Dayton Audio OmniMic V2 Precision Measurement System User Manual
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Determining Z-Offset (depth offset) between speaker drivers
A major advantage of Omnimic over other audio measurement systems is that it doesn't require a signal connection
between the computer and the audio system or speaker being measured. You can measure system response without
needing access to electrical connections simply by playing a test CD or audio file. That's very handy.
But there is a complication that comes with this convenience, which is that without a hard connection Omnimic has no way
of knowing the precise time at which a signal it is sensing left the loudspeaker. Omnimic determines timing by finding the
largest peak in the measured impulse response and referencing that in its phase determinations. When generating files
for loudspeaker design, it is important to know the relative delay between drivers in the loudspeaker system. If a
midrange driver is recessed in a baffle this will typically require a different crossover circuit than if the midrange is flush
mounted on a baffle. Different drivers have different amounts of depth, or "z position" of the originating point of the sound
waves they generate. What is important to know is not the absolute location of the acoustic origin of each driver, but the
relative "z offset" between two drivers for which a crossover section is being designed.
Omnimic now has a new feature to provide accurate determination of the z-offset between pairs of drivers. This feature is
based on a method described by Jeff Bagby for use with his
n which separate measurements are made
of each driver, and then those files are then combined mathematically and compared to a measurement that was made
with both drivers playing simultaneously. Bringing this process into OmniMic limits the number of files that have to be
exported from OmniMic to PCD and simplifies the steps for the user.
The feature uses a new math process for live ("Main") curve in Omnimic, which is a vector "Sum" function. This can be
found in the "Main Math" menu of the Frequency Response screen. When this is chosen (and an appropriate FRD file is
selected for summing with any new measured responses), the Main curve will be that of what is actually measured,
summed with the specified FRD file. For this to be reasonable, the saved FRD file must have valid phase information, so
make sure that the "show phase" checkbox is checked when saving FRD files to be summed with!
After a file has been chosen, a new menu option appears (see above), "Adjust Delay". Clicking this brings up a new form
with a control that lets you select the delay to be applied to the chosen file before it is summed with new measured
responses. If you check the box labeled "show Z-Offset Calculator", the form will expand to show a diagram and controls
which can calculate the proper Z-offset values to use in PCD.