Microphone test box

For several years I’ve had the idea of building a testing box for testing microphones, hearing aids, and other small sound devices, but I never got around to doing anything about it until recently.  My original plan was to buy some sort of solid box, line it with sound-reducing foam, and add a partition with a loudspeaker at one end.  At the other end I would mount the device being tested, with an extra microphone to monitor how loud the sound is outside the device.

A few months ago, one of our neighbors down the street was selling their house and had dumped a rather large, but ugly double wooden chest on the corner to give away.  This box seemed ideal for my project, and I’m a sucker for free things, so I went home to get my hand truck and wheeled the chest home.  It sat on the hand truck in my garage for months, waiting for me to get the loudspeakers, a hole saw, and sound-reducing foam.

This box has two lids, each of which opens into a separate compartment. The compartments are about 82 cm long, 40 cm wide, and 33 cm high. The wood seems to be fir or pine.

The box is solidly made, but rather roughly finished inside.

I finally started work on the project this week, having bought a 4¼” hole saw, a huge number of sound-reducing foam tiles, and two Valcom 936398 intercom loudspeakers. I chose the loudspeakers because they were cheap ($5 each) midrange speakers with 45Ω impedance.  Having a large impedance means that I can drive the speakers directly from the Analog Discovery 2 function generator with reasonable power, without hitting the 30mA current limit of the function generator.  By putting two in series I should get 90Ω, which would mean a maximum voltage of about 2.7V  at the maximum current and 81 mW of power.  With a sensitivity from the spec sheet of 85.95dB @ 2.83V and 1m, I should be able to get about 79dB at 1m.  Inside the box, the mic will be closer and there will be reflections of the walls, so the sound may be quite a bit louder, but there are likely to be weird resonances.

My first task was to do impedance spectroscopy on the loudspeakers just sitting on a table (no box), using the Analog Discovery 2 impedance analyzer.  For the individual speakers, I used a 100Ω sense resistor and a 1V amplitude, and I played with different voltages for the series connection.  The impedance measurements were pretty much the same for different voltages. I ended up using a 1kΩ sense resistor and 5V amplitude, to avoid any possibility of clipping when doing the short-circuit compensation.

The resonant frequencies are around 124Hz and 111Hz, with the series pair having a resonance around 122Hz.  The impedance is more like 40Ω than 45Ω, and the series impedance is around 80Ω, so my max power is about 72mW (or about 36mW for a sine wave).

I marked and drilled the 4 corner holes for each loudspeaker, then cut out the large hole with the hole saw.  I was surprised by how much the hole saw kicked when its teeth engaged the wood—the first time  it happened I was not holding the drill firmly enough and it broke the ¼” guide bit!

The bit snapped off very cleanly, but luckily I had a spare drill bit I could use.

I braced my arm against the edge of the box and managed to hold the drill steady enough to cut the circles, but I ended up with linear bruises from the edge of the box. I probably will not be using the hole saw with a hand drill in any future projects—I’ll clamp stuff very firmly and use the drill press!  Unfortunately, that was not really a possibility for this chest, as I was not about to try to disassemble and reassemble it.

The holes came out ugly (I couldn’t hold the drill steady when it kicked) and a bit too big.

Here are the holes from the side I started and finished on. In the middle I tried cutting from the other side, so as not to have massive tear-out when the blade finally came through.

Here are the holes from the other side. Despite the guide hole from the ¼” drill bit, the holes did not line up precisely, and I had to use a rasp to remove a little of the wood.

Although the hole from hole saw was a little too big and intersected with some of the holes for the bolts, it was still possible to bolt the loudspeakers in place with 1½” 10-24 machine screws and nuts.

Here are the loudspeakers viewed from the front

And here they are from the back.

My next step was to measure the impedance again with the loudspeakers in the box.

The resonant frequency is the same, but there are many smaller secondary resonances. These resonances are probably from acoustic resonances of the box, not just noise, as they were repeatable even with different voltages and sense resistance and redoing the short-circuit and open-circuit compensations

My next step is to try to remove some of the resonances of the wooden box by lining both compartments with sound-reducing foam.  The foam comes vacuum-packed, and it expands quite impressively when the plastic wrapping is removed.

Here are the foam tiles, completely filling one compartment. I’ll use over half the tiles lining each side.

I got a bunch of double-stick adhesive pieces to mount the foam tiles on the bottom, sides, and top of each compartment, but that is going to be fairly tedious cutting and sticking to make it all fit, so I did not have the energy to do that yesterday or today. Tomorrow my wife and I are talking one of our walks, and Sunday it is going to rain (we hope), so I won’t want to work on the porch where I have this box set up. So it may be a while before I get to the next step of testing with the foam in place. I won’t bother modeling the loudspeakers until I get the box lined, and I’ll probably only model up to about 2MHz—I’m pretty sure that there is not going to be any sound from the loudspeakers above about 40kHz (and I’d even be surprised to see much signal even at that frequency).

Once that is done, I can hook up a microphone and do network analysis to see what sort of frequency range I can get out of this setup.

The photos in this post were all done with my moto g(7) phone, and they required more color correction and exposure modification than I’m used to doing—I really have to get around to ordering a new camera!