Pure annoying tones during braking
An annoying noise (a pure tone) is produced when braking. During the braking sequence, the brake resistor cables are subject to acoustic and electric phenomena derived from the magnetostriction effect. In this case study, there was a resonance effect that amplified the acoustic signal.
To determine the root cause of the amplification and propose a design modification.
Accelerometers placed on points of support for brake resistor cables. A sound level meter for measuring sound pressure levels near these points and the passenger positions. Measurements were made under two conditions: with and without brake resistors in operation.
The brake resistor cables produced a pure tone due to an incorrectly damped magnetostriction effect not properly damped on the supports.
New cable supports with a greater structural damping effect were proposed. We also recommended covering the area with an absorbent material adjusted to the pure tone frequency.