Quiet That Measurement!

29-08-2023

Building your own portable measurement chamber (from May 2020)

We thought this might be fun to read - how we built a small test chamber for making headphone measurements.

With all things measured, it's not just a desire, but the gear that matters.  We've got a rack full of great gear - Audio Precision, CRYSound, Infomatica (CLIO), etc.  And we have great mics, couplers, headphone stands, etc.  But you need a place that has a controlled environment for measurements.

Well, we've had a few around here, but we thought we'd show you how we built a small, portable, and HIGHLY effective test chamber.  Combined with a Pocket CLIO and a nice 3D printed simulated IEC711 coupler, and you have a near-pro-level test system for well under $1000 - and most of that is the CLIO.

So, if you're interested in seeing how we build out a test box, keep reading!

First, secure your supplies.  This is what we did:

- A rolling, 12U road case.  You can find them on Amazon, Musician's Friend, Sweetwater.  We went to the local Guitar Center and bought a "scratch and dent" floor model for $150 OTD!  Rugged, metal construction, locking covers, wheels.  Yeah, it' $150 - but then, we actually want something that will last a while, and the lumber/hardware to build this ourselves would be close to $150 - and a solid day of labor.  So we chose the deal.

- Fiberglass board!  Rigid, 1" board is what we used.  Owens Corning 703 grade.  We buy a 12 sheet box fairly regularly (twice a year or so) for various projects.  Your local HVAC supply shop (or even installer) can get you 3 sheets (you'll need 3) for about $10 a sheet.  

- Acoustic foam.  We bought 1" black foam from Amazon, but you can find it all over the place.  Not so much needed for acoustics (it does help a little), but mainly to keep the fiberglass out of reach.

- Sprayable contact cement.  This is how you'll install the foam on the fiberglass.  No need to glue the fiberglass down, you can press it in and use smart order for boards to hold everything in place.

- NOT NEEDED, BUT RECOMMENDED: hair spray!  Coat each cut line on the fiberglass with a quick pass of hair spray.  REALLY keeps the fibers down to essentially zero!  This is a ProTip - something very few know, but it does work wonders.

For starters, we're going to be smart about how we do this.  Fiberglass first, then lay in the foam.  And the fiberglass will be free-floating!  Pressure only.  So do the panels in this order: top, sides, rear, bottom.  That means the sides and rear are supporting the top, and the bottom keeps the sides in rear in place - self supporting!  No glue needed.  Front presses in to the recessed section of the lid - again, self supporting!

We start with cutting.  We use a battery powered reciprocating saw with a FINE TOOTH METAL BLADE.  This one is 40 TPI.  You want FINE TOOTH, it cuts like butter and again - keeps the tearing (and fiber spread) to an absolute minimum!

Measure the size of the top - and WRITE THE DIMENSION ON THE FIBER BOARD!  That way you don't mess up!  Cut the panel and simply press it into place.  Measure the sides, do the same.  Back.  Then bottom.  Then the cap.  With a few cuts (12), you've got this:

Note the measurements on the board, indicating the direction of the dimension?  Anyway, at this stage we have a fiberglass lined road case, ready for the next step - acoustic foam!

Now, there isn't anything special about orientation, or sizing.  Just put it in.  We mixed up the pattern so it looks better, but you can do all one direction if you want.  Spray some glue on the fiberglass, some on the back of the foam, let it sit for a minute to tack up, then press together - it's not coming back off!

Cut pieces (leave them as big as you can - why do extra work) and cover.  we did top, sides, rear, bottom and front - same order as fiberglass.  Remember, this isn't adding a lot of noise control - it's really to control fiberglass migration, so cover as much as you can, but don't worry about tiny cracks.

After half an hour, we were done.  So we drilled a pair of 5/8" holes in the back (so we can run our BNC cables for testing) and this is what you end up with, over to the right.

Now THAT is a chamber!  So how do we test it?  Simple!  Put an IEC711 coupler inside, an identical IEC711 coupler on the top, and a loud speaker (in this case, a SONOS Play:5 at 2 meters distance, cranked and ramming out pink noise) and measure the differences:

WOW!  Big difference above 100 Hz - massive level!  Oh, and we're using calibrated microphones, so that SPL is actually dB SPL - it's a true SPL level.  YES - you can have SPL below 0 dB!  0 dB SPL is a 20 micropascal pressure level; it's not zero.  If you're at -10 dB SPL, that means you're around 6.7 micropascals of pressure.

We're seeing 20+ dB attenuation above 300 Hz, and more than 40 dB above 1 kHz.  This means that THD measurements (where 1% THD is -40 dB) above 500 Hz will be a LOT more accurate - no noise corruption!

And there you have it!  We spent $150 on the box, $30 on the fiberglass, $25 on the acoustic foam, and $15 on the glue.  So all-in we're around $220.  Not at all a budget-breaker, especially if you're at all serious (even at a hobbyist level) about making measurements.  And it is plenty robust - we're using the box daily in our regular operations here.

Hope this was instructive!