In-ear active noise reduction earphone

A method of configuring a feedback filter Kfb for use in an earphone having a feedback-based noise cancellation circuit, the method comprising:

• in a first processor,
• causing an electroacoustic driver of the earphone to output a calibration signal;
• receiving an output signal from a microphone acoustically coupled to the driver while the calibration signal is being output;
• computing a response of the earphone Gds based on the calibration signal and the microphone output signal;
• computing a target filter having a response Kloop/Gds and determining filter coefficients that will cause Kfb to have such a response;
• and
• providing the determined coefficients to a signal processor of the noise cancellation circuit.

An active noise reduction (ANR) earphone system comprising:

• a feedback microphone for detecting noise;
• digital feedback circuitry, responsive to the feedback microphone, for applying a filter to an output of the feedback microphone to produce an antinoise signal;
• an electroacoustic driver for transducing the antinoise signal into acoustic energy;
• a housing supporting the feedback microphone and the driver and maintaining the feedback microphone in a fixed position relative to the driver;
• a positioning and retaining structure for physically coupling the housing to the outer ear of the user;
• and
• an ear tip for acoustically coupling the feedback microphone and the driver to an ear canal of the user;
• wherein the ear tip and the ear canal form a front chamber containing the feedback microphone and bounded entirely by an interior surface of the ear tip, an interior surface of the ear canal, the user's ear drum, and a radiating surface of the driver, and
• a tube of air between the radiating surface of the driver and the ear canal bounded by the ear tip has a ratio of length L to effective area A no greater than 0.6 mm−1.

An active noise reduction (ANR) earphone system comprising:

• a feedback microphone for detecting noise;
• feedback circuitry, responsive to the feedback microphone, for applying a digital filter to an output of the feedback microphone to produce an antinoise signal;
• an electroacoustic driver for transducing the antinoise signal into acoustic energy;
• a housing supporting the feedback microphone and the driver and maintaining the feedback microphone in a fixed position relative to the driver;
• a positioning and retaining structure for coupling the housing to the outer ear of the user;
• and
• an ear tip for coupling the feedback microphone and the driver to an ear canal of the user;
• wherein a front shell of the housing, the ear tip, and the ear canal form a front chamber containing the feedback microphone and bounded by an interior surface of the front shell, an interior surface of the ear tip, an interior surface of the ear canal, the user's ear drum, and a radiating surface of the driver, and
• the interior surface of the ear tip makes up at least twenty percent of the bounding surface of the front chamber not including the interior surface ear canal.