Noise control fact sheet - Buying quiet
One of the most cost-effective ways of reducing noise in a workplace is to "buy quiet". Purchasing quiet products can reduce noise levels without additional modification to equipment or the workplace. This guide offers ideas for general equipment and for the specific purchase of saw blades.
Before you buy plant or equipment, ask yourself if there is a quieter way of doing the job. For example, before buying a pneumatic impact wrench, consider the various hydraulic and torque-controlled units now available. While these units may cost more, they last longer and cause less damage to the nut, as well as lowering noise and hand-arm vibration levels.
If you think the machine you are buying may be noisy, ask for noise level information from the supplier. Some suppliers may be reluctant to give out this information, so you may need to keep asking. The information is almost always available to the supplier from the manufacturer.
Under Section 23 of the Occupational Safety and Health Act, the supplier is obliged to provide adequate information at the time of supply, so the law is on your side.
To make sense of noise level information from a supplier, you will need to ask several questions:
Is this information complete?
The Code of practice Managing noise at workplaces defines adequate information on noise. The code sets out a 12-point checklist you can use to make sure the data you have been given is complete. If it is not complete, insist on the full data from the supplier.
Can I compare brands?
If two similar items of plant have been tested according to the same Test Standard, then you can probably do a straight comparison of the sound levels to see which brand is quieter. If the items have been tested to different Standards, or no Standards at all, comparisons may be hard. You would need to compare such things as the way the machine was mounted and operated, how far away the measurements were done and so on.
The European Community is busy preparing hundreds of Test Codes for many types of machinery, so that comparable test results can be published. Keep asking for this information.
Is there a quieter version of the same machine?
Sometimes the manufacturer will have a specially silenced model - or an add-on silencer - for a machine which may be suitable for your task. For example, you may buy a brick saw with or without a vibration damped blade (see section 3: purchasing saw blades). But don't be put off by higher costs. Look at the extra cost of the "add-on" and divide it by the number of decibels reduction achieved - you'll find that these add-ons usually amount to very cheap and effective noise control.
What will this noise level mean in my workplace?
You may be able to get a rough idea of the noise level of the item in your workplace, from the manufacturer's noise information. Ideally, the manufacturer will have tested the machine for noise level at the operator's ear position, while carrying out a typical task. In this case, the noise level in your workplace should be very similar.
Some manufacturers quote noise levels with their machines "free running", and these levels may change when the machine is actually working. For example, angle grinders are usually much noisier when working than when free running. From experience, you will know which machines are likely to be a lot noisier when working, and in these cases it may be too hard to estimate noise levels in the workplace. (Free running noise level is still important, however, as most machines spend a significant amount of time in free running mode).
You need to be aware that noise levels quoted on labels are measured at specified distances, e.g. 1 metre, and may not be the same as the level at your ear. As a rough guide, add 6 decibels (dB) to the noise level for each halving of the distance - the noise level at 0.5 metre will be about 6dB higher than at 1 metre, (in the case of a hand-held machine). A few machines (eg lawnmowers) have labels that state noise levels taken at 7 metres for neighbourhood noise, but the noise level is much higher for the operator.
Making your decision
Most people would choose the quieter machine, but the reality is, quieter machines usually cost a bit more. Manufacturing tolerances are tighter, gears are made to mesh better, quieter cooling fans are used, and so on. The question is: does the decibel reduction justify the extra cost? If a machine that is 5 decibels quieter costs $100 more, then the extra cost of $20 per dB can be considered cheap noise control. There are also the hidden benefits of a quieter machine - it is easier to hear warning sounds, it is less stressful and tiring to use, and you can be less reliant on personal hearing protectors.
If tradespersons and their employers don't demand quieter machines, there will be little incentive for manufacturers to produce them and in the end the users will be the losers.
In all types of sawing work, noise has to be taken into account, along with other safety points. When you select a sawblade, you choose how much noise you, and others, will be exposed to - noise that could damage your hearing and make the job unpleasant. The ideas in this section will help you to select the best sawblade for low noise, and should apply to all types of sawing work.
