Reducing occlusion effect in ANR headphones

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 feedback microphone acoustically coupled to the acoustic volume and electrically coupled to a feedback active noise cancellation signal path;

a feed-forward microphone acoustically coupled to an external environment and electrically coupled to a feed-forward active noise cancellation signal path;

an output transducer acoustically coupled to the acoustic volume via the first volume and electrically coupled to the feedback signal path and the feed-forward signal path; and

a signal processor configured to apply filters and control gains of the feedback signal path and the feed-forward signal path;

wherein the signal processor is configured to:

apply first feedback filters to the feedback signal path, the first feedback filters causing the feedback signal path to operate at a first gain level, as a function of frequency, during a first operating mode,

apply second feedback filters to the feedback signal path, the second feedback filters causing the feedback signal path to operate at a second gain level less than the first gain level at some frequencies during a second operating mode,

apply first feed-forward filters to the feed-forward signal path in conjunction with applying the first feedback filters to the feedback signal path to achieve effective cancellation of ambient sound in the first operating mode, and

apply second feed-forward filters to the feed-forward signal path, the second filters being selected to provide active hear-through of ambient sounds with ambient naturalness in the second operating mode;

the first gain level being a level of gain that results in effective cancellation of sounds transmitted through or around the ear piece and through the user's head into the acoustic volume when the ear piece is coupled to the wearer's ear,

the second level being a level of gain that is matched to the level of sound of a typical wearer's voice transmitted through the wearer's head when the ear piece is coupled to the wearer's ear, and

the second feedback filters and the second feed-forward filters are selected to provide active hear-through of a user's own voice with self-naturalness.

2. The headphone of claim 1 , wherein the second feed-forward filters applied to the feed-forward path is a non-minimum phase response.

3. The headphone of claim 1 ,

wherein frequency components of the typical wearer's voice below a first frequency are passively transmitted through the wearer's head and are amplified when the ear piece is coupled to the wearer's ear, and frequency components of the typical wearer's voice above the first frequency are attenuated when the ear piece is so coupled, and

the feedback signal path is operative over a frequency range extending higher than the first frequency.

4. The headphone of claim 1 , wherein the signal processor is a first signal processor and the feedback signal path is a first feedback 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 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 feedback active noise cancellation signal path; and

a second signal processor configured to apply filters and control gains of the second feedback active noise cancellation signal path;

wherein the second signal processor is configured to:

apply third feedback filters to the second feedback signal path, the second feedback filters causing the second feedback signal path to operate at the first gain level during the first operating mode of the first signal processor, and

apply fourth feedback filters to the second feedback signal path to operate at the second gain level during the second operating mode of the first signal processor.

5. The headphone of claim 4 , wherein the first and second signal processors are portions of a single signal processing device.

6. The headphone of claim 4 , wherein the third feedback filters are not identical to the first feedback filters.

7. A method of configuring 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:

for at least one frequency, measuring the ratio



G

cev





G

oev



with the active noise reduction circuit of the headphones inactive, where G cev is the response at a user's ear to environmental noise when the headphones are worn, and G oev is the response at the user's ear to environmental noise when the headphones are not present;

selecting a filter K on for the feedback path having a magnitude that results in the feedback loop having a desensitivity equal to the determined ratio at the at least one frequency;

selecting a filter K ht for the feed-forward signal path that will provide ambient naturalness;

applying the selective filters K on and K ht to the feedback path and feed-forward path, respectively;

at the at least one frequency, measuring the ratio



G

cev





G

oev



with the active noise reduction circuit of the headphones active; and

modifying the phase of K ht without altering the magnitude thereof to minimize deviation of the measured value of



G

cev





G

oev



from unity.

8. The method of claim 7 , further comprising iterating the steps of selecting K on and K ht , applying the selected filters, and measuring the ratio



G

cev





G

oev



,

and further adjusting the phase of K ht until a target balance of ambient response and own-voice response is reached.

9. The method of claim 7 , wherein selecting the filter for the feed-forward signal path comprises:

selecting a value of K ht that causes the formula

G

pfb

G

oea

+

K

ht

*

G

nx

*

G

ffe

G

oea

to be approximately equal to a predetermined target value.