A variable update step size is determined in proportion to a magnitude ratio or magnitude difference between a first residual signal and a second residual signal. The first residual signal is obtained by using adaptive filter coefficient sequence, where the adaptive filter coefficient sequence has been obtained in previous operations of the adaptive equalizer. The second residual signal is obtained by using a prior update adaptive filter coefficient sequence, where the prior update adaptive filter coefficient sequence is obtained by performing a coefficient update with an arbitrary prior update step size on the adaptive filter coefficient sequence having been obtained in previous operations of the adaptive equalizer.

Apparatus comprises one or more first microphones for providing audio signals from captured acoustic signals; one or more conductive terminals for connection to one or more conductive terminals of an external cover device for receiving audio signals provided by one or more second microphones included in the external cover device; and one or more processors coupled to the one or more first microphones and to the one or more conductive terminals and configured to perform audio processing on the audio signals from the one or more first microphones and/or from the one or more second microphones based on a control input. Cover apparatus comprises one or more microphones and one or more conductive terminals for providing audio signals provided by said one or more microphones to the electronic apparatus.

Equalizer circuitry includes a differential target voltage input, an equalizer output, a first operational amplifier, and a second operational amplifier. The differential target voltage input includes a target voltage input node and an inverted target voltage input node. The first operational amplifier and the second operational amplifier are coupled in series between the differential target voltage input and the equalizer output. The first operational amplifier is configured to receive a target voltage signal and provide an intermediate signal based on the target voltage input signal. The second operational amplifier is configured to receive the intermediate signal and an inverted target voltage signal and provide an output signal to the equalizer output. The first operational amplifier and the second operational amplifier are interconnected with one or more passive components such that a transfer function between the differential target voltage input and the equalizer output is a second-order complex-zero function.

Equalizer circuitry includes a differential target voltage input, an equalizer output, a first operational amplifier, and a second operational amplifier. The differential target voltage input includes a target voltage input node and an inverted target voltage input node. The first operational amplifier and the second operational amplifier are coupled in series between the differential target voltage input and the equalizer output. The first operational amplifier is configured to receive a target voltage signal and provide an intermediate signal based on the target voltage input signal. The second operational amplifier is configured to receive the intermediate signal and an inverted target voltage signal and provide an output signal to the equalizer output. The first operational amplifier and the second operational amplifier are interconnected with one or more passive components such that a transfer function between the differential target voltage input and the equalizer output is a second-order complex-zero function.

An example method of operation may include initiating an automated tuning procedure, detecting via one or more microphones a sound measurement associated with an output of one or more speakers at two or more locations, determining a number of speech transmission index (STI) values equal to a number of microphones, and averaging the speech transmission index values to identify a single speech transmission index value.

Example techniques involve calibration of one or more playback devices. An example implementation involves a playback device playing back audio content using a first calibration via the one or more audio transducers and the one or more amplifiers. The first calibration is based on a response of a listening environment to audio content playback by the playback device. The playback device records, via one or more microphones, at least a portion of the played back audio content. Based on the recorded audio content, the playback device detects a change in the response of the listening environment to audio content playback by the playback device. Responsive to detecting the change in the response, the playback device causes output of a prompt to initiate a calibration procedure for the playback device. The calibration procedure involves a mobile device recording playback by the playback device.

Example techniques involve calibration of one or more playback devices. An example implementation involves a playback device playing back audio content using a first calibration via the one or more audio transducers and the one or more amplifiers. The first calibration is based on a response of a listening environment to audio content playback by the playback device. The playback device records, via one or more microphones, at least a portion of the played back audio content. Based on the recorded audio content, the playback device detects a change in the response of the listening environment to audio content playback by the playback device. Responsive to detecting the change in the response, the playback device causes output of a prompt to initiate a calibration procedure for the playback device. The calibration procedure involves a mobile device recording playback by the playback device.

A limiter system for an active speaker may include at least one lowpass filter configured to receive an input signal and output a signal lower than a crossover frequency, at least one highpass filter, configured to receive an input signal and output a signal higher than the crossover frequency, a first allpass filter configured to adjust the phase of the signal lower than the crossover frequency, a second allpass filter configured to adjust the phase of the signal higher than the crossover frequency, a first limiter, configured to receive and limit the signal from the first allpass filter, a second limiter, configured to receive and limit the signal from the second allpass filter, and a mixer, configured to mix the signal lower from the first limiter and the signal from the second limiter.

The present application discloses playback devices and networks, software for operating playback devices and playback device networks, and methods of operating playback devices and networks, where an individual network device is configured to switch between operating various operating modes comprising a non-paired mode and a paired mode, where the playback device is configured to output various channels of audio content within various audio frequency ranges while operating in the various operating modes.

An in-ear noise dosimeter in the form of an earplug which senses sound in the ear canal using an eartip which has a sound delivery channel that couples sound at the end closest to the eardrum to an earplug microphone. The earplug can communicate wirelessly with a remote data collection and processing system. A dock unit for storing the earplugs when not worn can compensate for differences in unoccluded-ear versus occluded-ear responses by an acoustic compensator. An electronic compensation filter can be modified by a proximity switch in the earplug which changes state when the earplug is worn in the ear versus stored in a dock unit. The dosimeter can also have a temperature sensor for sensing human body temperature and remotely-located wireless LEDs used to alert the user of high noise dosage. Data can also be downloaded from the earplug using a reader unit.