In the case of electromagnetic noise, it occurs only when the electric motor is in an energized state. Therefore, to determine whether it is electromagnetic noise, one simply needs to perform on/off tests. If significant noise is present during motor operation and immediately disappears when power is cut, it can be identified as electromagnetic noise. If voltage regulation is possible, adjusting the voltage within the motor's rated voltage range can also help determine electromagnetic noise, as higher voltage corresponds to greater or more pronounced noise.

Ventilation noise pertains to the electric motor's airflow. Employing the method of blocking the air path during thermal testing can be particularly effective in determining whether it is ventilation noise. Blocking the motor's air inlet, if resulting in a significant reduction in noise, generally indicates a strong tendency towards ventilation noise.

As for mechanical noise in the electric motor, it primarily arises from friction among motor-related components or due to resonance. When the rotor part of the motor is stationary, whether powered or not, mechanical noise does not occur. The magnitude and acoustic effects of mechanical noise are directly related to the motor's rotational speed. At lower speeds, mechanical noise appears as periodic intermittent noise, while at higher speeds, the sound transforms into a continuous noise that is challenging to discern. Even after the motor is powered off, mechanical noise persists, but it undergoes changes as the speed decreases.

In the case of variable frequency electric motors, both electromagnetic noise and mechanical noise, along with ventilation noise, fluctuate with changes in the power supply frequency. For instance, severe electromagnetic noise and mechanical resonance noise may occur within specific frequency ranges.