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SI is a principal factor in human annoyance response. Activities where speech intelligibility is critical include classroom instruc- tion, personal communication, and leisure listening endeavors such as television, radio, and the like. SI can also be a critical factor in situations requiring a high degree of intelligibility essential to safety. Factors that can influence SI include location (outdoor or indoor), transmission loss (acoustical isolation) of structure, vocal effort, vocal frequency content (such as male or female), listening skill, hearing acuity noise frequency, and noise temporal characteristics. Most of the SI research involves steady-state or constant noise masking well-defined speech signals. The majority of this research was published well before 1985 and has not been expanded significantly since then. In 1999, the World Health Organization (WHO) published noise guidelines that included discussion of SI (Guidelines for Community Noise 1999). The guidelines do not discuss aircraft noise and SI expressly, but address the problems associated with speech comprehension, which result in a large number of personal disabilities and behavioral effects. WHO conclusions are applicable to steady-state noise, but do not address inter- mittent noise such as an aircraft flyover. SI is most rigorously defined using metrics that analyze signal-to-noise relationships defined in specific frequency bands. The Articulation Index and a related methodology, the Speech Interference Level, have been used since the 1950s for this purpose. In addition, the A-weighted sound level has 14 proven to be a good predictor of speech intelligibility and has often been employed in research findings. To that end, the ANSI has published a standard in terms of A-weighted decibels for rating noise with respect to speech (ANSI S12.65-2006). FICON, in a federal review of noise issues in 1992, explained that where speech communication is an issue, cer- tain specific analyses, such as the Time Above metric or the SEL, and/or the maximum A-weighted noise level (Lmax) may be useful (Federal Interagency Committee on Noise 1992). These metrics may be estimated using the Integrated Noise Model. The EPA Levels Document published in 1974 is one of the few documents to address the effect of intermittent noise on SI. The EPA questioned whether the results of SI relative to steady-state noise would apply to sounds that have fluctuating levels. The results demonstrate that, for 95% sentence intelli- gibility, normal vocal effort and a 2-m separation between talker and listener outdoors, the maximum Leq value associ- ated with continuous noise is less than the maximum value for an environmental noise whose magnitude varies with time, such as an aircraft flyover. Therefore, when interpreting the amount of SI using the ANSI method or the guidelines of FICON or WHO, these methods will overestimate that amount of SI. Little research has been published since 1985 on intermittent noise such as aircraft noise and its influences on speech; there- fore, more research is needed. CHAPTER FIVE SPEECH INTERFERENCE AND AVIATION NOISE
