Noise reduction for Generators; Part 1 – Noise

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When I set out to write this blog piece I thought it would be a straightforward matter of writing a few words about sound enclosures, construction materials and noise reduction. However, when I started researching these areas I quickly realized there is too much material for a single blog to be meaningful. This is therefore the first in a series of blogs on the topic of noise reduction for generators.

Part 1 – Noise

With the expanding use of diesel generator sets there has come an increased focus on managing their noise levels. Whether generator sets are in enclosures outside a facility or home, inside, on the roof or even on a yacht, designers are making more efforts to control generator set noise and vibration in order to reduce the impact on neighbours and building occupants. Whether generator sets run continuously, intermittently or occasionally (for standby power or testing), their sound levels nearly always require attention owing to the local environmental impact.

In order to understand what solutions are available for generator noise reduction, it is first necessary to understand noise. Therefore part 1 of this blog provides a simple summary of noise and its characteristics, and the sources of noise created by generators.

What is Noise? 

Moving or vibrating objects create pressure waves that travel through any medium in their proximity (most commonly air in the atmosphere) reaching our ears as sound. The definition of noise is simply undesirable sound. When the size (amplitude) of the pressure waves becomes too high, the amount of sound becomes uncomfortable. In addition to being annoying, excessive sound can cause permanent hearing damage. The following diagram explains the physiology of what happens when sound reaches the human ear.

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The human ear has such a wide dynamic range that the logarithmic decibel scale (dB) was devised to express sound levels in a convenient way. The ratio between the softest sound the ear can hear and the loudest sound it can experience without damage is approximately a million to one. By using a base-10 logarithmic scale, the whole range of human hearing can be described by a more convenient number that ranges from 0 dB (threshold of normal hearing) to 140 dB (the threshold of pain). The following diagram demonstrates this:

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 There are two dB scales used to describe sound: A and L.

  • The dB(L) scale is linear and treats all audible frequencies as having equal value. However, the human ear does not experience all frequencies the same way. Our ears are particularly sensitive to frequencies in the range of 1,000 to 4,000 Hz, and they are less sensitive to sounds in lower or higher frequencies. This is why dogs often start barking for reasons humans don’t understand – dogs have the ability to hear sounds of a much higher frequency than humans.
  • The frequency-weighted dB(A) scale has been adopted as the official regulated sound level unit to adjust the sound pressure levels to more accurately reflect what the human ear perceives.

It is also worth noting that the db(A) scale represents an “absolute” value. For example, stating that a generator produces 98 db(A) of noise isn’t particularly meaningful – remember by definition it can only be noise if it is undesirable to the recipient. Standing alongside a generator generating 98 db(A) of noise is completely different to standing 500 meters away (where it probably couldn’t be heard).

More useful therefore is to state at what distance from the generator the noise level is experienced. As a result most generator manufacturers state sound levels in terms of db(A) @ x meters, for example 85 db(A) at 1 meter. This in turn presents challenges in terms of measurement, but that is a topic for a later blog.

 

Now that I have discussed the basic physics of noise, part 2 of this blog will discuss sound / noise produced by generator sets.

References

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