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In an active noise reducing headphone, a signal processor applies filters and control gains of both the feed-forward and feedback active noise cancellation signal paths. The signal processor is configured to apply first feed-forward filters to the feed-forward signal path and apply first feedback fi

In an active noise reducing headphone, a signal processor applies filters and control gains of both the feed-forward and feedback active noise cancellation signal paths. The signal processor is configured to apply first feed-forward filters to the feed-forward signal path and apply first feedback filters to the feedback signal path during a first operating mode providing effective cancellation of ambient sound, and to apply second feed-forward filters to the feed-forward signal path during a second operating mode providing active hear-through of ambient sounds with ambient naturalness.

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1. An active noise reducing headphone comprising: an ear piece configured to couple to a wearer's ear to define an acoustic volume comprising the volume of air within the wearer's ear canal and a volume within the ear piece;a feed-forward microphone acoustically coupled to an external environment an

1. An active noise reducing headphone comprising: an ear piece configured to couple to a wearer's ear to define an acoustic volume comprising the volume of air within the wearer's ear canal and a volume within the ear piece;a feed-forward microphone acoustically coupled to an external environment and electrically coupled to a feed-forward active noise cancellation signal path;a feedback microphone acoustically coupled to the acoustic volume and electrically coupled to a feedback active noise cancellation signal path;an output transducer acoustically coupled to the acoustic volume via the volume within the ear piece and electrically coupled to both the feed-forward and feedback active noise cancellation signal paths; anda signal processor configured to apply filters and control gains of both the feed-forward and feedback active noise cancellation signal paths;wherein the signal processor is configured to: apply first feed-forward filters to the feed-forward signal path and apply first feedback filters to the feedback signal path during a first operating mode providing effective cancellation of ambient sound, andreplace the first feed-forward filters with second feed-forward filters in the feed-forward signal path during a second operating mode providing active hear-through of ambient sounds with ambient naturalness;and wherein the second feed-forward filters have value Kht selected to cause the formula GpfbGoea+Kht*Gnx*GffeGoea to be approximately equal to a predetermined target response, where Gpfb is the transfer function from external noise to the ear, through the headphones and with the feedback active noise cancellation signal path active, Goea is the transfer function from the external noise to the ear without the headphones, Gnx is the transfer function from the external noise to the feed-forward microphone, and Gffe is the transfer function of the filtered signal to the ear, through the output transducer, with the feedback active noise cancellation signal path active, andthe predetermined target response is the target hear-through insertion gain (Thtig), which is 0 dB within the hear-through pass band, and is the same as the insertion gain achieved by the first feed-forward filters outside the hear-through pass band. 2. The headphone of claim 1, wherein the second feed-forward filters cause the headphone to have a total system response at the wearer's ear that is smooth and piecewise linear. 3. The headphone of claim 2, wherein the second feed-forward filters cause the total system response to be smooth and piecewise linear in a region extending to frequencies above 3 kHz. 4. The headphone of claim 3, wherein the second feed-forward filters cause the total system response to be smooth and piecewise linear in a region extending to frequencies below 300 Hz. 5. The headphone of claim 1, wherein the difference in the overall noise reduction in speech noise between the first operating mode and the second operating mode is at least 12 dBA. 6. The headphone of claim 1, wherein the signal processor is further configured to apply second feedback filters different from the first feedback filters to the feedback signal path during the second operating mode. 7. The headphone of claim 1, wherein the feedback signal path and the ear piece in combination reduce ambient noise reaching the entrance to the ear canal by at least 8 dB at all frequencies between 100 Hz and 10 kHz. 8. The headphone of claim 1, wherein the feedback signal path is operative over a frequency range extending higher than 500 Hz. 9. The headphone of claim 1, wherein the feedback signal path is implemented in a digital signal processor and has a latency less than 250 μs. 10. The headphone of claim 1, wherein the second feed-forward filter defines non-minimum phase zeros in a transfer function characterizing the feed-forward signal path. 11. The headphone of claim 1, wherein the signal processor is further configured to: replace the second feed-forward filters with third feed-forward filters in the feed-forward signal path during a third operating mode providing active hear-through of ambient sounds with a different total response than is provided in the second operating mode. 12. The headphone of claim 11, further comprising a user input, and wherein the signal processor is further configured to select between the first, second, or third feed-forward filters based on the user input. 13. The headphone of claim 12, wherein the user input comprises a volume control. 14. The headphone of claim 11, wherein the signal processor is configured to select between the second and third feed-forward filters automatically. 15. The headphone of claim 14, wherein the signal processor is configured to select between the second and third feed-forward filters based on a time-average measurement of the level of the ambient noise. 16. The headphone of claim 15, wherein the signal processor is configured to make the selection between the second and third feed-forward filters upon receipt of a user input calling for activation of a hear-through mode. 17. The headphone of claim 15, wherein the signal processor is configured to make the selection between the second and third feed-forward filters periodically. 18. The headphone of claim 1, wherein the signal processor is a first signal processor and the feed-forward signal path is a first feed-forward signal path, the headphone further comprising: a second ear piece configured to couple to a wearer's second ear to define a second acoustic volume comprising the volume of air within the wearer's second ear canal and a volume within the second ear piece;a second feed-forward microphone acoustically coupled to an external environment and electrically coupled to a second feed-forward active noise cancellation signal path;a second feedback microphone acoustically coupled to the second acoustic volume and electrically coupled to a second feedback active noise cancellation signal path;a second output transducer acoustically coupled to the second acoustic volume via the volume within the second ear piece and electrically coupled to both the second feed-forward and second feedback active noise cancellation signal paths; anda second signal processor configured to apply filters and control gains of both the second feed-forward and second feedback active noise cancellation signal paths;wherein the second signal processor is configured to: apply third feed-forward filters to the second feed-forward signal path and apply the first feedback filters to the second feedback signal path during the first operating mode of the first signal processor, andreplace the third feed-forward filters with fourth feed-forward filters in the second feed-forward signal path during the second operating mode of the first signal processor. 