An elegant engineering solution to your noise or vibration problem may already be available. Search the largest database of noise control at source case studies in the world for best practice alternatives to conventional, high-cost palliatives such as acoustic enclosures, silencers, barriers etc.
The motor/gearbox on a large fermentation vessel suffered varying motor current and intermittent noise. Downtime would be costly due to the production schedule. Single point vibration analysis indicated excessive input shaft float – very unlikely (due to the construction) but confirmed later on partial strip-down (25 thou v 6 thou specification). Pinpoint accuracy.
We have developed a suite of innovative and elegant engineering impact noise control techniques that typically provide 10-20dB reductions. These are both much more effective than conventional technology and very much lower cost and rugged. We custom design the modifications either for new designs or for retro-fit to existing installations. We also have high hygiene versions for the food and pharmaceutical industries.
- Applications: chutes, hoppers, conveyors, bins, recycling plant, scrap yards, train/truck loading/unloading, quarries …
- Benefits: large attenuations, rugged (often harder wearing than the untreated surface); very low cost; hygienic
Additional Noise Control Examples
We have a host of other engineering noise control applications across a wide range of industries – contact us to discuss particular plant or machinery in detail.
High levels of impact noise were generated by coal falling into a delivery chute on an industrial site. Expensive noise barriers had been considered, but these would have made access difficult and would have reduced the noise by only around 5dB. Our alternative solution was to design highly damped chute modifications to reduce the noise at source.
These low-cost modifications were retro-fitted with very little downtime and not only reduced the impact noise by 20dB(A), but they also substantially reduced the dust generated (solving another problem in the area) and cut chute wear to reduce the maintenance requirements.
This engineering approach is very effective on all kinds of chutes and hoppers where there is impact noise from a wide variety of objects that include rocks and stones, scrap metal and even high hygiene products such as sweets, tablets and vegetables.
Why was this air source heat pump so noisy?
A newly installed air-source heat pump domestic central heating system immediately generated noise complaints abut low-frequency sound from a neighbour. The owner contacted us and subsequently provided the requested smartphone video clips (as per the recommended diagnostic proceedure) of the noise:
- at the neighbour,
- close to the units
- inside the adjacent garage
This allowed us to rule out structure-borne noise (a common noise issue for A/C and heat pump installations) and to diagnose that the cause of the noise problem was primarily sound reflections that created an amplified 93Hz pumping frequency due to standing waves.
How was the heat pump noise reduced?
This precise diagnosis coupled with the photos provided, allowed us to devise a simple solution involving low-cost components available in DIY stores (plywood and loft insulation). Once installed, these noise control measures broke up the standing waves and reduced the tone at the neighbour by about 30dB (as shown in this noise signature), eliminating the problem.
This was a fast, low-cost project completed remotely (without any site visits) via the supplied smartphone data.
We have provided a more detailed guide to heat pump noise reduction covering both air-source and ground-source noise control.
The traditional approach to granulator noise control is based on using palliative high-cost acoustic enclosures to reduce the spread of noise without tackling the noise generation at source. This approach is not only expensive, but it also has a substantial impact on access and therefore productivity and maintenance. Typical granulators generate noise levels close to, or above, 100dB(A). As such, they pose a very serious risk to hearing as it is not possible to guarantee adequate protection using PPE.
There is an alternative engineering approach that cuts the noise at source with the following benefits:-
- very low-cost compared with acoustic enclosure
- no effect on access, so no effect on productivity
- no effect on maintenance activities
- rapid deployment with minimum downtime
Granulator noise sources
Granulator noise is usually generated by 3 mechanisms:-
- Airborne sound from the blades or hammers, both aerodynamic (when run without material) and mechanical (impacts)
- Vibration from the granulator blades or hammers radiated as sound by the structure
- Web vibration radiated as noise on web-fed granulators
- Fan or blower noise from dust control and extract systems
Noise control modifications
The airborne sound path can be controlled internally, whilst high performance, rugged structural retrofit damping and impact control cut the vibration radiated noise. Blower noise is usually reduced using aerodynamic noise control technology that cannot clog and will last the lifetime of the fan without cleaning or maintenance.
