Active sound-cancellation system for an open fluid-duct

Abstract

The present invention is in the field of an active sound-cancellation system, an open fluid-duct comprising such an active sound-cancellation system, such as an air duct, and an active sound-cancellation computer program comprising instructions for operating such an active sound-cancellation system. The active sound-cancellation system reduces noise significantly.

Claims

1. An active sound-cancellation system for an open fluid-duct comprising a carrier, the carrier comprising at least one fixator for fixing the carrier to the duct, at least one axial array of n.sub.sm.sub.s audio sensors and n.sub.am.sub.a audio actuators, wherein, each individually, n.sub.a,s2, and m.sub.a,s1, wherein n.sub.s sensors and n.sub.a actuators are parallel to the axis of the duct, and wherein m.sub.a,s[2,2.sup.6], and at least one sound-cancellation controller, wherein the at least one sound-cancellation controller is configured to receive audio input from the at least one sensor array, configured to process said audio input, and configured to provide output to the at least one actuator array, wherein said output activates the actuator to reduce sound in a frequency domain of 10 Hz-100 kHz, wherein at least one sound-cancellation controller is adaptable, characterized in that in the longitudinal direction at least one sensor and at least one actuator are each individually spaced apart, at a distance of 1-25% of a duct diameter.

2. The active sound-cancellation system according to claim 1, comprising at least one of a clock operating at a frequency of 1 Hz-10 GHz, at least one low-latency high resolution sigma-delta analogue-digital converter (ADC) for providing at least one of a single and multiple-bit output stream, at least one ADC analogue input, at least one ADC digital output, the at least one ADC digital output being in electrical connection with a digital loop filter, at least one digital loop filter in digital connection with at least one ADC, having at least one digital output, the at least one digital loop filter operating in a time domain, and at least one pulse width modulating (PWM) controller for receiving digital output from the digital loop filter and providing PWM output, wherein the controller is programmable and adaptable, wherein the ADC latency in use is one clock cycle, and wherein the sound-cancellation controller is selected from an integrated circuit, an artificial intelligence unit, a trainable and adaptable artificial intelligence unit, and embedded software.

3. The active sound-cancellation system according to claim 1, wherein the at least one audio sensor is capable of receiving audio-signals at a frequency of 5-100000 Hz, and wherein the at least one actuator is capable of providing audio-signals at a frequency of 5-100000 Hz.

4. The active sound-cancellation system according to claim 1, wherein the at least one sensor each individually is configured to sample at a sample frequency of 100 Hz-100 MHz, and wherein each sensor individually comprises at least one field effect transistor, and wherein a series of n.sub.s and m.sub.s sensors is connected in series.

5. The active sound-cancellation system according to claim 1, wherein the at least one actuator each individually is configured to provide active sound cancelling at a cancelling frequency of 1 kHz-500 kHz, and wherein the at least one actuator each individually is configured to provide a sound pressure of 20-150 dB, and wherein in a row of n actuators the actuators are configured to be in phase at a given frequency.

6. The active sound-cancellation system according to claim 1, wherein the at least one sensor and at least one actuator are each individually a transducer, and wherein the transducer is selected from a MEMS, a moving coil, a permanent magnet transducer, a balanced armature transducer, and a piezo-element.

7. The active sound-cancellation system according to claim 1, wherein in an axial array, each individually, 50-100% of array elements is provided with a sensor and an actuator, respectively.

8. The active sound-cancellation system according to claim 1, comprising 2-10 axial arrays, and wherein axial arrays are at least partly provided along a horizontal axis.

9. The active sound-cancellation system according claim 1, wherein in the longitudinal direction the at least one sensor and at least one actuator are each individually spaced apart at a distance of 2-10% of a duct diameter, and wherein each individual sensor is coupled to activate at least one individual actuator.

10. The active sound-cancellation system according to claim 1, wherein the fixator is at least one fin.

11. The active sound-cancellation system according to claim 1, wherein each actuator individually is configured to provide at least one of a sound pressure perpendicular to the longitudinal axis of the system, and a sound pressure parallel to the longitudinal axis of the system.

