152 and the second control source arrangement 154. This may be referred to as “decentralised” control, or as “decentralised absorption control”. Each of the first controller 150a and second controller 150b employs a single channel Filtered Least Mean Squared (FxLMS) adaptive algorithm. The first controller 150a drives the first control source arrangement 152, which comprises an upstream facing first control source 152a, to minimise the reflected component. The second controller 150b drives the second control source arrangement 154, which comprises a downstream facing second control source 154a, to minimise the transmitted component. In this way, absorption of a primary disturbance signal in the primary source path 110 is maximised.

Referring to FIG. 3, an active acoustic control system 300 according to a first embodiment is shown.

The active acoustic control system 300 is for controlling an acoustic signal propagating along a propagation path. In particular, the system 300 is for actively absorbing the acoustic signal. In FIG. 3, the acoustic signal propagating along the propagation path is indicated at 310. The acoustic signal 310 comprises an uncontrolled portion 312 and a controlled portion 314. As shown in the figure, the controlled portion 314 has a reduced amplitude compared with the uncontrolled portion 312. This is by virtue of the active acoustic control system 300 being operated to absorb the acoustic signal.

The system 300 comprises an active control unit 350, a first sensor arrangement 360, a first control source arrangement 352 and a second control source arrangement 354. The first control source arrangement 352 is operable to generate a first control signal to control a first component of the acoustic signal 310. The second control source arrangement 354 is operable to generate a second control signal to control a second component of the acoustic signal 310. In this example the first control signal and second control signal are acoustic control signals/waves.

The first sensor arrangement 360 comprises pressure sensors. In this way, acoustic pressure may be sensed or detected, enabling control of acoustic signals.

The first control source arrangement 352 and second control source arrangement 354 comprise loudspeakers. In this way, acoustic signals propagating along the propagation path may be controlled by the provision of acoustic signals by the first control source arrangement 352 and second control source arrangement 354.

As above, the active control unit 350 may be retrofitted in a system or apparatus comprising the first sensor arrangement 360 and control source arrangements 352, 354.

The active control unit 350 is configured to receive information from the first sensor arrangement 360. The information provided by the first sensor arrangement 360 relates to the acoustic signal 310 propagating along the propagation path. The active control unit 350 is further configured to generate a control signal for controlling the acoustic signal 310 based on the information from the first sensor arrangement 360.

The active control unit 350 is arranged to control the first control source arrangement 352 and second control source arrangement 354. Notably, the active control unit 350 is arranged to independently control the first control source arrangement 352 and second control source arrangement 354. This may be referred to as “decentralised” control. The active control unit 350 is arranged to adaptively calculate the optimal voltage for controlling the first control source arrangement 352 and second control source arrangement 254 to maximise sound absorption.

By independently controlling the first control source arrangement 352 and second control source arrangement 354, a computational load advantage is realised. This is because it is only necessary to determine sets of filter coefficients for two filters for use in generating the control signals, one for each of the control source arrangements 352, 354.

The active control unit 350 can comprise a first controller 350a arranged to control the first control source arrangement 352 and a second controller 350b arranged to control the second control source arrangement 354. Advantageously, by providing separate controllers, computational complexity may be reduced as each controller is provided for a single operation or purpose. This may facilitate independent control. In FIG. 3, the wave separation algorithm used by the controllers 350a, 350b, or by active control unit 350 generally, is indicated generally at 324.

The system 300 further comprises an acoustic guide 330 in the form of a duct. The acoustic guide 330 provides, or defines, the propagation path for the acoustic signal. In this way, the acoustic guide 330 can isolate, or shield, the acoustic signal from outside disturbance, improving the ability of the system 300 to minimise an acoustic signal within the acoustic guide 330.

A primary disturbance source 320 is arranged at a first end of the acoustic guide 330. The primary disturbance source 320 may be, for example, a heating, ventilation and air-conditioning (HVAC) unit. It is desirable to minimise acoustic signals (e.g., unwanted noise) generated by the HVAC unit.

