| Publication: G.J. Pan, A.J. Brammer, J. Zera and R.A. Goubran, "Performance of Feedforward Adaptive Active Noise Control in a Circumaural Hearing Protector", 1995 International Symposium on Active Control of Sound and Vibration, Newport Beach, CA, July 1995. |
| Abstract: Hearing protection devices (HPDs), constructed from muffs that seal against the skin around the external ear (circumaural HPDs) and devices that rest against the pinna (supra-aural HPDS), reduce noise less at frequencies below 500 Hz than at frequencies above. In recent years, portable analog active noise reduction (ANR) HPDs have been developed to increase the total noise reduction at low frequencies. Commercial circumaural devices, most incorporating a signal channel for speech communication (communication headsets), are now available for military applications and for general aviation pilots, and supra-aural devices for noise reduction and audio headphones. An increase in noise reduction of commonly up to 15 dB for circumaural HPDs and10 dB for supra-aural HPDs is obtained at frequencies below1 kHz. The performance of some devices is, however, compromised by self-muting of the electronically-generated sound at high noise levels, and de-tuning of the analog feedback control loop to maintain stability. Adaptive control of the acoustic field in the vicinity of the ear canal entrance can be expected to accommodate changes in the coupling of sound from the (noisy) environment to the ear. Such changes in coupling may arise from air leaks around the edges of an ear cover, or by relative motion between a supra-aural ANR device and the ear canal. In addition, an adaptive digital ANR system offers the potential for tailoring the performance of the device to different acoustic environments, for example, to minimize the potential for noise-induced hearing loss. The signal processing algorithm and device that are optimum for this application remain to be established. The purpose of this paper is to describe the performance of a digital feed-forward active noise control system based on a commercial analog circumaural communication headset and a 32-bit floating point digital signal processor (DSP). Our initial work using this device has been described elsewhere. The critical features of this application of feed-forward active noise control are the presence of the ear cup between the reference and error microphones, and the short distance between these microphones. The latter leads to a correspondingly short time delay in which the controller must function to maintain casuality. |