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Old 17th April 2003, 09:06 AM   #1
Rarkov is offline Rarkov  England
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Question IDEA: Measuring FR of a driver with room acoustics?

Hi,
My lecturer visited my workplace yesterday (I'm doing a Sandwich year). He informed me that I would need a final year project idea ASAP. I had an idea about measuring frequency response using a laser pointed at the center of the dust cap. If the laser is at a known angle to the axis of the driver, as it moves back and forth, the laser point will move with Simple Harmonic Motion, across the center of the driver.

This could be used to find two things. If you know the Peak to Peak Excursion of the driver, you can work out the Limits of the laser point's movement. (For instance, if the angle from the axis of the driver to the laser is 30 degrees, and the peak to peak excursion of the driver was 20mm, the laser point would move 5.77mm either side of the central point). The other way of using this system would be if you do not know the Excursion. By measuring the displacement of the laser point, you could find the Excursion.

Now, what I want to know from you lot is: Will this work in practice? And more importantly, if the driver is in a known sealed box volume, can the driver excursion be used to calculate the FR?

In my mind, this system would totally ignore the effects of room response and diffraction etc etc. It would be there solely to measure the FR of a driver, not of a system. Incorperated with a system of measuring speaker impedance, it would be a very useful system!

Please tell me what you think!

Thanks,
Gaz
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Old 17th April 2003, 09:25 AM   #2
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The usual (and very accurate) method of measuring distance by the use of a laser is an interferometer. I.e. a device that allows "counting" of wavelengths.

To answer your questions:

Could it be used measure excursion: Yes

Could it be used to measure FR: restricted to the range where the driver acts as a solid piston.

Regards

Charles
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Old 17th April 2003, 09:43 AM   #3
Rarkov is offline Rarkov  England
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Quote:
Originally posted by phase_accurate
Could it be used to measure FR: restricted to the range where the driver acts as a solid piston.
Which is what range in a sealed enclosure? I take it, that it is largly driver dependant?

Thanks,
Gaz
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Old 17th April 2003, 09:50 AM   #4
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Yes it is driver dependant.
As a rule of thumb: A driver acts as a solid (whatever this is) piston up to the frequency where the driver circumference equals the wavelength of the input signal.

A suggestion: Why don't you develop something that allows the measurement of panel vibrations ?

Regards

Charles
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Old 17th April 2003, 10:00 AM   #5
Rarkov is offline Rarkov  England
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Quote:
Originally posted by phase_accurate
A suggestion: Why don't you develop something that allows the measurement of panel vibrations ?
Hi,

Can you ellaborate? Do you mean ESL panels or large baffle panels? I had thought about placing a lightweight device on a cone, but even this would effect sensetivity amongst many other T/S params. Measuring panel vibrations is a great idea, but what about "dead" boxes?!

Thanks,
Gaz
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Old 17th April 2003, 10:07 AM   #6
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How do you know how dead your box is without measuring ?

Regards

Charles
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Old 17th April 2003, 10:59 AM   #7
Rarkov is offline Rarkov  England
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That just puts us into a vicsous circle!
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Old 17th April 2003, 01:36 PM   #8
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You can do this using interferometry, but why? Near-field mike measurements work extremely well and are a snap to do. I'd go with the idea of panel measurements- no box is completely dead, and this can give you a quantitative measure of the deadness.

If you want to be rigorous, you can set the measurement up using a pair of interferometers with different pathlengths. That way, by comparing phase of the measurements, you can distinguish "out" motion from "in" motion, not unlike the system used in mice and trackballs.
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Old 17th April 2003, 03:22 PM   #9
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So, to measure panel vibrations I would want a "not too" sensetive acellerometer (or whatever they are called that measure ground movments) and a computer programme. If I can get hold of one of these things, I'll try it out to check out the theory...Unless anyone has (or could) try it already?!

Thanks,
Gaz
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Old 17th April 2003, 03:27 PM   #10
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I know that some manufacturers are using laser interferometry to make cone breakup etc visible, so it is nothing new at all.

But a lowcost version would be quite cool IMO.

Some secret services use such toys for eavesdropping by measuring the sound-induced movements of glass windows.

Regards

Charles
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