The first component of the acoustic signal 310 is a transmitted component of the acoustic signal 310. The second component of the acoustic signal is a reflected component of the acoustic signal 310. In this way, the first control source arrangement 352 is operable to generate a first control signal, which is a control signal for controlling the transmitted component. Accordingly, the second control source arrangement 354 is operable to generate a second control signal, which is a control signal for controlling the reflected component. By controlling the transmitted and reflected components, absorption is controlled accordingly. Advantageously, by independently driving the first and second control source arrangements to minimise the transmitted and reflected components respectively (by provision of appropriate control signals), absorption of the acoustic signal 310 is maximised. By doing so, only a residual noise component, indicated by controlled portion 314 of the acoustic signal 310, may remain. The residual noise component 314 may have zero amplitude.

The active control unit 350 is configured to receive information from a second sensor arrangement 370. The second sensor arrangement 370 is for sensing an error signal. By providing the second sensor arrangement 370, an adaptive system 300 is provided which is able to adapt operation based on the acoustic signal provided by the primary source 300. Advantageously, this facilitates broadband adaptive absorption control of acoustic signals.

The second sensor arrangement 370 is provided downstream of active control unit 350 along the propagation path. That is, the second sensor arrangement 370 is capable of sensing the acoustic signal after the active control unit 350 has controlled the acoustic signal, thereby to determine a residual noise component. In this way, the second sensor arrangement 370 can provide information to the active control unit 350 to inform further control of the acoustic signal. That is, the active control unit 350 may determine, based on the information (or feedback from the second sensor arrangement 370) that the control signals need to be adapted, changed or maintained, to control the acoustic signal. That is, the first sensor arrangement 360 may be referred to as a “reference sensor arrangement” and the second sensor arrangement 370 may be referred to as an “error sensor arrangement”.

The second sensor arrangement 370 comprises pressure sensors. In this way, acoustic pressure may be sensed or detected, enabling control of acoustic signals.

As shown in FIG. 3, the first control source arrangement 352 and second control source arrangement 354 are arranged in a one-dimensional manner along the propagation path. The first control source arrangement 352 and second control source arrangement 354 are linearly arranged along the propagation path. Such an arrangement may simplify signal processing, reducing computational load, and simplify determination of appropriate control signals.

Referring to FIG. 4, an active acoustic control system 400 according to a second embodiment is shown.

The active acoustic control system 400 is for controlling an acoustic signal propagating along a propagation path. In particular, the system 400 is for actively absorbing the acoustic signal. In FIG. 4, the acoustic signal propagating along the propagation path is indicated at 410. The acoustic signal 410 comprises an uncontrolled portion 412 and a controlled portion 414. As shown in the figure, the controlled portion 414 has a reduced amplitude compared with the uncontrolled portion 412. This is by virtue of the active acoustic control system 400 being operated to absorb the acoustic signal.

The acoustic signal 410 is generated in a first zone 416. The acoustic signal 410 is generated by, for example, an engine in the first zone 416. That is, the engine may be a primary disturbance source 420. The acoustic signal 410 is controlled such that only residual noise, or no noise, is present in a second zone 418. The second zone 418 may be, for example, living quarters. The system 400 is provided between the first zone 416 and second zone 418, for example at a wall or interface between the first zone 416 and second zone 418.

The active acoustic control system 400 according to the second embodiment comprises substantially the same components as the active acoustic control system 300 according to the first embodiment. Furthermore, the active acoustic control system 400 according to the second embodiment is configured to operate in substantially the same way as the active acoustic control system 300 according to the first embodiment, to maximise sound absorption.

The system 400 comprises an active control unit 450, a first sensor arrangement 460, a first control source arrangement 452 and a second control source arrangement 454. The first control source arrangement 452 is operable to generate a first control signal to control a first component of the acoustic signal 410. The second control source arrangement 454 is operable to generate a second control signal to control a second component of the acoustic signal 410. In this example the first control signal and second control signal are acoustic control signals/waves.

The first control source arrangement 452 and second control source arrangement 454 comprise loudspeakers. In this way, acoustic signals propagating along the propagation path may be controlled by the provision of acoustic signals by the first control source arrangement 452 and second control source arrangement 454.

Notably, the first control source arrangement 452 comprises a 2D array of control sources. That is, a number of control sources that extend across a two-dimensional plane. The second control source arrangement 454 comprises a 2D array of control sources. Together, the first control source arrangement 452 and second control source arrangement 454 define a dual layer array of control sources. Advantageously, in this way, acoustic absorption control is maximised in a two- or three-dimensional space.

