Dayton Audio OmniMic V2 Precision Measurement System User Manual
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Arrange and test whether a frequency response falls within a specified range
The "Evaluate within" function (in the "Math" section of the Frequency Response menu will show a small rectangle
at the top left of the frequency response graph that indicates whether measured responses fall within ("good") or
outside ("out") of the first two curves specified in the Curves menu. The evaluation is performed only over the
frequeny range currently being displayed. To use this to determine whether new measurements fall within (for
example) 3dB of a reference measurement, first make the reference measurement and save it to disk using the
"File->Save Curve" menu. Then, in the "Curves" menu, choose "Add" and then select the file you just saved and
then select an offset of 3dB on the form that appears. Use the "Curves->Add" menu again to choose the same
curve, this time with an offset of -3dB. The two curves should show on the graph. Next, go to "Advanced Mode"
(if you aren't already there) using the checkbox provided and in the "Math" menu, select "Evaluate within". You
can select whether to allow the program to try to fit the curve by shifting it up or down by up to some maximum dB
value. Finally, set the frequency range of the graph (using the
to display the frequencies
over which you wish the evaluation to be performed.
You can alternately define the two curve files to be used for limits by generating them using a text editor (simply
load an FRD file into your text editor to see the simple format. All frequencies need not be present, but they do
need to be in increasing order). Users familiar with spreadsheets such as Microsoft Excel can use these to edit
FRD data for special custom limit curves, after resaving the resulting data with an FRD extension (or saving with a
.txt or .prn extension and then changing the extension to .frd).
Measure frequency responses of Tweeters
OmniMic normally tests loudspeakers with a full range signal that briefly passes from below 2Hz to above 20kHz.
Some speaker drivers, particularly tweeters, compression drivers, ribbons, or electrostatics cannot be safely
driven at low frequencies. When testing devices such as these, you should always drive them through series
capacitors (at least) to limit low frequency drive. You can calculate the approximate value to use as
C=1,000,000/(6.2*f*Z) where f is the desired cutoff frequency, Z is the driver impedance, and C is the value in uF.
Smaller values of capacitor are safer
.
Using such a capacitor causes a problem, though -- it also affects the measured response. If you test with a
cutoff capacitor that acts far below where you tend to use the speaker then those effects might be ignored, but in
that case you would be less protected.
Another option is to test with a pre-filtered (but still syncrhonous with OmniMIc) signal that has a known highpass
response. Several of these are provided on the V3 CD (or DVD) test disks, some with "bass removed" and some
with "bass and midrange removed". The "bass removed" tracks roll off below 300Hz, and the "bass and midrange
removed" tracks roll off below 3kHz. The effect of the rolloffs from using these tracks can be corrected by
Normalizing the measured responses (either at measurement time, or after Adding an uncorrected file) with either
the "Bass Removed.frd" or the "Bass and Midrange Removed.frd" files that can be found in
your C:\Users\Public\OmniMic folder.
IMPORTANT NOTE: You should still use a series capacitor (perhaps something large such as 100uF) to
protect your sensitive driver from transients, hum, or other signals from the setup which might cause damage.
Note, however, that when you use those test signals, the measurements will be much more susceptible to noise
below the rolloff frequencies, so you should adjust the diplayed frequency range above the lower frequency
range.
Measure sensitivity of loudspeakers
Loudspeaker sensitivity is normally expressed as the SPL level that is sensed at 1 meter from the loudspeaker
when it is driven from a voltage of 2.83Vrms. When you are using a CD as the signal source with OmniMic, the
nominal level of the swept sine signals can be determined (or set) by use of the 50Hz reference tones that are at
the end of the Version 3 OmniMic Test Signal CD or DVD.
To use these, you may want to disconnect the speaker of the channel being tested (the output voltage of quality
amplifier should not be affected by whether a speaker is being driven). Then attach a Digital Voltmeter (also
known as a "DVM") to read AC Voltage across the amplifier terminals. Be sure to configure the voltmeter for AC
Volts and that the probes connect to the meter's Voltage (and not its Current!) reading terminals.