Granulator noise control case study
The hearing risk posed by the 100dB(A) noise level from this plastic granulator was reduced at source by 98% (18dB) by designing a web damper coupled with internal modifications to the infeed path. Neither modification had any effect on operation or access. The cost of the granulator noise control modifications was c £5k compared with the previously proposed acoustic enclosure cost of c £30k.
The driver noise level of this Manitou forklift truck was 93dB(A) at full engine speed and load. The detailed diagnostic measurements showed the dominant source to be the engine cooling fan.
The somewhat agricultural cooling fan was changed for a more efficient and quieter unit and tuned acoustic absorbent was fitted in the cab roof to reduce the standing waves. These measures reduced the hearing damage risk by 75% (6dB) at virtually no cost. When evaluated over the typical worst-case working day, these noise reduction measures ensured that the operator noise dose was well below 85dB(A).
The remaining noise was dominated by tones from the hydraulic pump as shown here. Interestingly, whilst the hearing damage risk was substantially reduced, at full engine speed you can hear from the recording that the noise actually sounds louder as the characterless broadband noise from the fan that had masked the tonal noise from the hydraulic pump had been reduced.
Whilst it would have been relatively easy to reduce the now dominant hydraulic noise by 5dB - 10dB, the client considered that further retrofit measures were unnecessary.
The manufacturer of air-source heat pumps approached us to use our diagnostic skills and technology help them to reduce the noise from their units without recourse to expensive, efficiency-sapping enclosures and silencers. The objective was to design-in low-cost engineering noise control features to make the units inherently quieter - without compromising efficiency.
Heat pump noise diagnosis
The diagnostic analysis showed that there were 4 major noise sources, all of which would need to be reduced in order to achieve the desired noise reduction. The sources identified were:-
- fan: tones at 70Hz + harmonic - cannot be reduced using silencers or barriers
- mains hum: 100Hz tone
- compressor: several tones
- air noise: broadband sound
Previous resources had been focused on reducing the overall dB(A). However, the low-frequency tones were the dominant noise complaint issue despite only contributing <0.5dB to the overall dB(A). These would not have been reduced by the conventional methods under consideration.
Noise-reducing modifications
These involved the following:-
- fan tones: aerodynamic modifications
- mains hum + compressor: high-performance damping of key components + local tuned acoustic absorbent
- broadband air noise: once the low-frequency components had been cut, a simple small carefully designed screen was very effective
The combined effect was to reduce the 70Hz fan tone by 97% (15dB) and the overall noise by 90% (10dB(A)) without affecting the efficiency of the heat pump as demonstrated by the above analysis and the sound file.
View a more detailed guide to heat pump noise reduction covering both air-source and ground-source noise control.
The hydraulic power pack driving a baler was the cause of both occupational noise issues on-site and noise complaints off-site. Whilst the former was simply a case of the overall noise level at 87dB(A) requiring the use of hearing protection, the latter was a result of the highly tonal - and therefore annoying - nature of the noise. We designed engineering noise control modifications that not only cut the overall noise level to 76dB(A) at 1m (no PPE required), but also eliminated the tonal noise issue.
How was the hydraulic noise reduced?
The noise sources were ranked and modified as follows:-
- Vibration from the submerged hydraulic motor/pump unit into the oil tank: unit vibration isolated from the oil tank to reduce the high-frequency tonal noise
- Valve block vibration into oil tank: high-frequency vibration isolation
- Thin steel panels: damped
- Small local acoustic screen: to reduce noise traveling off-site
Care was taken to ensure that there were no vibration isolation short circuits.
These modifications cut the overall noise from the power pack by 9dB(A). In addition, care had been taken to address the off-site tonal noise problem. The major problem tones were reduced by up to 17dB (98%) as shown by this narrow band frequency analysis.
The engineering modifications were low-cost, easy to install using a local contractor and had no effect on normal operation, access or maintenance.
View detailed hydraulic power pack, pump and motor noise control guidance >
