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. 2018 Mar 29;8(1):5403.
doi: 10.1038/s41598-018-23531-y.

Head Tracking Extends Local Active Control of Broadband Sound to Higher Frequencies

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Free PMC article

Head Tracking Extends Local Active Control of Broadband Sound to Higher Frequencies

Stephen J Elliott et al. Sci Rep. .
Free PMC article

Abstract

Local active sound control systems provide useful reductions in noise within a zone of quiet which only extends to about one tenth of an acoustic wavelength. If active control is required above a few hundred hertz, this generally limits the movement of a listener to unrealistically small changes in head position. We describe a local active sound control system using a fixed array of monitoring microphones, in which the pressures at the ear positions are estimated from these microphone signals using head position information from an optical head tracker. These signals are then actively controlled to give robust attenuation at the ear positions, even as the listener moves their head. Feedforward control provides selective attenuation of noise and broadband attenuation of around 20 dB is measured up to excitation frequencies of 1 kHz under favourable conditions, with head tracking achieved in a few seconds. The active control performance is thus comparable with that achieved with active headphones, but without the listener having anything attached to their head.

Conflict of interest statement

The authors declare no competing interests.

Figures

Figure 1
Figure 1
Physical arrangement used to demonstrate local active control at the ears of a listener while the head position is being tracked (a) and detail showing the position of the monitoring microphones and secondary loudspeakers (b), together with the grid, with a spacing of 5 cm, used to define the 20 measurement locations for the centre of the dummy head, including the nominal position, A, and the moved position, B, used in the following experiments (c).
Figure 2
Figure 2
Power spectral density of the signal measured at the microphones in the left and right hand side of the dummy head when it was in the nominal position and the primary source was behind the headrest before control, solid line, after control using the monitoring microphones and observation filter to estimate the signal at the ear position using the remote microphone technique, dashed line, and, for reference, when the microphones in the dummy head themselves were used as the error signals in the control algorithm, dot-dashed line (a) and (b). The results are also shown when the primary source was positioned in front of the headrest in the two lower graphs, (c) and (d), which illustrate the effect of the standard remote microphone technique, RMT, and when it includes a modelling delay of 0.7 ms.
Figure 3
Figure 3
Time history of the pressure signal measured in the right-hand ear of the dummy head, (a) when in position B before control, up to 30 seconds, when control is implemented with the observation filter and plant responses appropriate for the nominal head position, from 30 to 60 seconds, and when the head tracker is enabled so that the correct head position is identified, after 60 seconds. The power spectral density of the signal at this microphone is also shown, (b) before control, after control but without head tracking and after control with the head tracker enabled.
Figure 4
Figure 4
Active control with the integrated active headrest system with a human listener for reducing broadband random disturbance signals in real-time: (a) The test installation, in which the participant has two physical error microphones in their ears for evaluation purposes and (b) the measured signals at one of these error microphones during active control and head-tracking.
Figure 5
Figure 5
The block diagram of the adaptive control algorithm that uses the remote microphone technique to estimate the signals at the virtual error microphones from those at the fixed monitoring microphones, (a) and the arrangement used to implement the control system with a headtracker to schedule the transfer responses shown highlighted in the block diagram (b). The shaded blocks in both diagrams are the responses that are scheduled on the head position, as monitored by the headtracker.

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