Impact or pressure

Using impacts (short, sharp bursts of energy, e.g., a hammer) to do a job can be very effective, but is very noisy. Sharp impacts cause high peak noise levels, which can sometimes cause instant permanent hearing loss. Sometimes using a steady pressure can bring the same result, with a lot less noise. 

• bend metal in a press or a vice instead of hammering; and 
• screw type fittings are usually quieter to insert than nails. 

Choose quieter tools or machines

Ideally, your workplace should have noise level information (either from the manufacturers or from measurements done in the workplace) for each noisy piece of plant. Using this, you can select the quietest plant for the job. 

• an electric power tool may be quieter than a pneumatic one; and 
• choose a silenced compressor or power pack. 

Reduce noisy work 

The risk of noise deafness depends on both the noise level and the time you are exposed, so choose methods which reduce the amount of time spent on noisy work. 

• More accurate cutting, bending, welding and fixing means less need for noisy corrective work in hammering and grinding; and 
• if two machines produce roughly the same noise, use the faster one. 

Reduce noise output

Reduce Impact Velocity
Lowering the velocity of impact can reduce impact noise. 

• Lower materials slowly to the ground or floor, using the proper manual handling techniques; and 
• reduce the fall height of objects such as tools, offcuts etc. by using a bench, breaking the fall of the object or working close to the ground. 

Spread impacts over a longer time 
By spreading the impact over a slightly longer time, it is possible to do the same work with less noise. 

• use a 'dead blow' hammer (filled with lead shot) or a soft face hammer; and  
• place a wood block between the hammer and workpiece. 

Cushion impacts 
Many impact noises need never occur! 

• use rubber floor mats to cushion noise of falling or rolling objects;  
• treat work benches; 
• line scrap bins with rubber; and  use rubber buffers on truck trays, bin lids, gates etc.  

Reduce working forces 
Sometimes the system provides more 'grunt' than you need. 

• when using nail guns, air hammers, et cetera try to reduce the pressure in the air line (using a regulator) to the minimum needed for the task; and 
• avoid overloading machinery so that it "screams". 

Damp the noise radiation 
Damping the vibration, in the same way as you can damp the ringing of a wine glass by putting your hand on it can reduce the 'ringing' resonance of materials that have been impacted. The module on Building Quiet gives information on damping. 

• lay rubber blankets or sand-filled bags on the vibrating workpiece; and 
• use materials which radiate less noise than sheet metal, e.g., steel mesh, plastic for scrap bins, guards etc.

Separate the noise from the people

Increase 'distance'

Even if you have to be exposed to your own noise, others in a workplace can avoid unnecessary noise if you are careful where you position your plant. 

• try not to work in a corner, or an 'echo-ey' area, as this increases noise; 
• work further away from others, even outside;   
• use mobile screens or work behind partitions; and   
• locate your compressor or power pack further away, outside or behind a screen. 

Control access to noisy areas 

There are some simple ways of preventing people being in areas where noise is occurring.

• restrict access to noisy areas by cordoning off and/or warning signs, in consultation with the relevant persons on site; and  
• arrange to carry out very noisy tasks at times when other employees are not at the workplace. 

Why machines get noisier with use

1. Here are 9 reasons why machines of all types may get noisier with use.
2. Worn or chipped gear teeth - worn or chipped teeth will not mesh properly. The shiny wear marks are often visible on the teeth.  
3. Worn bearings - bearing wear will show up as vibration and noise, as flat spots or cracks appear in the balls. 
4. Slackness between worn or loose parts - this appears as rattling noises, squealing from slack drive belts, 'piston slap' in motors, air leaks, etc. 
5. Poor lubrication - this appears as squeaking noises due to friction or excess impact noise in dry and worn gears or bearings.  
6. Imbalance in rotating parts - just like your car wheels, any imbalance in a fan impeller or motor shaft will show up as excess vibration.  
7. Obstruction in airways - a build-up of dirt or a bent/damaged piece of metal in an airway or near a moving part, e.g., a bent fan guard, can cause whistling or other 'air' type noises.  
8. Blunt blades or cutting faces - blunt or chipped saw teeth, drill bits, router bits etc, usually make the job noisier as well as slower.  
9. Damaged silencers - silencers for air-driven machines or mufflers for engines may become clogged with dirt, rusted out or damaged, so losing their ability to absorb noise.  
10. Removal of a noise-reducing attachment - mufflers, silencers, covers, guards, vibration isolators etc., which reduce noise, should never be removed except during maintenance, and then must be replaced. 

