Providing ambient naturalness in ANR headphones

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;
• and
• a 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, and
• if 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.

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;
• and
• a 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
• G
• pfb
• G
• oea
• +
• K
• ht
• *
• G
• nx
• *
• G
• ffe
• G
• oea
• 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.