by Dave Jackson

Created on: February 10, 2009   Last Updated: February 16, 2009

Do you remember how quiet the skies were in the days following the 9/11 attacks? The level of noise we endure was impossible to ignore once airplane flights resumed. Few places on the planet are completely free of the noise of airplanes, day or night.

Even a jet flying at 30,000 feet will interrupt a peacefully quiet day. How can this be? Our atmosphere unfortunately transmits sound very well. Even high altitude air is dense enough to create noise when an object passes through it at speed. The structure of a small plane moving slowly will create very little noise. Noise will increase exponentially as speed and size increases until it seems the sky itself is being torn apart.

Humidity, weather and altitude greatly effect how noise reaches us. Moist, dense air transmits sound waves much better than dry or thin air. This effect causes the speed of sound to increase with altitude. Cloud cover will act as a blanket to hold noise down closer to us.

If an airplane exceeds the speed of sound a very loud sonic boom will result. The resulting shock wave produces a constant noise that thunders across the landscape behind the aircraft. Thankfully, restrictions on supersonic flights are so rigid that most of us have never heard a sonic boom.

Passengers inside aircraft must also endure "cabin" noise. The sounds of engines, propellers, and mechanical devices add to the constant din of air rushing past the wings and fuselage. Manufacturers reduce this as much as possible with sound deadening materials.

Many people actually enjoy the sounds of an airport. The scream of jet engines spooling up to thousands of rpm's and the dragster like roar of radial piston engines is music to my ears. To some, this "music" is a maddening problem that only gets worse.

The noise of a piston engine is easy to understand. Jet engine noise can be compared to a vacuum cleaner, with the added effect of a roaring fire in the combustion chamber. The speed of the turbines and super heated gas exiting are the main sources of noise.

Much of the noise from a piston engine is really from the propeller as it slaps through the air at high speed. This propeller noise can fool us into thinking a turboprop is a piston engine. The pilot can generate more noise than required by running the propeller too fast or with too much pitch. Propeller tips can be damaged by reaching supersonic speed, thankfully, as this is extremely loud.

Jet engine manufacturers constantly strive to reduce noise. Efforts to slow the turbines without

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