THE DECIBEL

The standard

unit used to define how much noise something makes is the decibel, dB. This unit dates back to the days of the telegraph, and is named in honor of Alexander Graham Bell. As the name also suggests, it is equal to a tenth of a bel, a unit you won’t see used anywhere these days. It was devised to quantify signal loss over the wires. Best known for its use in acoustics, it can also be applied to voltages in electronics, radar, radio field strengths, and more. Most importantly, it is not a linear scale. It’s logarithmic—a ratio of two values: the one you want to measure and a reference value, which, in the case of acoustics, is silence. A logarithmic scale may have suited its original purposes well, but makes it a confusing unit for everyday use. A difference of six decibels is equal to a doubling of air pressure. You might think that means a sound twice as loud, but because of the way the brain processes sound, that doesn’t follow. The perceived volume doubles at around 10dB difference.

Small differences in decibels equal a big difference in noise. A fan at full bore can make 30dB or more, an intrusive level in a quiet room, equivalent to somebody constantly whispering in your ear. What manufacturers consider “quiet”varies, but look for under 25dB. Properly quiet would be under 20dB, about the same as leaves gently rustling in the wind. Less than 10dB is very quiet indeed,

like somebody breathing. The human ear is more sensitive to certain frequencies, which makes a straight dB rating an imperfect measure of the obtrusiveness of a sound. It may only be as loud as rustling leaves, but it’s an annoying highpitched whine, too. It’s also worth remembering that adding dB values isn’t linear either—two fans of the same decibel level are 3dB louder in total. That’s another reason for going for two fans over a single more powerful, but louder, one.