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Chapter2. Theory 13 threshold of hearing p0, Eq. 2.11 [14, 25, 33]. p2 p Lp = 10 log10 p2 = 20 log10 p (2.11) 00 In order to represent the logarithmic sound perception, the unit of measurement for the SPL is the decibel dB. It uses a logarithmic ratio of the actual SPL to an arbitrary reference value, the threshold of hearing set at 20 μPa [33]. For easier management, acoustic energy is integrated over certain frequency bands. An oc- tave corresponds to the doubling of the frequency and can be further divided into one-third octave bands (TOB). The TOBs used for this work are in accordance to EN ISO Norm 266 [18] and are listed in Table A.2. In international standardisa- tion, approximately 11 kHz have been established as the upper limit of effective noise [33], taking hearing deterioration with age into account. The sinusoidal pressure fluctuation is characterized by its frequency and ampli- tude. The wavelength λ of a sound event is equal to the quotient of the speed of sound a and frequency f, λ = a/f. The sound intensity then is defined as the product of mean square sound pressure p and particle velocity v, I = pv. When sound propagates in the atmosphere, its intensity decreases due to two ef- fects. Firstly, as sound is emitted from a source, its intensity, i.e. its sound power per unit area, diminishes as it spreads out over a larger surface, the sound inten- sity quartered for each doubling of the distance to the source. Secondly, sound propagation is subject to atmospheric attenuation. Due to internal friction in the fluid, the sound wave amplitude is reduced. The amount by which the intensity of sound diminishes is mainly dependent on the frequency of the sound, being of greater magnitude for high frequencies as well as temperature, pressure and humidity, thus making air effectively a low pass filter over long ranges [29, 38]. 2.3.2 ICAO Noise Certification As explained in Chapter 1, there is no international legislature in aviation. The annexes published by the ICAO, however, are binding for signees of the ICAO convention, who comprise of almost all states conducting aviation. The ICAO is a special UN-agency, committed to the development of standardisation processes. The developed procedures (SARPs) are laid down as annexes to the Chicago Con- vention [15, 29]. As such, Annex 16 - Environmental Protection is the internationally agreed basis for noise related certification criteria during research, developmentPDF Image | Investigation of wing installation effects on the sound field of a model jet engine
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