Technical Notes - Environmental Noise

Sound file: typical chiller low-frequency noise reduction,

The noise from chillers, air conditioning plant, condensing units, HVAC, heat exchangers, dry and adiabatic cooler AHUs etc provides a host of opportunities to waste resources on inappropriate noise control measures that either don't work or that reduce efficiency - or both. In our experience, a substantial proportion of the noise reduction measures used on these units (acoustic enclosures, silencers and barriers) are simply ineffective as they address the wrong problem.

There are only 3 types of noise generated by these systems: broadband air noise, compressor tones and low-frequency fan hum. The hum, for example, cannot be addressed using silencers and barriers, and yet these are often mistakenly fitted at high capital and running costs (reduced efficiency).

10 minutes with a smartphone is all it takes to avoid costly mistakes by determining which sources require treatment and what is the best noise control option for the particular problem. Anyone can do this.

This approach not only dramatically reduces typical noise reduction project costs, but it can even substantially improve system efficiency (by 23% on one data centre application). Find out how best to reduce the noise from chiller, condensing, cooler and related units here >

Data center noise poses an increasing problem across the world as more and more are built to provide the digital services we all use. In addition, there are similar issues with crypto mining noise control. The 2 major noise sources on these types of site are the cooling systems (banks of fans on chillers) and standby generators. Whilst the latter are run to order for testing and are not usually much of a noise problem (as long as the low-frequency exhaust noise is properly attenuated), the former run 24/7.

Conventional noise control techniques such as acoustic louvres and barriers reduce cooling efficiency and increase running costs. Moreover, they cannot solve the very common low-frequency data center hum complaint problems experienced by many sites.

However, there are innovative new low-cost techniques that not only cut noise, but also improve cooling efficiency thus making many noise control projects self-financing. As they also kill the low-frequency tones (hum), they represent a seriously disruptive noise control technology that is often worth implementing for the efficiency gains alone, even if there is no noise issue.

In addition to cooling systems that run 24/7, there will also be power supply systems - either online (e.g. CHP turbines) or on standby (e.g. noisy diesel generator sets that are only run to a test schedule). As with cooling systems, low-frequency generator set tones may be a problem.

A data center noise control project that pays for itself is no longer an oxymoronic concept...

Find out how best to reduce data center and crypto mining noise here >

The key to effective cooling tower noise reduction is accurate diagnosis. Far too often, noise control measures are selected without a clear understanding of -

• the type of noise problem: is it simply the overall dB(A) level or is there tonal character such as a low-frequency hum?
• the source of the noise: fan (broadband or tonal); falling water; pumps (often tonal) and/or gearbox noise (tonal)

As a result, far too many noise control projects end in costly failure or go through a number of iterations before the problem is solved. Accurate diagnosis is not rocket science. Forward a few photographs and a smartphone video clip to us by email and we can provide a precise and accurate remote noise control diagnosis plus the optimum costed noise control options within an hour. At no cost.

There is also new low-cost aerodynamic technology that cuts the noise from cooling tower fans whilst often improving efficiency (unlike silencers and barriers that reduce efficiency).

Find out details of the best options to control the noise from cooling towers here >

Conventional cyclone separator and dust extract collector noise attenuation measures are not only high-cost, but are often not very effective due to poor (or no) accurate diagnosis. This wastes resources and delays noise complaint resolution. Moreover, dust clogs conventional silencers, requiring regular maintenance or replacement.

There are simple, effective and low-cost control techniques to resolve cyclone separator and dust collector noise issues quickly and efficiently. The best option(s) can be selected by answering the following questions:-

1. Are there tones e.g. a hum? Is it fans and/or resonances?
2. Is the noise problem overall dB(A)? Is it air or material impact noise?
3. Intermittent noise? Reverse jet filters?

If you are not sure of the answers to any of these questions, you can simply email us smartphone video clips and photos and we will get back to you with the options.

Find out about all the cyclone separator and dust collector noise control options here >

Sound file: typical diesel generator set tonal noise

Diesel generator set noise often causes environmental noise problems in ways that are not necessarily obvious. The classic headline noise figures provided by suppliers of gen sets are usually expressed in terms of "XdB(A) at Y metres". This data can hide a multitude of hidden sins...

Take 2 generator sets with the same overall noise levels. One causes no noise problems. The other generates complaints over a wide area. The difference? The problem set is radiating low-frequency tonal noise from either the engine exhaust or the cooling fans that has little or no influence on the overall dB(A) as the "A" weighting attenuates low frequencies.

