Top 10 Noise Control Techniques

The noise from mechanical sources is caused by the vibration of plant elements such as machine frames, panels, gears, casings, floors, castings etc. Damping involves dissipating a proportion of the vibration energy in relatively thin panels to reduce the level of vibration and hence the radiated noise. Very widely applicable, it also reduces fatigue which can reduce maintenance costs significantly.

Typical applications

Chutes, hoppers, machine guards, panels, conveyors, tanks…

Benefits

Low cost, large noise reductions, rugged, hygienic.

Unconstrained layer damping involves gluing proprietary high-damping material to thin metal panels. As the panel vibrates, it bends, stretching the damping material and some vibration energy is dissipated as heat. Constrained layer damping, on the other hand, traps the high-damping material between two metal sheets to create a laminated sound-deadened panel. Under vibration, the whole volume of the elastomeric damping material is sheared, dissipating most of the vibration energy. As a result, constrained layer sound damped steel (SDS) is c x30 more efficient than conventional damping treatments. SDS also avoids the hygiene, wear and "peeling" problems associated with stick-on damping.

Damping chiller panels

This chiller noise included tonal components radiated by the frame and panels. The conventional treatments for these types of unit are silencers plus (often) an extensive acoustic barrier. Not only are these high cost, but they can reduce the efficiency of the chillers and cause access problems for maintenance.

In this case, we used a combination of acoustic absorbent and retro-fit constrained layer damping to reduce the noise by 8dB(A) at very low cost and with no effect on system performance or efficiency.

Download the Top 10 noise controls pdf with sound files here >

Vibration transmitted from machinery into “lively” structures such as steel beams and plates is then radiated very efficiently as noise. In effect, you have bolted your vibration source to a loudspeaker which amplifies the sound. Isolation pads provide a very simple and low cost way to reduce the transmission of vibration.

Vibration isolation applications

Machine feet, motors, pumps, bolting sources to steel beams or mezzanine floors…

Examples

Power press noise control

A high-speed strip-fed press used to blank electrical trip-catch components generated operator noise levels of 101 dB(A) Leq (at 270 strokes/minute) in a relatively quiet area. The manufacturer had fitted an enclosure round the top of the press to little avail as it reduced the press noise by only 3dB(A). Thus was because the press legs themselves are welded boxes that were behaving as large loudspeakers, radiating the vibration from the blanking process very efficiently as noise. Consequently, half of the noise source on the press was outside the fitted acoustic enclosure.

The press frame was isolated from the fabricated legs by inserting 6mm composite pads between frame and legs as shown in the photograph. This process took a couple of fitters half a day and reduced the operator noise level by 9dB(A).

Motor-pump unit vibration isolation

These units are well isolated from the concrete floor using conventional steel springs. However, the pumps and motors are rigidly bolted to thin steel formed plates which behave as noise radiating loudspeakers.

Provided the coupling alignment is not a problem (if it is, then you can fit a ladder frame to maintain alignment), fitting AV pads under the motor and pump typically reduces the radiated noise by 10dB or more. Alternatively, laminate the steel plate to damp the vibration.

Download the Top 10 noise controls pdf with sound files here >

Suppliers: effective isolation pads and materials are available from:-

http://www.tiflex.co.uk/tico/tico_s.html

http://www.fabreeka.co.uk/Products&productId=39

https://www.farrat.com/anti-vibration-materials-industrial

The geometry of duct runs into and out of fans has a very marked effect both on fan efficiency and on the noise produced. Noise is generated by turbulence in the flow – which is wasted energy.

Typical applications

Axial flow or centrifugal fans - ID, extract, LEV, chiller, HVAC, cooling tower etc. More fan noise reduction examples >

Benefits

Increased efficiency reduces both running costs and noise.

Fan Noise Reduction - installation techniques

Running a fan at maximum efficiency coincides precisely with the minimum noise level. Any fan installation feature or geometry that tends to reduce fan efficiency is therefore likely to increase noise

Two of the most common examples are bends close to the fan (on the intake side in particular as fans tend to be more sensitive to inlet flow conditions) and dampers (close to the fan intake or exhaust).

Ideally, for maximum fan efficiency and minimum noise, make sure there is at least 2 - 3 duct diameters of straight duct between any feature that may disturb the flow and the fan itself. In addition, you can make use of bell-mouth intakes to smooth intake flow. 3 - 12dB(A) noise reductions are often possible by ensuring that this approach is implemented.

Taking these two examples or poor fan installation geometry:-

• the bend immediately into the intake will significantly affect fan noise and efficiency
• fitting the damper directly to the fan outlet means that the higher air velocity through some of the vanes will increase noise and reduce efficiency

Fan noise is roughly proportional to the 5th power of fan speed. Consequently, in many cases it is possible to achieve a large noise reduction by reducing fan speed slightly via invertors, by changing control systems or pulley sizes and re-setting dampers. This process can also reduce running costs as well as noise levels.

