,17(//,*,%,/,7<,1$&7,9(&20081,&$7,21+($'6(76 5ROHRI(UURU3DWKLQ$FWLYH1RLVH5HGXFWLRQDQG6SHHFK5HSURGXFWLRQ A.J. Brammer, 1,2 R.B. Crabtree, 3 D.R. Peterson, 2 M.G. Cherniack 2 1 National Research Council, Canada; 2 University of Connecticut Health Center, U.S.A.; 3 DRDC-Toronto, Canada ,QWURGXFWLRQ A primary expectation of communication headsets is to maintain speech intelligibility under all operational conditions, and especially in circumstances in which a loss of intelligibility may have serious consequences, e.g., military operations, air-traffic control. This requirement may be difficult to maintain in noisy environments and for persons with hearing loss, and also when the communication system is operated at sound levels sufficient to induce temporary threshold shift from the speech or spurious electronic signals. Effective application of active control to maintain or improve speech intelligibility in communication headsets requires an understanding of the limitations imposed by physical acoustics, control theory, signal processing, and electronics, as well as the physiological and psycho-acoustic mechanisms influencing speech perception. Almost all of the current generation of commercial communication headsets equipped with active control to reduce environmental noise employ a miniature microphone and loudspeaker mounted in a circumaural earmuff, with the electro-acoustic components connected to a feedback control system. The device is usually designed to provide active noise reduction (ANR) at frequencies below 1 kHz, as the passive attenuation of the circumaural earmuff provides sufficient noise reduction at higher frequencies. The interaction between the performance of the control system and speech communication in ANR headsets has been infrequently examined.(Gower et al., 1994; Nixon et al., 1992; Steeneken et al., 1997; Brammer et al., 1998) The purpose of this paper is to explore this relationship with particular reference to the control structure and, more specifically, the error path. While the role of most factors in the performance of a communication headset with ANR has been extensively discussed, the role of the error path has received comparatively little attention. &RQWURO6WUXFWXUHDQG(UURU3DWKSimplified block diagrams for one circumaural earmuff of an active headset containing the essential elements of a single-input, single-output, feedback, or feedforward, control system are shown in Figs. 1A and 1B, respectively. A fixed-filter feedback control structure is shown, as it is commonly employed commercially, and an adaptive-filter feedforward control structure that has been applied to a communication headset. The latter has been described in detail elsewhere.(Brammer et al., 1997) The complete headset consists of two such earmuffs with cushions to provide an air seal between the earmuff and the head, connected by a sprung headband. Each ear cup contains an independent ANR system. The block diagrams show the signal paths (continuous lines for the control system, and dashed lines for the communication channel) with directions (arrows), and signal summation and subtraction (S + , and S - ). In Fig. 1A, the control filter processes the input signal in a prescribed manner intended to reduce environmental noise at the location of the microphone. An integral part of the process of sound cancellation is the transformation of the electrical signal to sound by the loudspeaker, the propagation of sound from the loudspeaker, S, to the microphone, E, and the transformation