Note that there is no calibration of audio hardware, so the level of usefulness of any such software would be strictly limited.
Would just be good for relative measurements?
I don't know, but I doubt that the frequency response of a mobile phone microphone is either linear or consistent across sound level.
I don't even think you could compare two sounds with different frequencies, but I don't know.
I suspect that calibration of any such thing would require a whole lot of infrastructure, consider for example the angle of the phone in relation to sound and the impact of holding the phone in how it affects vibration and noise damping.
You might be able to use a calibrated sound level meter and pair it via Bluetooth with your phone, but I think that's going to be as close as you might get.
In the past I've tried a wired USB microphone, but the OS isn't real-time, so the jitter was horrendous. A pi would give you a more consistent result.
There is "Oscilloscope" which can show frequencies and their intensity
OpeNoise is one.. it will need calibration but if you weren't concerned about actual dB readings you could use it uncalibrated
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