`microphone.sound_level()` values are too low

[microbit-carlos]

Running this programme with the latest CI build and the max value it prints is around 7:

from microbit import *

while True:
    print(microphone.sound_level())
    sleep(10)

Likely from 4f369bb, although I haven't tested the parent commit to confirm if that's the cause.
For reference, the microphone min and max values used by MakeCode for scaling are bit a different:

• https://github.com/microsoft/pxt-microbit/blob/d80514bda223f729d452c1e51047f3272a2317fb/libs/microphone/microphone.cpp
• https://github.com/lancaster-university/codal-microbit-v2/blob/864e7b87d2968c91c84e8eb204836b67accabda4/model/MicroBit.h#L323-L325

@dpgeorge could you have a look at this one once you get a chance? I've added it to the 2.1 milestone as well.

[dpgeorge]

It's not due to the commit 4f369bb, I tested prior to that and it's the same output (max I could get was 5).

[dpgeorge]

Using uBit.audio.level->getValue() the return values are in the range 0 to 480 in steps of 8 (according to my testing).

Using uBit.audio.levelSPL->getValue(), the return values are in the range 52 to 86 in steps of 1.

These different values don't have a 1-1 mapping, the levelSPL seems to be more noisy/sensitive.

Which one should we use in MicroPython? How should we scale the range?

[microbit-carlos]

We'll have to check this with @JohnVidler & @finneyj when they are back next week, but in the meantime let's use the levelSPL with the min and max ranges you've found on your testing.

MakeCoded uses a larger max rate, but that is likely the cause of this:

• Sound level in MakeCode never reaches high values lancaster-university/codal-microbit-v2#225

[microbit-carlos]

We had a chat with @finneyj and for MicroPython and MakeCode we'd like to use levelSPL.
For consistency CODAL should provide the min and max values, and it's possible CODAL we'll need to recalibrate the current internal calibration, but that will come in a future CODAL release.

In the time being, let's use levelSPL with the min and max values you've found via testing @dpgeorge.

[dpgeorge]

Commit 9047314 switched to use levelSPL with a range of 52-86.