How quiet are silent generators?

[Gordon Brown 1Participant @gordonbrown1]

It comes as no surprise to me that this thread is riddled with errors and misunderstandings regarding the measurement of noise and expressing noise levels, so perhaps now is the time to put things straight. Firstly, there is definite confusion regarding the manner in which manufacturers express the noise emissions from their machinery. The reputable ones, such as Honda, express the noise level as a Sound Power Level (LWA) which is a function of the machine and is independent of the environment in which it is placed. Any particular machine operated in the same manner will have the same sound power whether placed in a field or in a room and the determination of sound power level should have been carried out under very specific operating conditions, usually full duty under representative load. Despite what Clive Foster says, LWA is used for all manner of equipment, air conditioning units, fans, lawnmowers, household appliances, hand held tools, cranes, excavators, bulldozers, backhoes, generators etc. It is most definitely NOT “pretty much noise level when you are standing next to it rather than the usual out in the middle of a (small) field situation”. The point is that LWA can be converted to Sound Pressure Level (LPA) provided you know the circumstances under which the equipment is to be deployed and where you want to estimate the LPA in relation to the machine.

However, some less than helpful manufacturers have a nasty habit of specifying the noise output of their machinery by giving the LPA, which will depend on the environment and distance, which are very rarely given. The 10m and 7m distances referred to by Clive are rarely given, sometimes 1m is used and I have come across 2m and 15m as well. The measurement conditions are never described and the difference between free field (suspended in space) and hemispherical radiation (on a hard surface) will be 3dB. I have also seen measurement results where the sound level meter was placed facing the quietest side of the machine. On one occasion I was actually asked to do this, the previous consultant having been asked to do this – my response was fairly blunt…

The difficulty that most folks have regarding this is that both LWA and LPA are expressed as decibels (dB not Db) but they use different reference values as they are describing different parameters. It is possible to convert between the two, the simple relationship is LP = LW – 20log r -14 if the machine is in free field and LP = LW – 20log r -11 if it is on hard ground. The situation is more complex if you move the machine indoors, but conversion is still possible. So, taking the Clarke 94dB generator as an example; the 94dB is definitely LWA, it says so on the Clarke website, it’s a guaranteed sound power level and should have been measured at full duty. Its sound pressure level at 7m over soft ground is therefore 63dB and at 10m it is 60dB. The Wolf generator’s noise output of 58dBA appears to be measured at 7m, no further details given anywhere that I can find, but I wouldn’t trust it as you can more or less guarantee that the measurement was done under the best possible conditions for minimising noise levels . The 5dB difference between noise levels at 7m is likely to be noticeable but my advice would be to only consider noise levels to be reliable and directly comparable if they are expressed as LWA.

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