Representative frequency approach to the effect of bandpass filters on evaluation of sound attenuation by the atmosphere

Simplified procedure for computing the absorption of sound by the atmosphere

“…A base-10 designed third-order Butterworth 7 filter shape was assumed. The same assumption was made by Joppa et al 10 , and appears to be reasonable, since the traditional third-order Butterworth algorithms are used in common filter designs by many analyzer manufacturers. The Volpe Method predicts band level attenuation (␦ B ͑f i ͒ in decibels) by atmospheric absorption using two equations as follows: For mid-band attenuation levels, ␦ t ͑f m,i ͒ Ͻ 150 dB…”

“…4 depicts an approximation, at the source, of a true one-third octave-band spectrum shape for typical commercial jet aircraft at high engine power settings. This test spectrum, used by Joppa et al 10 in the development of the Approximate Method, was also used herein. Six conditions were tested as follows:…”

“…The points were selected as representative of the attenuation characteristics within the window after comparing the band level versus mid-band level attenuation for each of the 22 T/H points. The selected points were at the extremes and at the center of the FAR 36 accepted T/H window and include the FAR 36 reference T/H ͑25°C/70%RH͒ and the ͑6°C/35%RH͒ T/H point used by Joppa et al 10 in the development of the Approximate Method. The eleven selected T/H points are:…”