The modest low-frequency attenuation of the conventional passive hearing protector can be improved electronically by active noise cancellation techniques. This paper presents the theory and some of the actual limitations of an active noise cancellation hearing protector. Three prototypes with similar types of controller, but with different mechanical construction were made and their performance was measured. The electronics of the system were implemented using analog electronics and feedback construction. The measurement results were compared with the results of an equivalent passive hearing protector with no internal electronics and with an intact earcup. The results show that the integration of the controller inside the earcups degrades the passive attenuation of the hearing protector at frequencies below 200 Hz. With proper design, an active noise cancellation hearing protector can still have 15 dB more noise attenuation at 100 - 200 Hz range than an equivalent passive hearing protector.
When designing an active noise cancellation hearing protector, it is relatively easy to implement a laboratory prototype with high performance. However, when the device is taken out of the laboratory into real extremely noisy situations, it must be capable of producing very high sound pressure levels. In this paper, the factors, which limit the low frequency performance and dynamic range are analysed. The results show that the voltage swing of the loudspeaker amplifier and the force factor of the loudspeaker are two important limiting factors in extremely loud situations. A prototype of an active noise cancellation hearing protector was implemented and its performance was measured in an authentic noisy situation. The developed device attenuated 125 Hz tonal noise by 18 dB. The active noise cancellation system attenuated noise even at a sound pressure level of 118 dB SPL. This can be considered sufficient for most situations.
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