We take the sound level in commercial aircraft to be a necessary evil associated with the flying experience. Much however can be done to quiet aircraft noise. While the day of the quiet flying boardroom now exists, application of advanced materials and sound ripple measurement techniques technology is beginning to trickle down to the design of commercial airliners. He has been chief laboratory engineer for Goodmans Loudspeaker Co.
Studio Engineer for Polygram Records U. Preface Much progress has been made in recent years in the “eternal quest” for quieter passenger-carrying-aircraft interiors. At least one “trickle down” benefit of the above to the “average” flyer is evident in the gradual emergence of quieter airliners. Brief history Much of the work leading to our current understanding of speech intelligibility and corresponding interference factors began after World War II. Beranek along with French, Steinberg and others, studied factors affecting speech intelligibility and devised a number of ways of quantifying these factors. One such quantity was named the Articulation Index. Although the original studies used a greater number of narrower bands, Beranek eventually concluded that use of the 3 octave bands, 600 -1200 Hz.
2400 -4800 Hz, was sufficient for useful analysis under most circumstances and determined that a simple arithmetic average of the sound pressure levels within the three bands yielded a meaningful number. Beranek did, however, find it important to add information from the 300 Hz. In 1967 Webster extended Beranek’s work with a slight modification to the method of determining SIL’s so that the “preferred” octave bands of 500 Hz. This method became known as the PSIL or the three-band Preferred-octave Speech Interference Level quantity.
Although similar, calculated SIL’s based on the redefined octave bands yield numbers typically about 3 dB less than with Beranek’s original bands. It is interesting to note Beranek’s suggestion of the need to add information from the 300 to 600 Hz. SIL calculations when conditions so required. We should view this as an indication of the importance placed on the lower frequency components of speech in the formative work in this field! Application It is important to realize, particularly in view of the work upon which the SIL scale was based, that an erroneous understanding of the this quantity persists throughout the aircraft cabin noise suppression industry. The SIL scale was not meant to measure perceived “noisiness”.
SIL measurements particularly when based on 1000, 2000 and 4000 Hz. When used as a measure of “noisiness”, the SIL scale breaks down in meaningfulness when the steps between the octave bands become large. This typically results in much lower numbers than the predominant octave band noise would justify! The omission of the 500 Hz.