Buy quiet

SOME say privacy is the greatest luxury. Your correspondent believes silence is even more golden.

Like others, he puts up with the hubbub of daily life as a trade-off for the convenience of living in a city rather than the wilderness. In doing so, he accepts there is no escape from the noise of vehicles on the road, aircraft overhead, construction workers hammering away down the street, and gardeners everywhere using lawn-mowers, hedge-trimmers and leaf-blowers. In-doors, the dish-washer, washing-machine, vacuum-cleaner, refrigerator and air-conditioning add to the cacophony. The last thing anyone needs is yet another man-made contraption capable of emitting copious quantities of decibels. Yet, that is precisely what, unthinkingly, your correspondent has just inflicted upon himself and his good neighbours.

Regular readers may recall your correspondent wrote earlier this year about the virtues of air tools over electrical ones (see “Air superiority”, April 30th 2010). Since then, he has watched the price of air-compressors continue to fall. So much so, he could not resist buying a new one at a local tool-store sale. It offered twice the horsepower and storage capacity of his existing model, all for little over half what he paid previously. The only problem is that the new air-compressor makes a truly horrendous racket—over 100 decibels (dBA), by one measure, compared with the previous model’s 85dBA or so. He is now wondering how best to silence it—ideally, by at least 10-15dBA.

Everyone knows—or thinks they know—what a decibel is. The unit itself was devised back in the 1920s by engineers at Bell Telephone Laboratories to describe the amount of signal lost over a length of standard telephone cable. Because a decibel is ten times the logarithm of one quantity relative to a reference quantity, it can be used to measure the level of practically anything—say, the gain of an amplifier, the signal-to-noise ratio of a wireless transmission, or the level of air pressure above or below one atmosphere. The most common usage is as a measure of relative sound pressure—in short, loudness.

Ah, yes, loudness. That is not so simple a matter, either. On the emission side, it depends on the frequencies of the various components in the sound as well as their amplitudes. On the receiving end, the human ear—a non-linear device with enormous dynamic range—adds to the complication. From sounds so loud they can cause permanent loss of hearing down to the quietest the ear can detect spans a frequency range of a trillion to one. The base-10 logarithm of one trillion is 12. That equates to 120 decibels—the level, in sound terms, of a thunderclap immediately overhead, or the blast received in the front row of a rock concert.

Because it is non-linear, the ear is not equally sensitive to all frequencies. For good reason, it works best between one kilohertz and five kilohertz—the range where the sounds of predators and other hazards are most likely to be heard. A “weighting curve” has therefore to be used to make sounds measured by microphones and other acoustical instruments more representative of what people actually hear. Of the four different weighting schemes (A, B, C and D curves) developed since the 1930s, the one used most widely today is the so-called A-weighting—hence the term dBA.

In general, people find sound becomes annoying when the level in the community, averaged over 24 hours, exceeds 65dBA. Since the introduction of high-bypass jet engines, with their big, slower turning compressor fans, airports have managed to keep more of their 65dBA sound contours within their own perimeters. Likewise, since the advent of welded track and electric locomotives, railways have become less of an annoyance.

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