Understanding noise levels for silent Honda generators

The first victim of marketing is the truth

As the battle between Persil and Ariel developed, the advertisers claimed an ever whiter outcome. Eventually, throwing off the shackles of reality, they gave us whiter than white, the window test and the bluey whiteness you’d never noticed before. Here in Suffolk we call that light blue.

Those same marketeers have applied their talents to the subject of generator noise in the battle between Honda and Yamaha. They are rapidly approaching the point where generators will be marketed for their ability to absorb and suppress noise from annoying pin dropping accidents. It is the cold war writ hushed. The result is a series of noise level figures based on any one of the following, depending on which gives the most pleasing outcome.

Peak energy levels

Averaged energy levels

Sound pressure levels(SPL)

Weighted mean average energy levels

Logarithms

Fibbing

As the battle for the lowest numbers progresses, the manufacturers seem to wander ever further from their equipment. Measurements are taken at 1m, 3m, 5m, 7m and so on. Just massaging the figures would never satisfy the creators of whiter than white so they have given us, semi silent generators, silent generators and super silent generators. If these marketeers would but see my potential I would offer up super duper silent suitcase generators as the logical next step.

Added to all this is ECO throttle. Modern inverter type Honda generators have a feature called ECO throttle which alters engine speed to suit the applied load. As a result they can be a lot quieter at low loads than at full load.

Confused? Help is at hand. Regulation has forced manufacturers to use a common noise standard displayed on a silver sticker on the machine. This is the LWA rating and is measured in decibels (dB) at 1m. What does LWA stand for? The following explanation is courtesy of Jason Morgan:

“LWA is a Logarithmic Watt (in some text ‘W’ is ‘weighted’) method ‘A’.  It is a fudge that takes in to account human hearing is non linear over frequency and applies a graph shape (one is called ‘A’, there’s also a ‘B’, ‘C’ etc) to the sound pressure level (actually power level, SWL) to convert pure energy content (rather than the air pressure) into what we perceive as relative sound loudness regardless of frequency. Compared to SPL it’s more useful unless you are a dog or worried about vibration being the transmission method (where you get harmonics, shifting some of the power into another frequency which we might hear as louder despite losses).”

This does at least provide a common yardstick allowing a comparison between machines. Manufacturers are still free to serve up confusion and fabulous claims like silent suitcase generator in their advertising literature and other blurb but on the machine itself is that darned little silver sticker.

Sound energy and the inverse square law

The futility of all this concern over generator noise levels is illuminated by consideration of the relationship between sound energy and distance. Condensing a phenomenally complex subject to a single sentence, one can say that when the distance from the source is doubled, the sound energy is reduced by a factor of four. This equates to a difference of 6dB for every doubling of the distance at which the measurement is taken. Looked at in another way, the noise from the quietest generator, the Honda EX650 silent suitcase generator (79dB@1m) and an un-silenced frame generator at around 100dB@1m would be roughly equal if the frame generator were 7m further away.

 

 

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by Peter Noble