A new developed powerful battery-driven tool (similar to a drilling machine) emitted an unacceptable noise. The origin of that noise was not known.

The tool consists of an electro motor which is mounted on an alloy plate and connected to a gear shaft. In order to identify the origin the noise was measured and analysed in different conditions:

  1. only the lectro motor

In this conditions everything sounds “normal”. The motor thus is not the origin of the noise problem.

  1. the motor on the alloy plate

Compared to the first sound now a tonal component can be remarked which reduces the sound quality. In the difference of the spectra shown on the left it can be seen that this component raises the level for up to 8 dB in the region around 2 kHz. This is due to a resonance frequency of the alloy plate, which can be avoided by a change of the geometry of the plate.

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  1. the motor on the alloy plate with the gear

The noise now sounds very rough. Information about the origin can be derived from a modulation analysis (graph on the right). A clear modulation (red zone) can be observed. Using the modulation frequency, the speed in rpm, and the number of teeth the responsible gear wheel was identified. A change of the bearing and of the tooth scheme avoided the problem.

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