Building quiet

The materials you use for noise reduction can work in four basic ways.

• Sound isolation - stopping sound from passing through, by reflecting it. 
• Sound absorption - soaking up sound so it is not reflected. 
• Vibration isolation - stopping vibration energy passing from one point to another. 
• Vibration absorption - soaking up the vibration by damping.

Stiff heavy materials will stop a lot of sound, although limp, heavy materials are also good. Common materials include sheet metal, timber, masonry, plasterboard, glass and loaded vinyl. These can also be used in double leaf constructions, e.g., plasterboard on timber studs, doubled glazed windows, in which case the air space needs to be as wide as possible. 

Porous materials, perforated materials or very light materials are poor performers for noise isolation, although they may be good sound absorbers. 

Porous materials such as open cell foams and fibrous materials (fibreglass, rockwool) act as very good sound absorbers by converting sound energy to a small amount of heat. Note however that most sound absorbing materials are poor performers for noise isolation, as they allow sound to pass through easily. This is why sound absorbing material is usually fixed onto a solid noise isolating material. 

Closed-cell foams, eg. polystyrene, are poor sound absorbers. 

Sound absorbing materials often need to have a protective facing to prevent damage. Common facings include perforated sheet metal (10% open area), perforated foil (sisalation), or perforated vinyl. 

When it is necessary to prevent moisture entering or fibres leaving, eg. in the food industry, a thin layer (typically 25 microns) of a tough material such as polyethylene is often used as a facing. 

Materials which are 'springy' can be used to isolate a vibrating machine from a floor, wall, or ceiling. These can take the form of springs, rubber mounts, air cushions, pads or mats of rubber, cork or fibreglass. Selection of the best isolator for a job is a specialised task. 

Springy materials are also very good as buffers for absorbing impacts. 

Materials which have good internal 'damping' can be used to absorb vibration energy. A material which will not 'ring' when tapped has good internal damping, and can be used to absorb the ringing of lightly damped materials. 

Example: Why does a wine glass which has been tapped stop ringing when you place your hand on it? 
Answer: Your hand is made of highly damped material! 

Damping materials include foams, rubbers, soft wood and granular materials such as sand.

You can screen noise by building a noise barrier from a sound isolating material. 

An additional sound absorbing layer stops sound echoing in the area near the source and bouncing over the barrier. It can be secured at its edges in metal channel sections or with timber studs and given a protective facing (see 'sound absorption' above). 

Some ideas

• make it a mobile screen by adding wheels
• add a glass or perspex viewing window
• sound absorbing material on the ceiling above and the wall behind - it will work better. 

An acoustic enclosure can range in size from a small box or cover to a tractor cab, up to a large machine enclosure or sound proofed room. 

Points to watch

• The wall materials need to be heavy enough to isolate the required amount of sound (see 'sound isolation' above). 
• Use an absorbing layer on the inside to reduce sound build-up. 
• Seal off any gaps or openings by block-off pieces and gunned sealant, or flexible flaps where openings are necessary. 
• Make sure any doors, hatches or removable panels have full seals on each contacting edge. 
• Make sure the enclosure is not touching any vibrating part of the machine, or it may vibrate as well - isolate any pipework which penetrates the enclosure, sealing the gap with soft sealant. 
• Don't forget that the machine may need ventilation - provide silenced airflow ductwork (see below). 

Acoustic ducts can allow enough airflow while absorbing noise as it passes along the duct. Note also that noise travels along intake ducts against the airflow. 

Points to watch

• The outer ductwork layer needs to be thick enough to prevent noise 'breakout'. 
• The absorbing material needs to be thick enough to absorb sound over a wide range of frequencies, e.g., 50 mm thick fibreglass is a typical lining thickness. 
• The duct cross-section needs to be large enough to allow air to flow with little restriction. 
• The length of duct which is lined with absorbing material needs to be long enough to reduce sound sufficiently - the larger the duct size, the longer the lined length needs to be. 
• You may need to insert a flexible 'vibration break' between the lined duct and the noise source (see below). 

Situations where you may see a need for vibration isolation include: 

• a vibrating machine rigidly mounted on a floor above ground
• pipework (from a vibrating machine) rigidly fixed to a wall or ceiling
• part of a machine touching or connected to a large panel which can vibrate (and radiate noise), e.g., a ceiling-mounted air conditioner touching the ceiling grid
• While selecting the right vibration isolators is often a specialist task, you could be called on to install the isolators.

Points to note

• make sure you don't 'bottom' the isolator by over-tightening or squashing it flat - it should be free to move a little
• make sure the sideways movement of the isolated machine, when running up or down, is not excessive (some isolators will 'resonate' at lower speeds, requiring additional vibration dampers)
• watch out for other rigid connections between the isolated machine and other surfaces, e.g., a rigid electrical conduit which can convey vibration around the isolator, effectively 'short-circuiting' it. 

In summary 

Trade skills can be used to reduce noise in the workplace. Having a basic knowledge of acoustic materials and how they work, and knowing some of the pitfalls in 'building quiet' will help. For further assistance, you can contact the material suppliers or look up any good text on architectural acoustics.  

• Managing noise and preventing hearing loss at work: Code of practice
• NASA's buy quiet program and guide