12. The active sound-cancellation system according to claim 1, wherein an n+1.sup.th sensor is positioned adjacent along a horizontal axis of the sound-cancellation system of an n.sup.th actuator.

13. The active sound-cancellation system according to claim 1, wherein the system comprises at least one of a primary feedforward path and a feedback path for cancellation, the feedforward path receiving output from a sound shaper and providing input to a second adder, the sound shaper is configured to shape propagation of a sound wave, phase, and frequency of sound, after noise filtering above 100 kHz, the feedback receiving output from the at least one sound-cancellation controller output and providing input to the at least one first adder.

14. The active sound-cancellation system according to claim 1, wherein the system comprises at least one of a first adder receiving input from the feedback path and a reference path, respectively, wherein the first adder provides input to a first subtractor of the at least one sound-cancellation controller, wherein the at least one sound-cancellation controller comprises a feed forward sound-cancellation controller receiving input from the first subtractor and providing output to a loop-shaping filter, the loop-shaping filter providing input to the sound-cancellation controller output and to an estimator in a sound-cancellation controller feedback path, the sound-cancellation controller feedback path providing input to first subtractor for subtracting from the first adder.

15. The active sound-cancellation system according to claim 1, wherein the system further comprises at least one secondary path receiving input from the sound-cancellation controller output and providing output to a second adder, the second adder optionally providing output to an error sensor.

16. An open fluid-duct comprising an active sound-cancellation system according to claim 1, wherein the fluid duct is an air-duct, selected from a ventilation, a pump, a heating installation, a cooling installation, a window, an exhaust, a motor of a ship, a motor of a heavy engine, an internal combustion airbreathing engine, an internal combustion airbreathing jet engine, a jet-engine, a turbojet, a turbofan, a ramjet, a pulse jet, and a pipe-line.

17. The open fluid-duct comprising an active sound-cancellation system according to claim 16, wherein the active sound-cancellation system is positioned on a longitudinal axis of the duct, and wherein the active sound-cancellation system is positioned in the 10-40% downstream section relative to a length of the longitudinal axis part of the duct, and wherein the active sound-cancellation system is positioned at a junction in the fluid duct, and wherein a frontal surface area of the active sound-cancellation system is 2-75% of the cross-sectional area of the duct.

18. An active sound-cancellation computer program comprising instructions for operating an active sound-cancellation system according to claim 1, the instructions causing the computer to carry out the following steps: activating the at least one sensor, receiving input from the at least one sensor, the input comprising sound spectral and sound pressure information, activating the at least one actuator, therewith performing one of reducing sound pressure in the duct and leaving the duct for at least one sound frequency.

19. The active sound-cancellation computer program according to claim 18, comprising instructions to activate at least two sensors simultaneously, and to activate at least two actuators simultaneously, to measure the sound pressure over a longitudinal axis of the duct and over a cross-sectional area of the duct, and to reduce a sound pressure leaving the duct by >20 dB.

20. The active sound-cancellation computer program according to claim 18, comprising instructions to calculate and predict a sound pressure over a longitudinal axis of the duct and over a cross-sectional area of the duct, and comprising instructions to feed forward activate at least one actuator, and comprising instructions to activate an n+1.sup.th actuator by an n.sup.th sensor, based on the input of the n.sup.th sensor.

Description

SUMMARY OF FIGURES

(1) FIGS. 1, 2a-c, 3a-b, 4-6, 7a-e and 8 show details of technical features.

DETAILED DESCRIPTION OF FIGURES

(2) In the figures: 1 sound-cancellation system 10 carrier 11 fixator 12 array 13 sensor 14 actuator 20 sound cancellation controller
FIG. 1 shows an experimental set-up, wherein All paths from actuators and disturbance actuator to all reference sensors and the error sensor are determined, one actuator at the time. The other actuators are turned off. These paths correspond to G, H, P, X in FIG. 1.

(3) FIG. 2a shows schematics of a prior art single-input-single output set-up. FIG. 2b shows schematics of the present multiple-input-multiple-output system. FIG. 2c shows schematics of a prior art single-input-single output set-up.

