Electronic – Understanding microphone sensitivity

Most phone mics have automatic gain control, active noise cancellation, and other processing which makes having a sensitivity spec somewhat useless.

Additionally you don't care about a mV/Pa spec since you do not have access to the analog signal anyway. You might want something like dbFS/Pa.

And to make matters worse, there is likely a wide variation from phone to phone. Even with phones of the same make and model. Phones are not known for having high quality microphones. It wouldn't surprise me to see a 6 dB variation between the same model of phone.

You'll have to do some experiments and measure the sensitivity. Even then, don't expect things to be good or consistent.

From diagram 3, I could think of two possibilities:

1. The microphone interface is polling for an existence of a mic periodically and turning the line off when not found. Probably not correct.

2. The mic input is doubled as another digital interface. Some phones have a way to detect whether the plugged in ear phones are manufacturer's original models or not, for example. If such a earphone pair is detected, the internal audio equalization may be tweaked to produce best possible fidelity.

The pulse shows narrow, I could not sit and figure out it's width, but it does look like a start bit for me. Assuming it is a start bit, it is probably followed by, say some bits and a stop bit - all are zeroes (like a UART). Beyond that, the controller probably is expecting a response.

The first diagram probably is the mic decoupling capacitor acting as weird RC filter with oscilloscope input impedance. Open drain outputs usually require a remote termination. That's another way to detect if remote is connected, somewhat like USB.

Which is the phone on the other end? I might not have access to the same model and I definitely don't have a similar cable handy to figure exactly out what is happening.

It might be interesting, though, to try sending an UART response on the line, taking the high pulse width as bit time: wait for ten or 11 bits and send high start bit and probably try some combinations. Even if we get a response, it might take a while to figure out the protocols etc. I would advise extreme caution, though - if it is a dangerous interface like a firmware backdoor, a trial sequence could potentially erase the flash.