Speech intelligibility in tracked vehicles and pink noise under active noise reduction and passive attenuation communications headsets

Abstract

Speech intelligibility tests using three headset systems, two passive and one with active noise reduction technology, were conducted. Nine listeners, six males and three females, responded to 180 50-word Modified Rhyme Word Lists presented under two noise conditions, two conditions of bilateral phase reversal of the speech signal, and the three headset systems. These three independent variables were studied in a full factorial within-subjects design. The noise conditions were pink noise and recordings from the troop carrying compartment of a U.S. Army M-2 Bradley Fighting Vehicle (tank noise having substantial low-frequency energy). Phase reversal of the speech signal was also tested for its effect on speech intelligibility. The three headset systems were the David Clark Noise Attenuating Aviation Headset, and the Bose Aviation Headset used in both its passive and active mode. Active noise reduction technology employs the physical principle of constructive and destructive interference by creating signals which are identical to the noise under the earmuff but out of phase by 180 degrees. This technology is most effective at frequencies below 1000 Hz. Results showed that the Bose headset in its active mode required a significantly higher speech-to-noise ratio in both noise environments than the two passive headset systems to achieve the 70% level of speech intelligibility, the dependent variable in this study. The mean speech level under the Bose headset in its active mode was 2.8 dB higher in tank noise and 3.5 dB higher in pink noise than under the David Clark headset. The Bose headset in the active mode, however, provided a greater degree of broadband attenuation especially in the lower frequency range, e.g. < 630 Hz.

Phase reversal proved to be of no benefit to performance in either noise environment. The pink noise proved to be the harsher environment for speech intelligibility than did the tank noise, primarily due to the increased levels in the speech bandwidth. Articulation Index scores for the three headset systems evidenced that the differences in performance among the three headset systems were in part the result of better earphone response characteristics in the principal speech bandwidth (600 - 4000 Hz) in conjunction with the strong attenuation performance of the David Clark headset in that same range.

It is suggested that, in the absence of better earphone response characteristics and a broader bandwidth of active noise reduction performance, speech intelligibility is no better under active noise reduction headsets than under quality passive headsets. Further, the levels of speech intelligibility attained by active noise reduction headsets are at a cost of increased signal strength and higher purchase price.