Final answer:
The AC hum in electromagnetic devices is caused by physical vibrations in transformers due to alternating current and can be reduced by grounding the device's case and mechanical fixes.
Step-by-step explanation:
The AC hum or chatter in AC electromagnetic devices such as air conditioning units installed on rooftops usually originates from the physical vibrations induced in the transformers by alternating current (AC) power. These vibrations occur at twice the frequency of the AC power because the magnetic field produced by the AC reverses direction with the current, causing the components of the transformer to expand and contract rapidly. This repeated and rapid movement can cause a resonant humming or buzzing sound. Additionally, electromagnetic induction can induce an electromotive force (emf) on the cases of these devices. If the case is not properly grounded, it may carry a voltage that can cause additional electric noise, vibrations, or even a shock hazard.
To reduce the hum or chatter, ensuring proper grounding of the device case is crucial. This helps to keep the induced emf near zero, minimizing leakage currents and associated noise. Moreover, mechanical fixes such as reinforcing the mounting of the AC unit, adding vibration dampers or noise-reducing insulation, can also significantly lessen the hum experienced in the upper floors of the homes.