The first control source arrangement 452 and second control source arrangement 454 are provided downstream of the first sensor arrangement 460. In this way, information provided by the first sensor arrangement 460 is indicative of the acoustic signal prior to control by the system 400, which simplifies processing and calculation of appropriate control signals.

The first control source arrangement 452 faces downstream relative to the propagation path. The second control source arrangement 454 faces upstream relative to the propagation path. In this way, control of the transmitted component and reflected component of the acoustic signal is targeted and thereby improved.

The first sensor arrangement 460 comprises pressure sensors. In this way, acoustic pressure may be sensed or detected, enabling control of acoustic signals. In particular, the first sensor arrangement 460 comprises a dual layer array of sensors.

The active control unit 450 is arranged to control the first control source arrangement 452 and second control source arrangement 454. Notably, the active control unit 450 is arranged to independently control the first control source arrangement 452 and second control source arrangement 454. This may be referred to as “decentralised” control. The active control unit 450 is arranged to adaptively calculate the optimal voltage for controlling the first control source arrangement 452 and second control source arrangement 454 to maximise sound absorption.

By independently controlling the first control source arrangement 452 and second control source arrangement 454, a computational load advantage is realised. This is because it is only necessary to determine sets of filter coefficients for two filters use in generating the control signals, one for each of the control source arrangements 452, 454.

The active control unit 450 can comprise a first controller 450a arranged to control the first control source arrangement 452 and a second controller 450b arranged to control the second control source arrangement 454. Advantageously, by providing separate controllers, computational complexity may be reduced as each controller is provided for a single operation or purpose. This may facilitate independent control. In FIG. 4, the wave separation algorithm used by the controllers 450a, 450b, or by active control unit 450 generally, is indicated generally at 424.

The second sensor arrangement 470 comprises pressure sensors. In this way, acoustic pressure may be sensed or detected, enabling control of acoustic signals. In particular, the second sensor arrangement 470 comprises a dual layer array of sensors.

As shown in FIG. 4, the first control source arrangement 452 and second control source arrangement 454 are arranged in a three-dimensional manner across the propagation path. The first control source arrangement 452 and second control source arrangement 454 are distributed across the propagation path. Such an arrangement may enable control of acoustic signals in two or three dimensions.

Referring to FIG. 5, an active acoustic control system 500 according to a third embodiment is shown.

The active acoustic control system 500 is for controlling an acoustic signal propagating along a propagation path. In particular, the system 500 is for actively absorbing the acoustic signal. In FIG. 5, the acoustic signal propagating along the propagation path is indicated at 510. The acoustic signal 510 is an acoustic signal propagating by virtue of a vibrating structural component, for example a wall or other structural member. That is, the propagation path may be defined by the structural component. That is, vibrations may occur in the structural component, and it is desirable to control the acoustic signals by virtue of the vibration of the structural component. The acoustic signal 510 comprises an uncontrolled portion 512 and a controlled portion 514. As shown in the figure, the controlled portion 514 has a reduced amplitude compared with the uncontrolled portion 512. This is by virtue of the active acoustic control system 500 being operated to absorb the acoustic signal.

The active acoustic control system 500 according to the third embodiment comprises substantially the same components as the active acoustic control system 300 according to the first embodiment and the active acoustic control system 400 according to the second embodiment. Furthermore, the active acoustic control system 500 according to the second embodiment is configured to operate in substantially the same way as the system 300 according to the first embodiment and system 400 according to the second embodiment, to maximise sound absorption.

The system 500 comprises an active control unit 550, a first sensor arrangement 560, a first control source arrangement 552 and a second control source arrangement 554. The first control source arrangement 552 is operable to generate a first control signal to control a first component of the acoustic signal 510. The second control source arrangement 554 is operable to generate a second control signal to control a second component of the acoustic signal 510. In this example the first control signal and second control signal are acoustic control signals/waves.

The first control source arrangement 552 and second control source arrangement 554 comprise actuators. In this way, acoustic signals propagating along the propagation path may be controlled by the provision of vibrational input control signals by the first control source arrangement 552 and second control source arrangement 554.

The first sensor arrangement 560 comprises accelerometers. The accelerometers can sense, or detect, deformation of the structural members to which they are attached, connected or arranged to sense. In this way, acoustic pressure generated by the acoustic signal may be indirectly sensed, by reference to the deformation of the structural member.

