Why Calibrate? If you calibrate your microphones, your audio processing will be able to look at the in-coming digital data and know what sound level is happening in the real world. Knowing the true sound level in the different frequency bands allows you to tailor your algorithms (amplification, compression, noise reduction) to better respond to a person's specific hearing loss.
Truth Microphone: It is important that the truth microphone be placed very close to the Tympan microphone during this calibration. Ideally, they'll see exactly the same sound levels. Pictures showing my arrangement is shown below.
- Sony Mic: Like many lapel microphones, this mic comes nicely packaged with a 1/8" (3.5 mm) phono plug. We have a mic jack on the Tympan PCB just for this purpose! I programmed the Typman to supply a 2.5V bias voltage for this microphone.
- PCB Mic: This microphone is just a raw element. We designed the Tympan to have two of these mics right on the circuit board so that they'd be easy to use. Therefore, this mic is already wired and simply needs to be enabled in the Tympan software.
- Knowles Mic: This is a raw hearing aid microphone. I soldered some wires to it and connected the wires to the Tympan's "line in" holes that are on the edge of the Tympan PCB. I provided the bias voltage from a pair of AA alkaline batteries.
Example Data: Below is a spectrogram of the audio data that I recorded during one of these calibration tests. The top plot shows the audio from the truth microphone. The bottom plot is the signal recorded by the Tympan, in this case for the Sony mic. You can see that I had a loop of audio that I was playing over-and-over into the room. The audio loop alternates between white noise and a 1 kHz test tone. In the bottom plot, you can clearly the see the effect of increasing the gain of the Tympan input.
- Sony Mic at 120 dB SPL yields a digital level of: (120-94) + -48.6 = -22.6 dBFS. This means that it has 22.6 dB of excess headroom. I can safely set the gain to +20 dB.
- PCB Mic at 120 dB SPL yields a digital level of: (120-94) + -47.4 = -21.4 dBFS. This means that it has 21.4 dB of excess headroom. I can safely set the gain to +20 dB.
- Knowles Mic at 120 dB SPL yields a digital level of; (120-94) + -28.9 = -2.9 dBFS. This means that it only has 2.9 dB of headroom. For this mic, I would leave the gain at 0 dB.
We're Calibrated! With this testing, we've now calibrated these three microphones with the Tympan. We know the frequency response and we know how to relate our recorded digital values to real-world SPL numbers. We also have some guidance as to what gain value we should use for each microphone. We got a lot done!
Follow-Up: I've added the raw data and my Matlab analysis files to my GitHub repo. You can get them here.