A simple diagnostic process using a smartphone is all that is required to avoid problems by diagnosing and ranking the gen set noise sources. The results can then be used to determine the optimum low-cost noise control techniques using the latest innovative technology, technology that can even improve cooling efficiency... Find out how best to reduce diesel engine noise >

Environmental Health officers and Environment Agency staff (not to mention the companies with noise complaint issues) are making increasing use of our services to cut the time, resources and costs associated with the identification and resolution of industrial noise complaints. This simple process outsources most of the hassles and costs, providing: -

• regulator costs: a dramatic cut in the required regulator staff project time and hence costs
• complainant stress: typical industrial noise complaint resolution times are reduced from months or years to weeks, reducing unnecessary local resident stress
• mitigation costs: a 50% - 90% cut in typical industrial noise generation mitigation project costs - a substantial benefit to the company involved

This win-win-win scenario is possible due to an innovative approach and technology that allows us to provide accurate diagnoses from smartphone data via email linked to the largest global database of best practice noise control solutions. This gives you access to a virtually instant Best Practicable Means cost/benefit analysis.

...frankly abysmal modern diagnostic and noise control expertise expressed in most consultant noise reports ...

It also provides a 2nd opinion as to the potential best practice alternatives to the frankly abysmal modern diagnostic and noise control expertise expressed in most consultant noise reports (independent surveys indicate that c 80% - 90% of noise reports are seriously inadequate).

Find out more about outsourcing industrial noise complaint problems...

All environmental noise control projects require 3 types of feature evaluation:-

1. Broadband noise (dB(A)): the simple default - quite often used inappropriately
2. Low-frequency tonal noise that does not contribute significantly to the dB(A) value
3. Intermittency/variability: impulsive sound, sources switching on and off

Despite tonal noise issues being very common causes of complaints, objective identification of the tones using narrow-band frequency analysis is incredibly rare in consultant reports. They present, at best, octaves and 1/3 octaves. Octaves are totally inadequate and 1/3 octaves are not much more useful. As there are very capable free smartphone narrow-band frequency analysis apps, there is simply no excuse for this lack.

This is rank incompetence that wastes time, money and resources

Including objective tonal analysis in assessments and environmental noise control reports is best practice. It not only accurately reflects the subjective effect of the noise, but it is also an invaluable diagnostic resource to determine the cause - and hence the optimum mitigation options.

How to use narrow-band analysis to evaluate and eliminate tonal noise problems >

We provide extensive specialist technical support, information and training on noise and vibration to both regulators (Local Authorities (EHO) and Environment Agencies (EA)) and to industry. As we regularly deal with the same noise and vibration problems encountered, we have probably solved your problem already.

Our acclaimed range of EHO / EA specific noise workshops can help transfer time saving best practice techniques and technology in-house. This approach typically reduces the time to resolve noise complaint and other problems from months or years down to weeks - or even as little as days in some cases... This dramatically reduces the resources spent (freeing them up for other work) and also the time that local residents spend suffering unnecessary stress from unacceptable environmental noise.

We provide a worldwide service via the internet and smartphone technology. Make use of our Remote Control of noise service for a 2nd opinion and evaluation of best practice - or contact us for advice.

You can also outsource much of the time and resources required to resolve industrial noise complaints making the process much faster and more cost-effective.

Best Practicable Means / Best Available Technology evaluation service

Complaints and noise nuisance: how do you determine what constitutes “best practicable means” to mitigate. Companies employ consultants to generate reports claiming the cost of mitigation is too high to be practicable. In most cases, the suggested noise reduction technology is not best practice. Our definitions of low-cost BPM are game changers for regulators as problems can usually be resolved within weeks, saving time, resources and money.

We are quite happy to provide definitive definitions of what constitutes BPM/BAT with complete technical justifications, in court if necessary.

Noise and vibration training workshops

We provide very effective noise training for EHOs and Environment Agencies, from workshops to full CIEH environmental noise competency (some councils have profited from hosting in-house versions by inviting other authorities). These include:-

• BPM / BAT noise workshop
• Fan noise control masterclass

The Delegate View: “one of the most relevant and informative courses in a long time..” .. “I’d hate to come up against you in court! .. the best value seminar we’ve ever been on” – “standard of presentation was excellent .. humorous and entertaining .. highly recommended”

As far as we know, the INVC is the only carbon negative noise consultancy in the world. We have achieved this by means of 2 major innovations...

