DIY Portable Anechoic Chamber

Question

I would like to perform some measurements of the microphones (i.e. frequency response, SNR, etc.). First and best option would be to use the Anechoic Chamber- obviously this is not an option because of access and price. Another way would be to perform measurements in quiet room and then window the Impulse Response. Unfortunately this will limit lower frequency of my measurement - obviously I do not want that. I believe that the best way to do that is by using small Anechoic Box and putting speaker and microphones in it. Prices of such are very high, therefore is there any good way of creating one? Maybe someone already did it. Mostly I am curious with:

• what kind of absorbent to use inside
• what to use as the skeleton (maybe some kind of server cabinet?)
• how to insulate it in a best way from outside noise

I would be thankful for any help. Regards.

---

Copied from comment so everyone could see:

Indeed I thought about performing measurements in open field or in a duct (as proposed in IEC 60268-4). Of course doing outdoor recordings in the UK city is rather complicated, therefore I immediately dropped this idea. Although sequential averaging of IR's from sweep-sine measurement is doable.

I've been also thinking about this "duct" solution, but there is no information regarding it's specification. I suspect it is more like impedance tube, but probably it is not enough to make it on your own. Hence I've chosen to go for the Anechoic Box.

Answer 1

Consider using an anechoic chamber from others. Every microphone and speaker manufacturer will have one, most Universities have huge EMI measurement chambers (working perfectly). They might let you use it for an hour at a time, or you can rent it for a day for some more money.

If you need to build one by your self, start with a noise floor measurement box. Use some heavy metal or concrete box with lid(getting the cable out), at least 20-50 litres of volume, and stuff almost completly with damping material like cotton wool. Use in an very quiet enviroment - cellar. This will be very accurate. Mic must not touch anything inside, of corse.

If you need to measure frequency response, you need a perfect speaker. Or something close to it, and a close-to-perfect measurement mic with known freq response. Sweep and record both mics, calculate diffs. That is not possible in less then a real room - or outside, with no ambient noise like cars or animals, and absolutely no wind. Measuring >50 times will average out some disturbances, though.

Answer 2

There are pre-existing facilities that regularly measure the frequency response of microphones. you should call one of them (try pcb) and talk to them about your project... A chamber might not be the best way to measure this.

But if you do want to build a chamber, a lot depends on what the lowest frequency you want to measure. If you have a quiet space, you could probably build something decent if you don't need to measure below 500Hz.

For structure construction, you could use standard wall construction... Put two layers of plywood on either side of a metal stud with a 2-4" air space. Be sure to fill the air space with fiberglass insulation or something similar.

Here's a useful pdf: Design and implementation of a small anechoic room and sound-actuation system.

Answer 3

Unfortunately "small" and "anechoic" are mutually exclusive, at least at low frequencies. The length of the wedge absorbers needs to be at least a quarter of a wave length plus some extra space for the field dissipate.

If you need to do less-echoic measurements, your best shot may be to find a large open space, get as far away from reflecting surfaces as possible and then time gate. This will impact low frequency resolution but any small-ish box would do this too and probably be worse.