There are very accurate meters to measure magnetic field. We used them to investigate the effect of fields on our inertial missile guidance system, and it is a fact that the earth's magnetic field affected instrument performance (Gyro drift, Accelerometer Scale Factor) and that the system performace varied depending on which way it was pointed (geographically). This was in spite of the fact that the system itself included very strong magnets in the gimbal torque motors. An earlier generation of guidance system had a vidicon (tube) camera for the star sensor, and this was also affected by which way it was pointed when being tested.
A CRT (TV picture tube) is very sensitive to DC magnetic field because the electrons "fly" a long way between the gun and the tube face, and the whole design is aimed at allowing the electron beam to be easily deflected so as to make the picture. An amplification tube, on the other hand, has closely spaced emitter and plate, and a little deflection of the electrons doesn't matter as long as they all end up on the plate.
Loudspeakers have a DC field. AC magnetic fields are another matter and can cause hum by interacting with any element of the amplifier circuitry, most often interconnects.
A CRT (TV picture tube) is very sensitive to DC magnetic field because the electrons "fly" a long way between the gun and the tube face, and the whole design is aimed at allowing the electron beam to be easily deflected so as to make the picture. An amplification tube, on the other hand, has closely spaced emitter and plate, and a little deflection of the electrons doesn't matter as long as they all end up on the plate.
Loudspeakers have a DC field. AC magnetic fields are another matter and can cause hum by interacting with any element of the amplifier circuitry, most often interconnects.