Active noise reduction : (over-the-ear) headset and (in-the-ear) earphone

• Main Author: Zhou, Jia.
• Other Authors: Chang Joseph Sylvester
• Format: Final Year Project
• Language: English
• Published: 2009
• Subjects: DRNTU::Engineering
• Online Access: http://hdl.handle.net/10356/17927
• Institution: Nanyang Technological University

Noise is an annoying problem in most environments and in some cases, it poses a hearing health hazard when the noise exposed to a person exceeds a given duration and intensity, or the noise dosage. In many cases, earmuffs headsets are used for hearing protection. The noise protection offered by earmuffs is typically based on having a physical barrier to attenuate/block the noise from the external environment to the ear canal of the user, which is called passive noise reduction. Passive noise reduction is very effective to attenuate noise at high frequency, but less effective at low frequencies. In view of the limited passive attenuation, many current state-of-the-art earmuffs and other hearing protectors employ both passive and active noise reduction approaches. At this juncture, ANR is applicable only to low frequencies (<1 kHz) because it is very difficult to obtain a 180¡ã phase reversed waveforms at high frequencies. It becomes immediately apparent that a combination of active and passive attenuations would be very effective and advantageous. The highest cost components in ANR headsets are the transducers, the microphone and loudspeaker, which are required to be well matched both in phase and magnitude frequency responses. Interestingly, the basic algorithm/design approach of and ANR headset is negative feedback design which requires well matched transducers, resulting high cost. To reduce the need for well matched transducers, thereby reducing the cost of these transducers, we have investigated the possibility of combining feedforward and feedback ANR designs for earmuff/headset (based on poorly matched microphones and loudspeaker). The investigation shows it is possible to obtain high active noise cancellation wherein the active noise cancellation equals or exceeds that of commercial negative feedback designs based on well matched microphone and loudspeaker. Final Year Project Abstract vi Pertaining to the objective, we have constructed and measured a feedforward-cum-feedback ANR earmuff/headset based on poorly matched (hence low cost) microphones and loudspeaker. The designed headset shows a comparable ANR result compared to feedback-only commercial designs based on well-matched (hence high cost) microphones and loudspeaker, thereby verifying the proposed design. Some users of ANR headsets complain that the headsets are heavy and uncomfortable when used over long durations. In light of these complaints, some manufacturers have now introduced earplug-type ANRs where the miniature loudspeaker and microphone are embodied in a ear plug housing (compared to these in a headphone/earmuff). Our investigation has shown that these earplug ANRs are effectively based on feedforward designs and as in the ear muff-type ANRs, there is a need for well matched transducers ¨C hence their associated high cost. Further, our literature shows that there is no established feedforward ANR design methodologies ¨C the literature for negative feedback ANR show that negative feedback designs is a well mature art. In view of the no established feedforward ANR design methodologies, a generalized feedforward ANR design methodology is proposed. The proposed design methodology provides the ANR designers a systematic (rather than trial-and-error) way to design the feedforward ANR. A feedforward ANR design for a earplug-type ANR (based on poorly matched microphone and loudspeaker) is investigated and propose. The investigation shows that it is possible to obtain a cancellation equals or exceeds that of a commercial feedforward design based on well matched microphone and loudspeaker. A feedforward earplug ANR is constructed and measured. The designed earplug ANR shows comparable noise cancellation to a commercial (with well matched microphone and loudspeaker) design, thereby verifying the proposed design. Final Year Project Abstract vii In conclusion, this FYP has proposed the design of earmuff and earplug ANRs with reduced transducer matching requirements, thereby reducing the cost of manufacturing these ANRs, and also proposed a generalized feedforward design methodology for ANRs.