The active control unit 550 is arranged to control the first control source arrangement 552 and second control source arrangement 554. Notably, the active control unit 550 is arranged to independently control the first control source arrangement 552 and second control source arrangement 554. This may be referred to as “decentralised” control. The active control unit 550 is arranged to adaptively calculate the optimal voltage for controlling the first control source arrangement 552 and second control source arrangement 554 to maximise sound absorption.

By independently controlling the first control source arrangement 552 and second control source arrangement 554, a computational load advantage is realised. This is because it is only necessary to determine sets of filter coefficients for two filters for use in generating the control signals, one for each of the control source arrangements 552, 554.

The active control unit 550 can comprise a first controller 550a arranged to control the first control source arrangement 552 and a second controller 550b arranged to control the second control source arrangement 554. Advantageously, by providing separate controllers, computational complexity may be reduced as each controller is provided for a single operation or purpose. This may facilitate independent control. In FIG. 5, the wave separation algorithm used by the controllers 550a, 550b, or by active control unit 550 generally, is indicated generally at 524.

A second sensor arrangement 570 comprises accelerometers. In this way, acoustic pressure may be indirectly sensed or detected, enabling control of acoustic signals. The accelerometers may sense or detect structural acceleration of a component to which they are connected or mounted.

Referring to FIG. 6, an active acoustic control system 600 according to a fourth embodiment is shown.

The active acoustic control system 600 is for controlling an acoustic signal propagating along a propagation path. The active acoustic control system 600 is for controlling an acoustic signal propagating along a propagation path. In particular, the system 600 is for actively absorbing the acoustic signal. In FIG. 6, the acoustic signal propagating along the propagation path is indicated at 610. The acoustic signal 610 is an acoustic signal propagating and incident on a structural component 602, thereby to cause the structural component to vibrate or deform. The acoustic signal 610 comprises an uncontrolled portion 612 and a controlled portion 614. As shown in the figure, the controlled portion 614 has a reduced amplitude compared with the uncontrolled portion 612. This is by virtue of the active acoustic control system 600 being operated to absorb the acoustic signal.

The active acoustic control system 600 according to the fourth embodiment comprises substantially the same components as the active acoustic control system 300 according to the first embodiment, the active acoustic control system 400 according to the second embodiment and the active acoustic control system 500 according to the third embodiment. Furthermore, the active acoustic control system 500 according to the fourth embodiment is configured to operate in substantially the same way as the system 300 according to the first embodiment, the system 400 according to the second embodiment and the system 500 according to the third embodiment, to maximise sound absorption.

The system 600 comprises an active control unit 650, a first sensor arrangement 660, a first control source arrangement 652 and a second control source arrangement (not shown). The first control source arrangement 652 is operable to generate a first control signal to control a first component of the acoustic signal 610. The second control source arrangement is operable to generate a second control signal to control a second component of the acoustic signal 610. In this example the first control signal and second control signal are acoustic control signals/waves.

The first control source arrangement 652 and second control source arrangement comprise actuators. In this way, acoustic signals propagating along the propagation path may be controlled by the provision of vibrational input control signals by the first control source arrangement 652 and second control source arrangement.

The first sensor arrangement 660 comprises accelerometers. The accelerometers can sense, or detect, deformation of the structural component 602 to which they are attached, connected or arranged to sense. In this way, acoustic pressure generated by the acoustic signal may be indirectly sensed, by reference to the deformation of the structural component 602.

The active control unit 650 is arranged to control the first control source arrangement 652 and second control source arrangement. Notably, the active control unit 650 is arranged to independently control the first control source arrangement 652 and second control source arrangement. This may be referred to as “decentralised” control. The active control unit 650 is arranged to adaptively calculate the optimal voltage for controlling the first control source arrangement 652 and second control source arrangement to maximise sound absorption.

By independently controlling the first control source arrangement 652 and second control source arrangement, a computational load advantage is realised. This is because it is only necessary to determine sets of filter coefficients for two filters for use in generating the control signals, one for each of the control source arrangements.

