BACKGROUND SPEECH VARYING IN INTELLIGIBILITY – EFFECTS ON COGNITIVE PERFORMANCE AND PERCEIVED DISTURBANCE

We present three experiments, which tested the impact of background speech varying in intelli-gibility and/or level on basic cognitive functions (verbal short-term memory, sustained attention, verbal-logical reasoning). Variations of background speech intelligibility were obtained by aurali-sation based on room- and building acoustics. In each experiment, performance was measured during low background speech (35 dB(A)) of good and poor intelligibility, respectively, and com-pared to silence as well as to highly intelligible speech not lowered in level (55 dB(A)). Besides performance data subjective disturbance ratings were collected. Our results show that highly intelligible background speech of 55 dB(A) impairs cognitive per-formance. In order to diminish its disturbance impact significantly, a sole level reduction does not suffice – an additional reduction of speech intelligibility is necessary. Subjective disturbance ratings, however, value lowering of the background speech’s level to a greater extent than re-ducing its intelligibility. Furthermore, although cognitive performance coincided during soft and poorly intelligible speech with those during silence, it was still rated as significantly disturbing. Thus, our experiments demonstrate that objective performance tests and subjective ratings complement but cannot substitute each other with regard to a comprehensive evaluation of acoustic office environments and their alterations