Stig Ingamansson’s Noise Control:  Principles and Practice 

Sound Behavior:  A7 – A8 

A7 – High frequency sound is greatly reduced by passing through air 

High frequency sound is reduced more effectively than low frequency sound by passing through air. In addition, it is easier to insulate and shield. If the noise source does not cause problems in its immediate vicinity, it may therefore be worthwhile to shift the sound toward   higher frequencies. 

Principle: 

The boat siren, which seems extremely loud to those on board, is much less loud at a distance. 

Example 

The low frequency noise from roof fans in an industrial building disturbs residents of houses 400 meters away. 

Control Measure 

The rooftop fan is replaced by another of similar capacity but with a larger number of fan blades.  This produces less low-frequency noise and more high-frequency noise. The low-frequency noise no longer causes disturbances, and the high-frequency noise is adequately reduced by distance 

A8 – Low-frequency noise is less disturbing 

The human ear is less sensitive to low-frequency noise than to high-frequency noise. If it is not possible to reduce the noise, it may be possible to change it so that more of it is at lower frequencies. 

Principle: 

Two passing trains make equal amounts of noise, but one is more disturbing because it moves faster, causing more rapid impacts and creating higher frequency noise. 

Example: 

The diesel engine in a ship operates at 125 rpm and is directly connected to the propeller. The noise from the propeller is extremely disturbing on board. 

Control Measure: 

A differential gear is installed between the motor and the propeller so that the propeller can revolve at 75 rpm. The propeller is replaced by a larger one. The noise is shifted to a lower frequency making it less disturbing.