Instability mitigation in an active noise reduction (ANR) system having a hear-through mode

1. A method comprising:

receiving an input signal captured by one or more first sensors associated with an active noise reduction (ANR) device;

processing the input signal using a first filter disposed in an ANR signal path to generate a first signal for an acoustic transducer of the ANR device;

processing the input signal in a pass-through signal path disposed in parallel with the ANR signal path to generate a second signal for the acoustic transducer, wherein the pass-through signal path includes a second filter configured to allow at least a portion of the input signal to pass through to the acoustic transducer in accordance with a variable gain associated with the pass-through signal path;

detecting, using one or more second sensors disposed in a feedforward signal path, an existence of a condition likely to cause instability in the pass-through signal path;

responsive to detecting the existence of the condition likely to cause instability in the pass-through signal path, adjusting the variable gain associated with the pass-through signal path; and

generating a driver signal for the acoustic transducer using an output of the pass-through signal path, the output being based on the adjusted gain.

2. The method of claim 1 , wherein the ANR signal path generating the first signal is a feedforward ANR signal path.

3. The method of claim 1 , wherein the one or more second sensors comprise an infrared (IR) sensor.

4. The method of claim 1 , wherein the one or more second sensors comprise a proximity sensor of the ANR device.

5. The method of claim 4 , wherein detecting the existence of a condition likely to cause instability in the pass-through signal path comprises detecting by the proximity sensor that an object is within a predetermined distance from one of: the proximity sensor or one of the first sensors.

6. The method of claim 1 , wherein adjusting the variable gain associated with the pass-through signal path comprises reducing the variable gain.

7. The method of claim 1 , wherein adjusting the variable gain associated with the pass-through signal path comprises setting the variable gain to substantially equal to zero.

8. The method of claim 1 , wherein adjusting the variable gain associated with the pass-through signal path comprises adjusting a variable gain amplifier (VGA) disposed in the pass-through signal path.

9. The method of claim 1 , wherein adjusting the variable gain associated with the pass-through signal path comprises selecting a set of coefficients for a filter disposed in the pass-through signal path.

10. The method of claim 1 , further comprising:

detecting, using the one or more second sensors, that the condition likely to cause instability in the pass-through signal path is no longer in existence; and

responsive to detecting that the condition likely to cause instability in the pass-through signal path is no longer in existence, increasing the variable gain associated with the pass-through signal path.

11. The method of claim 1 , wherein the condition likely to cause instability in the pass-through signal path comprises a condition likely to cause coupling between the acoustic transducer and at least one of the one or more first sensors.

12. An active noise reduction (ANR) device comprising:

one or more first sensors configured to generate an input signal indicative of an external environment of the ANR device;

an acoustic transducer configured to generate output audio;

a first filter disposed in an ANR signal path of the ANR device, the first filter configured to process the input signal to generate a first signal for the acoustic transducer of the ANR device;

a pass-through signal path disposed in parallel with the ANR signal path, the pass-through signal path configured to generate, based on the one or more first sensors a second signal for the acoustic transducer, wherein the pass-through signal path is configured to allow at least a portion of the input signal to pass through to the acoustic transducer in accordance with a variable gain associated with the pass-through signal path;

one or more second sensors disposed in a feedforward signal path of the ANR device; and

a controller comprising one or more processing devices, the controller configured to:

detect, based on input from the one or more second sensors, an existence of a condition likely to cause instability in the pass-through signal path, and

responsive to detecting the existence of the condition likely to cause instability in the pass-through signal path, adjust the variable gain associated with the pass-through signal path,

wherein the acoustic transducer is driven by a driver signal that is based on an output of the pass-through signal path, the output being based on the adjusted gain.

13. The device of claim 12 , wherein the ANR signal path is a feedforward ANR signal path.

14. The device of claim 12 , wherein the one or more second sensors comprise a proximity sensor of the ANR device.

15. The device of claim 14 , wherein the controller is configured to detect the existence of the condition likely to cause instability in the pass-through signal path by detecting, based on an input from the proximity sensor, that an object is within a predetermined distance from one of: the proximity sensor or one of the first sensors.

16. The device of claim 12 , wherein adjusting the variable gain associated with the pass-through signal path comprises reducing the variable gain.

17. The device of claim 12 , wherein adjusting the variable gain associated with the pass-through signal path comprises setting the variable gain to substantially equal to zero.

18. The device of claim 12 , wherein adjusting the variable gain associated with the pass-through signal path comprises adjusting, by the controller, a variable gain amplifier (VGA) disposed in the pass-through signal path.

19. The device of claim 12 , wherein adjusting the variable gain associated with the pass-through signal path comprises selecting, by the controller, a set of coefficients for a filter disposed in the pass-through signal path.

20. The device of claim 12 , wherein the controller is further configured to:

detect, based on input from the one or more second sensors, that the condition likely to cause instability in the pass-through signal path is no longer in existence; and

responsive to detecting that the condition likely to cause instability in the pass-through signal path is no longer in existence, increase the variable gain associated with the pass-through signal path.

21. One or more non-transitory machine-readable storage devices having encoded thereon computer readable instructions for causing one or more processing devices to perform operations comprising:

receiving an input signal captured by one or more first sensors associated with an active noise reduction (ANR) device;

processing the input signal in an ANR signal path to generate a first signal for an acoustic transducer of the ANR device;

processing the input signal in a pass-through signal path disposed in parallel with the ANR signal path to generate a second signal for the acoustic transducer, wherein the pass-through signal path is configured to allow at least a portion of the input signal to pass through to the acoustic transducer in accordance with a variable gain associated with the pass-through signal path;

detecting, using one or more second sensors disposed in a feedforward signal path, the existence of a condition likely to cause instability in the pass-through signal path;

responsive to detecting the existence of the condition likely to cause instability in the pass-through signal path, adjusting the variable gain associated with the pass-through signal path; and

causing generation of a driver signal for the acoustic transducer using an output of the pass-through signal path, the output being based on the adjusted gain.