All else being equal, this table provides a guide to the trade-off that can be expected between noise level and fan speed.

For example, if you can reduce the speed of a fan by 20%, then you reduce the noise output by 5dB – which is a 68% reduction in the noise.

Limitations

The main limitation to this approach as far as geometry is concerned is that it will often take up more space as you need to ensure that there are straight duct runs into and out of the fans.

Fan installations generate tonal and broadband noise. Tonal noise can be an occupational (overall level and “nuisance”) and an environmental noise problem – annoying nearby residents. Aerodynamic noise control techniques are based on retro-fitting aerodynamic and acoustic elements inside fan casings and associated ductwork. This is low cost and can provide significant improvements in fan efficiency compared with conventional silencing – or even compared with the unmodified fan i.e. it is worth fitting for efficiency gains even if noise is not an issue.

Examples

Can extract and chopper fan noise attenuation

The high level of noise from 3 new fan extract systems for aluminium cans was reduced by 22dB(A) and the tonal noise by 23dB (99.5%) by designing aerodynamic inserts that were fitted inside the fan casings. Despite the requirement for the cans to pass through the system to be shredded by the fan blades, these modifications reduced the tonal content of the noise without any loss of efficiency (unlike silencers) and with no maintenance requirements for the lifetime of the fans.

This solved both the environmental and occupational noise problems at a stroke.

Moreover, the installed cost was only c 10% of the previously proposed conventional noise control measures that included bespoke silencers, acoustic lagging and enclosures. The modifications were installed with a downtime of less than 1 day.

Drier Fan Silencing

Post modification, the blade pass tone was reduced by 20dB (99%), eliminating the noise problem. In addition, there was a 6% (16kw) reduction in fan energy consumption (saving £7625 p.a.) – which meant that the noise control project paid for itself in a few months. This is in stark contrast to the £100k of conventional noise control measures originally proposed. This capital cost was reduced to <£7k and the £8k or more additional running cost from the reduced fan efficiency due to additional silencing was turned into a total running cost saving of around £16k p.a. over the original proposal using conventional techniques.

Combustion fans; HVAC Chillers; Extract fans etc…

Combustion fan modified overnight to reduce the tonal noise by 22dB and the overall noise by 9dB(A); 15dB(A) chiller noise reduction at a cost of £40k compared with £300k conventional without affecting efficiency…

More details, including sound files are available at Fan Noise Reduction

Poorly or unsilenced pneumatic exhausts are a very common feature on many sites. Rumours also abound that silencers can introduce unacceptable back-pressure, reducing performance. There is no technical reason for unsilenced exhausts – zero back-pressure silencing giving attenuations that are 10dB – 20dB more than that achieved via commonly used silencers are easily achievable.

Typical Pneumatic Exhaust Attenuation Applications

Pneumatic cylinders, valves, pumps, pressure release valves… Note: a well-designed silencer can have zero back pressure

Benefits

Low cost, large noise reductions, no impact on performance

Another common feature of many sites is the use of “nozzles” that are simply open ended pipes run at full factory line pressure, whatever the application.

Typical applications

Cooling, drying, blowing, cleaning, component ejection…

Benefits

Low noise and reduced compressed air use for the same duty, intrinsically safer…

The generic types of nozzle to look out for are entraining units (see schematic and photograph) from various manufacturers. These accelerate ambient air to provide less turbulent mixing – which means less noise – and are available in a variety of sizes. As the outlet cannot be blocked with, for example, a finger, they are intrinsically safer than a copper tube outlet.

Examples

Standard camshaft washer drying nozzles were replaced with suitable high efficiency entraining units after performance testing. These not only reduced the noise at source by 12dB(A), but they also reduced the drying cycle time by around 20% and reduced the air consumption by a similar percentage.

Hand-held non-entraining nozzles used for cleaning and drying can always be replaced with entraining equivalents that reduce noise levels, improve safety and reduce air consumption (win-win-win). The same goes for vibratory bowl feeders and conveyors that use air to aid component movement.

Chain rattle (particularly on guides as the chain wears) and the banshee tonal wail of timing belts are not uncommon noise sources that can often be reduced at source quite easily.

Technique

Noisy chain drives can often be replaced directly with quieter timing belts. Within the range of timing belts available, there are quiet designs that use modified tooth profiles and pulley geometries to minimise noise. For applications where noise is critical, there are chevron (or helical) tooth patterned belts that can provide very quiet running. Noise reductions in the range of 6 - 20 dB(A) are often practical via this approach.

The noise from timing belts is also very sensitive to the correct belt tension. If the belts are over-tightened, the noise increases dramatically (5 – 10dB). Ensure that maintenance engineers know the correct tension – and make it easy for them to adjust the tension to the correct value. This also extends belt life.

The noise from chain drives can also sometimes be reduced by fitting laminated steel guides (the same material as recommended above for vibration damping) at key positions to smooth the path of the chain.

