Binaural recording is a technique for producing realistic “3D” audio. It is accomplished by configuring microphones in an arrangement that approximates the hearing mechanisms in the humanCom head. When these recordings are played back with headphones, they can sound remarkably lifelike.
Human hearing is affected by the way that sound is transmitted through the head, the position of the ears, and the shape of the ears. Therefore, so called “dummy head” microphones are built to have the same shape and density of the human head. In this project, we will create a dummy head microphone in the shape of an infant’s head, and it will be filled with a silicone compound to achieve a density similar to a human head.
Professional dummy head recorders can cost upwards of $8000. This project should only cost about $100, assuming you some standard tools and materials (such as drills and tape). I can't guarantee it will sound as good as an $8000 microphone, but it certain won't sound 80 times worse!
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Step 1: Gather Materials
Materials To Purchase
- Vinyl head - I got mine from Dolls by Sandie for $20. The specific model is "Sleeping Newborn Unpainted Head - Euro Vinyl by Berenguer". I picked this head because the eyes are closed. Other vinyl heads of this type have eye sockets. The sockets are sealed, so those models won’t leak silicone, but I would still feel like I needed to buy some eyeballs so that it wouldn’t look creepy(!).
Stereo binaural tie lapel electret miniature microphone – Found on Ebay for $40.
2 ½ lb kit of Smooth-On Silicone Mold Making Rubber OOMOO 30 - $30 on Amazon
3/4" x 2' Wooden Dowel - $2
Small, cheap plastic drinking cup. $0.05
Tools, Misc Supplies
- Sharp knife
- Drill bits (preferably Brad Point Dowel bits used for wood)
- Hot glue gun + hot glue
- Electrical Tape
- Small plastic cup
- Largish cereal Bowl
- Throw-away towel
- Spring clamp
- Large disposable plastic drinking cups
- Plastic spoons for mixing
Step 2: Prepare the Head
Gain access to the interior of the head by cutting away the attachment disk at the neck with a knife. The hole should be large enough to admit the dowel, some wire, and a stream of liquid silicone compound.
Step 3: Mount the Microphones
Note: It is very important that the microphone fits snugly to prevent silicone compound from leaking out of the ears during the filling process.
- Select an appropriately size drill bit. Since the vinyl stretches during drilling, pick a drill bit slightly larger than the microphone. I picked a brad-pointed bit because the drilling needed to be accurate in this soft material.
Drill holes in the ears to admit the microphones
Attempt to insert the microphone from the inside of the head. Ideally, The hole should be loose enough so the microphone goes in easily and snug enough that it will hold the mic in place. If the hole is too small, the easiest thing to do is ream the hole with the drill bit to make it a little larger.
When in place, the microphone will be just flush with the inside of the ear and invisible when the head is facing your eyes straight on. The reason for this positioning is to enhance the realism of the recordings. Your own eardrum is deep inside the ear and the shape of the ear has an effect on the sound.
Step 4: Secure Microphones With Hot Glue
Note: It's a good idea to practice this step a couple of times because you pretty much get one chance to do it right and you have to be able to do it in about 5 seconds.
- Push the microphone out of the ear about ½” so that you can put a bead of hot glue right at the ear hole.
- Once the glue is applied, quickly push the microphone back in so that it is flush with the inside of the ear. Remember that you want the microphones to be invisible when the face is viewed from the front.
At this point you might want to make a test recording to make sure that the microphones are working before you make the big step of pouring in the silicone compound.
Step 5: Create an Overflow Catch at the Neck
The overflow catch is necessary because the process of filling the head with silicone is prone to making a mess. Sealing a cup around the opening will help prevent silicone compound from flowing over the ears.
- Cut off the bottom off of a small disposable cup.
- Thread the microphone cable through the cup.
- Run a bead of hot glue around the neck hole and attach the cup.
- Add more hot glue to spots where you think it might leak.
Step 6: Prepare and Secure the Dowel
The only thing holding the dowel in the head will be the silicone compound, which is stretchy and not very adhesive, so this step will ensure there is a solid mechanical connection keeping the dowel in place.
Note: I chose a wooden dowel instead of a metal or plastic dowel because it will do a better job at insulating the microphones from vibrations coming from whatever object the head is attached to.
- Cut small 1/8” wedges into the top 3” of the dowel, 3 wedges on two sides
- Position the dowel inside the head so that the top of the dowel is about ½” away from touching the top of the inside of the head. This will ensure the microphone wire has a little slack left in it after taping it to the dowel. (Very important for later)
- Using electrical tape, secure the microphone wire to the dowel about 8” from the top of the dowel. This helps to keep the microphone wire neatly along the dowel. Later it will help keep the microphones secure.
Step 7: Fill the Head With Silicone Compound
Note: it might be a good idea to put a little tape over the ears to protect the exposed microphones from any silicone compound leaking out of the catch cup.
- Prepare a cereal bowl with a towel in it. This is where you will rest the head while the silicone sets. The towel helps secure the head and keep it from tipping over. Use a towel you don't mind throwing away, because you WILL get silicone on the towel.
- Mix all of the silicone compound. You will probably need two or three large plastic disposable cups to do this.
- Stir for a few minutes, scraping the sides to make sure everything mixes well.
- Place the head upside down in the bowl with the dowel pushed off slightly to one side to make room for the pour.
- Pour the silicone into the head slowly. The stream should be small enough to go directly into the head without clogging the neck hole and filling the catch cup. If you pour too fast, the escaping air will get trapped and create a big mess for you as the bubbles come up.
- Keep pouring until the silicone is about ¼” away from the top of the catch cup. There may be some leaks at this point. Don’t worry about them except to make sure that silicone compound is not getting into the ears. It will be very easy to clean up the drips after the compound is hardened.
Step 8: Cure Silicone and Clean Up
- The compound takes 6 hours to cure, so find a warm secluded spot to place the head/bowl arrangement.
- To keep the dowel from tipping to one side, carefully draw it up about ½” (Careful not to pull out the microphones!) and secure a clamp so that the clamp is resting on the rim of the catch cup. The weight of the dowel transmitted through the clamp should hold it in the correct place by friction. If this is not working, you might try cutting out a little piece of cardboard for the clamp to rest on instead of the edges of the cup.
- After curing, Peel away any drips or leaks. (Easy and fun!) Remove the catch cup by nicking the rim of the cup with a knife or scissors and peel it away.
Now you are done! Take it out and make some recordings!
You can swaddle up your baby head with a small pillow to make it look less creepy.