The active control unit 650 can comprise a first controller 650a arranged to control the first control source arrangement 652 and a second controller 650b arranged to control the second control source arrangement. Advantageously, by providing separate controllers, computational complexity may be reduced as each controller is provided for a single operation or purpose. This may facilitate independent control. In FIG. 6, the wave separation algorithm used by the controllers 650a, 650b, or by active control unit 650 generally, is indicated generally at 624.

Referring to FIG. 7, a plot of frequency (x-axis) against energy (y-axis) is shown to indicate the performance metric of the systems 300, 400, 500. The transmission coefficient (indicative of the transmitted component) is indicated at 710. The reflection coefficient (indicative of the reflected component) is indicated at 720. The absorption coefficient (indicative of a theoretical absorbed component) is indicated at 730.

As will be understood from FIG. 7, the transmission and reflection coefficients indicate zero energy across a broadband frequency range. This illustrates that the transmitted and reflected components are minimised across a broadband frequency range. Accordingly, the absorption coefficient indicates perfect absorption of the acoustic signal across the same broadband frequency range.

Referring back to FIG. 1, in non-reciprocal acoustics, the aim is to allow acoustic transmission in a first direction, whilst preventing, or minimising, acoustic transmission in a second, opposite, direction. Similarly to absorption control, in non-reciprocal acoustics it is aimed to maximise the absorption, which can be achieved by minimising the downstream pressure, whilst produce zero upstream sound radiation due to the control sources 52a, 54a.

The first control source arrangement 52 is for generating a first control signal to control a first component of the acoustic signal. The second control source arrangement 54 is for generating a second control signal to control a second component of the acoustic signal. Here, the first component of the acoustic signal is a transmitted component, and the second component of the acoustic signal is a reflected component. The active acoustic control system 10 is configured to control the transmitted component and reflected component individually. By minimising the transmitted and reflected components, absorption is maximised accordingly.

Referring to FIG. 8, a feedforward control block diagram of an active acoustic control system 800 is shown. Specifically, FIG. 8 illustrates a “fully-coupled” control system. Nevertheless, the skilled person will appreciate that non-reciprocal control may instead incorporate a decentralised control strategy or system, as described above.

The positive primary source path is indicated at 810. The negative primary source path is indicated at 820. The active control unit is indicated at 850. The secondary path is indicated at 822. The active control unit 850 is configured to drive a first control source arrangement to minimise the reflected component B. The active control unit 850 is configured to drive a second control source arrangement to minimise the transmitted component C. Positive incident wave A is used as the reference signal. In FIG. 8, the wave separation algorithm used by the active control unit 850 is indicated generally at 824.

Referring to FIG. 9, an active acoustic control system 900 according to a fifth embodiment is shown, alongside a coordinate system. The active acoustic control system 900 is for controlling an acoustic signal propagating along a propagation path. In particular, the system 900 is for non-reciprocal acoustic control. As shown in the figure, A is the positive incident wave component, B is the reflected wave component, C is the transmitted wave component and D is the negative incident wave component.

The active acoustic control system 900 is a non-reciprocal active acoustic control system. The system 900 comprises an active control unit 950, a first sensor arrangement 960 and a first control source arrangement 952. The first control source arrangement 952 is operable to generate a first control signal to control a first component of the acoustic signal. In this example the first control signal is an acoustic control signal/wave.

The first sensor arrangement 960 comprises pressure sensors. In this way, acoustic pressure may be sensed or detected, enabling control of acoustic signals. Here, the first sensor arrangement 960 comprises a dual-layered 2D array of pressure sensors.

Furthermore, in this example, the system 900 comprises a second sensor arrangement 970. In this way, acoustic pressure may be sensed or detected, enabling control of acoustic signals. Here, the second sensor arrangement 970 comprises a dual-layered 2D array of pressure sensors.

Whilst in this example, the system 900 comprises a first sensor arrangement 960 upstream of the active control unit 950 and a second sensor arrangement 970 downstream of the active control unit 950, benefits of the invention may still be realised by providing only a first sensor arrangement 960. In particular, this may still facilitate control of a transmitted component.

