WO1994029847A1

WO1994029847A1 - Three dimensional sound control with active noise cancellation

Info

Publication number: WO1994029847A1
Application number: PCT/US1994/004901
Authority: WO
Inventors: Sen M. Kuo; Mary K. Christensen; Robert A. Herold
Original assignee: Caterpillar Inc.
Priority date: 1993-06-11
Filing date: 1994-05-02
Publication date: 1994-12-22
Also published as: AU6824894A

Abstract

A three dimension localized quiet zone (3) suitable for a workplace (1) of an operator. An active noise cancelling signal is projected into the zone (3) where monitoring microphones (6, 7) are present. Input sound is acoustically sensed at the sources of a plurality of sound sources and normalized. The active noise cancellation is adjusted through an adaptive Filtered X Algorithm with modification for multiple monitoring microphones (6, 7) and multiple speakers (4, 5).

Description

THREE DIMENSIONAL SOUND CONTROL WITH ACTIVE NOISE CANCELLATION

Technical Field

The invention relates to the control of sound using active noise cancellation and in particular to the formation of a localized three dimensional quiet zone with a reduced sound level.

Background and Relation to the Prior Art

Active noise cancellation involves superimposing on a noise acoustic wave an opposite acoustic wave that destructively interferes with and cancels the noise wave. The cancelling acoustic wave is of equal amplitude but of opposite phase to the noise acoustic wave. The generation of the proper interference signal to produce cancellation, in the proper position at the right time, requires talcing into consideration a number of variables resulting in elaborate signal processing. The active noise cancellation principle is most useful at frequencies below 500 cycles per second (Hz) . Above that frequency range, noise attenuating materials applied to surfaces are more effective.

The implementation of the principle of active noise cancellation generally involves sensing of the characteristics of the noise acoustic wave, generating the cancelling acoustic wave and through monitoring the combined waves developing a feedback signal that keeps the cancelling wave in adjustment. The monitoring signal is frequently called an "error" signal.

Most implementations of the active noise cancellation principle also accommodate changes in the frequency and the intensity characteristics of the noise. This involves incorporating adaptability into a feedback from a monitoring microphone that provides information used in adjusting the cancelling wave. The computations involved in determining the adjustments to the cancelling wave are performed under a procedure known in the art as an algorithm. A number of algorithms with adaptability have evolved in the art. A survey article by J.C. Stevens entitled "An Experimental Evaluation of Adaptive Filtering Algorithms for Active Noise Control", Georgia Institute of Technology, GRTI/AERO, Atlanta, GA. 1992 Pages 1-10, provides an illustrative description of the current capabilities in the art. The implementation of the algorithms has been achieved in the art using digital signal processing and fabricated in the form of single semiconductor chip active noise controller devices.

Active noise cancellation systems exhibit instability under conditions where the cancelling signal gets into the noise prior to the sensing of the characteristics of that noise. Care in constructing a system is employed to prevent the situation or a modification of the algorithm can be used to accommodate it.

The active noise cancellation principle has been applied extensively in the art in arrangements where the noise source is localized and the operations of sensing, cancelling and monitoring can be positioned serially such as in reducing noise in ducts and pipes. An illustrative example is U.S. Patent 4,987,598.

When, however, there is an attempt to apply the active noise cancellation principle to three di ensional space many interdependent considerations are encountered. In three dimensional space the source of the noise is usually not localized, the complexity of the sound fields, where reflections from enclosures of various shapes may be involved, is usually significantly higher and the sensing, cancelling and monitoring operations in serial order may not be readily achievable.

There has been some effort in the art toward reducing the noise in a three dimension enclosure on a vehicle. In U.S. Patent, 4,506,380 a system is shown wherein a possible noise condition, such as a particular engine speed that has been measured with a tachometer is countered with a cancelling signal in the vehicle enclosure. In this system the cancelling signal is selected through a table look up operation of previously stored tachometer vs. noise data. In an article by Perry et al, entitled "The Use of DSP for Adaptive Noise Cancellation for Road Vehicles", Paper No. 3 Session 3, Pages 331 to 338, tachometer or ignition based indirect sensing of noise is processed in a controller with a cancelling signal being provided in a vehicle enclosure employing a plurality of peripherally mounted speakers and with the monitoring being through a distributed plurality of microphone pairs positioned at each seat. A seat for a vehicle with an active noise cancelling capability is shown in U.S. Patent 4,977,600 wherein cancelling speakers are positioned in wrap around portions that extend toward the occupants' head and flexibly mounted monitoring microphones are positioned in the vicinity of the occupants' ears.

