Building the Isolation Box
There are a number of reasons why we need a box to measure headphones in:
- You want to measure in a quiet environment to keep the noise in measurements low.
- We need to expose the head to a known broadband noise environment to get isolation measurements.
- You need an repeatable and damped acoustic environment for measurement repeatability. Not only in the isolation measurement, but open back headphones allow sound to leak out, reflect of walls/objects, and potentially affect measurements.
- Now, I could modify a closet or room in my house to do this, but I kinda don't have the room, and I'd like to be able to bring the measurement system to CanJam, so it has to be a box to be portable in this case; sized somewhat between a washing machine and refrigerator at most.
To make the box quiet, you have to build its walls that don't pass sound. There is two ways a wall stops sound transmission: by being immovable (massive and stiff) and reflecting it; or by being absorptive (fiberglass and foam disturbing the passage of air) and attenuating it. The very best killer isolation rooms are a “room within a room” where the room's walls are concrete/cinderblock, and the space between the rooms is filled with absorbing material. Here's an interesting Wiki page
The room I built at HeadRoom, was a normal room that I put multiple layers of drywall on, and then built an air space and then a wall of fiberglass and foam absorption material. It was more of an anechoic chamber than an isolation chamber. Also it was hard to make a door that sealed well out of a standard door. Bottom line, it didn't isolate very well, and we had to do all the measuring at night or early in the morning so that it was quiet in the building when we did the measurements.
Unfortunately, the “room within a room” method makes for thick walls and very difficult problems making doors that seal. To make the box small enough to fit through a door and still have thick enough walls, I'd end up with not enough space on the inside for the head. So, I've opted for a box with stiff and massive walls using 3/4” plywood and a layer of drywall heavily glued on the inside. I would have gone with two layers of sheetrock, but 200 lbs of box seemed more portable than 300 lbs of box.Broadband Noise Environment
To get the broadband noise environment we simply mount a speaker in the box and play pink noise. Now, there's a bit of a problem that you need to solve which is that you want the noise to be coming at the head from as many directions as possible. When we had a speaker in the HeadRoom anechoic chamber the sound came from one direction only, and if you look at our isolation mesurements they look a bit lumpy. I'm guessing that may be in part from the point sound source exciting a small number of discrete resonances in the acoustic system being measured. So in the case of this box, I want the sound to be able to bounce around in the box some so that it provides more of a diffuse field of sound and hopefully will provide a more representative measure of the headphone's attenuation.Repeatable and Damped Acoustic Environment
Lastly, since open back cans radiate noise outward, it's possible for that sound to reflect off the walls of the box and get back into the headphones to interfere with the measurement. So, while you want some “liveliness” (reverberation) on the inside of the box for the isolation measurement, you also want to damp it and prevent direct reflections to keep the measurement of open cans clean; and since resonances and reflections are positionally sensitive, you want to be able to position the head in the same place in the box for every measurement.
The bane of any technical development is the need to compromise. We're simply not going to get 30dB of isolation and make it portable. I was hoping for 20dB; looks like I'm currently getting about 15dB … I think I might be able to improve that just a bit with some tweaking, but I think it will be adequate since my house is pretty quiet.
Let's look at some pix:
Spring in a Montana driveway ....
Measure twice, cut once.
Putting the sides together.
Starting to make the box. (Man, I need a tan.)
Mad man at work.
Time to add some drywall.
Gluin' and screwin'.
Looks like a good avatar!
Got to make it look pretty.
The door has to close air tight, so I put noeprene on the door as a gasket ....
... and beefy latches.
Have to get the cables in there somehow; a hole filled with clay works well.
I'm working on thinking up another entry at the door for temporary cables at the show that doesn't use clay.
I've used partially inflated wheelbarrow inner tubes underneath as vibration isolation.
Inside is the head on it's stand, and it's a bit difficult to see but there is some 1x1 screwed into the bottom of the box that the stand indexes against for positional repeatability. You'll also note the absorbing foam to either side of the head and that the head is on an angle; that reduces reflected sound from the headphones getting back into the cans.
Here's a clearer view of the speaker; it's pointing sideways somewhat to cause more reflected than direct signal for the isolation measurement. The 2x4 on the side walls stiffens the walls and also stirs up the sound a bit.
And here's the whole kit and kaboodle.
Next time we'll talk about the gear, eh?