It would appear at first that compressors or fans should not radiate sound due to blade motion unless the blade tip speed is supersonic, but even low-speed turbomachines do in fact produce a great deal of noise at the blade passing frequencies.
4 Noise Measurement and Rules
Human response sets the limits on aircraft engine noise. Although the logarithic relationship represented by the scale of decibels is a first approximation to human perception of noise levels, it is not nearly quantitative enough for either systems optimization or regulation. Much effort has gone into the development of quantitative indices of noise.
4.1 Noise Effectiveness Forecast (NEF)
It is not the noise output of an aircraft per se that raises objections from the neighborhood of a major airport, but the total noise impact of the airport’s operations, which depends on take-off patterns, frequencies of operation at different times of the day, population densities, and a host of less obvious things. There have been proposals to limit the total noise impact of airports, and in effect legal actions have done so for the most heavily used ones.
One widely accepted measure of noise impact is the Noise Effectiveness Forecast (NEF), which is arrived at as follows for any location near an airport:
For each event, compute the Effective Perceived Noise Level (EPNL) by the methods of ICAO Annex 16, as described below.
For events occurring between 10 PM and 7 AM, add 10 to the EPNdB.
Then NEF = , where the sum is taken over all events in a 24-hour period. A little ciphering will show that this last calculation is equivalent to adding the products of sound intensity times time for all events, then taking the dB equivalent of this. The subtractor 82 is arbitrary.
4.2 Effective Perceived Noise Level (EPNL)
The perceived noisiness of an aircraft flyover depends on the frequency content, relative to the ear’s response, and on the duration. The perceived noisiness is measured in NOYs (unit of perceived noisiness) and is plotted as a function of sound pressure level and frequency for random noise in Fig. 4.1.
Figure 4.1 Perceived noisiness as a function of frequency and sound pressure level
Pure tones (frequencies with pressure levels much higher than that of the neighboring random noise in the sound spectrum) are judged to be more annoying than an equal sound pressure in random noise, so a “tone correction” is added to their perceived noise level. A “duration correction” represents the idea that the total noise impact depends on the integral of sound intensity over time for a given event.
The 24 one-third octave bands of sound pressure level (SPL) are converted to perceived noisiness by means of a noy table.
Figure 4.2 Perceived noise level as a function of NOYs
Conceptually, the calculation of EPNL involves the following steps.
Determine the NOY level for each band and sum them by the relation
where k denotes an interval in time, i denotes the several frequency bande, and n(k) is the NOY level of the noisiest band. This reflects the “masking” of lesser bands by the noisiest.
The total PNL is then PNL(k) = 40 + 33.3 log10N(k).
Apply a tone correction c(k) by identifying the pure tones and adding to PNL an amount ranging from 0 to 6.6 dB, depending on the frequency of the tone and its amplitude relative to neighboring bands.
Apply a duration correction according to EPNL = PNLTM + D, where PNLTM is the maximum PNL for any of the time intervals. Here
where Dt = 0.5 sec, T = 10 sec, and d is the time over which PNLT exceeds PNLTM – 10 dB. This amounts to integrating the sound pressure level over the time during which it exceeds its peak value minus 10 dB, then converting the result to decibels.
All turbofan-powered transport aircraft must comply at certification with EPNL limits for measuring points which are spoken about in the next chapter.
5 Noise Certification
The increasing volume of air traffic resulted in unacceptable noise exposures near major urban airfields in the late 1960s, leading to a great public pressure for noise control. This pressure, and advancing technology, led to ICAO Annex 16, AP-36, Joint Aviation Regulation Part 36 (JAR-36) and Federal Aviation Rule Part 36 (FAR-36), which set maximum take-off, landing and “sideline” noise levels for certification of new turbofan-powered aircraft. It is through the need to satisfy this rule that the noise issue influences the design and operation of aircraft engines. A little more general background of the noise problem may be helpful in establishing the context of engine noise control.
Реферат опубликован: 28/08/2007