Comparison of the speech transmission index and the modified rhyme test in simulated cockpit ambient noise.

The military aircraft test and evaluation community has recently introduced a much quicker, less obtrusive method for evaluating speech intelligibility of a communication system. The method uses the Speech Transmission Index (STI) to predict intelligibility using 15 s of flight time. Previous research has developed a correlation between this method and subjective intelligibility scores for the Phonetically Balanced (PB) word test. This research has not yet shown an experimental relationship to the Modified Rhyme Test (MRT), one preferred method of the military. In addition, the research has not accounted for ambient noise with a spectrum that represents an in‐flight aircraft cockpit environment. In the present study, the relationship between the STI and MRT in simulated noise was investigated. Two types of noise were used. The first type was noise with a spectrum to represent in‐flight ambient cockpit noise. The second was pink noise (equal energy per octave), a commonly used representation of environmental cockpit noise. The study was two phase. Phase one measured the intelligibility of a communication channel degraded with the noise presented at different signal‐to‐noise ratios. The intelligibility test was the MRT. Phase two used the same communication channel and signal‐to‐noise ratio conditions to measure objectively the intelligibility using the STI. The findings of the study showed that the MRT scores were significantly differ ent between the different noise presentations while the MRT scores and STI yielded approximately the same values.