Nathan Gallardo

Sound Design || Music || Audio DIY || Game Dev

Binaural dummy-head microphone [Part 1 – Casting ears with alginate and plaster]

So, for my Binaural Microphone project I need a pair of ears.

There are 4 steps to creating the silicone ears:

1) Create a mould from the human model using alginate.
2) Create a positive cast using fine casting plaster.
3) Create a silicone mould from the plaster cast.
4) Create the final silicone positive from the silicone mould.

In this post I cover the first two steps. (I did a trial run on my own finger in this post.)

Get all your equipment together. Here you can see the alginate powder, fine casting plaster, a roll of cling-wrap, two cling-wrapped strips of cardboard and various mixing containers.

Casting materials

Read the instructions on the alginate and casting plaster before you begin work. You have to work very quickly so make sure you’re prepared.

Roll the card around on itself to create a cylinder a few inches deep; make sure it’s wide and deep enough to make a nice solid mould. This will be the dam around the ear into which you pour the alginate. Have your guinea pig stop their ear up with cotton-wool; if you’re concerned about losing it in there tie some cotton around it so it can be pulled out. Also, make sure they coat their ear and hair in a thin layer of vaseline: the alginate won’t stick to skin but it will grip hair. Place the dam on your pal and pour the alginate in.

Guinea Pig

Pour it out in as thin a stream as possible to avoid bubbles and make sure to get it in all the crevices etc. Start from the bottom up and flood the well, rather than just pouring it straight over the ears.

You’ll see it change colour, and within a few minutes it will be ready to remove. Again, check the instructions on your alginate for timing. Mine was quick casting alginate, so I had about 30 seconds to mix, 30 to pour, and then 2 or 3 minutes before it was set.

Loosen it up by gently wiggling the mould, then pry it off from front to back. Be gentle with it and do it slowly, you don’t want to tear the alginate.

Initial alginate mould Initial alginate mould Initial alginate mould

Now you want to construct another dam around this new mould. Have your plaster ready to mix and pour as quickly as possible.

Getting ready to pour the plaster into the alginate mould

Pour it in a very thin stream into the mould. Fill it up enough so that you end up with a nice thick base of plaster.

Plaster going into alginate mould plaster going into alginate mould

Once it’s in there gently tap the sides and wiggle it around to make sure the plaster fills the entire mould and to get any air bubbles out.

Plaster poured into alginate mould

The plaster will set quite quickly. When it’s ready, remove the cardboard.

Plaster/Alginate freed from case

Use a razor to score the alginate and slowly tear it away from the ear. Use tweezers and a needle to get the alginate out of the crevices, being careful not to scratch your plaster.

Ear emerging

Piece of cake right?

Two plaster ear casts

Once you feel the plaster has fully cured, about 24 hours, you can finalise the cast.

You need to think about how you’re going to mount the final ears onto your dummy head. I sanded down the base so the ears kind of sat on platforms. When mounted, the ears will sit flush with the surface of the head. Think about your hole placement now too, don’t leave any overly thin sections in either the moulds or the head.

You’ll need to seal your plaster. I originally used Plasti-Kote Krystal, which inhibited the silicone.

MAJOR PROBLEM.

I wasted a load of silicone due to this. Don’t use Plasti Kote. The clear spray lacquer from Halfords (in the UK, Google it to see what’s best elsewhere) is perfect. Dries rock hard and doesn’t inhibit the silicone.

Photobucket

Next: Building the dummy-head.

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