Stig Ingemansson’s Noise Control – Principles and Practice

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.

A1 Sound Behavior – Causes of Sound Production

Changes in force, pressure, or speed produce noise

Principle:
Sound is always produced by changes in force, pressure, or speed.  Large changes produce loud noises, small changes produce less noise.  In many processes the same result can be achieved with the application of high power for a short time period, or with less power over a longer period.  The former results in high noise levels, while the latter produces much less noise.

Example:
In a box-making machine cardboard is cut with a guillotine.  The knife must strike rapidly and with great force for the cut to be perpendicular to the direction of motion.  Much noise results.

Control Measure:
Using a knife blade which travels across the production line the cardboard can be scored with minimal force over a longer time period.  Since the cardboard strip is in continuous motion the knife must travel at an angle with the moving production line for the cut to be perpendicular.  The cutting is practically noise-free.

A2 Sound Behavior – Causes of Sound Production

Airborne sound is usually produced by vibration in solids and fluids

Principle:
When we use the word ‘sound’ in everyday speech we usually mean airborne sound.  Airborne sound is normally produced by vibrations in solid materials – structureborne sound – or pressure variations in fluids – fluidborne sound – which are coupled to a surface that radiates airborne sound.  For example, vibrations of the strings of a stringed instrument are transmitted through the bridge to the sound box.  When the sound box vibrates sound is transmitted to the surrounding air.  A circulating pump produces pressure variations in the water of a heating system.  The fluidborne vibrations are transmitted to the radiators whose large surface areas radiate airborne sound.

Example:
The radiation of sound from a pipe with a small diameter is usually negligible.  However, a rigid connection of the pipe to an efficient radiator like a wall or ceiling may convert the pipe into a noise problem.

Control Measure:
If flexible supports are substituted for rigid connections the pipe vibrations will not be transmitted.  This type of isolation is usually necessary for refrigeration and hydraulic lines.

A3 Sound Behavior – Causes of Sound Production

Structure-borne travels great distances

Principle:
Vibrations in solids can travel great distances before producing airborne sound.  This problem is especially pronounced in concrete buildings and on ships.  When the structure-borne sound reaches a large surface, the airborne sound radiated can become a problem.  The best solution is to block the vibrations as close to the source as possible.

Example:
Vibrations and stop/start shocks from an elevator drive are transmitted throughout a building.  Structure-borne sound is carried hundreds of meters in the concrete skeleton, virtually without attenuation.

Control Measure:
The elevator drive can be isolated from the building structure by flexible elements.  Further reduction can be achieved by constructing the elevator shaft and installing the drive so that they are completely isolated from the rest of the building structure.

A4 Sound Behavior – Low and High Frequencies

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.