Testing the acoustical noise of a generator set and interpreting the data

March 10th, 2019, Published in Articles: EE Publishers, Articles: Energize, Articles: Vector

Large generator sets produce high levels of noise that must often be mitigated to meet national regulations and directives.

This usually involves key decisions about the optimal generator set location and installation, how to design noise control systems and other factors, which is why it is important to conduct accurate noise level assessments timeously, before investments are made and facility and enclosure designs are locked in. However, obtaining precise, consistent and comparable data can be difficult.

Not all manufacturers measure generator noise the same way, and some use less precise standards for their data. As generator set acoustic performance grows in importance, it is important to understand how to compare the data in a way that’s “apples to apples”.

Why precision acoustical testing matters

Noise levels in workspaces are regulated by governments to protect workers’ safety. In addition, communities are increasingly enacting ordinances aimed at reducing sounds that are disruptive to nearby residents.

Fig. 1: Generator undergoing acoustic testing at Cummins ATC.

For example, in North America, typically permitted noise levels during the day and at night are around 60 dB(A) and 50 dB(A) respectively. However, local authorities have set even lower noise limits in special zones, which is why it is important to measure noise levels using appropriate and applicable standards to establish an accurate platform for comparison.

These precisely measured noise levels are also useful for understanding the noise characteristics of generator sets so that noise control strategies such as acoustic enclosures and acoustic walls can be implemented successfully.

To bring a generator’s noise levels into compliance with ordinances and regulations, facility design engineers must carefully evaluate where to locate the generator and what noise reduction strategies will be most effective. These strategies may include the construction of sound-attenuating enclosures or the addition of acoustic insulation or barriers, exhaust silencers or isolation mounts. Power system designers equipped with precise acoustic data are better able to design noise reduction systems that meet noise restrictions.

Understanding acoustical data measurements

When comparing generator noise and sound data, it helps to understand some of the most common units of measurement used for industrial products. Sound levels are often expressed in terms of sound pressure level and sound power level. Both are important measurements, but they’re quite different.

SPLA vs. SPWLA

A-weighted sound pressure levels (SPLA or LA) are specific to the distance of the receiver from the sound source. SPL decreases as distance from the sound source increases and vice versa. SPL data is meaningless without the context of distance from the source.

Fig. 2: Location of microphones to achieve 360° baseline noise data.

A-weighted sound power level (SPWLA or LWA) is a measure of acoustical energy produced by a sound source, with no regard to its distance from the point of observation. Sound power level is calculated based on SPL measured by using parallelepiped or hemispherical array methods as described by ISO or ANSI standards.

For electric power generator sets, noise levels also depend on two major operating parameters: engine speed and load. It is important to know at what speeds and loads the noise levels were measured since noise levels increase as speed and load increase.

Cummins conducts noise testing at its acoustical testing centre (ATC) in Minnesota, USA. This carefully engineered and environmentally controlled facility provides reliable acoustic data through precision-grade acoustical equipment and rigorous measurement techniques.

The ATC is among the largest in the diesel and power generation industries, featuring a hemi-anechoic chamber certified as “precision grade,” the maximum accuracy level according to ISO 3745:2009. Its vast size is needed to accommodate large generator sets and the many microphone positions required for precision-grade testing, including those 7 m  from the face of the largest generator set the company manufactures.

Measurements are conducted around the generator set with the microphone set-up according to ISO and ANSI standards so that the unique noise characteristics of the components are captured and the directionality of the noise sources studied. This precision-grade noise data increases the operator’s confidence level and eventually helps in devising noise control strategies quickly and economically

Recommendations for evaluating acoustical data

With the growing focus on generator set noise and its impact on workers and nearby communities, noise mitigation is an essential component of generator set installation. Obtaining precise acoustical data allows for the design of effective and cost-effective noise control solutions that bring generator sets in compliance with community and federal noise limits.

However, comparing generator set acoustical data isn’t as simple as it seems. Not all testing facilities meet the latest ISO, ANSI and SAE standards. In addition, various manufacturers may offer different types of data, which can make difficult comparing like with like. Some questions to consider when comparing generator sets acoustical data from different manufacturers include:

  • Were the generators tested at the same engine load/power node?
  • Were the generators tested at the same engine speed?
  • Was the data collected with or without a set-mounted radiator?
  • Are the units of measurement the same (SPLA or SPWLA)?
  • Were sound pressure level measurements taken at the same distance?
  • Was the same standard or measurement method used for the noise measurements?
  • How reliable is the data?
  • Were the measurements conducted at precision or engineering-grade test facilities? Precision grade is more precise than engineering grade.

Testing the Cummins QSK95 Series generator set

The QSK95 Series generator set is equipped with a 16-cylinder diesel engine capable of producing approximately 3,5 MW. It was tested at a variety of operating conditions and a range of acoustic parameters to determine how different aspects of normal operation would contribute to overall noise levels.

Testing focused on sounds which are most impactful to the human ear and structures surrounding the generator sets, as well as on frequencies with higher potential to disturb surrounding communities.

The company strived for precision and reliability in its numbers, calculating average readings from an array of 71 microphones. This covered the entire human audible frequency range, from 20 to 20 000 Hz.

Engineers conducted 58 different test configurations over a total of 321 hours in the testing facility and acquired almost 360 GB of noise data. By using advanced noise measurement techniques developed at the ATC, the company was able to determine the noise contributions from individual components of the generator set to overall noise levels. This included the noise characteristics of the engine; cooling system; exhaust system; air intake system; fuel injection system and alternator.

Acquiring noise data in this manner helps to ensure that precise information is incorporated in product specification sheets. It is also helpful for quickly devising noise control strategies without the need for extensive engineering resources, whether it’s to meet regulation or directive-based noise limits or customer-specific requirements.

Some highlights of the QSK95 Series generator set testing:

  • Baseline noise data was acquired at 1 m using the parallelepiped method according to ISO 3744:2010 with 59 microphone locations around the generator set (engine and alternator ends, sides and top of the generator set).
  • Baseline noise data was also acquired at 7 m using eight microphones (eight locations spanning 360° around the generator set, each microphone placed at 45° increments) based on internal noise measurement standards derived from the US Department of Defence power generator set noise measurement requirements. Noise data collection at 7 m from the surface of the generator set is important because sound levels are generally stable at that distance, enabling reliable extrapolation and noise level prediction beyond 7 m. Very few hemi-anechoic chambers in the world are large enough to conduct testing at that distance.
  • Cummins used 71 microphone locations to measure noise levels at 1 and 7 m from the generator set, 1 m from the line of exhaust and fuel pump and at operator location to achieve a very high degree of reliability.
  • The average SPLAs at 1 and 7 m reported in the data sheets is an average of 59 and eight microphone locations at a distance of 1 and 7 m from the generator set respectively.
  • The SPWLA reported in the data sheets was calculated using the average SPLA measured by means of 59 microphones placed at 1 m from the generator set, in line with the parallelepiped method described in ISO 3744:2010.

These extensive testing configurations resulted in highly detailed, three-dimensional (sound field) sound data. This data provides a valuable resource to customers who seek to design effective and cost-efficient generator enclosures and noise attenuation systems.

Contact Kenneth Gaynor, Cummins, Tel 011 589-8400, kenneth.gaynor@cummins.com

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