Typical Applications

Noisy chain or belt drives and conveyors.

Benefits

Reduced noise and wear rates.

Most companies have large numbers of electric motors across a wide range of applications.

Technique

Some electric motors can be very noisy (>90dB(A) at 1m), particularly higher speed (2950rpm) units. However, it is not necessarily common knowledge that general duty motors are available (at little or no cost premium) that are up to 10 dB(A) or more quieter than typical units as direct replacements. The best approach is to feed these motors into the purchasing system over a period of time so that all replacement motors are quiet motors.

In many cases, the quieter motors are also more efficient units which means that the running costs are also reduced.

Typical applications

Noisy motor drives – pumps, gearboxes, fans etc…

Benefits

Lower noise levels with no effect on performance.

Hydraulic power pack noise is a common problem as they often seem to have been specifically designed to be as noisy as possible... The classic example is a hydraulic motor-pump unit that is not intrinsically noisy when suspended in free space, but is then bolted to a large sounding-board in the form of an oil tank that behaves as a very efficient loudspeaker. The conventional acoustic enclosures sometimes used cause both thermal and maintenance problems (small leaks go unnoticed). However, the noise from most hydraulic power packs can be dramatically reduced using low-cost engineering techniques that have no effect on temperature or access.

Typical Applications

Hydraulic power packs that include oil tanks, submerged pumps or pumps bolted to large steel structures.

Benefits

No enclosure (thermal and maintenance problems), large noise reductions at low cost.

Hydraulic power pack noise reduction techniques

The problem is that the vibration source (hydraulic pump) is rigidly connected to a large radiating surface that amplifies the sound. The simplest solution is to install a vibration break between the hydraulic pump (or the motor-pump unit) and the tank. This can be achieved by fitting a carefully designed vibration isolation pad (e.g. rubber-bonded cork – see section 2 above) between the pump and the tank and by ensuring that there are no rigid connections i.e. fit flexible hydraulic hoses and ensure that the heads of the bolts are also isolated as shown in section 2. This reduces the classic high-frequency tonal noise from these units. Note that it may also be necessary to isolate valve blocks in a similar manner.

Where the motor-pump alignment could be an issue, then it may be necessary to construct a simple steel frame (angle iron) to which both motor and pump are bolted to maintain alignment and then isolate the combined structure from the tank. In addition, if there is any residual high-frequency vibration radiated by the tank or associated steel mounting panels for control systems, it can be damped by creating a laminate in situ (as per vibration damping in section 1 above).

Noise reductions of 10dB – 20dB can often be achieved using this approach at low cost and without affecting temperature, access or visibility (potential oil leaks). Example of hydraulic power pack noise control using vibration isolation and damping >

Limitations

Few as this approach almost invariably provides an elegant engineering solution to a common noise problem.

Download the Top 10 noise controls pdf with sound files here >

Provided the noise from a machine is dominated by airborne components, existing safety guards can often be converted into effective acoustic screens. If a component of the noise is vibration radiated as noise, then the guards can be damped (as section 1 above).

Typical Applications

Machine tools, power and printing presses, packing and filling machines, conveyors etc etc…

Benefits

Simple and low-cost upgrade to reduce noise levels.

ii) Acoustic absorbent: lining a significant proportion of the inside of the guards with acoustic absorbent (foam, rockwool / fibreglass, sealed hygienic absorbent modules) will reduce the reverberant noise “trapped” by the guards. Consequently, the noise level inside the guards will fall and therefore less noise will escape through any gaps. There is no necessity to cover the whole of any panels, just fit patches of absorbent on convenient flat surfaces or even hang small absorbent panels inside the volume.

"Blue Peter" Testing

In most cases, both sets of modifications can be tested in mock-up form using cardboard (and wide tape) to cover gaps or to extend the guarding. You can then fit areas of acoustic foam inside the guards to test the results (taking care to account for the background noise from other sources).

Not only does this process help with the practical aspects (assessing the potential effects on access, visibility etc), but it usually also provides a very good indication of the noise reduction that can be expected in practice.

Note that you can cover open guards (mesh etc) with polycarbonate sheet if visibility is required. This may seem a very “Blue Peter” exercise, but it is a quick and effective way to assess the acoustic performance of the options before spending money on the final version made from steel, aluminium, polycarbonate or even plywood. Remember that guard vibration radiated as noise can also be treated via damping (as section 1 above).

The noise control diagnostic process detailed above must always be followed to evaluate what constitutes best practice. The basic process does not require a high level of engineering skill, but you can also plug-in to our expertise to update a noise risk assessment with the noise control element or evaluate the options for a noisy machine.

We have a very extensive noise control best practice knowledgebase that can sometimes be accessed by email (send us noise data, photos and smartphone recordings) or we can carry out the noise control audit following the procedure shown here.

You can also download the Top 10 noise controls pdf with sound here >