Pointing a microphone to a connected loudspeaker creates a feedback path and usually results in acoustic resonance, which manifests in a sustained oscillation: https://youtu.be/_XTtjZ8aZbc. A similar effect can be observed with guitar feedback, where the guitar coils pick up the sound coming out of the amplifier.
This survey paper provides a great insight into the phenomenon and its mitigation approaches: https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.231.9808. It refers to the Nyquist stability criterion on p.5 (292) and explains the effect from the usual closed-loop system stability analysis prespective.
My question is: what are actual the physical factors that define the frequency of resonant oscillation in microphone feedback?
My inexperienced guesses include:
- frequency responses and resonant frequencies of both the microphone and the loudspeaker, which in turn depend on their internal circuitry
- distance between microphone and loudspeaker
P.S. Please let me know if my question can be reformulated in a better way using the correct terminology. I am a non-English EE undergrad without formal control theory education (yet) and I form my assumptions based on my personal research in this exciting field.