(4) FIG. 3a-b shows an exemplary MIMO system with 4 sensors and actuators (numbered accordingly), and a duct in FIG. 3a, wherein the MIMO-system partly is incorporated, for visibility only.

(5) FIG. 4 shows a simulation of sound pressure level (dB) of a parasitic mode shape at 4226 Hz and possible reference sensor positions A,B, respectively. In the figure, only actuator 2 is active.

(6) FIG. 5 shows performance comparison for best performing set-ups of the prior art system (SISO), the present system (array), in comparison to an empty and two passive setups.

(7) FIG. 6 shows the noise reduction of the present system compared to ear-buds and head-phones.

(8) FIG. 7A-E and FIG. 8 show experimental results.

(9) The figures are further detailed in the description.

(10) The invention although described in detailed explanatory context may be best understood in conjunction with the accompanying figures.

Experimental Results

(11) The experimental results have been a result of an MSc-Thesis program of C. Jansen, with title ACTIVE NOISE CONTROL IN VENTILATION DUCTS USING A DISTRIBUTED LOUDSPEAKER AND MICROPHONE ARRAY, under embargo until Jun. 17, 2021, which document and its contents are incorporated by reference.

(12) A silencer (sound cancellation system) is designed with the following performance goals in mind: over its intended bandwidth, it should only couple to plane waves, minimize acoustic feedback from actuators to reference sensors, have sufficient output capability for a typical residential application, have its actuators and reference sensors remain within their linear regimes, have no or well damped resonances in its transfer functions related to actuators or reference sensors, fit inside a duct of 125 mm diameter and have no passive damping material. It is desirable to extend the bandwidth to a frequency as high as possible. Initially, an array length of four elements has been chosen arbitrarily. Later on, computational limitations resulted that only three reference sensors and actuators could be used. Simulations with COMSOL Multiphysics have been performed to optimize the geometry. A schematic drawing of the finished silencer is shown in FIG. 3a-b. Each identical element contains one Tymphany 830970 loudspeaker (actuator) and three Kingstate KECG2742TBL-A microphones (together forming a reference sensor) having a flat frequency response of 20-8 k Hz 1 dB connected in parallel. The dimensions are kept as small as possible, with a diameter of 65 mm and length of 53 mm, resulting in an internal volume of 0.09 L which is lightly stuffed with polyester wadding. The actuators, typically a speaker, are selected on the basis of swept volume and compact dimensions and have a resonance frequency of about 260 Hz when mounted. They are driven by a Caliber CA75.4 car audio amplifier. The enclosure wall that opposes the actuator of the adjacent element follows has a conical shape, while following the shape of the dome at the centre of the actuators diaphragm, to increase the frequency of the parasitic resonance of the air gap.

(13) As a sample, the mode shape is investigated when actuator 2 is active and the other actuators are turned off. The resulting mode shape is shown in FIG. 4 and has been simulated with anechoic duct terminations.

(14) All paths from actuators and disturbance actuator to all reference sensors and the error sensor are determined, one actuator at the time. The other actuators are turned off. These paths correspond to G, H, P, X in FIG. 1. The actuator under investigation is fed a pink noise signal, while both this signal and all reference and error sensor signals are measured. Ten seconds of signals are captured, after having allowed the system to settle for two seconds. Then FIR filters are fit to them, using a least mean squares fit that minimizes the difference between the output and filtered input signal. The disturbance signal and the error sensor are only part of the experimental set-up, not of the present sound-cancellation system.

(15) The controller structure is feed forward with feedback cancellation. A schematic of the controller and relevant acoustic paths is shown in FIG. 1. Grey areas represent digital signal manipulation in the Micro-LabBox, for which the code is generated by Simulink. These are: feed forward controller C. loop shaping filter F, estimate of feedback path Gest and noise shaping filter R. Blocks outside of the grey areas represent electro-mechanical paths, which all include, in the following order: digital to analogue conversion, amplifier, loudspeaker, acoustic path, microphone, micro-phone preamplifier, analogue to digital conversion and a discrete second order Butterworth high pass filter at 1 Hz. These are acoustic feedback path G, secondary path H. primary path P and reference path X. Note that all inputs and outputs of the MicroLabBox are signals to actuators and from sensors. Therefore reference path X not only contains the path from sound field to reference signal, but also the path from disturbance actuator signal to sound field.