Heretofore in the art the considerations of active noise cancelling of background noise encountered by operators in workplace locations have not been addressed.

Summary of the Invention The invention provides a localized region where the level of sound is controlled, that is three dimensional, and is of a size suitable as the workplace of an operator.

The controlled sound region is achieved by projecting an active noise cancelling signal into a localized region in the area with the background noise. An input to the cancelling signal is the product of a collection of individual noises that are part of the background noise each of which individual noises is collected acoustically at a location proximate to the source of the individual noise component, by placing at least one of a plurality of individual microphones adjacent each source of component noise. The cancelling signal is monitored by microphones placed in the controlled sound region and the cancelling signal is maintained in effectiveness by processing that performs the special functions of normalization of multiple inputs and in an adaptive filtering type algorithm accommodation of multiple cancellation signal speakers and prevention of component saturation.

Brief Description of the Drawings

Figure 1 is a schematic view of the sound control system of the invention.

Figures 2, 3 and 4 are cryptic sketches of the remote acoustic sensing of exemplary individual types of sources of noise that make up the background noise. Figure 5 is a sketch illustrating the acoustic paths and electrical component wiring of the invention.

Figure 6 is a diagram of a functional block representation of the algorithm employed in processing the active noise cancellation signal.

Description of the Invention

The invention involves structural and processing modifications that permit the principle of active noise cancellation to be used in producing a three dimensional quiet zone in a noisy area that has a size suitable for an individual such as an equipment operator. While the shape of the quiet zone and the amount of sound level reduction in it may be affected by complex sound fields involving speaker positioning frequency of the noise being cancelled and reflection from nearby objects; the quiet zone should have at least a theoretical lOdB sound level reduction within an essentially spheroidal shape with a diameter about 1/5 of the wavelength of the particular noise frequency. Expressed another way, the at least lOdB reduction or about 1/100 of the mean square pressure of the primary noise source, should extend about 0.1 of the wavelength from the centerpoint of cancellation. The quiet zone is about the size of the head and shoulders of an equipment operator.

The invention is particularly useful in construction equipment, wherein the primary noise source is the equipment's drivetrain (not shown) . The invention includes a means 29 for reducing the sound level in the quiet zone which is a result of the primary noise source. In the preferred embodiment, the primary noise reducing means includes an acoustic input or primary microphone 30 located generally in the vicinity of the drivetrain and particularly within an enclosure of the drivetrain. Alternatively, the primary noise may be sensed by other types of sensors, e.g., tachometers.

The invention is particularly valuable in controlling low frequency, up to 500 cycles per second (Hz) sound. At those low frequencies, passive sound barriers are minimally effective.

The structural modifications involve the positioning of the speakers or cancelling signal transducers; the positioning of the monitoring microphones or error transducers; and the positioning of the sensing or input microphones for acoustic sensing of sounds, that are components of the background noise, at the location of the source of that sound.

The processing modifications involve normalization of multiple input signal levels, and accommodation of the presence of several speakers and several monitoring microphones and the prevention of component saturation in the algorithm used in developing the cancellation signal. Referring to Figure 1 there is shown a schematic view of the sound control system of the invention wherein in a space 1 surrounded by background sound 2, shown with dashed lines, a quiet zone, or zone of reduced sound level 3 is produced by projecting into the zone 3 that is to be quiet an active noise cancellation signal through an acoustic transducer means shown in this illustration as speakers 4 and 5. The active noise cancellation signal is monitored through a monitoring sound pickup means shown in this illustration as microphones 6 and 7.

The background sound 2 is made up of at least one and often several sources of sound with the location of the sources being outside the area of the quiet region and usually outside of an enclosure not shown.

