on Mechanical Transmissions

Acoustic Design

The acoustic design of mechanical transmissions is a neglected but of great importance. Due to the growing emphasis on regulations and an increase in customer needs in various industrial sectors (transmissions for electrically-powered vehicles), attention to the acoustic design of mechanical transmissions has grown in recent years.

In physics, a wave is a perturbation that propagates in space and can transport energy from one point to another through the variation of a physical quantity. The sound wave is a particular type of wave in which the perturbation is the pressure variation induced by the vibrating body in the medium (usually the air). This pressure variation is able to propagate in the medium as a succession of rarefactions and condensations (i.e. variations in density). Since sound is a wave, it can be defined as a physical quantity that measures the amount of energy transported by the wave, which passes through a section of unit area in the unit of time. This physical quantity is called sound intensity; on a perceptive level it is decisive for grasping what we call the volume of sound.

Sound Intensity

The sound intensity I (dB) is a physical quantity, which objectively measures the flow of energy carried by the sound wave. However, this magnitude does not correctly describe the perceived intensity (loudness), as it depends decisively on the frequency of the sound and to a lesser extent on the timbre.

The perceived intensity has a complex link with the sound intensity; to adequately describe it, it was decided to resort to representation using isophonic curves that report, as the frequency varies, the geometric location of the points for which the perceived intensity is constant. These curves are commonly called A, B, C weight scales; the sound intensity filtered according to these curves is defined respectively dB (A), dB (B) and dB (C).

The weighting curves A, B and C constitute filters that adapt the sound intensity measured in dB to the response of the human ear respectively for sounds of less than 55 dB (scale A), between 55 dB and 85 dB (scale B) and above 85 dB (scale C).

The weighting curve A is however generally used for the measurement of sound intensity up to 115 dB, due to OHSA regulations. Note how the A weighting scale, commonly used for analyzing the noise of transmissions, significantly attenuates low frequencies: a sound at a frequency of 95 Hz is attenuated by about 20 dB (100 times).

Why Gears Count

In most cases, the noise of the gears that originate in the meshing is due to the imperfect (non-conjugated) action of the gear wheels. This imperfect action results in dynamic forces on the teeth which are transmitted structurally through the bearings and shafts to the walls of the casing. The vibrations of the walls of the casing translate into sound pressure waves (aerial propagation) that reach the ear of the listener.

The human ear filters sound as a physical phenomenon (oscillation of air molecules) adapting it to the capabilities of the human brain, transforming it into what we know as sound on a perceptual level (psychoacoustic sensation). Based on the flow of acoustic energy and vibration, several approaches are possible to reduce gear noise:

1. Reduce the excitation of the meshing;
2. Reduce the transmission paths of dynamic actions and vibrations between the mesh and the casing;
3. Reduce the acoustic radiation efficiency of the carcass.

Courtesy Davide Marano - Organi di Trasmissione

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