The Wanderer
Dometic TEC 29 generator
Noise is unwanted sound. As many an offender now rapidly finds, few people perceive a loud generator in an otherwise quiet campground as anything but that. But while defining noise is easy, reducing it is extraordinarily hard. Understanding why, necessitates knowing how hearing actually works.
The human ear must cope with massive extremes of sound levels and its associated energy. One can (literally) hear one’s heart beat in mid-Sahara (I’ve done so), yet withstand massive thunderclaps, etc. without damage. This effect relates not just to sound but also to light and pain, etc.
The explanation is that human physiology has evolved to cope with the surprisingly high spread of energy levels in various areas (including nature) via non-linear responses.
Sound is the movement of energy through air and other substances. It does so in waves of longitudinal compression and refraction. Its energy level can be expressed in units such as joules or watts. These waves impinge on a diaphragm in the ear and are perceived as sound or if unwanted – as noise (as by many a teenager with Bach).
The range of sound energy intensities that the human ear can detect without damage is extraordinary. It extends from that of a just audible heart beat (or pin drop) of which the associated sound energy is a tiny 0.000000000001 watts per square metre to a close-by thunder clap of 1.0 watt per square metre (twelve orders of magnitude higher).
The associated physiology somehow evolved such that much of human perception is logarithmic. In plain English, that means that to double (or halve) perceived sound, light or pain, etc. the source generating it has to increase or decrease energy intensity 10 times. Reducing the perceived effects to one quarter requires a 1000 times: an often impossible decrease in the source energy.
As the ear’s response (as with all such physiological functions) is thus logarithmic, a convenient way to avoid juggling up to 12 or so noughts is to use the associated logarithmic scale. This corresponds to a sound range from the threshold of hearing (0) to a maximum of about 130. This is expressed as such in decibels (dB).
In effect we perceive loudness as doubled when energy intensity is increased 10 times, 60 dB thus seems twice as loud as 50 dB, and 70 dB seems four times as loud as 50 dB. The smallest change most people can detect is about 1 dB.
A minor complication is that the human ear is far more responsive to frequencies from 2000-3000 Hz than others. An unamplified operatic soprano can produce max volume in that area, whereas the orchestra is mainly playing around 500 Hz where the ear is far less sensitive. She can thus be clearly heard over the top of a full orchestra at its maximum intensity (e.g. Violetta’s aria in Verdi’s La Traviata).
To allow for this non-linear response the dB scale is weighted to refl ect what humans hear most of the time (it is called dBA.) There is some variation in sound comfort levels. The frequency response also narrows with age (particularly mine!).
Sound intensity is inversely proportional to the square of the distance from its source. In Australia, unless otherwise stated, sound measurement is undertaken at seven metres from the sound’s source. Shonky vendors may quote it as measured from further away or expressed in dB not dBA.
SOUND MASKING
Because halving sound energy makes only a minor perceived difference, if the noise from the exhaust has much the same level as the clatter typical of air-cooled engines, even reducing the exhaust noise to zero makes little or no difference to the sound level. It sounds (and is) different but is perceived as loud as before: it may even be more annoying. With diesel heaters, it is necessary to silence the air intake as well as the exhaust – as each masks the other.
REDUCING NOISE GENERATOR NOISE LEVEL
The only way to quieten a $99 chain store special is to take it to the tip. Its sound energy needs to be reduced by an impossible 100-1000 times to make any worthwhile difference. Even were that done, the noise level would still be unacceptable. They are also hideously polluting.
One needs to start with a generator that is quiet already. Inverter generators are by far the quietest – partly because they are better made, but also because they are slower running and with engine speed varying with load.
Most 1000-2000 watt inverter generators emit 52-60 dBA at seven metres. They are quietest on part load, but as the bigger they are the noisier, the optimum is one that works at about 60% load most of the time. At 54 dBA at seven metres (at 75% of maximum output) by far the currently quietest (for its size) is Dometic’s 2600 watt (2900 watt peak) TEC 29 – about the same as two people conversing a metre apart.
Conventional generators are much noisier. To generate ac at 50 Hz they must run at a constant 3600 rpm. Most of 1000-2000 watts generate 65 to 75 dBA. Some two-stroke imports exceed 80 dBA.
While inverter-generator noise levels may seem quiet in urban areas, this is mainly due to the masking effect. Urban background noise tends to be much the same (or greater) and thus masks it. As the typical background noise in a quite campsite is typically 35-40 dBA, the generator totally masks that background and may be the only noise heard. A night bird’s call may exceed it, but is usually perceived as sound – not noise.
| Typical Sound Scale | |
| Threshold of hearing | 0 dBA |
| Slightly rustling leaves | 10 dBA |
| Whispering | 20 dBA |
| Quiet room | 35-40 dBA |
| Urban background | 50-57 dBA |
| Inverter generators | 52-60 dBA |
| Conventional generators | 70-75 dBA |
| Vacuum cleaners | 80-85 dBA |
| Front row rock concert | 110 dBA |
| Threshold of pain | 130 dBA |
ABSORBING OR ENCLOSING
It is rarely possible to quieten the generator as such, but one can usually reduce its transmitted noise. It is usually not so much the air intake or exhaust that is offensive, but mechanical clatter – particularly of those that are air cooled.
The generator may be contained within a soundproof enclosure that blocks the noise, and/or by converting the noise energy into heat.
If built into a motorhome or coach its encasing can usually be enhanced by using rubber generator mountings and lining the enclosure as explained below. This is likely to mask exhaust and air intake noise – but that’s fi xable via car exhaust and intake silencers.
It is vital that the door closes fully. A lot of high-frequency noise will pass through a tiny gap – as opening a car window by only a millimetre will show.
Converting noise energy into heat is done by absorbing and dissipating the energy via a dense but limp material. The most effective is a curtain of thin sheet lead, or that lead adhered to a soft foam backing. For portable generators it is feasible to make up make a smallish tent of such material. Even having a sheet of that material shielding the generator from neighbouring RVs assists. (What does not is using a 50 metre lead and placing it nearer the next RV.)
Whitworth Marine stocks such (self adhesive) sound absorbing materials. The catalogue 80204N is the most expensive - but by far the more effective. It costs about $120 for an 800 by 1370 by 25 mm sheet. (Whitworth’s founder, Craig, is the brother of the now sadly deceased CMCA founder, Don Whitworth.)
The often suggested egg cartons are handy for reducing reverberation (echo) within a room but are useless for blocking sound.
As applications and needs vary it is not feasible to suggest specifics, but once the basic issues are understood this article will hopefully assist.
This and a vast number of RV and solar related topics are covered in Collyn’s books. Most are available directly (and post free) from CMCA NHQ. They are also available (but plus postage) from caravanandmotorhomebooks.com.
Category: Technology
Written: Thu 01 Sept 2016
Printed: September, 2016
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