Instrumentation makers Bruel and Kjaer are working with many airports to measure and display sound levels in the neighborhoods they adjoin. B&K’s Webtrak tool for LAX is available at http://ems02.bksv.com/webtrak/lax4
See flights coming in and out of the greater Los Angeles area in real time. See sound levels in Playa del Rey, Hawthorne, El Segundo, and Inglewood. There’s also a replay feature. Use Historical Mode to pick a day and time and see what happened; speed it up if you like. If you’d like to complain about a particular flight, select the icon of the suspected flight, get its number, and click a link to file a report.
The Beach Cities are usually quiet despite LAX but there are many nights where the engine roar is powerful despite distance and intervening structures. So, what can cause this? Thermal inversion layers in the atmosphere are known to refract sound waves due to the index of refraction difference at the boundary. Acoustical engineer Mike O’Connor offers a nice summary of the physics, analogous to optical refraction, at http://www.mocpa.com/inversion.html
“It is not only noise from vehicular traffic on distant segments of roadways that is boosted in strength by temperature inversions. Emissions from distant trains and commercial aircraft (during takeoffs) are also amplified. The reader who has never before taken note of this phenomenon might now try to see if it’s ‘real’. Just listen to the background noises from distant trains, distant aircraft (just during takeoff runs) and vehicles on distant roadway segments, preferably after dark or shortly before or after dawn, and note the atmospheric conditions. Disregard windy conditions, but take note of the loudness of such noises under all other atmospheric conditions. It should become apparent that the noise levels from distant sources are clearly higher on some days than on others, even though the observations were made at approximately the same time each day. With some effort it should also be apparent that the calm and clear conditions that are said here to give rise to temperature inversions are in effect when the distant sources seem loudest.”
Henry Robinson offers a more detailed treatment at http://www.lochlyn.org/atmophys/sound/sound.pdf with an interesting observation about amphitheatre design.
“On a day when convection near the ground causes the temperature to decrease rapidly with height, sound heard on the ground from a departing aircraft will be muffled because the sound rays, and energy, will be forcing the sound upward. Indeed, balloonists often can hear conversations of people on the ground but be unable to make themselves heard by the same people on the ground. The Greeks constructed their theaters, as in Figure 3, so everyone could see, but, in doing so, they also used the natural curvature of sound to enable everyone to hear as well in afternoon plays.
Takeoff during well mixed conditions is relatively quiet at the ground but takeoff through an inversion concentrates the sound under the aircraft. Observe sound abatement procedures while taking off in an inversion situation.”
So, is that the mechanism at work on noisy Redondo Beach nights? This is worth some observation and checking against weather data.