In order to control the transmitted and reflected components, the components are separated from the total acoustic pressure using a wave separation method. To separate the positive and negative propagating waves in 3D, a two-dimensional spatial Fourier transform (SFT) may be used to decompose the spherical incident wave into its directional plane wave components. The two-dimensional SFT can be defined as

$P (k_{x}, k_{y}, z) = \int \int_{- \infty}^{\infty} p (x, y, z) e^{j (k_{x} x + k_{y} y + k_{z} z)} dx dy$

$where$

$k_{x} = k_{0} \cos φ \sin θ,$

$k_{y} = k_{0} \sin φ \sin θ,$

$k_{z} = k_{0} \cos θ,$

k_x, k_yand k_zare the spatial frequencies in the x, y and z directions, θ and φ are the spherical angles and k₀is the acoustic wavenumber. To simplify implementation of the two-dimensional SFT, a two-dimensional discrete SFT may be used, which sums the weighted pressures measured at several sampling points, where the weighted pressure at the i-th pressure sensor in each pressure sensor array can be expressed as

$P (k_{x}, k_{y}, z_{i}) = p (x_{i}, y_{i}, z_{i}) W (x_{i}, y_{i})$

$where$

$W (x_{i}, y_{i}) = e^{j (k_{x} x_{i} + k_{y} y_{i} + k_{z} z_{i})}$

W(x_i, y_i) is the complex weighting factor that applies the appropriate phase shift required from the pressure sensor location to the plane perpendicular to incident wave direction.

The weighted pressures can then be used to calculate the positive and negative propagating wave components between the two closely-spaced pressure sensor arrays in the upstream and downstream spaces via the integration wave separation method. The wave components at the i-th pressure sensor can be calculated as

$A_{i} = \frac{p_{1_{i}} W (x_{i}, y_{i}) + p_{2_{i}} W (x_{i}, y_{i})}{4} + \frac{c_{0}}{2} \int_{0}^{T_{s}} p_{1_{i}} W (x_{i}, y_{i}) - p_{2_{i}} W (x_{i}, y_{i}) dt,$

$B_{i} = \frac{p_{1_{i}} W (x_{i}, y_{i}) + p_{2_{i}} W (x_{i}, y_{i})}{4} - \frac{c_{0}}{2} \int_{0}^{T_{s}} p_{1_{i}} W (x_{i}, y_{i}) - p_{2_{i}} W (x_{i}, y_{i}) dt,$

$C_{i} = \frac{p_{3_{i}} W (x_{i}, y_{i}) + p_{4_{i}} W (x_{i}, y_{i})}{4} + \frac{c_{0}}{2} \int_{0}^{T_{s}} p_{3_{i}} W (x_{i}, y_{i}) - p_{4_{i}} W (x_{i}, y_{i}) dt,$

$D_{i} = \frac{p_{3_{i}} W (x_{i}, y_{i}) + p_{4_{i}} W (x_{i}, y_{i})}{4} - \frac{c_{0}}{2} \int_{0}^{T_{s}} p_{3_{i}} W (x_{i}, y_{i}) - p_{4_{i}} W (x_{i}, y_{i}) dt,$

where c₀is the speed of sound in air, T_sis the sampling period, p₁_ito p₄_iare the pressures measured at the i-th pressure sensor in the first to fourth pressure line arrays.

Once the wave components A, B, C, D have been separated using the pressure measured at each pressure sensor, a feedforward FxLMS algorithm may be used to adaptively control the transmitted wave component C and reflected wave component B, where the reference signal for the calculation is the positive propagating incident wave A.

As introduced above, the system 900 comprises a first control source arrangement 952. Notably, the first control source arrangement 952 comprises a finite 2D array of first control sources for generating a first control signal for controlling a first component of the acoustic signal. Advantageously, by this construction, non-reciprocal acoustic control is realised in a two-dimensional or three-dimensional environment. Furthermore, such a system is fully adaptable, and the direction of the non-reciprocal behaviour can be reversed by changing the reference and error signal for this controller. Importantly, the present system does not introduce distortions or non-linearities into the signal, which avoids problems associated with distortions or non-linearities. Furthermore, broadband non-reciprocal control is realised.

As above, the active control unit 950 may be retrofitted in a system or apparatus comprising the first sensor arrangement 960 and first control source arrangement 952.

The active control unit 950 is configured to receive information from the first sensor arrangement 960. The information provided by the first sensor arrangement 960 relates to the acoustic signal propagating along the propagation path. The active control unit 950 is further configured to generate a control signal for controlling the acoustic signal based on the information from the first sensor arrangement 960.

