Question # 6
See also: Physics for the Coffee Table (PFTCT)
A major city (population 5 million) is planning to build a new airport at a new location inside the city. Two sites are proposed: one beside the ocean and one twenty kilometers inland from the ocean.
Consider the noise levels originating from aircraft taking off from both airports. Ignore the noise from airborne aircraft.
Which airport provides the quietest noise footprint for any single resident who lives an equal distance from either airport? Why?
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Inland airports have larger noise footprints than coastal airports.
This phenomena occurs due to the effects of temperature inversions that develop more intensely inland than they develop along the coast. The warm oceans reduce the incidence of Radiative Temperature Inversions.
This is one of the many reasons why coastal airports are quieter than inland airports.
Food for thought.
Incorrect answer . Although it is true that land and sea breezes affect the sound footprints, these two air flows generally alternate every twelve hours and so the opposing effects cancel each other out over the period of a day.
See also: Temperature Inversions and thermoclines
Atmospheric air temperature normally decreases with height in the lower atmosphere.
A Temperature Inversion exists when the air temperature increases with height.
Radiative Temperature Inversions (or thermoclines) form on clear nights when the radiation from the Earth’s warm surface travels out to space rather than being reflected back to the surface by clouds. When this terrestrial heat energy is lost, the surface cools faster than the insulating blanket of air that lies above that surface. In the extreme cases of cooling, thin layers (a few metres high) of Radiative Fog form when the surface temperature of that air lowers to its dew point.
The speed of sound is faster in warm air than in cool air. Similar to the effect of light being refracted (bent) as it passes through a lens, sound is refracted (bent) as it passes though air masses of changing temperatures.
Surface sound is refracted (bent) skywards on a normal day in conditions where the temperature decreases with altitude. Conversely, on a clear night when a Radiative Temperature Inversions exists, this temperature “lens effect” refracts surface noise back towards the ground (colder air). So on a clear night, the sound of an aircraft taking off is guided down towards and tunneled along the ground. This effect may be so pronounced that an observer above the airport might hear nothing!
Oceans reduce the incidence and severity of Radiative Temperature Inversions. On a clear night, water cools at about half the rate as land. So Radiative Temperature Inversions will form faster and more intensely over land than over water. So the noise footprint from coastal airports is less that the noise footprint from inland airports.
There are many other practical examples of thermoclines:
Submariners survive by relying on changes in water density to refract and tunnel their submarine’s sound, keeping the sound in deep layers below the detection range of surface craft. For example, the “SOFAR” tunneling channel is normally found at about 600 metres depth where the slowest local speed of sound in water is located. Additional tunnelling channels are also created at other boundaries where the water density changes (a function of temperature, pressure, salinity …). In the case of flight data recorders, the channeling layers refract the “pinging” sounds rising from the deep ocean floor, bending them to align with what is best described as an effective sound equivalent of an optical “fibre cable” or transmission “wave guide”. The result is that the rising “pinger” sound flattens out and radiates outwards horizontally (akin to the top of a thunderstorm that rises to the tropopause) rather than continue upwards to reach ocean surface. This theory explains why sea microphones must be lowered down below the depth of the channels to hear the sound from flight data recorders and submarines.
- Views of a mirage (of a lake of water) in the desert is really just visions of light from the blue sky light that has been splayed (refracted) horizontally into your eyes.
On a clear night, a thermocline forms when the Earth cools, the lens effect funnels sound to you from cars, trains, and aircraft located tens of kilometers away. Everyone has heard the unmistakable and confusing sounds from trains that are known to be normally too distant to hear. During my childhood in Melbourne, I clearly remember hearing the sound of aircraft taking off from Tullamarine Airport that was located over twenty kilometers away.