I have a setup similar to this one: amp, 100uF cap leading to headphones, nothing more. Powered from a Pi-Ion battery @3.9V.
It had worked fine with MAX4466, but after replacing that with MAX9814, I get VERY loud high-pitched noise. The mic is responsive, and the noise even dies for a fraction of a second when I am providing it with very loud sounds myself (like tapping the mic)
The noise appears to be feedback from the (very small) 32 ohm Philips headphone speaker:
Moving the speaker closer and further from the module changes the noise frequency (Doppler effect) Works even at 80cm (my cables don't reach further).
[edit] Tried different headphones, 140cm away – same.
I have tried 2 different MAX9814 modules, replacing caps, even ditching the breadboard and mounting on a piece of PCL plastic (insulator):
No change.
Dropping max gain from the default 60dB to 50dB makes is a little better (until the autogain gets there again) and at 40dB makes the noise cease at distances over 20cm.
Similar with a pair of Panasonic headphones (not disassembled), but there, noise can disappear at over 35cm at 50dB. At 60dB, still appears at 140cm.
Granted, the module was purchased from ebay not from adafruit, but the board design appears to be the same.
The question, again. is whether all that is perfectly normal, or not.
The fact I have not read about it on the sites which decribe how to connect a headphone jack to the MAX9814, on the default gain setting, no less, leads me to expect it is not. The effect happens with the (Panasonic) headphones a full cable length away, after all.
Best Answer
The MAX9814 has an automatic gain control - it jacks up the gain to try to get a constant output level. It also has several selectable gain settings - the highest gain is 60dB
The MAX4466 doesn't have automatic gain. It has fixed (or manually variable) gain. It is a simple op-amp that can be used for other purposes.
What you are hearing is feedback - the microphone picks up a sound, the amplifier makes it louder, and sends out back out through the speaker. The microphone hears the output from the speaker and makes it louder. Round and round it goes, getting louder all the time. This feedback loop is same thing that makes an oscillator oscillate - and that's what your amplifier does. It oscillates at a frequency that depends on the time it takes the signal to make one round trip - and since that depends on the distance from microphone to speaker, you get a tone whose frequency gets lower as the distance from microphone to speaker grows.
This happens with the MAX9814 and not the MAX4466 because the circuit with the MAX9814 amplifies more (has a higher gain) than the circuit with the MAX4466.
To stop the feedback squeal, you can select a lower gain on the MAX9814. Download the datasheet, and have a look at it. It tells you how to set the gain. Try it. See how far you get. If you have trouble, come back and ask for help changing the gain (new question, reference this one if needed.)
This has nothing to do with RF or the doppler effect.
You have no transmitter or receiver, so no RF.
The sound changes with distance, not motion so it is not the doppler effect.
And finally:
Yes, this is what you should expect when connecting a microphone to a speaker through a high gain amplifier.