Improving the Effectiveness of Communication Headsets with Active Noise Reduction: Influence of Control Structure
Abstract:
For communication headsets equipped with active noise reduction ANR, the performance of the control system may influence the communication signal reaching the ear. Conversely, the communication signal may perturb the operation of the ANR system. The interaction between the communication and control signals depends primarily on the control structure, and on the bandwidths and frequency responses of the signal channels. The effects are described for two circumaural communication headsets with similar passive, and active, noise reductions, one with an analog feedback control system and the other with an adaptive digital feed-forward control system. Measurements were conducted in a diffuse sound field with the headsets mounted on a head and torso simulator. The frequency response of sound reproduced by the communication channel was measured when the ANR system was not operating, and when the control system was operating, with swept pure-tones and broadband noise. Speech intelligibility was estimated for environmental noise shaped to represent the spectrum of speech, the noise within a tank, or the noise within an aircraft cockpit, by the Speech Transmission Index STI. The STI and fidelity of sound reproduced by the communication channel of the device with a feed-forward control structure tended to exceed that of the more common feedback control structure. This appeared to be a consequence of the compromised frequency response of the earphone and drive electronics employed in the feedback control system to maintain stability of the feedback loop, as well as the presence of communication sounds sensed by the control microphone that were fed back into the controller. The lack of corruption of the communication signal by the feed-forward control system, together with the possibility of using electro-acoustic components with flat frequency responses, suggests that this control structure may be more consistent with the audio fidelity requirements.