The rate of change determines the amount of high frequency noise

Principle:

The more rapid the change in force, pressure, or speed, the more dominant the high frequency noise.  A very rapid change produces a shorter pulse which as higher dominant frequencies.  The rate of change is often determined by the resilience of the two impacting surfaces – the more they deform, the longer they are in contact and the lower the dominant frequencies.  When bounding a basketball on the floor, the ball is in contact with the floor for a relatively long time and the dominant frequency is low. The ping pong ball is in contact with the table for a very short time, and the dominant frequencies are much higher.

Example

With a rough gear design (rectangular teeth), the forces on the teeth rise and fall rapidly.  Much high frequency noise is generated.

Control Measure

With a smooth gear design (rounded teeth), the teeth fit more smoothly together, the force transfer is more continuous and the high frequency noise is reduced.  Because the maximum force is reduced when the teeth engage, the sound level is lower at all frequencies than it is with the rectangular tooth design.

More about this series

The illustrations in this feature article appeared first in a book published in Swedish by Arbetarskyddsfonden, the Swedish Work Environment Fund. The Fund was established by national legislation in Sweden to conduct research and education in the field of work environment improvement, and is jointly operated by Swedish employers and labor unions.

The book was translated into English and disseminated as a guide for workers and employers by the U.S. Department of Labor. It has also been translated into other languages, notably Finnish and Danish, but its overall circulation has been limited.  The illustrations are being republished here to give them wider circulation, particularly among noise control engineers.

The formulation of the principles, the choice of examples and the preparation of preliminary sketches was the work of Stig Ingemansson. The original illustrations were prepared by Claes Folkesson.

The principles are presented in eight sections:

A. Sound behavior (8)

B. Sound from vibrating plates (8)

C. Sound production in air or gases (7)

D. Sound production in flowing liquids (2)

E. Sound propagation indoors (5)

F. Sound propagation in ducts (6)

G. Sound from vibrating machines (6)

H. Sound reduction by enclosure walls (5)