How wear and tear affects noise output

We can understand how wear and tear affects noise output by looking at the 'bathtub' curve (see graphic in PDF version of this document). This shows what happens to the noise output of a machine over its working life. 
 
Servicing needs to be carried out at Point C, before the wear and tear becomes serious. By doing this, we may not achieve a large noise reduction, as the noise level will drop back to the level at Point A or Point B. But what we will do is to avoid the period of high noise exposure up to Point D. 

Not only this, but repairs carried out at Point D are going to be much more expensive. 

WorkSafe study on noise output

WorkSafe carried out a study to see how this works in practice. (Reports ENC-6-94 and ENC-7-94 available in the WorkSafe library). 

• A total of 11 power tools, including drills, angle grinders, sanders, a circular saw and a planer were tested for noise "before" and "after" routine servicing at a commercial power tool service centre.  
• The average noise level 0.5 m away was 93.9 dB(A) 'before' and 92.0 dB(A) 'after'.  
• Noise reductions ranged up to 7 dB(A).  
• The average reduction was 1.6 dB(A).  
• As expected, this average reduction is small in terms of decibels. However, it means that sound energy output before maintenance was 45% higher, showing that wear and tear was clearly on the rise.   
• One machine with badly chipped gear teeth. Its noise output had risen to 100 dB(A), mostly because of a dramatic rise in gear noise level (it was not included in the 11 machines tested as it was not able to be repaired and re-measured at the time.)  
• The average cost of repairs related to noise reduction was about $50, or about $30 per decibel of noise eliminated. This is cheap noise control, especially when you think of the expensive repairs and the noise you have avoided! 

In other studies on specific machines, the Department noted significant noise reductions through careful maintenance work: 

• The noise level of a common type of reciprocating compressor was reduced by about 8 dB(A), by adjusting the valve seating to improve the seal and adding "Molyslip" additive to the lubricating oil to reduce roughness in the piston stroke. 
• The noise level of a pneumatic knife used in abattoirs was reduced by about 8 dB(A), by improving the balance of the rotor vanes and replacing bearings and a worn collar which allowed parts to rattle.  
• The noise level of an electric motor and belt drive for an aluminium docking saw was reduced by 15 dB(A) when free running, by replacing squealing belts and worn motor bearings and drive pulley bearings. 

How can I tell when my machine needs servicing?

There are three simple ways you can check whether any of the nine problems above are starting to appear. 

Look - can you see signs of wear? Is the machine's performance down? Remember, however, that only qualified people may take an electrical machine apart. 

Feel - some of the problems will show up as looseness or increased vibration, which you can learn to recognise by feel, especially in hand-held machines. Again, only touch the machine where you can do so safely. 

Listen - your ears are a good fault detector - even with hearing protectors on. Listen for new noises, especially tonal ('whining') sounds, repeated impacts, or high frequency ('screech') sounds. Also, slipping belts will cause a screech at start-up, while a damaged bearing may appear as a 'clunk' during run-down.

Eliminate or reduce noise at the source by using a quieter method for doing the task, using a quieter machine, and doing regular maintenance on plant.

Instead of having to think of these points every time you do a job, work through the most useful ideas in consultation with others in your workplace to come up with a set of procedures for working quiet.  Ideally, your workplace should have in place a system for checking and servicing the various machines and power tools and a set of procedures for quiet working methods.