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Old 27th September 2011, 02:35 AM   #541
IG81 is offline IG81  Canada
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Is there a way to measure the absolute distance from a driver's acoustic center to the microphone?

IG
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Old 27th September 2011, 02:53 AM   #542
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Take an on axis measurement. Move the mic a known distance away from the first measurement point along the axis and take another measurement. Look at the difference in time it took the signal to arrive between the two measurements. Divide the distance between measurements by the time difference. Now you know the speed of sound, although the accuracy of your measurement depends on your previous measurements. Now look at the time between zero and the signal start at the point you're interested in and divide the speed of sound you measured by this time. I would not use holm for this - I'd use a 2 channel system where I could take the reference channel to be the voltage at the driver terminals or something like that. Then you could compare the time between the same points on the time signal (if they looked recognizably the same, which is a big if).

However I'm not sure I'd ever find a use for this in speaker design. Everything is relative to everything else - as long as you keep a constant time origin and measure your drivers in their final positions at one mic position, it's not necessary to known any absolute distances. In addition, the acoustic center of a driver will typically change with frequency, so depending on what you're doing, you might want to modify the above technique to use a band-limited signal.
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Old 27th September 2011, 03:08 AM   #543
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Thanks! It's close to what I had in mind, but just could not wrap my head around a few things here. I'll try it this way.

Of course relative measurements is where it's (mostly) at in design; but if I can get somewhat consistent results, even with HolmImpulse for this, it could be quite educational for me.

IG
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Old 27th September 2011, 03:26 AM   #544
IG81 is offline IG81  Canada
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I actually had the delta-distance over delta-time thing right, but I'm not certain how to locate "signal start" position in the IR window. The Holm Acoustic website is down at the moment to make matter worse, so i cannot access the user guide and perhaps answer myself...

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Old 27th September 2011, 03:34 AM   #545
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Also look at the number of samples the impulse is offset. That includes some latency in the system, tho.
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Old 27th September 2011, 12:29 PM   #546
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Quote:
Originally Posted by IG81 View Post
Thanks! It's close to what I had in mind, but just could not wrap my head around a few things here. I'll try it this way.

Of course relative measurements is where it's (mostly) at in design; but if I can get somewhat consistent results, even with HolmImpulse for this, it could be quite educational for me.

IG
There is no way to get the precise point, you can only get close. As you pointed out, the only thing that actually matters is relative offset.

I don't recall if this has been mentioned in the thread. One way to do it is shown here. You have to be sure to have the same absolute setting for the window start time marker for each measurement. The system latency will be the same, so that isn't an issue for determining relative offset.

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Old 27th September 2011, 12:35 PM   #547
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Originally Posted by ByronInLawrence View Post
I found this old post from March 2010, in which someone renamed a text file "FRD" file and SpeakerWorkshop imported it. I remember long ago file types were defined by the file extension. The modern versions of windows don't seem to use that system anymore. Files don't have extensions. Changing a text file, and adding ".FRD" on the end does not allow Speakerworkshop to import it. SW still recognizes it as a text file. Does anyone know how to get HOLM Impulse to export a FRD file with the modern versions of windows? HOLM Impulse is pretty worthless without this ability.
An FRD is a text file. What you may need to do is open the file with a plain text editor and delete any leading comment lines, leaving only the data.

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Old 27th September 2011, 01:30 PM   #548
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As for finding the "signal start" time, is it indicated by either the 11-10-9-8-etc markers or the dashed line that precedes the moveable gating marker?

IG
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Old 27th September 2011, 02:47 PM   #549
dlr is offline dlr  United States
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As for finding the "signal start" time, is it indicated by either the 11-10-9-8-etc markers or the dashed line that precedes the moveable gating marker?

IG
Let me correct myself. If you use the method shown at the link, the start time marker isn't necessary other than what is required to get a good window for the FR. You'll be using the measurement output from HOLM to import into design software (since HOLM doesn't do that). The method shown does not require measured phase, you create the driver models and generate phase for each driver in your design software, such as the PCD. The summed response of these models is compared against the measured summed response to get the proper offset.

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Old 27th September 2011, 02:59 PM   #550
IG81 is offline IG81  Canada
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Quote:
Originally Posted by dlr View Post
Let me correct myself. If you use the method shown at the link, the start time marker isn't necessary other than what is required to get a good window for the FR. You'll be using the measurement output from HOLM to import into design software (since HOLM doesn't do that). The method shown does not require measured phase, you create the driver models and generate phase for each driver in your design software, such as the PCD. The summed response of these models is compared against the measured summed response to get the proper offset.

Dave
Yes, but I'm not trying to find relative distance, but absolute, within reasonable accuracy (only as good as I can take physical measurement and system latency I suppose). This is mostly for educational purpose, not for system design.

So is the start time marker the first numbered marker or a point on the dashed line?

IG

Last edited by IG81; 27th September 2011 at 03:03 PM.
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