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Old 15th December 2003, 03:23 AM   #1
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Default anechoic jig

i was stuck in traffic today and got to thinking about how to best take a freq response graph indoors without an anechoic chamber. i thought maybe if you could make a square with 4 pieces of wood, cover both the inside and the outside with 2 inch thick egg crate foam, open front and open back box, then mount your meter or mic in the center of the box. would this work well enough to cancel out room reflections. I think it would be good idea but would need some work from several minds to make it work well. i have already thought about the wood may resonate at the wavelength that equals its length. but i can't think of much else that would be wrong with it. maybe even add a back with a hole where the mic or meter could be put inside the box to reduce rear reflections. any ideas? is this way too far "out there" to get working?
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Old 15th December 2003, 07:26 AM   #2
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here is a quick drawing since nobody has responded yet. it probably has been done before. i just think a portable anechoic chamber sounds like a good idea to me.
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File Type: jpg anechoic jig.jpg (27.1 KB, 328 views)
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Old 15th December 2003, 09:13 AM   #3
azira is offline azira  United States
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That's kind of a cool idea. I was looking at my living room which is kind of small though.

I think it would work good for mids and highs but the bass reflects off walls really easily. Like the wall behind your speakers. I think that it would also reflect from the wall behind your jig into your jig. But that probably wouldn't matter if you used a directional mic, no reason to use an omni. You probably don't want to enclose the box either.

On the other hand, your going to be measuring at like 1-ft away, say your speaker is in the middle of the room or something, you might not have these issues....
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Old 15th December 2003, 09:27 AM   #4
sreten is offline sreten  United Kingdom
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Sorry SG,

But it simply won't work very well and its not needed.

You can take measurements in-room.

You use near field techniques in the bass, and gated impulse
techniques in the far-field over say 300Hz. The gating is set
to be shorter than the first in-room reflection.

You can also use multiple averaging techniques for in-room
response including the bass response.
Outdoor measurements are also a possibility.

Even large anechoic chambers are inaccurate in the bass,
and don't represent the bass response you will get in room.

/sreten.
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Old 15th December 2003, 11:49 AM   #5
Shaun is offline Shaun  South Africa
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speekergeek

The absorptive foam would only be effective down to frequencies where it's thickness is greater than the wavelength of the signal (or thereabouts; someone give us more accurate info here, please). Below that, the signal will go straight through, and get reflected off the side walls.

This is why anechoic chambers are (1) generally large and (2) have such huge (deep) absorptive panels.

It's much easier to get semi-anechoic response with a gated MLS signal (at home).

The next best thing to an anechoic chamber is to measure on an elevated platform. About 7 metres high will get you down to 20Hz. Next best to that is ground plane measurement on a reflective surface (eg, marble) with 7m clearance on all sides.

Good luck
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Old 15th December 2003, 06:33 PM   #6
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I am in the process of building an impedence jig (not to change the subject! ) and i have downloaded speaker workshop recently also. can speaker workshop do the averages and gating that you speak of?
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Old 15th December 2003, 06:56 PM   #7
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On the subject of damping. Egg boxes only work above 14kHz and the foam not much less. 100mm acoustic foam measures ok (it says in the manual). But I have enough panels to make the box you describe and I can't notice the difference.

I damp my room down when recording by throwing bedding arround. I read somewhere about making a bass trap by filling open cabinets with duvets. By far the most even acoustic damping material across frequencies is the human body. With that in mind, I suggest that you throw an acoustic measurement party and invite as many humans as possible.

OK, not practical, but a lot more fun.
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Old 15th December 2003, 08:04 PM   #8
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Speekergeek,
Speaker Workshop can do averaging and gated measurements, although the averaging only repeats the tests several times and averages the result. Select this by clicking options/preferences/measurements and enter the number of measurements you wish to average in the repeat count section of the MLS signal box.

For gated, you create a driver by clicking on resource/new/driver then give it a name. Double click on your new driver in the tree on the left side of the screen then click on measure/freq resp/gated. After a few seconds of noise, a frequency plot appears under your driver in the tree so double click to see it. I usually apply smoother to remove the hash: transform/smooth/third octave.

You need to adjust the time markers so the SW knows when the echos are due so that it can deal with them. I think Eric Wallin describes this best in his tutroial (see the gated section, about two thirds down)...
http://www.gti.net/wallin/audio/audua/audua.html

I've been in a couple of anechoic chambers and the cones were about 1m long. It's a very strange and unsettling feeling when all echos are taken away!

Nice one,
David.
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Old 15th December 2003, 08:53 PM   #9
Bose(o) is offline Bose(o)  Canada
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Why don't you forget the box and just create the skeleton for the like. You could simply bridge the gaps with layers of foam. This way it doesn't create any pressure, reflections and resonances for measurement. Of course, large amounts of SPL would penetrate the area and you would probably have included the room modes within your measurement.

I also agree with just doing "in-room" measurements. There is nothing wrong with this type and it more resembles what you are listening or will be listening to through music.
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Old 15th December 2003, 10:10 PM   #10
usekgb is offline usekgb  United States
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I had the fortunate experience to go in to Electro-Voice's anachoic chamber a couple of years ago. I couldn't even see the back of the fins they used in this chamber. I'm not sure how low this chamber can go accurately, but I would say the fins were at least ten feet (~3m) deep. It's a really creepy feeling to be inside one of those things. You can actually hear the blood flowing through your ears!

Cheers,
Zach
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