19. The headphone of claim 18, wherein the first and second signal processors are portions of a single signal processing device. 20. The headphone of claim 18, wherein the third feed-forward filters are not identical to the first feed-forward filters. 21. The headphone of claim 18, wherein only one of the first or second signal processor applies the respective second or fourth feed-forward filters to the corresponding first or second feed-forward signal path during a third operating mode. 22. The headphone of claim 21, further comprising a user input, wherein the third operating mode is activated in response to the user input. 23. The headphone of claim 1, wherein the signal processor is further configured to apply a single-channel noise reduction filter to the first feed-forward signal path during the second operating mode. 24. The headphone of claim 1, wherein the signal processor is further configured to: detect high-frequency signals in the feed-forward signal path,compare the amplitude of the detected high-frequency signals to a threshold indicative of a positive feedback loop, andif the amplitude of the detected high-frequency signals is higher than the threshold, activate a compressing limiter. 25. The headphone of claim 24, wherein the signal processor is configured to detect the high-frequency signals using a phase-locked loop monitoring a signal in the feed-forward signal path. 26. The headphone of claim 1, wherein the ear piece provides a volume enclosing the feed-forward microphone, and further comprising a screen covering an aperture between the volume enclosing the feed-forward microphone and the external environment. 27. The headphone of claim 26, wherein the aperture between the volume enclosing the feed-forward microphone and the external environment is at least 10 mm2. 28. The headphone of claim 26, wherein the aperture between the volume enclosing the feed-forward microphone and the external environment is at least 20 mm2. 29. The headphone of claim 26, wherein the screen and the feed-forward microphone are separated by a distance of at least 1.5 mm. 30. The headphone of claim 1 wherein the second feed-forward filters provide active hear-through of ambient sounds with ambient naturalness over a hear-through pass band comprising a frequency range extending at least from 300 Hz to 3 kHz. 31. An active noise reducing headphone comprising: an ear piece configured to couple to a wearer's ear to define an acoustic volume comprising the volume of air within the wearer's ear canal and a volume within the ear piece;a feed-forward microphone acoustically coupled to an external environment and electrically coupled to a feed-forward active noise cancellation signal path;a feedback microphone acoustically coupled to the acoustic volume and electrically coupled to a feedback active noise cancellation signal path;an output transducer acoustically coupled to the acoustic volume via the volume within the ear piece and electrically coupled to both the feed-forward and feedback active noise cancellation signal paths; anda signal processor configured to apply filters and control gains of both the feed-forward and feedback active noise cancellation signal paths;wherein the signal processor is configured to:apply first feed-forward filters to the feed-forward signal path and apply first feedback filters to the feedback signal path during a first operating mode providing effective cancellation of ambient sound,replace the first feed-forward filters with second feed-forward filters in the feed-forward signal path during a second operating mode providing active hear-through of ambient sounds with ambient naturalness,detect high-frequency signals in the feed-forward signal path,compare the amplitude of the detected high-frequency signals to a threshold indicative of a positive feedback loop,if the amplitude of the detected high-frequency signals is higher than the threshold, activate a compressing limiter;when the amplitude of the detected high-frequency signals is no longer higher than the threshold, decrease an amount of compression applied by the limiter gradually, andif the amplitude of the detected high-frequency signals returns to a level higher than the threshold after reducing the amount of compression, increase the amount of compression to the lowest level at which the amplitude of the detected high-frequency signals remain below the threshold. 32. A method of operating an active noise reducing headphone comprising an ear piece configured to couple to a wearer's ear to define an acoustic volume comprising the volume of air within the wearer's ear canal and a volume within the ear piece;a feed-forward microphone acoustically coupled to an external environment and electrically coupled to a feed-forward active noise cancellation signal path;a feedback microphone acoustically coupled to the acoustic volume and electrically coupled to a feedback active noise cancellation signal path;an output transducer acoustically coupled to the acoustic volume via the volume within the ear piece and electrically coupled to both the feed-forward and feedback active noise cancellation signal paths; anda signal processor configured to apply filters and control gains of both the feed-forward and feedback active noise cancellation signal paths, the method comprising: in the signal processor, during a first operating mode, providing effective cancellation of ambient sound by applying first feed-forward filters to the feed-forward signal path and applying first feedback filters to the feedback signal path, and in the signal processor, during a second operating mode, providing active hear-through of ambient sounds with ambient naturalness by replacing the first feed-forward filters with second feed-forward filters and applying the second feed-forward filters to the feed-forward signal path, wherein the second feed-forward filters have value Kht selected to cause the formula GpfbGoea+Kht*Gnx*GffeGoea to be approximately equal to a predetermined target response, where Gpfb is the transfer function from external noise to the ear, through the headphones and with the feedback active noise cancellation signal path active, Goea is the transfer function from the external noise to the ear without the headphones, Gnx is the transfer function from the external noise to the feed-forward microphone, and Gffe is the transfer function of the filtered signal to the ear, through the output transducer, with the feedback active noise cancellation signal path active, and the predetermined target response is the target hear-through insertion gain (Thtig), which is 0 dB within the hear-through pass band, and is the same as the insertion gain achieved by the first feed-forward filters outside the hear-through pass band.