What is the reason for mutual inductance?

Mutual inductance: If two coils are close to each other, part of the magnetic flux generated by the current in the first coil is linked with the second coil. When the current in the first coil changes, the magnetic flux between it and the chain of the second coil also changes, and the induced electromotive force is generated in the second coil. This phenomenon is called mutual inductance. The mutual inductance of the two coils is related to their respective self-inductance coefficients. When the magnetic flux generated by each of the two coils is equal to each turn and passes through each turn of the other coil, this situation is called no magnetic leakage. This can be achieved by closely arranging and winding two coils together. In this case, the relationship between mutual inductance coefficient and corresponding self-inductance coefficient is relatively simple.

As shown in fig. 2, there are two adjacent loops (1) and (2) which carry currents I 1 and I2 respectively. The magnetic field generated by I 1 passes through the loop of (2), and the magnetic flux is φ 2 1, which should be proportional to I 1, φ 2 1. Similarly, the magnetic field generated by I2 passes through the loop of (1), and the magnetic flux is φ 12 = M 12 * I2. Both M 12 and M2 1 can be called mutual inductance. It can be proved that when M 12=M2 1=M, it is called mutual inductance for short. The amount of mutual inductance depends on the geometry, size, relative position, turns and permeability of the surrounding medium. Changing the distance, relative position or direction of the two coils will change the mutual inductance of the two coils, so it will cause the magnitude of the induced current, which will make the sound louder or louder. The addition of iron core will greatly increase mutual inductance, thus increasing the sound several times.