Stig Ingamansson’s Noise Control – Principles and Practice: A5 and A6
Editors Note: We are republishing Eric Ungar’s Acoustics from A to Z and Stig Ingemansson’s Noise Control: Principles and Practice from previous NNI issues as part of our initiative to include more educational articles. Their lessons are just as valid today. Look for more in future issues and on noisenewsinternational.net.
Sound Behavior: A5 – A6
A5 – The Slower the Repetitions, The Lower the Frequencies of the Noise
The amount of low frequency noise produced by a sound source depends primarily on the rate at which changes in force, pressure, and speed are repeated. The longer the interval between repetitions, the lower the frequencies of the noise generated. The level of the noise depends upon the magnitude of the changes.
Principle
Example
Two gear wheels are the same size, but they have different numbers of teeth. The principal source of noise in a gearbox is the contact of one tooth with the corresponding tooth on the gear wheel in mesh with it. If the gear wheels rotate at the same speed, the gear with fewer teeth will produce a lower frequency noise.
A6 – High Frequency Sound is Highly Directional and easy to Reflect
When high frequency sound strikes a hard surface, it is reflected just as light is reflected from a mirror, but passes directly through any holes in the surface without change of direction. High frequency sound does not bend around corners.
Principle
Example
High frequency noise travels directly from the high-speed riveting machine to the worker’s ears.
Control Measure
A sound-isolating enclosure, open at the bottom of the side facing the operator, is installed around the machine. The inside surfaces of the enclosure are lined with sound absorptive material, for example, foam plastic. The upper portion of the side facing the operator is fitted with safety glass. The glass reflects the sound directed at the ears of the operator to the sound-absorptive lining. The sound level at the ears of the operator is reduced accordingly.