The active noise cancellation signal projected by the speakers 4 and 5 is a signal designed to cancel the primary noise and the selected component noises sound frequencies in the background noise 2. The monitoring or error signal sensing is accomplished with microphones 6 and 7 positioned in the quiet zone 3 generally close to the location of the ears of the operator. A processor 8 is provided that performs the mathematical operations and signal generating operations required in translating the monitoring and input signals into a constantly updated cancellation signal. The input sounds are acoustically sensed at positions that are proximate to the sources of those sounds. In this illustration three, 9, 10 and 11 are shown that would pick up the sound at the source of origination from such items as running machinery and air conditioning pumps.

The speakers 4 and 5 and the microphones 6 and 7 are connected to the processor 8 by conductors 14, 15, 16 and 17, respectively.

In accordance with the invention the system removes from the background sound as many component sounds as practical while permitting the operator or occupant of the quiet zone the freedom to move and the ability to hear the sounds necessary such as emergency indications. In operation, signals indicating the condition of the cancelling signal being projected into the quiet zone 3 are transmitted by the monitoring or error microphones through conductors 16 and 17 to the processor 8. The processor 8 processes and normalizes the input signals from the selected sources of noise to be cancelled delivered through microphones 9, 10 and 11. The processor also processes the signals from the monitoring microphones using an adaptive least means squared type of algorithm known in the art with modifications to accommodate several cancelling speakers to improve stepping in developing filter values and to prevent component saturation in developing a constantly updated cancellation signal, which is delivered to the speakers 4 and 5 through conductors 14 and 15.

The quiet zone 3 is about the size of the head and shoulders of a human, shown dotted in Figure 1. The zone 3 can be positioned by selecting the location of the speakers 4 and 5 so that the operator would not be confined to a particular standing or sitting type of position. The monitoring microphones are positioned in the quiet zone in the vicinity of where the ears of the operator are likely to be. An aspect of the invention is the sensing of sound outside the area of the background sound region where the operator must work to reduce the sound level in the quiet zone 3. In Figures 2, 3 and 4 cryptic sketches are provided of the remote acoustic sensing of three different types of sound sources. The noise components usually desired to be removed are those that raise the general level but have a low probability of containing information useful to the operator. In accordance with the invention the input microphone is positioned close to the source of the sound but the sound travels through the air to the microphone. In some prior art applications indirect sound indicators such as tachometers and ignition pulse counters have been used. It has been found that acoustic pickup not only cancels harmonics but also can cancel broad background noise and is a more faithful replica of what the ear would hear. Referring to Fig. 2, the sound of a liquid in a pipe 18 in a remote location symbolized by a wall portion of an enclosure is shown with the pick up microphone 9 being separated by air from the pipe 18. Referring next to Fig. 3, a sound source having a steady noise of a conveyor 19 and an intermittent percussive noise of a stamping operation 12 is shown separated by an enclosure again symbolized by a wall and with the pick up microphone 10 separated by air from the sound sources. Referring next to Fig. 4 the sound of rotating machinery 21 located in a location remote from the quiet zone region is illustrated. In this illustration as in the others the pickup microphone 11 is separated by air from the source 21. The sensing of the input noise from source locations that are remote also reduces any acoustic feedback from the cancelling signal and permits the effect from having to be taken into consideration in the algorithm. Referring next to Figure 5 wherein a sketch is shown illustrating the acoustic paths and electrical component wiring in the invention. The same reference numerals for like components as in Figure 1 are employed. In Fig. 5 the acoustic paths between the speakers 4 and 5 and the monitoring microphones 6 and 7 are shown as dash-dot lines to illustrate that where there is more than one cancelling speaker and more than one monitoring microphone the algorithm in the controller 20 of the processor 8 must accommodate the fact that each monitoring microphone will receive input from each speaker.

In the processor 8 for the monitoring microphone signals enter the controller 20. There is also an input signal normalization stage consisting of an input signal normalization stage consisting of a summing amplifier 23, and input operational or reference amplifiers 24, 25, 26 and 31 for the remotely sensed noise signals from microphones 9, 10 and 11 and for the primary noise sensing means 29, respectively.

In the controller 20 stage of the processor 8 there is an adaptive least means squares type of algorithm embodied in integrated circuit form. The output stages 27 and 28 deliver the cancelling signal from the controller 20 through conductors 14 and 15 to the speakers 4 and 5 respectively.

