The principle of the generator is of course electromagnetic induction... Magnetism generates electricity. It's a popular saying... that can't be written...
Microphones, microphones, cassette recordings, etc. , all input sounds are electric. Magnetic induction
Speakers, tape playback and other sound playback are all current magnetic effects
You can only understand the principle by clearly distinguishing the signal conversion
Microphones are When a sound signal is converted into an electrical signal, it turns into electricity (generating a specific current), so it is electromagnetic induction
Playback equipment such as speakers convert existing electricity into vibration (through changes in the magnetic field), produces vibrations, produces sound), it is electricity that generates magnetism, it is therefore a current magnetic effect
Ordinary electret condenser microphones use the sound pressure of sound waves to cause the pole pieces to move, and the capacitance between the pole pieces therefore produces changes. The relationship between the voltage U between the pole pieces, the capacitance C between the pole pieces and the quantity of constant charge Q is: U=Q/C. Therefore, changes in sound pressure can be detected by detecting changes in voltage between the pole pieces.
Judging from this process, there are no factors that allow low-frequency infrasound waves to escape detection. In fact, as long as the subsequent amplification circuit uses a DC amplifier, infrasound waves can be detected with a condenser microphone. On the other hand, the frequency of ultrasonic waves is too high, and the vibration speed of the pole piece cannot keep up with the speed of change of sound pressure, ce which makes it difficult to detect with ordinary microphones.