(16) Due to limitations in processing power in the experimental set-up, only actuator 2-4 and reference sensor 1-3 are used for the array. These are chosen to maximize the time lead between reference sensors and actuators. The filter lengths could not be shortened, because the impulse responses of the real paths they describe takes some time to decay. The SISO system uses actuator 4 and reference sensor 1. These are chosen because this results in the same maximum time lead as for the array and because it is expected that the coherence between these is good, as the acoustic feedback path has a smooth transfer function as compared to otherreference sensor pairs.

(17) Stability robustness is determined by the feedback path caused by acoustic feedback from actuators to reference sensors. This cannot be completely cancelled, leaving a residual that can lead to instability. There may be several causes for imperfection of the feedback estimate, such as a change in temperature or air velocity.

(18) Robustness is pursued by trying to keep the gain of all individual feedback paths CF(G-G.sub.est) below 1 at all frequencies. This is implemented in the following way. It is assumed that effort weighting causes C to have a flat amplitude response, of which the level is dependent on the amount of effort weighting. Therefore F(G-Gest) must have flat amplitude responses as well. First, the transfer functions G-Gest from actuators to reference sensors including imperfect feedback cancellation are estimated. Then for each actuator, the worst transfer function to the reference sensors is picked. An FIR filter F with a length of 801 taps, having an inverse amplitude response and minimum phase behaviour, is designed, and added to the relevant actuator. Its transfer function is the inverse of G-Gest multiplied by the desired open loop response. To avoid compensating for narrow-band notches in the frequency response, F is taken to be the lowest value of this inverse and the smoothed inverse. Actuator overload at low frequencies is limited by limiting the gain of Fat f<300 Hz to that of a first order high pass filter. The resulting filter is made minimum-phase. Performance robustness additionally is determined by noise gain. Noise is generated by turbulence and by the circuit inside the microphones.

(19) Steel spiro ductwork of 125 mm diameter has been used. Airflow is not taken into account and the air is at room temperature. The disturbance signal is generated by a Tang Band W2-2040s loudspeaker mounted in the centre leg of a T joint. The cavity between loud-speaker housing and duct wall is filled by melamine foam. At one side of the joint is a straight duct of 111 cm, containing the silencer, and terminated by an exit. At the other side there is an anechoic termination, made from a straight duct of 150 cm, with a closed end, loosely filled with polyester wadding. The exit has three microphones in parallel to capture the residual signal, together forming the error sensor. For calculation of noise shaping filter R, the exit was replaced by another straight duct of 150 cm, with a closed end. Both straight ducts are loosely filled with polyester wadding to make them appear as anechoic terminations. The silencer was removed, and a single microphone was placed inside the duct, midway between duct top and bottom at a node of the first vertical mode, and 39 mm towards the side wall at a node of the first axisymmetric mode. The microphone is not at a node of the first lateral mode between the duct side walls, which is not a problem, because this mode is not excited due to symmetry.

CONCLUSION

(20) An array of three reference sensors and three actuators has a larger insertion loss than the SISO system, by coupling to the sound field over a wider range of frequencies and avoiding the necessity of having large gains in the controller at some frequencies. The advantage in this experiment is caused by the arraying of the reference sensors, while the setup was such that the results are not suitable to draw conclusions about arraying of the actuators. At the same value of effort weighting parameter /3, the array performs similar to the SISO system. The array obtains a higher insertion loss than the SISO system, because more effort weighting can be applied, without the system becoming unstable. The maximum insertion loss for the array was 6.7 dB(A) and for the SISO system it was 3.9 dB(A), yielding a difference of 2.8 dB(A), and the array has the added advantage that the residual has a more even spectrum. For the specific disturbance of shifting the reference sensors and actuators by 3 cm, the performance robustnesses are similar.

