Bass Removed.FRD file

[4thtry]

Would anyone know where I could go to find the "Bass removed.frd" file that is used to remove the bass when testing tweeters. I just downloaded the latest version of OmniMic (5.03), but I can only find the file "Bass and Midrange Removed.frd" when I look in the C:\users\public\OmniMic subdirectory. Thanks.

[jhollander]

Bill, these are wav files that are on the CD. Send me a PM if you can find them.

[4thtry]

John, I have the two audio wav files, one for "bass removed" and the other for "bass and midrange removed" So I can take the measurement. But what I am looking for is the bass removed.frd file that is used later to "normalize" the measurement.

[guitar maestro]

What is this FRD you speak of? Where is it supposed to be found originally?

[4thtry]

What is this FRD you speak of? Where is it supposed to be found originally?

When you install OmniMic, this file should be automatically placed in the C:\users\public\OmniMic subdirectory. For some unknown reason, this FRD was missing from my latest update (version 5.03). However, I recently rolled my PC back to a previous version of OmniMic and the bassremoved.frd file strangely reappeared, right where it was supposed to be. Apparently, the file disappeared during one of the updates.

[dcibel]

Yes, it would appear to be missing from the latest installation. Unfortunately I can't find my original CD, and only have the latest installer saved. 4thtry, can you post the file for others to use?

Just an FYI, this is the excerpt from the manual:

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 synchronous 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. ALWAYS use a blocking cap if you are testing a ribbon tweeter without its crossover!

When you use those test signals, the measurements will be much more susceptible to noise below the rolloff frequencies, so you should adjust the displayed frequency range to avoid the lower frequencies.

The idea here is that you roll off the low end using the "bass removed" track, then you bring the signal back by normalizing the measurement to the filter transfer function. This will inevitably bring a bunch of noise into the low end response, but the general signal trend and phase should be a lot closer to what the measurement would have been without the filter.

[guitar maestro]

I saved the contents of my original CD, but it's not there as a raw file. Perhaps it is extracted when you run the executable for the Omnimic installation.

EDIT: I just tried installing OmniMic v4.7 as a fresh install on another computer, and no dice. That FRD must be from an earlier version of OmniMic.

[guitar maestro]

Bass Removed.frd

Bass and Midrange Removed.frd

Question: Upon Normalizing, it asks which frequency to keep the same level. What should be selected if the measurement was originally taken using the bass removed sound file? The Help file doesn't go into as much detail.

[dcibel]

Bass Removed.frd

Bass and Midrange Removed.frd

Question: Upon Normalizing, it asks which frequency to keep the same level. What should be selected if the measurement was originally taken using the bass removed sound file? The Help file doesn't go into as much detail.

I'll provide the same response here as I did via your email.
Essentially what we are doing here is removing the filters effect from the measurement by applying an inverse filter if you will on the measured response. If you load up a measurement you took using the "bass removed" track, you just hit the normalize button, it will ask you to open the file to normalize with, so you are selecting the "bass removed.frd" file here. The frequency reference is basically asking what frequency in the file you want to be referenced as 0dB, then everything else is normalized to that reference. Since the reference file is a flat line response with a high pass roll off at 300Hz, the defuault of 1kHz will work just fine here.

Keeping in mind that the bass removed track is a high pass at 300Hz, for most tweeters the response is likely unaffected since you likely have next to no output below 500Hz. You should hopefully see the response lifted below 300Hz, and the phase would be affected by this change as well.

If you had used the bass and mid removed track, this one has a high pass at 3kHz which would have great effect on most tweeters, needing the response normalized to remove the effect of the filter from the frequency response. When you normalize using this track, I would pick 5kHz for the reference frequency.

[guitar maestro]

10-4. Saw zero change below 300Hz, but I guess it doesn't matter in this case, since like you said, there is little to no output down that low. Thanks.

[4thtry]

I see you found the missing file. The reason that I was originally looking for this file was because I was running into noise problems when trying to use the "bass and midrange" removed file. When I normalized the measurement, this boosted the noise too much in the midrange. So I was hoping that if I measured and normalized with the bass removed file, noise would be less of an issue. But before I found the missing file, I decided to go ahead and measure my CX150-8 tweeter without a protection capacitor, keeping the full range signal level at a fairly low level. This worked. I did not damage the CX150-8 tweeter using the full range test signal (at least I don't think I did).

[guitar maestro]

Glad you worked it out! But yea, there it is if you need to download it and keep a safe copy.