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+ 0 – Distance l Wavelength Sound wave. 1 cycle n Frequency Noise Sound waves are characterized by a wavelength, frequency, and amplitude. The wavelength, λ (the Greek letter lambda), is the physical distance between adjacent identical points on the wave, such as crest- to-crest. The frequency, ν (the Greek letter nu), is the number of cycles per second. Multiplying the wave- length times the frequency gives the speed of sound, c = λν. The height of the wave is called the amplitude and is the loudness of the sound. off or land. Finding ways to make airplanes fly more quietly, however, is one of the most effective ways to reduce airport noise pollution. The various sound intensities listed in the chart below show that a jet airplane can generate an extreme amount of noise. There are many parts of an aircraft that can make noise, but since the aircraft engine is one of the noisiest parts of the aircraft, designing quieter engines continues to be a priority. Intensity, W/m2 Decibel level (sound intensity level, SIL) Examples 0.000000000001 0 Threshold of hearing 0.00000000001 10 Rustling of leaves 0.0000000001 20 Quiet whisper 0.000000001 30 Whisper 0.00000001 40 Mosquito buzzing 0.0000001 50 Normal conversation 0.000001 60 Air conditioner at 6 m 0.00001 70 Vacuum cleaner 0.0001 80 Alarm clock 0.001 90 Lawn mower 0.01 100 Subway 0.1 110 Auto horn at 1 m 1 120 Threshold of pain 10 130 Machine gun 100 140 1000 150 Nearby jet airplane Sound is a major concern with aircraft propulsion. Aircraft engines, whether internal combustion piston engines or jet turbine engines, generate power from expanding gases. Propellers and turbine fans move large volumes of air backwards in order to push air- craft forward. All these activities push on the surround- ing air causing compression and rarefaction of the air molecules. This produces pressure waves, which we perceive as sound if they are strong enough and of the right frequencies. Generally, an aircraft achieves its best perfor- mance at cruise and spends most of its operating life at high altitude, away from populations, so the noise it produces at this time is not a great concern. It is when an aircraft is landing, taxiing, and especially taking off at full power that noise becomes an issue. As air traffic grows, communities surrounding airports experience an increase in noise pollution. There are many ways to combat airport noise pollution including residential noise insulation programs, land use planning, modified takeoff and landing procedures, and by restricting the number of flights and the time of day flights may take Researchers in the Acoustics Branch at NASA Glenn, working with engineers at universities and in the aircraft industry, are looking for ways to reduce the noise of aircraft engines. Finding ways to reduce engine noise without sacrificing aerodynamic performance of the engine is a real challenge. You are aware that cars control exhaust noise by means of a muffler, but you may not be aware that this causes backpressure in the exhaust system and reduces the engine’s effective horsepower. This is why dragsters and racecars do not have much in the way of mufflers; they need maximum horsepower. The same is true in aircraft. Altering the geometry of an exhaust nozzle may reduce noise, but it can also reduce engine thrust. Likewise, altering the number or orientation of fan blades or stator vanes in an engine may help reduce noise, but it may also affect the structural integrity of the engine. Since an aircraft engine is so complex, it helps to study parts of the engine individually. An engine’s exhaust jets are one dominant source of noise, and the noise generated by the rotating parts of the engine—like the fan and the compressor—are another. Pushing the Envelope: A NASA Guide to Engines 63 PHYSICS AND MATH Pressure AmplitudePDF Image | NASA Guide to Engines
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