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Old 13th October 2004, 07:37 PM   #1
Bose(o) is offline Bose(o)  Canada
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Default Measuring Subwoofer Response

Hello all!

Haven't cruised the forums in a while, nice to be here again. Anyway, I am in the design and soon, the building process for a dual Silver Flute 8" sealed subwoofer. It will be push-push, dipolar.

My first question concerns measuring the response of the subwoofer. Where do I place the mic? With a driver on either side, you can't get the summation of the subwoofer by placing the mic in front of one woofer can you? Do I just measure it in-room and tailor the x-over and such for my full-rangers to suit the response of the woofer?

My second question is in regard to building FRD files. How does one create FRD files and input them into the computer? I am willing to purchase the necessary components, yet the internet isn't to direct as to what is exactly required. I have Xoversim already and will get other software if I must. Just tell me what to get.

Thank you for your time, I appreciate your help!
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Old 13th October 2004, 08:08 PM   #2
johnnyx is offline johnnyx  United Kingdom
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For the first question, I have a similar problem, two woofers in a box. In my case they are on the same side, but what is the true response of them both?

I measured frequency response with the mic very close to one of the woofer cones. A nearfield measurement such as this gives a good frequency response curve mostly devoid of room effects. In this way, you can see what it's doing. The other driver will be the same. This would be good for setting crossover frequencies.

To get an overall view, I use an RTA measurement which is all woofers and the room effects from the listening position. This would be a good choice for loudness comparisons, it includes room gain etc. In other words, make a few measurements to get an overall picture.

This is what I do, maybe someone knows better?

Cann't answer the second question, sorry.
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Old 13th October 2004, 09:53 PM   #3
Bose(o) is offline Bose(o)  Canada
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Default One driver, then other...

Alright, measuring one woofer then assuming the other is doing exactly what the other is (especially in this case: push-push) is valid. Since, both woofers are moving out when the wave is positive and since both drivers are very similar and are in the same box "feeling" the same forces.

Now, can I assume that the woofers add-in to each other ? Making the total theoretical response that I measure at one woofer +3dB. than the SPL actually produced by the one woofer.
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Old 13th October 2004, 10:41 PM   #4
johnnyx is offline johnnyx  United Kingdom
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I don't think it's possible to measure only one woofer, unless the mic is very close to the cone, 1cm or so. Further out you get the woofer output you are measuring, and that of the other woofer, plus extra gain because of mutual loading effects, plus reflections from the room boundaries. If you do measure @ 1cm, you would have to convert this to spl @1m @1w for comparisons.

This is the reason I make a few different measurements to get an overall picture. It isn't easy, and I would like to hear of a simpler way.
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Old 13th October 2004, 11:26 PM   #5
Bose(o) is offline Bose(o)  Canada
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Quote:
Originally posted by johnnyx


This is the reason I make a few different measurements to get an overall picture. It isn't easy, and I would like to hear of a simpler way.


Yes, so would I.... *tiwddles thumbs*
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Old 14th October 2004, 02:11 PM   #6
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Nearfield measurements are almost impossible as the output of the second driver is going to influence the result in any case, assuming distance from cone to cone of a half-wavelength or less. But since the output of both drivers is going to be basically omnidirectional within the usual passband of a sub measuring anechoic (outdoors) with the box on the ground and the mic suspended a meter above will give a reasonable result as far as half-space is concerned.
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Old 14th October 2004, 02:42 PM   #7
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Default Re: Measuring Subwoofer Response

Quote:
Originally posted by Bose(o)
Hello all!

My first question concerns measuring the response of the subwoofer. Where do I place the mic? With a driver on either side, you can't get the summation of the subwoofer by placing the mic in front of one woofer can you? Do I just measure it in-room and tailor the x-over and such for my full-rangers to suit the response of the woofer?

My second question is in regard to building FRD files. How does one create FRD files and input them into the computer? I am willing to purchase the necessary components, yet the internet isn't to direct as to what is exactly required. I have Xoversim already and will get other software if I must. Just tell me what to get.

first question:
in ideal cases you'll get +6db for two drivers. any other cases should be less response
second question:
http://filext.com/detaillist.php?extdetail=FRD
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Old 14th October 2004, 03:56 PM   #8
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Measure everything in room at the listening position. Measuring anywhere else just results in a rough estimate and with a sub it is almost meaningless.
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Old 14th October 2004, 05:55 PM   #9
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Quote:
Measuring anywhere else just results in a rough estimate and with a sub it is almost meaningless.
Perhaps, but 1m/1w anechoic half-space is the industry standard, and remains the only way to compare two different boxes under the same conditions.
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Old 14th October 2004, 06:37 PM   #10
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Quote:
Originally posted by BillFitzmaurice


Perhaps, but 1m/1w anechoic half-space is the industry standard, and remains the only way to compare two different boxes under the same conditions.
While this standard is a good tool for comparing drivers in making your decision about what drivers to buy, once you have the box and driver(s), all that should matter is FR measurements at the listening position. This is especially true with subs because the room and placement have such a tremendous impact on response. Measurements from anywhere else have little useful meaning.
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