DIY anechoic chamber!

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I have spent a few days and a few bucks making my own anechoic chamber. It is not finished yet as I am still waiting for some 3" wedges.

This is the chamber from the outside:

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Down left you can see the cable inslet. I am planning another inlet on the right side of the door:

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It is made out of a 3" plastic pipe with a 90 degrees bend:

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Let's look inside:

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The door is off course at thick as the rest of the walls:

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This wire pullt the handle on the inside of the door:

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Just pull the wire and push the door:

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The door is fitted with 4" rock wool and 4" glass wool sound barrier plates as all the other walls.

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The lightening consist of 3 pcs 12V/20W halogene light bulbs hanging down thru the roof in rubber cables:

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The floor has been fitted with even 2" more damping:

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The floor is made with a 20"x6' removeable "corridor" in the middle:

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The door fills the opening with about 5-10mm tollerence. It will be fitted with rubber sealings to reduce the pressure on the door blade:

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The front edge og the door is angeled to not crash when opened:

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The frame is screwed to a cheap standard door:

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The frame around the door opening is angeled just like the door to fit perfectly:

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The mike stands straight on the sound barrier plates. It will be replaced with a lighter and smaller stand that will be fitted with an absorbing hoose:

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The ceeling is kept in place by a fence fitted in steel wires hanging from the original roof. The wires will also act as fastenig for the studio wedge panels.

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Here is the other end of the cable inlet. It will be filled with absorbent matherial in both ends to avoid noise from the outside:

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All the surfaces will be covered with studio wedge panels.

In the roof and on the door blade I am planning to use this one:
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This is a 2" plate with very little reflection.

The floor and all other surfaces will be fitted wit this one:
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It is a 3" "Z-foam"-model with very high grade of absorbtion and very little reflection. The panels will be organized with all the ribs crossing the direction of the measurement.

The corners has as much as 2' thick damping.

Right inside the door I will make a small rotating plattform for the speaker to be measured.
 
Lionel said:
Nice job but take care of your lungs, wear a mask.

Yep, I second that. Just looking at the room made me short of breath, and made my lungs hurt. ouch!

Use a mask!!! I hope you are doing so already.

I've made my own anechoic chambers before. You see, I have these piles of PZA's lying about, and I arrange them 'just so'....
 
MethMan said:
IMO it cannot be anechoic chamber under say 200 Hz but as a small room improvement should work well.


Yep - it won't work well at wavelengths longer than twice the shortest dimension, but it will be better than measurements in a reflective room. I'd like to see some measurements results at different sample lengths.
 
That is why the room is built so heavily absorbant. I did a few tests yesterday. It seemed like the room worked well even down under 100 Hz, When i turned the box around (this was a BR) and measured the port the sum of the port and the cabinet seemed to sum relatively close to the simulated response down to about 30 Hz.

I will have to use a known driver with reliable parameters in a closed box to evaluate the performance of the room.

I am going to make a test box and measure the response both here and in the anechoic chamber at Seas with the same equipment. (Their chamber is reliable down to about 70 Hz).
 
Yep - it won't work well at wavelengths longer than twice the shortest dimension, but it will be better than measurements in a reflective room. I'd like to see some measurements results at different sample lengths.

True. This is not only limitation. On the low frq. chamber works as "in-box" Further:

Snickers-is That is why the room is built so heavily absorbant. I did a few tests yesterday. It seemed like the room worked well even down under 100 Hz, When i turned the box around (this was a BR) and measured the port the sum of the port and the cabinet seemed to sum relatively close to the simulated response down to about 30 Hz.

I don't think so. It is hard to believe. It is probably about what you mean with words "seem" an "relatively close". Look at enclosures which are fulfilled with wool and they have resonant modes. Low freg. standing wave "goes thru" 5 times more wool and still exist.
Mineral+glass wool has relatively small ability to absorb longer wavelengths. Under say 500Hz the ability falls down steeply. This demerit is usually improved by coating it with another fibrous material (usually some kind of paint with porous structure or another structure on the surface). The second, not less neccesary, construction tip is to change the "speed of sound". This is made by offsetting sound absorbant material from the sound reflective structure. There is a non-linear dependence between the offset and low freq. absorbant ability.
You have made a good thing when using two different materials - heavy weight? mineral wool and lightweight? glass wool, but in your configuration cannot affect long wavelenghts.

I am going to make a test box and measure the response both here and in the anechoic chamber at Seas with the same equipment. (Their chamber is reliable down to about 70 Hz).

I'm not familiar with Seas chamber, but usually are build with cca. 70 cm long wedges coated with fabric (dust separation is secondary virtue) mounted on the grid.

What is the surface material you have? Polyurethane plastic foam?
 
I have not received the surface materials yet. You can read more about them here:

http://www.primacoustic.com/specs_phy.htm

I am aware of the standing wawe problems, but they did not show on the mesaurement i did.

Regarding speaker boxes it is the large boxes that have those problems with standing wawes. I would believe that the largest meassure of the room should represent the lowest frequency of standing wawes, but I will get a small ammount of room gain. The corner traps is supposed to handle that.

All the damping is pretty heavy, and the outer plates are designed for sound absorbation. The tests have been done inside walls, so the properties of the bare material is not so well tested.
 
Thanks for link; urethan open cell foam. Look at physical merit of the material and its behavior under 500Hz.

Regarding speaker boxes it is the large boxes that have those problems with standing wawes. I would believe that the largest meassure of the room should represent the lowest frequency of standing wawes
This is not true.

The corner traps is supposed to handle that.
Hardly IMO. Corner traps are primarily used to supress "billiard reflection" and it is not your case.

I am aware of the standing wawe problems, but they did not show on the mesaurement i did.
I wrote it as an illustration of wool absortion capability. If not shown in measurement, IMO you probably measured coarsly or with a too small signal amplitude with consequence to measurement mistake. By the way, look at or other producer's site (Rockfon, AMF...). In product site you can nicely see on the graphs dependence of the overal construction depth and acoustic-absorbant behaviour. Without alpha(zero) very near 1, it is not anechoic. With construction you made and which have, I guess, alpha(p) 0,7 at 250Hz and 0,4 at 125Hz it is hard to believe it will have a sufficiently small influence (+-1dB say) on acuracy of the future loudspeaker measurement. Last word: anaechoic chambers are huge.
 
I know such chambers are large. But I am going to use it primarily above 200Hz. When measured nearfield (0,5m) the response graph seems to be within +0/-0,5dB down to 80 Hz. But the room is sensitive to different directions and possitions.

Now I have added more sound panels about 25 cm from each wall, and that made the response more smooth. Unfortunately I am not able to do that in the ceeling or at the floor, but I have fath in the uretane wedges too.
 
SAFETY!!!

I recommend you eliminate the latch.. Make the door close via springs.

Another possibility is to re-build the jamb section so that it can be removed from inside.

Or, compromise the jamb sufficiently that you can break it open. Or, a spring loaded ball bearing assembly.

Nice job...I wouldn't want to get stuck in there if your wire broke or disconnected from the latching mechanism..

Cheers, John
 
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