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- Roland CS-10EM Binaural Microphones/Earphones ~100€ (not produced anymore)
- Semi-custom pre-amplifier design based on ELV circuit with 36dB amplification ~35€
- Audioinjector.net (stereo) £17.50(w/o shipping) ~20€
- Raspberry Pi 3 model B ~35€
- SanDisk Ultra 32Gb microSDHC UHS-I card ~20€
- Intenso Powerbank Q10000 ~15€
- Belt bag ~15€
Total: approximately 240€ plus shipping
First, a few words of warning:
Hearing aids are medical products! You use these instructions and the software at you own risk. The described device can produce very high sound levels. Exposure to high sound levels can permanently damage your hearing! You are responsible for the configuration of the device and the protection of your hearing.
Please read about the consequences of noise induced hearing loss before proceeding to the fun part: https://www.nidcd.nih.gov/health/noise-induced-hearing-loss
I (Marc René Schädler) sign the released images with GPG. The signatures can be found in the repository: https://github.com/m-r-s/hearingaid-prototype/tree/master/signatures
- Connect the hardware according to the image
- Turn fully to the right all knobs of the sound card
- Download, extract, and copy the image to the SD card
- Make sure that you don't wear the headphones!
- Boot the hardware
- Wait a minute (the first boot takes a bit longer)
- Try carefully if the loudspeakers of the headphones produce sound
- Carefully plug-in the in-ear headphones
- Take out the headphones immediately if you feel uncomfortable
- Experience an altered perception of sound
The default wifi-password is "openmha-wifi". The default ssh user name is "pi" and the default password is "openmha". The default openMHA configuration includes:
- Soft clipping
- Binaural coherence filter
- Multi-band dynamic compression
- Frequency shifter
The default calibration assumes a pre-amplification of 36 dB. The sound card is by default limited to -25 dB full scale (FS) which should be roughly 100 dB SPL. The openMHA configuration is a modified version of the 2017 openMHA prerelease configuration for offline audio file processing, where the adaptive differential microphones (ADM) plugin has been removed and the default gain table (amplification scheme) has been set to "no amplification" (starting from SD card image version 0.2). In earlier versions, the default gain table was taken unmodified from the prerelease configuration, which applied high gains at some frequencies.
Please read the Instructions.
Welcome to the world of hearing aids!
We will have to solve (or should recognize) some very typical problems before you can really "use" the device. This includes the calibration of the microphones and loudspeakers, the processing latency, the limitations due to feedback, and finally the individualization, i.e., "fitting" to a specific hearing profile.
The goal of calibrating the setup is to achieve an agreement between physical values and internal values of sound pressure level (SPL). It is mainly required to correctly determine which sound levels can be heard by the prospective user, and which cannot.
The signal processing after recording a signal and before playing it back takes some time, called latency. If the latency is too high it results in undesired perceptual artifacts.
Feedback is the portion of an emitted signal which is recorded by the microphones. It mainly depends on the acoustical paths from the loudspeaker to the microphone, i.e., on the physical connection between both. If feedback-times-amplification exceeds 100% for a certain frequency, the amplitude diverges which results in a loud and unpleasant tone. Hence, feedback limits the maximum amplification.
Hearing impairment is very individual. This requires fitting the provided amplification to the individual hearing profile. Excessive compression makes the distinction of soft and loud sounds difficult and also can affect speech intelligibility.