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Old 2nd September 2013, 02:43 PM   #1
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Default Where to do ground plane measurements?

There's no way i'm even attempting a 3-way speaker without ground-plane measurements. I've had enough trouble with my current 2-way speakers - Doing the usual near-field spliced with far-field guesswork.

I was thinking of hiring a school sports hall because they are large, have a smooth, flat floor and high ceilings and there should be a socket somewhere near for power for my amplifier.

Before I do, i'd like to check with you guys to make sure that i'm doing the right thing. If not, what do you think I should do?
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Old 2nd September 2013, 06:51 PM   #2
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I do my measurements outdoors, away from buildings.
Requires timing the captures between wind gusts, but no hire fees...
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Old 3rd September 2013, 11:55 AM   #3
Juhazi is offline Juhazi  Finland
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Grass is excellent to absorbing waves! Outdoors distance of 1,5 - 2m is fine. It is good to use at least 2 different distances and analyze/compare the results right on. Also a 1,5-2m high stand for the speaker helps. If the bass is downfiring, it must have a solid base plate to simulate foor placement.

I have seen ground plane level (speaker and mic on ground) used only for subwoofer comparison. The response is valid as such (like nearfield), but again, very different from what happens indoors. It is difficult to guess what kind of response would be best for your room.

It is important to do indoor measurements too, with the speaker at or near the final placement. Listening tests are important too eg. for bass level 2dB difference is a lot!
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Old 3rd September 2013, 03:18 PM   #4
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You can actually get a quasi-anechoic response of the bass indoors. The plots below are of a Tang Band W5-704 (shielded) Bass Reflex. Green trace is of the Port taken at the mouth, Red trace is Nearfield Response of the W5-704 and the Violet trace is the complex sum of the Green and Red traces. Trouble is, you'll need a software like LMS.

Another option is to lay the speaker on the floor with the drivers facing the ceiling. This is a variation of a Pit measurement, except that the front baffle is not flush with the floor, so it will not be exactly 2 Pi. This is quite easy to do outdoors. As with all outdoors measurements, wind and ambient noise is something you'll have to contend with.

Regards
Mike
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Old 3rd September 2013, 03:36 PM   #5
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Quote:
Originally Posted by fatmarley View Post
There's no way i'm even attempting a 3-way speaker without ground-plane measurements. I've had enough trouble with my current 2-way speakers - Doing the usual near-field spliced with far-field guesswork.

I was thinking of hiring a school sports hall because they are large, have a smooth, flat floor and high ceilings and there should be a socket somewhere near for power for my amplifier.

Before I do, i'd like to check with you guys to make sure that i'm doing the right thing. If not, what do you think I should do?
I want to address your premise that splicing indoor near- and far-field measurements requires "guesswork", etc.

Recently I released version 2.0 of my "FRD Blender and Minimum Phase Extractor" Excel based software tool. This takes at least one measurement (like a gated-impulse derived frequency response) and combines it with modeled or measured low frequency data (e.g. a box model or nearfield measurement) plus a model of the baffle step response. The tool allows you to scale the data, and blend the responses together to make a seamless, wide-band frequency response. Fixed slope tails are used to represent what the FR is doing in the high and low frequency limits (e.g. 12dB/oct for a woofer in a closed box at low frequencies). Finally, you can extract the minimum phase from the wide-band response plus tails data.

The new version includes a box modeler and a diffraction modeler right in the workbook. These were contributed by Jeff Bagby, based on some of his Excel tools. File importing and exporting can be done easily using some buttons for that purpose.

Note: If you use OpenOffice Calc, you can still do everything except the buttons will not work since OO does not support VBA.

-Charlie
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Old 3rd September 2013, 05:47 PM   #6
SY is offline SY  United States
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I had an anechoic chamber (actually, my company did and they weren't using it, so...). Unfortunately, during a rebuilding of the production area, I came in on a Monday morning and found that it had been dissembled and discarded. Ah, well, there's still plenty of smooth factory floor, perfect for ground planes. And I still have the use of the B&K/DPA mikes and preamps.
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Old 3rd September 2013, 06:52 PM   #7
Juhazi is offline Juhazi  Finland
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My plant
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File Type: jpg AINOgradient outdoor.jpg (408.7 KB, 121 views)
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Old 3rd September 2013, 08:33 PM   #8
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Old 3rd September 2013, 08:45 PM   #9
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Quote:
Originally Posted by Michael Chua View Post
Another option is to lay the speaker on the floor with the drivers facing the ceiling. This is a variation of a Pit measurement, except that the front baffle is not flush with the floor, so it will not be exactly 2 Pi. This is quite easy to do outdoors. As with all outdoors measurements, wind and ambient noise is something you'll have to contend with.

Regards
Mike
I wonder if that would be a good way of measuring a speaker that's designed to go up tight against a wall?
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Old 3rd September 2013, 08:51 PM   #10
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Quote:
Originally Posted by CharlieLaub View Post
I want to address your premise that splicing indoor near- and far-field measurements requires "guesswork", etc.

Recently I released version 2.0 of my "FRD Blender and Minimum Phase Extractor" Excel based software tool. This takes at least one measurement (like a gated-impulse derived frequency response) and combines it with modeled or measured low frequency data (e.g. a box model or nearfield measurement) plus a model of the baffle step response.

-Charlie
Brilliant! This is just the thing I didn't know I was looking for.

I've had a quick play and things were looking good but when I copied and pasted the text files for LspCAD, the frequency response looked the same as my botched nearfield/farfield splicing job ( bass hump ). Not sure what I did wrong but i'm going to have another go...
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