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