Diesel engine generator set noise control techniques
Diesel generator sets (however packaged) are very common sources of occupational and environmental noise problems. A common issue is that the headline overall dB(A) noise level does not tell the full story and hence large sums of money are often wasted on the wrong solutions. Luckily, there are elegant, low-cost retrofit engineering noise control solutions that can even increase system and cooling efficiency.
There is a simple process to diagnose the best approach to the attenuation of noise from these units. Diesel generator noise control must be based on a detailed understanding of the typical noise sources, each with its own characteristics and each with a different ideal silencing solution.
How to reduce diesel generator set noise - best practice
The noise from generator sets is produced by a limited number of sources. The key to effective noise reduction is to diagnose and rank these sources and their characteristics. The sources include:
- engine exhaust - low-frequency hum: this may not contribute to the overall dB(A) but is a very common cause of complaints over large distances., even when simple dB(A) targets have been met. Also applies to specifying noise from hired generator sets.
- engine and alternator - overall dB(A): diesel engines are very noisy (often well over 100dB(A) at 1m) over a wide range of frequencies. Acoustic enclosure is the only practical option.
- cooling system noise - internal or external radiators: the issue is usually engine noise (broadband dB(A)) radiated out through the silenced cooling air intake and exhaust paths. Where external radiators are used, this is coupled with a combination of broadband air noise (conventional barriers or silencers can effectively reduce dB(A) - if that is the problem...) and low-frequency tonal noise (fan blade pass sound) for which aerodynamic noise control is the only practical option (silencers, barriers or enclosures are not effective for this noise component).
The noise characteristic diagnosis and control options for each of these sources are detailed below. If the diagnostic process described below is not followed, then it is very easy to specify and install costly, ineffective noise reduction measures that can also affect performance.
Generator noise control options
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Diesel engine exhaust noise attenuation
The following audio recordings illustrate the common engine exhaust tonal hum issue. Both generators in the recordings produce the same overall dB(A) noise level but sound very different. The unit with the exhaust tone is much more likely to generate complaints.
Diesel generator noise with and without low-frequency tonal exhaust components recorded outside building
Diesel generator noise with and without low-frequency tonal exhaust components recorded inside building
Narrowband analysis provides objective evidence for the subjective experience. The passage of the noise through windows progressively attenuates higher frequencies, making the sound "bass-heavy". This frequency analysis of the sound files shows an overlay of the noise signatures both outside the building (blue trace) and inside (red trace).
The low-frequency 75Hz (+ harmonic) engine exhaust tones are little unattenuated by passing through a window, the attenuation increasing progressively with frequency.
In another similar example shown here, the 75Hz diesel engine exhaust tone from a packaged CHP installation was reduced by 18dB using a tuned exhaust silencer.
Engine and alternator overall broadband noise reduction - dB(A)
The only practical noise control option is an acoustic enclosure to cut the spread of noise, but we can improve the attenuation. Many generator sets are supplied as packages that include a purpose-designed enclosure - or a converted "off-the-shelf" container that may or not be totally effective.
As acoustic enclosures are very efficient at thermal insulation, well-designed and silenced cooling is required as below.
Cooling system noise control - internal or external radiators
For enclosed diesel generator sets, it is necessary to push large volumes of air through enclosure openings to manage the internal temperature. These openings are an acoustic weak link that must be adequately silenced using techniques that do not cause excessive back-pressure. This is usually achieved using either acoustic louvers or compact labyrinth silencers, the choice depending on the space available and the required attenuation.
We have been very successful in designing world-leading high-performance silencing within very tight space constraints using new technology coupled with Computational Fluid Dynamics (CFD) to model and optimise airflow to maximise cooling efficiency.
We use the same techniques to determine the optimum cooling geometry for generator acoustic enclosure packages (fan and airpath locations).
For external radiators, then the issues are typically broadband noise (dB(A)) and low-frequency fan blade pass tones (hum) control. The former can be reduced using:-
- innovative retrofit aerodynamic modifications that improve fan efficiency so they can be run at lower speed, cutting the noise and running costs
- innovative acoustic barriers that attenuate the noise without the free-cooling penalty (and hence reduced cooling efficiency) associated with conventional screens
- conventional silencers on the outlets - if this is the main source
The latter cannot be addressed using traditional enclosures, silencers or barriers. The only practical solution is retrofit aerodynamic technology to reduce the tonal noise at source coupled with improved fan efficiency. This signature analysis shows the relative contributions to the noise from a typical diesel generator set of the engine tones and the cooling fans (red - unweighted, blue -"A" weighted). In this case, the cooling fans dominate the overall "A" weighted tonal content.
There is an optimum approach to reducing noise from external cooling systems that is the same as that used to attenuate the noise from chillers.
Diesel generator set noise control: the process
We follow this simple procedure to ensure that the optimum environmental noise control package is applied. You should always do the same. This approach applies to all generator set installation noise problems.
- Analyse the noise environment: measure overall levels and use a free smartphone frequency analysis app to assess tonal content - or email smartphone noise or video recordings to us for a free analysis
- Diagnose the noise source(s): ascribe tones and overall noise levels to specific generator components and acoustic paths as detailed above
- Source control: select the appropriate noise control measures (as above) for the problem source(s)
- Evaluate the noise attenuation performance: reanalyse frequency content and noise levels
This process is the only way to ensure that the selection of, and expenditure on noise control measures is not based on costly guesswork.
We can provide the whole process as a service, normally without a site visit via remote control of noise. The initial analysis and diagnosis provide you with the results and a detailed evaluation of the costed noise control options using current best practice followed by detailed recommendations for implementation by local contractors. We can also use noise modelling to generate noise maps for new or modified installations to predict noise levels in the surrounding area (and inside the facility if required).