The active control unit 950 is arranged to control the first control source arrangement 952 to generate a first control signal for controlling a first component of the acoustic signal. The first component is a transmitted component of the acoustic signal. In this way, a single layer of first control sources is advantageous in achieving non-reciprocal acoustic transmission.

The first control signal is used to control the first component of the acoustic signal thereby to minimise the first component of the acoustic signal. In this way, a single layer of first control sources is advantageous in achieving perfect non-reciprocal acoustic transmission.

The first control source arrangement 952 comprises a 2D array of monopole control sources. Advantageously, by this construction, non-reciprocal acoustic control is realised in a two-dimensional or three-dimensional environment.

As mentioned above, the system 900 may further comprise a second control source arrangement 954. The second control source arrangement 954 is operable to generate a second control signal to control a second component of the acoustic signal 910. In this example the second control signal is an acoustic control signal/wave.

Notably, the second control source arrangement 954 comprises a finite 2D array of second control sources for generating a second control signal for controlling a second component of the acoustic signal. Advantageously, by this construction, non-reciprocal acoustic control is realised in a two-dimensional or three-dimensional environment. Furthermore, such a system is fully adaptable, and the direction of the non-reciprocal behaviour can be reversed by changing the reference and error signals for this controller. Importantly, the present system does not introduce distortions or non-linearities into the signal, which avoids problems associated with distortions or non-linearities. Furthermore, broadband non-reciprocal control is realised.

The active control unit 950 is configured to receive information from the second sensor arrangement 970. The information provided by the second sensor arrangement 970 relates to the acoustic signal propagating along the propagation path. The active control unit 950 is further configured to generate a control signal for controlling the acoustic signal based on the information from the second sensor arrangement 970.

The active control unit 950 is arranged to control the second control source arrangement 954 to generate a second control signal for controlling a second component of the acoustic signal. The second component is a reflected component of the acoustic signal. In this way, a single layer of first control sources and dual-layer consisting of second control sources are advantageous in achieving non-reciprocal acoustic transmission and absorption.

The second control signal is used to control the second component of the acoustic signal thereby to minimise the second component of the acoustic signal. In this way, a single layer of first control sources and dual-layer consisting of second control sources are advantageous in achieving perfect non-reciprocal acoustic transmission and absorption.

The second control source arrangement 954 comprises a 2D array of monopole control sources. Advantageously, by this construction, non-reciprocal acoustic control is realised in a two-dimensional or three-dimensional environment.

The first control source arrangement 952 and second control source arrangement 954 comprise loudspeakers. In this way, acoustic signals propagating along the propagation path may be controlled by the provision of acoustic signals by the first control source arrangement 952 and second control source arrangement 954. Here, the first control source arrangement 952 and second control source arrangement 954 each comprise a 2D array of control sources. In this example, the first control source arrangement 952 and second control source arrangement 954 each comprise a finite 2D array of monopole control sources. The first control source arrangement 952 and second control source arrangement 952 are correspondingly arranged to provide pairs of control sources comprising one control source from the first control source arrangement and one control source from the second control source arrangement.

As above, the active control unit 950 is arranged to control the first control source arrangement 952 and second control source arrangement 954. In an example, the active control unit 950 is arranged to independently control the first control source arrangement 952 and second control source arrangement 954. This may be referred to as “decentralised” control. The active control unit 950 is arranged to adaptively calculate the optimal voltage for controlling the first control source arrangement 952 and second control source arrangement 954 to maximise sound absorption.

By independently controlling the first control source arrangement 952 and second control source arrangement 954, a computational load advantage is realised. This is because it is only necessary to determine sets of filter coefficients for two filters for use in generating the control signals, one for each of the control source arrangements 952, 954.

The active control unit 950 can comprise a first controller 950a arranged to control the first control source arrangement 952 and a second controller 950b arranged to control the second control source arrangement 954. Advantageously, by providing separate controllers, computational complexity may be reduced as each controller is provided for a single operation or purpose. This may facilitate independent control (e.g., decentralised control). In FIG. 9, the wave separation algorithm used by the controllers 950a, 950b, or by active control unit 950 generally, is indicated generally at 924.

Referring to FIG. 10, an active acoustic control system 1000 according to a sixth embodiment is shown.