FURTHER EXAMPLES

(21) It is noted that two phenomena are considered to affect the design of ducted active noise cancellation systems and are explained. The present array approach covered manages both effects. For non-linearities other non-array techniques are also listed.

(22) Observability and Controllability

(23) The acoustic observability and controllability issue is considered a fundamental property of a duct and single microphones, and speakers, are typically unable to detect and or correct for specific frequencies if present in the unwanted disturbance. The present array of strategically placed elements overcomes this fundamental limitation.

(24) Non-Linearities

(25) Non-linearities are considered inevitable in real systems and may originate within any domain and propagate to other domains. Moving coil actuators (speakers) are prone to non-acoustic non-linearities at lower frequencies. It is noted that in a ducted system nonlinearities are amplified significantly at specific frequencies and significantly impact noise cancellation performance.

(26) Observability and Controllability

(27) With reference to FIGS. 7A-E (Transfer functions for the SISO system shown (Prim, Sec1 are actuators; Ref1 and err are micro-phones).

(28) FIG. 7E shows three forward paths and one feedback path Feedforward: Disturbance source (Prim) to optimisation point (Err) (FIG. 7A) Feedforward: Audio controller speaker (Sec1) generating anti-noise in the centre to optimisation point (Err) (FIG. 7B) Feedforward: Disturbance source to sensing microphone (Prim to Ref1) (FIG. 7C) Feedback: Audio controller speaker (Sec1) to sensing microphone (Ref1) (FIG. 7D)

(29) The four graphs 7A-D show the transfer functions for each of these paths.

(30) Natural Characteristic for a Duct:

(31) FIG. 7A shows a typical transfer function for the disturbance to error path for an open duct at the disturbance end with a closed receiver end (as in an ear-canal).

(32) Controllability Issue: FIG. 7B shows the transfer function of the Sec1 to Err. Note the null at 322 Hz. The speaker Sec1 has limited ability to control this frequency at the Err microphone. The controller may correctly drive the speaker to generate the required correction signal however the transfer function shows that its amplitude of the anti-sound at the Err microphone will be low and unable to cancel a disturbance at this frequency.

(33) Observability Issue: FIG. 7C shows the transfer function from the prim to the ref1 microphone. There are three nulls in this experimental configuration at 178, 546, 987 Hz. Any frequency components at these frequencies are significantly attenuated at the reference microphone. They are not detectable (observable) by the microphone. This prevents the controller from taking corrective actionit simply cannot see an issue.

(34) The observability and controllability sensitivity are fundamental for a single element system. Using the present array of sensors and actuators with enough elements can properly observe and control the whole frequency range.

(35) Non-Linearity

(36) The speakers generate the non-linearities in this experimental data. At low frequencies the actuator generates the solid line curves measured at 1 m in free space. When the same transducer is placed in a duct the energy no longer dissipates as it is contained with the duct leading to higher sound pressure levels. Harmonic content falling at critical frequencies is disproportionally amplified by the duct (The sensitivity is at the duct eigenfrequenciesSee Prim to Err transfer function above).

(37) Consider a 56 Hz source date in free space. The 5.sup.th harmonic is at 280 Hz and is relatively inconspicuous at about 50 dB below the fundamental. The same frequency generates the largest component at 280 Hz in the ducted system (dashed lines). It is now only about 20 dB below the fundamental and would significantly impact the overall noise cancellation performance if the disturbance contained 56 Hz.

(38) In a single actuator system, a much higher quality lower distortion device must be used to avoid generating the non-linearities. The actuator is likely to be larger and more expensive. Achieving high performance at high SPL will remain challenging.

(39) In the present array-based system specific actuators are configured to avoid operating at frequency that generate high distortion allowing lower cost components without compromising performance. Alternatively, local feedback control linearises (reduce) the distortion, or pre-compensation (or sometimes post compensation) reduces the distortion.

(40) It should be appreciated that for commercial application it may be preferable to use one or more variations of the present system, which would be similar to the ones disclosed in the present application and are within the spirit of the invention.