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. Car engines, too, have been hushed to well below their annoyance levels. Electric vehicles have become almost too quiet. There is talk of having to equip them with automatic beepers, or even simulated engine sounds, in order to warn pedestrians of their approach. While sound barriers along busy highways have helped make life more tolerable for local residents, research still needs to be done on making road surfaces quieter. The bulk of the traffic sound today comes from the screech and whine of rubber on tarmac or concrete. Transport industries aside, machinery makers in general have been slow to make their products quieter. Cost is the main reason. It takes a lot of engineering effort and extra materials to deaden intrusive sounds at source. To save on materials, for instance, consumer goods like washing-machines tend to have enclosures that are light and stiff—a perfect drum-like combination for radiating sound. A quieter model might have its motor installed in isolated mountings, the pump on a completely separate structure, the tub spinning in automobile-grade bearings, and the enclosure made of layers of material each with a different acoustical impedance. Naturally, the finished product would be more expensive. But, then, an increasing number of consumers around the world are willing to pay for such improvements in the quality of life. Making products quieter is a tougher sell in industry. Most countries long ago set 85dBA as the maximum level of sound permitted in the workplace for periods up to eight hours at a time. America, along with India, allows 90dBA, while the Netherlands permits no more than 80dBA. Above those set limits, employers have to adopt “engineering controls” to deaden the emitted sound. That can involve altering the design of the equipment, adding noise-cancelling devices, cladding it in sound-absorbing materials, or even housing it in a special enclosure. Back in 1983, the Occupational Safety and Health Administration (OSHA) relaxed the rules for employers in America still further, allowing them to increase the noise level of unsilenced machines in the workplace from 90dBA to 100dBA, provided operators were issued with ear muffs. Because of the logarithmic nature of the measurement, a 10dBA increase is equivalent to a doubling of the level of loudness allowed without resorting to engineering controls. According to a recent study (“Engineering a Quieter America”) by the National Academy of Engineering, OSHA’s weak enforcement policy “signaled the death knell for the engineering control of noise in all but the most progressive and innovative companies.” Ever since, equipment makers in America have had little incentive to make their products quieter. Not so within the European Union, where more demanding standards for quality of life have forced manufacturers to engineer quietness into many of their products. The National Academy of Engineering worries that this is putting American manufacturers at a competitive disadvantage, especially now that “Buy Quiet” is becoming a trend in wealthier parts of the world. As for your correspondent’s anti-social air-compressor, there is much to be said for equipping it with an active noise suppressor, using a loudspeaker driven by an electric signal that has the same magnitude as, but the opposite phase to, the compressor’s acoustical signature. Active noise control is particularly effective at deadening sounds below 200 hertz—precisely where large reciprocating compressors produce some of their most offensive hammering noises. But that would involve a level of tinkering your correspondent does not have time for at the moment. Instead, he plans to build a simple acoustic enclosure—in short, a perforated box lined with layers of sound-damping material. Such passive noise controllers are pretty good at reducing sounds in the middle and upper frequencies, though less so in the lower registers. If, after that, the neighbours still complain, he’ll get out the lawn-mower and sheepishly use it to mask the noise of charging up his big new compressed-air tank. http://www.economist.com/blogs/babbage/2010/10/noise_reduction