The active acoustic control system 1000 is for controlling an acoustic signal propagating along a propagation path. The active acoustic control system 1000 is for controlling a first acoustic signal propagating along a propagation path. In particular, the system 1000 is for actively absorbing the first acoustic signal to achieve non-reciprocal acoustic behaviour. In FIG. 10, the first acoustic signal propagating along the propagation path is indicated at 1010. The first acoustic signal 1010 is an acoustic signal propagating and incident on a structural component 1002, thereby to cause the structural component to vibrate or deform. The first acoustic signal 1010 comprises an uncontrolled portion 1012 and a controlled portion 1014. As shown in the figure, the controlled portion 1014 has a reduced amplitude compared with the uncontrolled portion 1012. This is by virtue of the active acoustic control system 1000 being operated to absorb the acoustic signal.

Additionally, a second acoustic signal is indicated at 1016. The second acoustic signal 1016 is an acoustic signal propagating and incident on the structural component 1002, but is not controlled so as to reduce its amplitude. In practice, the second acoustic signal 1016 may be an external (e.g., remote from the system) acoustic signal which it is desired to detect, for example an incoming acoustic signal. The first acoustic signal 1010 may be an internal acoustic signal, that is, a signal generated in the proximity of the system 1000, or at a vehicle in which the system 1000 is employed. In practice, the system 1000 is operable to receive and detect the second acoustic signal 1016, whilst preventing or inhibiting the first acoustic signal 1010 from propagating (e.g., being emitted) to the outside.

The active acoustic control system 1000 according to the sixth embodiment comprises substantially the same components as the active acoustic control systems according the above-described embodiments. Furthermore, the active acoustic control system 1000 according to the sixth embodiment is configured to operate in substantially the same way as the active acoustic control system according to the above-described embodiments, specifically the system 900 according to the fifth embodiment, to exhibit non-reciprocal acoustic control behaviour. That is, the interior is indicated at 1004 and the exterior is indicated at 1006, and the interior sound field is absorbed by the nonreciprocal device, whereas the exterior sound field is perfectly transmitted and can be detected.

The active acoustic control system 1000 is a non-reciprocal active acoustic control system. The system 1000 comprises an active control unit 1050, a first sensor arrangement 1060 and a first control source arrangement 1052. The first control source arrangement 1052 is operable to generate a first control signal to control a first component of the first acoustic signal 1010. In this example the first control signal is an acoustic control signal/wave.

The first sensor arrangement 1060 comprises accelerometers. The accelerometers can sense, or detect, deformation of the structural component 1002 to which they are attached, connected or arranged to sense. In this way, acoustic pressure generated by the acoustic signal may be indirectly sensed, by reference to the deformation of the structural component 1002. Here, the first sensor arrangement 1060 comprises a 2D array of accelerometers.

Whilst not illustrated in FIG. 10, the system 1000 may comprise a second sensor arrangement. In this way, acoustic pressure may be sensed or detected, enabling control of acoustic signals. Here, the second sensor arrangement comprises a 2D array of accelerometers.

Nevertheless, benefits of the invention may still be realised by providing only a first sensor arrangement 1060. In particular, this may still facilitate control of a transmitted component.

As explained above in relation to the system 900 of the fifth embodiment, a two-dimensional SFT may be used to decompose the signal components. Again, once the wave components A, B, C, D have been separated using the structural acceleration measured at each accelerometer, a feedforward FxLMS algorithm may be used to adaptively control the transmitted wave component C and reflected wave component B, where the reference signal for the calculation is the positive propagating incident wave A.

The system 1000 comprises a first control source arrangement 1052. Notably, the first control source arrangement 1052 comprises a finite 2D array of first control sources for generating a first control signal for controlling a first component of the acoustic signal. Advantageously, by this construction, non-reciprocal acoustic control is realised in a two-dimensional or three-dimensional environment. Furthermore, such a system is fully adaptable, and the direction of the non-reciprocal behaviour ca be reversed. Importantly, the present system does not introduce distortions or non-linearities into the signal, which avoids problems associated with distortions or non-linearities. Furthermore, broadband non-reciprocal control is realised.

As above, the active control unit 1050 may be retrofitted in a system or apparatus comprising the first sensor arrangement 1060 and first control source arrangement 1052.

The active control unit 1050 is configured to receive information from the first sensor arrangement 1060