08/09/2010

The sound of silence measured in decibels

Scientifically, noise is measured in decibels and can be averaged out over a period. Human perception is different: we are sensitive to short but loud noises, in particular when there is a large difference between the noise and the level of background sound.


The Sound of Silence: the title of a beautiful song of Simon and Garfunkel. In the days of the ash cloud, residents around Geneva airport, like those around other airports, rediscovered the concept for a few brief days. Perhaps the memories of these days, like (for Geneva residents of my generation) those three Sundays long ago when motor traffic was banned on three consecutive Sundays and we walked freely in the middle of the roads, will be nostalgic events that we talk about to our descendants.

I rediscovered it this last weekend high up in the mountains of the Valais. On the Sunday the loudest noise, apart from a very occasional helicopter, was probably the massed alpenhorns welcoming the winner of the Sierre-Zinal race. Overnight, the loudest perceived sound is that of the mountain stream several hundred metres away: my IPhone sound meter application shows around 50 decibels for this.

Since sound is a form of energy, it can be measured as a quantity of decibels. However, there are different types of measurements depending on which range of sounds are being measured. Animals hear high-piched sounds which humans do not, and our ears gradually hear a more narrow range of sounds as we get older (especially if we frequented loud discos in our youth!).

Aviation sound is registered in a type A modulation, so measurements are referred to in short as decibels A (dBA). As well as measuring instantaneous peaks, noise can be averaged over a period of time, to give an equivalent noise for that period, known as Leq (equivalent level). This has the obvious disadvantage that a very loud noise for a short time (a loud aircraft that passes after midnight!) averages out at a much lower value for the hour.

Because it is impossible to have zero sound (except at absolute zero temperature), sound is measured on a logarithmic scale. This means that every extra 3 decibels of sound is approximately a doubling of what we hear. In a very quiet area during the night the background  sound might drop to around 40 dBA, but will rise by an extra 10 dBA (ten times the noise) during the day.

The Swiss noise regulations for zones around airports define different levels of noise for different types of zones (residential, industrial, agricultural) and times of day (day, early night, late night), and then calculate contour lines for the various combinations by using a sophisticated noise simulation computer program. Then, if you are within a high noise contour line you very likely find restrictions on what you can do (I won't even start to talk about the difficulties to get indemnities, nor the possibility of zones being declassified by the authorities!)

In France, as in many other European countries, the Leq noise level is calculated as one single number for the whole day.The 24-hour day is considered as 12 hours of daytime (usually 6am to 6pm), four hours of evening and 8 hours of night (in contrast to the Swiss idea that the day lasts from 6am to 10pm, or even 11pm). To calculate the daily average, called Lden evening measurements are incremented by 5 dBA and night measurements by 10 dBA. It is understood that the authorities in France want to used Lden values whan calculating the contours within which the soundproofing measures, paid for by the Geneva Airport environmental funds), will be implemented.

The Geneva airport authorities do include in a monthly nuisances report (overall, night flight and noise measurements) the Lden values for each of their 15 microphones which are installed around Geneva canton, but they have no legal significance in Switzerland. Regrettably, neither these reports nor the underlying measurements are available to the general public on their Web site, leaving the impression of an absence of openness about aircraft noise.

So, if you want to rediscover the sound of silence, remember the publicity of another mountain village, Gstaad

Come up and slow down!

11:57 Posted in Noise around Geneva airport | Permalink | Comments (2) | Tags: silence, noise, geneva, airport | |  del.icio.us | | Digg! Digg |  Facebook

Comments

Good evening,

interesting story,may I add some important aspect of human sound perception?, (sorry first, my English is not perfect). One of the important aspect of a sound, in addition to its level and its duration is its tone a.k.a "frequency", the human ear is more sensitive to a certain range of frequencies, measured in KiloHertz (KHz). For example, a sound at let's say 80bd at 1000 KHz will be felt as louder than a sound of 80db at 500 KHz.A simple example to illustrate that: the TV commercials, every time your TV show stops to give place to the commercial, we all think that the sound has been pumped up, actually that's wrong, it's just that the middle-high frequencies have been boosted to obtain an aural excitement, the human ears are more sensible to mid-high frequecies, the amount of decibel stays the same. An other aspect of the sound perception is cultural: if you like planes, well you will have the feeeling that the noise isn't such a big deal, not the die hard ecologist. Other example, a military bande produces a higher level of decibel than a rock concert (limited to 92 db in Switzerland), ask an elderly which plays louder.. mmm I assume you guessed the answer. As for your concern about planes, limiting the level of decible is not the only solution, the concern would be to find a good way to reduce frequencies the most agressive to human aural perception. and lastly but I won't talk about it, the distance of the sound is important too, but that's quiet obvious, it's just important to say that the low frequencies need more distance to devolope, that's the reason we only hear low noise of far flying planes.

And, yes, I use to come and walk around Gtsaadt, and I must admit it that it's a beautiful and quiet place.

Posted by: Nico | 08/15/2010

Thanks for the comments: you could also have written them in french. There is also a very recent article (I forget where) about how different people may be genetically programmed to be more, or less, disturbed by noise while they are asleep.
The duration of the sound is also a relevant factor: an aircraft taking off may be heard for several minutes.
The choice of the A-weighted decibels is apparently standard for aircraft noise. It would indeed be nice if we could somehow change the frequencies of aircraft engines to make them less noisy to our ear. However, the current aircraft will still be flying in 10 or 15 years (or more!).
I was not actually at Gstaad. I think they had a tennis tournament there, plus a music festival, so it would have been much noisier than high up in the Val d'Anniviers)

Posted by: Mike Gerard | 08/15/2010

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