Note: If this is your first venture into binaural recording, you can skip the ear canals and still make great binaural recordings. Once you get comfortable and want to experiment, this is a fun addition to your dummy head. Instead, simply mount the microphones in the opening of the ear canal from the inside pointing out and keep the front of the mic flush with the opening of the hole.

One extremely important aspect to building a good binaural dummy-head, in my opinion, is the ear canal. Soundwaves hit our outer ear (pinna) and are reflected and/or funneled into the ear canal. The ear canal delivers the incoming sound directly to the eardrum.

The ear canal is basically a tube, and we all know what sound through a tube sounds like. So sound traveling up the ear canal to the ear drum is distorted by the shape of the ear canal itself. We don't hear a "tube-like" sound of course, because our hearing system is accustomed to this distortion. In effect, we "cancel" it out.

After several experiments using the SP-TFB-2 binaural mics mounted in my own ears, I was puzzled as to why I didn't get any vertical directional cues in my recordings. I decided to model some ear canals in my dummy head to see if that had any effect. After weeks of trial and error, I finally came up with an ear canal design that actually significantly improved the vertical localization of sounds. At that point I (informally) concluded that modeling the ear canal is a critical step in designing a good dummy-head. Without the ear canal, you will lose vertical information.

You can laugh all you want at how simple my design ended up, but I've got to say that it works like a charm. Below is a graph of three frequency response curves taken from different studies of actual human ears with a sound source at 0 degrees azimuth. This plot includes the frequency response of the ear canals.

From [Wiener and Ross (1946), dotted line; Shaw (1974), dashed line; Mehrgardt (1977), solid line]

Here is a frequency response snapshot of my silicone ears and make-shift ear canals with white noise at 0 degrees azimuth.

Notice the peak at around 3 kHz and the valley at around 10 kHz? If you look at the relative dB shift between peaks and valleys, you'll notice that there is about a 20 dB peak at 3 kHz and about a -10 dB valley at 10 kHz. This matches up fairly well with the actual human response curves. But more importantly, the listening experience is significantly better.

Let's start construction now with building the ear canals...

The ear canals I built and still use in my dummy head today are made from plastic tubing I stole from a homebrew syphon tube I had lying around. It just happens to have a 7 mm inside diameter, which matches the human ear canal. You should be able to find similar plastic tubing at any hardware store. One foot will be plenty. Just make sure the inner diameter is 7 mm and that the outer diameter is 10 mm.

In this section you will use the following parts:

• Plastic tubing (1 foot)
• Metric ruler
• Scissors

As you can see in the diagram on the left, mark the main tube at 30 mm.

Cut the tube on your 30 mm mark.

We will now cut an additional small piece of tubing to go inside the outer tube. This will help obtain the frequency response curve shown above. I won't go into technical details on this. Just trust me and mark and cut an additional 7-8 mm piece of tubing off your source tube as shown on the left.

Now we need to trim the small length of tubing. Cut some "circumference" off of the tube so that when it rolls back up, it makes a smaller diameter tube. Continue shrinking the small piece until it fits snuggly into the larger tube.

Slide the smaller length of tube into the larger tube so there is about 5 mm from the end of the larger tube to the beginning of the smaller tube.

You might have to use a small screwdriver to push the smaller tube in far enough.

Do this process again for the other ear canal and make sure to match the dimensions the best you can.