Fan efficiency

Across the world, organisations using our aerodynamic fan noise control technology are currently saving megawatts of power as a result of the efficiency gains they have realised, compared with both conventional noise reduction technologies and in some cases, compared with unmodified fans. Our current record is a 22% improvement in the efficiency of cooling fans as part of a noise control project.

Remote noise control - hold the site visits...

We have developed technology that allows us to complete an increasing proportion of our noise control projects anywhere on the planet without a site visit. This not only reduces our carbon footprint (flights and other transport costs etc), but it is also faster and lower cost.

Our remote control of noise services uses email and remote working (e.g. Whatsapp) to carry out diagnoses and the development of best practice noise control solutions. We also use innovative technologies to avoid traditional resource-heavy noise control measures such as barriers and acoustic enclosures in favour of elegant engineering that helps clients to reduce their carbon footprints.

Environment Agencies (EAs), environmental health (EHOs), noise consultants and organisations in many countries regularly use our free online noise diagnosis and control best practice service to eliminate time-consuming hassles. All you need to access unique expertise and technology that reduces time and resources spent on projects by an order of magnitude is a smartphone on-site.

E-analysis: this approach not only cuts typical noise project investigation and resolution time from months or years down to a few weeks - or even a matter of days (our current record is 2 days from initial contact to noise control implemented and problem solved), it also reduces noise control costs by 50% - 80%.

You can take a "virtual noise expert" to site anywhere via Whatsapp "telepresence" to provide instant feedback and analysis.

E.g. for a suspected chiller noise problem: "Record now at the site boundary, then move close to the plant and record again. Ok, there are 2 series of tones. Check the fan speed and take a close-up. Ok, the low-frequency tone is fan blade pass and the higher frequency tones are from the compressor. The former would be an £800 fix that would improve efficiency and the latter around £1k."

Regulators and consultants routinely send smartphone data from site visits for a free expert opinion. We can usually provide source identification and costed noise control options based on current best practice within 1 or 2 days.

Get more detailed information and case studies about the remote noise analysis and control options >

The noise from heat pumps, be they air-source or ground-source units, often poses a serious problem as not only can they be loud, but they are also installed very close to or on noise-sensitive premises.

The result: noise complaints

Unfortunately, current heat pump noise specifications and limits are often inadequate as they do not take into account the common low-frequency tonal nature of the noise generated by many heat pumps.

Ineffective heat pump noise control is far too common...

Mis- (or no) diagnosis of the characteristics of heat pump noise is extremely common, leading to money wasted on noise control measures that are either ineffective or that reduce efficiency - or both. Our experience indicates that a substantial proportion of the noise mitigation measures applied to these units (acoustic enclosures, attenuators, barriers) are either over-engineered or ineffective as they address the wrong problem. This is supported by the fact that the Environment Agency and EHO surveys indicate that 95% of noise consultant reports are inadequate.

There are 2 categories of noise generated by heat pumps:

• broadband noise - mid-high frequency noise that controls the overall dB(A)
• low-frequency tonal noise e.g. fan or compressor/pump hum

The latter cannot be addressed using the silencers and barriers so often installed due to simple physics. Moreover, these can reduce system efficiency.

A few minutes with a smartphone is all that is required to carry out the proper diagnosis to determine and rank the noise sources and hence to the optimum noise control options. Anyone can do this.

This approach dramatically reduces the costs of typical noise reduction projects and can even simultaneously improve system efficiency.

Important note: private individuals

If you are a private individual with a heat pump noise problem, then as a commercial consultancy we have to charge for our time and advice. However, we have put together a guide to help most people deal with heat pump noise problems themselves whilst avoiding the common (and often costly) mistakes. Please go to to our guide here >

Find out the optimum heat pump noise control methods here >

Heat pumps can cause noise complaints from neighbours. The person who has installed the heat pump to do the right thing to mitigate CO2 emissions and to reduce power consumption and bills may then have to contend with noise complaints. This may happen even when the headline noise level planning requirements have been met.

Unfortunately, we cannot provide specific help to private individuals. The INVC is a commercial organisation and we do not have the resources - we have to charge! However, the following guidance should allow you to solve most problems yourself or in conjunction with the Local Authority Environmental Health Officer (EHO).

What to do if you have noise complaints about your heat pump?

Current heat pump noise specifications are based on the overall noise level measured in dB(A). This is often seriously inadequate as it only addresses part of the issue. It does not take into account the low-frequency hum often produced by heat pumps. Nor does it take into account vibration that may be transmitted into the building that is then heard as a low-frequency humming noise. Don't guess! Use the following diagnostic process to understand the problem.