Sawblades do their work through the impact of each tooth on the workpiece. Some saw teeth break off small pieces of the material, as when cutting aluminium; others, like timber rip saws, slice their way through the material.
The force which each saw tooth applies to the material causes fracture of the material, but also causes shock waves to travel through the material and through the blade. These waves, or vibrations, radiate as noise.
Some people think a noisy machine must be doing a good job, but in a way it's just wasting energy. The ideal sawblade is one which directs maximum energy into cutting, and very little into vibration and noise. So a quiet blade should also be efficient in cutting. The noise made by a sawblade when cutting depends on a number of factors, including:
- number of teeth - more teeth usually causes less noise, as there is less impact force per tooth;
- size of teeth - smaller teeth also cause less noise, for the same reason; and
- shape of teeth - generally, the sawblade manufacturers have adopted what they think are best all-round tooth profiles for efficient cutting.
Selection Rule No.1 - Choose a sawblade with the greatest number of teeth, of the smallest width, suitable for the job.
In many cutting processes, vibration of the sawblade is a major noise source - even when you have followed Rule 1 and selected a blade with the largest numbers of small teeth. If you strike a sawblade, it will "ring" like a bell, because of its elastic properties. In the same way, each tooth striking the workpiece will cause the sawblade to "Ring". The amount of ringing depends on the vibration "damping" of the sawblade. If you put your hand on a ringing bell, the sound stops, because you have "damped" the vibration.
Some good sawblades have vibration "damping" built in. This may be in the form of slots cut into the body of the sawblade (to stop vibration energy running around the blade).
Note that the normal expansion slots which are cut into tungsten carbide tipped blades do not go deep enough to eliminate vibration.
Another form of vibration damping is an internal damping layer built into the blade.
You can tell whether a blade has any built-in vibration damping by tapping it - a well damped blade will respond with a dull "tick", rather than a "ting".
Selection Rule No.2 - Choose a sawblade with built-in vibration damping.
When free running or idling, a sawblade can still make a lot of noise. This aerodynamic noise is caused by pockets of air being trapped in the saw gullets (the gaps between the teeth). As these pockets of air speed past the still air - often at speeds of over 200 km/h - the shearing effect of air against air creates noise. The larger the gullet size, the more noise is created.
Selection Rule No.3 - Choose a sawblade with gullets as small as possible, while still allowing for removal of material.
Here are some good general pointers for keeping your sawblade noise at the lowest level:
- sharpen the blade regularly - blunt or chipped teeth reduce cutting efficiency and increase noise;
- ensure side to side runout ("wobbling") is small, when attaching the blade;
- keep the saw machine itself in good order through regular maintenance of bearings, belts etc; and
- select a running speed that gives least noise (a high speed causes more air noise but often gives less cutting noise).
Sound level tests* on different sawblades under comparable conditions, show that these three Selection Rules really do make a difference. Here are some examples:
dB(A) at operator position
10 second average
Tooth number and size Cutting lengths of aluminium - 350mm dia. TCT blade, 84 teeth, 3.5mm wide
- 350mm dia. TCT blade, 108 teeth, 3.2mm wide
Vibration damping Cutting bricks - 350mm dia. "standard" masonry blade, 20 teeth
- 350mm dia. "damped" masonry blade, 20 teeth
Air noise "Dummy cut" (run up to 3400 rpm, run down), without cutting - 350mm dia. TCT blade, 84 gullets, 10mm x 7mm
- 350mm dia. TCT blade, 108 gullets, 8mm x 4mm
* WorkSafe Western Australia
Engineering Noise Control Reports No's. ENC-2-93, ENC-4-93. available in the WorkSafe Library.
These noise reduction results are significant. A 10 decibel reduction means the amount of sound energy has been reduced to a tenth (10%) of its original value, and to the ear, sounds about "half as loud".
There are three rules for selecting a sawblade:
- Choose a sawblade with the greatest number of teeth, of the smallest width, suitable for the job.
- Choose a sawblade with vibration damping built in.
- Choose a sawblade with gullets as small as possible, while still allowing for removal of material.