The distance that traffic noise can travel is dependent on the frequency of the noise, for example, the low frequency rumbling of heavy goods vehicles compared with the high frequency squeaking of brakes.
It's an extraordinary low-frequency rumble, not so much piercing as vibrating.
Numerous studies have found that urban birds sing louder and at higher pitches than rural ones, perhaps to overcome the low-frequency rumble of traffic.
In the latest work, Dr. Okal examined the sound recordings from Wake Island, which captured a low-frequency rumble (measured at seven hertz) that lasted 45 seconds.
So en route from the United States to Asia, for example, he recommended trying to eliminate the low-frequency rumble in the aircraft.
The PNC curves, developed in 1971, add limits on low-frequency rumble and high-frequency hiss; hence, they are preferred over the older NC standard.
Black-chinned hummingbirds, for instance, communicate in higher frequencies that may still be audible against the low-frequency rumble of the compressor.
Continuous low-frequency rumbling noises above eighty-five decibelsuchuch as those produced by helicopters and armored vehicles, are attenuated by more than thirty decibels.
In close proximity they employ a range of vocalizations, from low-frequency rumbles to higher-pitched screams and trumpets, along with a variety of visual signals, from the waving of their trunks to subtle anglings of the head, body, feet and tail.
In a telephone interview Friday, Mr. Vallance said that one of his team's solutions was to calibrate the engine to run more frequently in the 1,000 to 1,500 r.p.m. range, where the engine wouldn't burn much fuel but where potentially annoying, low-frequency rumbles or booms tend to reside.
