The High Accuracy Binaural Pressure Zone Microphone

This is a device which I first created about 10 years ago. While considering how the human brain perceives sound, it occurred to me that the brain might consider the two human ears as located in one point in space, rather than separated by the few inches that actually exist. If this is so, I thought, then a binaural microphone with two elements located as close as is physically possible might produce some striking acoustic illusions. I must say, the results I've had with this creation, even using the cheapest materials, have been amazing. I have been completely unable to tell if a voice or sound I hear while wearing headphones is a recording playing back or an actual human talking or walking up to me. Frequently when listening to a recording made with the mic, I'll hear something and turn around only to find nothing there. I've let people of all ages try this, and all of them have been fooled, even with cheap headphones.

So, now I present to you how to build my microphone.

Parts Inventory
- A pair of tiny condenser microphones with clothing or tie clips.
- One sheet of 1/4" plexiglas, 24" square. Larger is better, but you may not be able to mount it on mic stand.
- One #6 or #8 screw, a matching nut and two washers.
- One standard microphone grip, threaded for a typical mic stand.
- One typical mic stand.

Comments on the parts:

I used a pair of inexpensive Radio Shack tie clip condenser microphones. Surprizingly enough, they still sell the same mic: catalog # 33-3013. The metal parts appear to be anodized black now, but otherwise it seems to be the same item. They dont have a terrific low end, but they do have excellent response otherwise. I would love to try this with DPM miniature mics (DPM, formerly known as B&K, make most of the finest microphones in the world.) Let me know if you go that route, ok? You'll probably make history if you do.

I chose to use plexigas. You could use a sheet of masonite or anything a quarter of an inch thick, but it must be glass-smooth. Since the finished mic is large and often used indoors, I think it looks a heck of a lot less intrusive when one can see through it. And while you are assembling it, you can easily see when the two mics are lined up with each other.

Tasks to create the the High Accuracy Binaural Pressure Zone Microphone

Here's a drawing of the overall goal:

construction overview

For the RS mics, the first task is to modify the tie clips so that they can be secured to the plexiglas in such a manner as to hold the face of the mic towards the sheet of plexiglas. What I did was remove the hinge and also the sprung clamp. That leaves you with the part of the tie clip that has a mount for the microphone to clip into. Then take a pliers and bend the end where the hinge was around the screw at least three-quarters of the way around the screw. The goal is to mount the clip in such a way that when the microphone is placed in the clip, the face of the mic is flat against the plexiglas. The face of the mic element must be very close to the surface of the plexiglas, but not touching the surface. I think that the thickness of a business card is just enough. Remeber, we are making a "Pressure Zone" microphone, here. (which see) The mic needs to "hear" the pressure zone only.

Drawing of tie clip bent around screw

If you have another style of mic element, you may have to fashion a clip. Whatever you use, make sure it does not interfere acoustically with the mic. In other words, make sure that the clip doesn't block the sound to the mic. If it was me, I'd probably bend a paper clip into position. It's small, cheap, easily available, and would do the job.

Next, place the bent clip on the plexiglas so that the mic element is in the center of the sheet. Mark where you will need to drill a hole in the plexiglas. This hole is where the screw goes that holds the clip to the plexiglas.

Photo of bent tie clips mounted to the plexiglas

The photo above shows how it looks when they are mounted properly.

The mics simply slide into the plastic holders on the tie clips:

photo of mics mounted in the clips

photo of mics mounted in the clips

photo of mics mounted in the clips

The fragile mic cables really ought to be strain-relieved. I will leave that as an excersize to the reader!

The next step is to add a threaded coupler to allow the plexiglas to be mounted on a mic stand. I used one of these:
photo of mic clamp/grip/holder/thing

Remove the screw. This is the hinge. The two parts will separate, given enough force. Trust me on this. Toss away the mic clamp part. What you want to keep is the part that has the threaded insert and the two screws that held the clamp on. Mark the exact midpoint of one of the edges of the plexiglas (that would be 12" if you have a 24" square sheet). Slide the mic stand mounting on to the edge of the plexiglas at the mark. Make sure that the plexiglas seats at the bottom of the slot in the mount. Mark where the screw goes through the mount. Remove the mount, and drill a hole:

Stand mount ready to be attached to the plexiglas

Slide the mount onto the plexiglas again, thread the screws and tighten them. Make sure they are extremely tight.
It will look like this:

mic stand mount secured to plexiglas

Now at last, you are ready to go!

Loosen the shaft of the mic stand so it can spin freely. Hold the plexiglas with one hand, and spin the mic stand shaft so that it threads into the mic mount you just screwed into the plexiglas. Make it tight.

And here is the end result:

finished product

I think it's actually striking to look at. People are generally concerned about what it is and what it does, and, oh, is it dangerous?????

Oh, yeah, I forgot to mention – I made one other modification. I removed the connectors on the mic cables and replaced them with quarter inch phono jacks. This works for me, as the mics are unbalanced anyway and my mixer will take either balanaced (XLR) or unbalanced (1/4" phono) connectors.

Have fun!

Make a joyful noise,
andrew at noisefloor.org