drivers you will be
using. For passive crossovers, you will also need impedance data for the drivers -- for that, an electrical
impedance measurement such as the
is ideal. See
for a technique
that provides accurate phase responses for crossover designs using
and similar programs.
You can also do the adjustments within the PCD program using files made with OmniMic. For this you need three
measurements of each pair of drivers -- one of each driver alone, both with phase response, and one of both
playing together (without a crossover, which isn't designed yet). You should use a blocking capacitor on the
tweeter to avoid hitting it with very low frequency energy, and you should also keep the test level down to the
lowest level that provides a clean measurement. The phase response can be made using OmniMic in its
"Advanced Mode" and with phase enabled, but without any "added tweeter" -- this gives a true phase response
measurement, though the delay between driver and microphone isn't accounted for -- and that is what Jeff's
technique addresses. For the measurement with both drivers playing, try both with the tweeter inverted and with
it non-inverted to try to find a configuration which gives a null where both drivers are playing. Then using the
measured response FRD files from each of the individual drivers, simulate in PCD the results of both playing (and
with the tweeter polarity as used for the combined measurement). In PCD, vary the simulated delay between the
two drivers to make the results (in dB curve shape) match as closely as possible the shape of the dB curve
measured when both drivers were playing. This delay value then will correct the relative delay between the two
drivers for complete crossover design in PCD.