Date: 17 Dec 97 03:41:29 From: email@example.com (Darren Rhodes) Organization: Daz Technology References: 1 2 3 4 5
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On 10 Dec 97 04:05:03 , Chris Dickson <firstname.lastname@example.org> wrote: >I later talked to the car park attendant, and he told me that he'd >noticed that the noise was much louder on days with dense low-level >cloud cover (as was the case at the time). He said that the car alarms >didn't generally go off on clear days, but almost always did on cloudy >ones. > >Can anyone suggest an explanation for this? I've speculated that it may >have somthing to do with the air density/temperature/moisture content. >Does anyone know how/if these factors affect noise propagation, and if >they're something that have to be, or ought to be considered in aircraft >noise monitoring/prediction? Temperature, humidty and windspeed/direction have a big effect on noise levels. The standard used for certification noise measurements is 15 degC and 70 percent humidity with windspeed less than 10 knots. This is often not representative of a given airfield including heathrow and thus higher noise levels are quite likely in practice. As regards the clear day vs cloudy day I suspect the biggest factors here are humidty and temperature. These affect sound sbsorption rates and since we're talking of large propagation distances a small change in absoprtion rate can be quite a few decibels in noise levels on the ground. I am involved with measuring noise levels around Heathrow airport and we do see wide variations in measured noise levels from day to day that are not attributable to how the aircraft is flown but mostly down to the weather. Recorded Data that is not close to the standard day conditions is normally filtered out prior to further analysis.