A low cost alternative to expensive commercial models.
Introduction
The principle is to use small electret microphone capsules, mounted on a flexible support made in 3D printing that one places in the ears, as for earphones. The microphone capsules are connected to a battery powered preamp using a stereo mini jack plug. The preamp output can then be used on any sound card input such as smartphone, computer, portable recorder. For a very limited budget, it becomes possible to make binaural 3D sound recordings!
List of Material
For this project you will need:
Binaural microphones
- A 3D printer,
- 1 m of flexible Thermoplastic Polyurethane (TPU) filament for the supports. Rigid plastic such as Polyactic Acid (PLA) is also possible, although less comfortable to wear in the ears,
- 2 omnidiractional electret microphone capsules of 6 mm diameter x 5 mm height. Other dimensions are possible but will requires to change the support CAD accordingly,
- 1 old pair of earphones,
- some liquid neoprene glue.
Microphones preamplifier
- 1
MAX9812Ldual channel electret microphone preamplifiers board, - 1 3xAA battery holder with swith on/off,
- 2 AA 1.5V batteries,
- 2
3F073.5mm stereo jack socket, - 1 m of PLA or other rigid plastic filament for the adapter part,
- 1 soldering iron,
- a few drops cyanocrylate superglue,
- some metal wire.
Procedure
Printing the supports
The first step is to print with flexible filament the microphone supports to wear in the ear. The Freecad part and corresponding STL file are available here:
The STL file binaural_mic_support.FCStd corresponds to the microphone support for the right ear. To print the left ear support, just mirror the STL in your 3D printer slicer. A layer height of 0.1 mm is recommended.
Besides, the STL file is adjusted for a 6x5 mm microphone capsule, for flexible filament. If the microphone capsule you use has different dimensions, don’t hestitate to modify the Freecad binaural_mic_support.FCStd file.
The microphones and their supports, once printed are shown in Fig.1.
Mounting the microphones
Soldering
To solder the microphones, the wires of old earphones are used (Fig. 2). For easier mounting, the wire should be passed through the ring of each support before soldering the microphones. Pay attention to the polarity of the microphones when soldering.
Gluing
Once in place in the support, pass the connecting wire through the opening made in the support for this purpose and fill the rest of the cavity with liquid neoprene-type glue as shown in Fig. 3
Microphones preamplifiers
Choice of the model
The electret microphones need a power source to operate, around 3V. Also, their signals require preamplification to bring them to a line-type signal level. Several options are possible to perform these operations:
- Design a dual channel electret preamplifier with discrete components using two mono channel circuit.
- Use two mono channel preamplifier breakout board, such as the
MAX4466. - Use a dual channel preamplifier board such as the
MAX9812L.
In this tutorial, the latter solution, based on the MAX9812L board, is used.
Preparation of the box
The preamplifier board is powered with 3 Volts, using 2x 1.5V AA batteries connected in series. A box that can connect 3x 1.5V AA batteries in series is used to hold the whole system (Fig. 4): the preamplifier and the inputs/outputs connectors are mounted in the third battery slot of the box.
To do this, the battery box is modified to receive only 2x AA batteries: The central connector between battery 1 and 2 is removed. The connector of battery 1 takes its place, as seen on the right of Fig. 5.
Then, two ⌀ 6 mm holes are drilled on the side of the box to mount the stereo jack connectors 3F07 as shown in Fig. 6.
Finally, the jack connectors and the preamplifier board are mounted and held in place using a 3D printed adapter part as shown in Fig. 7. The Freecad part and corresponding STL file are available here:
Depending on the position of the holes and the configuration of the battery box, you may need to modify the binaural_mic_preamp_adapter.FCStd file, and use small plastic shims to hold the jack connectors in place.
Soldering
The final step is to solder the preamplifier board to the jack connectors and the power supply. To do so, remove the preamplifier card from its slot to facilitate the soldering. Once soldered and reassembled in its slot, a small spot of superglue glue will block the assembly. The circuit once soldered is visible on Fig. 8.
Example of use
An illustration on how to wear the microphones is shon in Fig. 9. An example of recording, a “virtual haircut” is provided below (listen with headphones):