on 12-18-201404:56 AM - last edited on 03-10-201503:52 PM by Deb_EP
Generator set sound levels vary greatly across the world because of different regulations regarding maximum noise levels. They can differ even further depending on other factors including the technology used by the generator set manufacturer, the measuring techniques and how the data is reported. This makes comparing noise levels from similar-sized generator sets confusing and in a lot of cases, imbalanced.
We’ve identified two major areas that make sound level comparisons easier. First, knowing where the generator set was tested is important because some countries require strict reporting standards while others do not. Secondly, testing techniques play a critical role in measuring sound accurately as the recording setup, equipment used and the amount of load applied to the generator set all impact the results.
Sound Levels by Location
In Europe, the Directive 2000/14/EC on noise must be implemented to provide an accurate sound power level (SWL or LWA) at a specified percentage of maximum load. Therefore, comparing generator set SWLs in Europe is fair because the measurements are taken from a standard production sample, and the measurement details are provided automatically. Additionally, the European directive requires three production units for certification, and periodic re-certification is required, which guarantees precise and up-to-date SWLs.
Non-European generator set manufacturers can also report SWLs, but they aren’t necessarily comparable because not all manufacturers are required to test generator sets with a standard production sample. Instead, the generator set manufacturer could report average noise levels that may not capture noise levels above the generator set where the exhaust effects can be significant. Additionally, manufacturers may report the noise levels at differing loads making comparisons between brands difficult.
Sound Power vs. Sound Pressure
Sound power is a measure of the energy that’s transferred constantly per second from the sound source in all directions and is independent of any external influences. Sound pressure is measured from a point near the sound source and varies by distance and is affected by external influences such as sound waves reflecting off walls.
An electric heater is a useful analogy for explaining sound power and sound pressure levels. The heater’s power is constant and just like sound power, the room conditions don’t matter. This means the heater will emit the same amount of power if the room is hot, windy, cold or damp. On the other hand, temperature will vary in the room and will generally increase as a thermometer is moved closer to the heater. Similarly, sound pressure will vary and increases as the recording equipment is moved closer to the generator set. This becomes especially significant when determining the optimum location for the generator set at a specific site.
Sound Testing Techniques
Noise levels are typically measured worldwide at 1m and 7m from the sound source. Noise should be measured using a sound level meter and an octave band filter set for a detailed analysis, but these tools are not always used. Advanced technologies including specially designed hemi-anechoic chambers for testing can increase accuracy, improve data analysis and provide clearer measurements, but not all generator set manufacturers have this luxury. The spectrum of recording equipment and test facilities is vast; therefore not all sound pressure levels are comparable.
Figure 1: Sound pressure tested at 1m.
Additionally, equipment configuration can have a large impact on noise testing. The number of microphones, the height from ground level and the load application all directly impact the results. For example, Figure 1 shows 11 microphones in various positions at 1m away from the generator set. Three overhead microphones are used to capture the noise from the roof/exhaust discharge, which may be significant, but some manufacturers may not measure there.
Caterpillar utilizes one of Europe’s largest anechoic chambers (Figure 2) located in the Larne Engineering Centre of Excellence in Northern Ireland. The chamber permits detailed analysis of generator set noise under free field conditions meaning there are no reflective surfaces above ground level to obscure measurements. The results are used to maximize product performance and reliability.
All in all, sound level comparisons can be overwhelming for those trying to choose between different manufacturers. However, fair assessments can be made if the correct information is provided. It is important to ask where noise testing took place, which techniques were used to measure the sound and what equipment was used for recording. These three areas will help clarify what the noise levels really mean and how they can be applied to individual electric power system needs.