Step 1: diagnostic process

The first step to eliminate the complaints is to understand the nature of the noise that is the cause. There are 3 possibilities:-

• broadband noise measured as dB(A). This is the single figure overall noise level provided by the supplier. However, measuring dB(A) means that low frequency noise components have been filtered-out, posing a problem if there are low-frequency noise components from the pump.
• airborne low-frequency tonal noise from the fan and/or from the compressor, the "hum".
• structure-borne vibration from pipes and/or the mounted heat pump that cause walls or joists to vibrate and that is then heard inside as a low-frequency hum. This can be diagnosed by comparing the noise with the window open and shut - there won't be miuch difference if the problem is vibration.
• structure-borne vibration from mounting the heat pump on a flat roof. The pump has been mounted on a very large loudspeaker. Conventional anti-vibration mounts are often not suitable for this type of installation as they are designed for floor mounting.

Go through the following process to determine which of the above is the dominant issue. Be aware that heat pump noise varies considerably with load. They are much noisier in very cold weather.

1. Ask: ask the complainant about the nature of the noise about which they are complaining. This may or may not provide useful information as many people struggle to describe noise in words. Specifically, ask if they hear a hum or drone. In addition, ask them where and when the noise is worst - outdoors, indoors, at night etc. Note that listening to and assessing heat pump noise near the pump will not usually give you a useful impression of the problem.
2. Listen: if at all possible, listen to the noise yourself at the worst complaint location(s) and try to describe it to yourself as above.
3. Analyse: download a free noise spectrum analysis app to your smartphone (there are many, for example https://www.keuwl.com/SpectrumAnalyser/ for android). Set the maximum frequency range to around 1000Hz and analyse the noise signature at the worst complaint location. If there are large peaks at particular frequencies (see example below), then there are tones (hum or drone). More tonal noise analysis information here >

Heat pump noise analysis example

In this example, as a result of complaints from a neighbour, the owner spent >£4k on an acoustic enclosure that had no effect on the complaints. Whilst the enclosure reduced the overall dB(A), it did not reduce the cause of the complaints which was a hum at 58Hz. This noise signature narrow band frequency analysis from video clips provided by the owner clearly shows that the complaint was not about the dB(A), but about a very low frequency hum at 58Hz that could not be attenuated by an enclosure.

If the above process had been followed, it would have been obvious that the enclosure would not have worked.

The lack of any diagnostic process to determine the characteristics of the heat pump noise that are the cause of the complaints is extremely common. This leads to time and money being wasted on the installation of ineffective noise control measures (Environment Agency and EHO surveys indicate that 95% of noise consultant reports are inadequate).

Step 2: evaluate the heat pump noise attenuation options

The optimum noise control solution depends on the nature of the noise problem as diagnosed above.

1. Broadband noise measured as dB(A). Depending on the site geometry, simple acoustic barriers or an off-the-shelf acoustic enclosure may well be sufficient. Even DIY enclosures (marine plywood and acoustic absorbent foam or rockwool) can be very effective, but be careful not to compromise the airflow.
2. Airborne low-frequency tonal noise. The acoustic enclosures or barriers that are often installed are rarely effective due to simple physics. If it is the fan, then the only real solution is aerodynamic noise control (for which we would have to charge) or a very carefully designed, non-standard enclosure/silencer. If the problem is the compressor, then the solution is improved vibration isolation and/or damping of the heat pump casing.
3. Structure-borne vibration. Ensure that the compressor inside the casing is properly vibration isolated with no hard links (you should be able to wobble the compressor quite easily) and with flexible connections to the pipes. In addition, if the pipework into the building is hard mounted to structures such as joists (or even the brickwork in some cases), this will transmit pressure pulsation vibration that will then be heard as noise. If the pump has been mounted on a flat roof, then make sure the pump is above a joist and mounting it on a large paving slab that is then isolated from the roof with 50mm or more of rockwool may be a solution.

More on the optimum heat pump noise control methods here >

Step 3: contact your local EHO

Your neighbour may already have done this! They will have the instrumentation to evaluate a complaint, but be aware that they will often only consider the overall dB(A) - which gives you a problem if that is not the primary cause of complaints. Some will also be able to carry out the diagnostic measurements and analysis described above, but we would recommend that you should already have carried out your own evaluation as above.

Hope you find this useful...