In a preferred embodiment the controller 20 is a commercially available integrated circuit. A satisfactory model for controller 20 is the TMS 320C30 Floating Point DSP manufactured by Texas Instruments, Inc., Dallas, Texas. A satisfactory model of a microphone suitable for an input microphone is a model PZM distributed by the Radio Shack Co.; and a satisfactory model of a micrphone suitable for a monitoring microphone is a model SM98A made by the SHURE, CO. A satisfactory model of a cancelling speaker is the Rockford Fosgate PR0128 12" Subwoofer. A satisfactory algorithm is the Multi Channel Filtered X Least Mean Squares type with modifications to accommodate multiple speakers component saturation and small steps. A preferred embodiment example is shown in Fig. 6.

Referring next to Figure 6 there is shown a diagram of a functional block representation of the algorithm employed in processing the elements of the active noise cancellation signal of the invention. The algorithm is the Multi Channel Filtered X Least Mean Squares type known in the art with the modifications of the invention. In the diagram of Fig. 6 the elements represent the functions of variables that influence the cancelling signal and the monitor or error signals. The algorithm operates by calculating an error correction for the cancelling signal, applying the correction and repeating in a series of cycles till a minimum variation is achieved. The selected input noise signals delivered through terminals 9, 10 and 11 have been normalized by providing weighting factors for the input microphone paths and algebraically adding them in a summing amplifier as shown in Fig. 5 and the combined signal enters the algorithm as a normalized input. The cancelling signal as shown from the acoustic paths in Figure 5 travels in the main paths directly from the speakers to the monitoring microphones 6 and 7. The adaptive filters of the controller provide adjustment of the cancellation signals. A portion of each signal in the acoustic path goes direct to the nearest microphone and across to the other microphone. The main, the direct from cancellation signal 1, and the cross from cancellation signal 2, are acoustically superimposed to form error signal 1, which in turn, is introduced into the controller labelled FILTERED X LMS. A similar situation takes place to produce error signal 2. The changes in the adaptive filter as the iteration steps take place is indicated by a dashed arrow. The size of the steps taken will influence how soon the algorithm will converge on the optimum minimum error in the cancelling signal. The weight vector is changed every iteration. The step size is influenced by input signal and error signal power. The algorithm accommodates in the filter weighting for the multiple paths from the speakers to the monitoring microphones and for component saturation by reducing step size.

What has been described is the formation of a quiet zone in three dimensional space that is suitable for an operator workplace by an interdependent positioning of the elements of active noise cancellation, the remote acoustic sensing of background noise sources and the modification of a standard processor algorithm.

Claims

1. A noise control system comprising in combination: at least one sound cancellation speaker

(4,5), each said speaker (4,5) being positioned to project sound into a three dimensional zone (3) suitable as an operator workplace (1) ; means (29) for acoustically sensing a primary noise source and responsively producing a primary noise signal; at least one monitoring microphone (6,7), each said monitoring microphone (6,7) being positioned in said three dimensional zone (3) ; at least one input microphone (9,10,11), each said input microphone (9,10,11) being positioned proximate to and acoustically sensing an individual source of background sound located separate from the area of said zone (3) ; signal translation means (23,24,25,26,31) operable to normalize the signals from each of said at least one input microphone (9,10,11) and primary noise signal into a single signal, and processor means (20) responsive to said normalized single signal and to signals from each said at least one monitoring microphone (6,7) and operable to provide an iteratively corrected cancellation signal to each said at least one sound cancellation speaker (4,5) .

2. The noise control system of claim 1 wherein said at least one sound cancellation speaker (4,5) is two sound cancellation speakers (4,5).

3. The noise control system of claim 2 wherein said at least one monitoring microphone (6,7) is two monitoring microphones (6,7).

4. The noise control system of claim 3 wherein said at least one input microphone (9,10,11) is three input microphones (9,10,11).

5. The noise control system of claim 4 wherein said signal translation means (23,24,25,26,31) includes an operational amplifier (24,25,26,31) for each input microphone (9,10,11) with the output of the operational amplifiers (24,25,26,31) summed in a summing amplifier (23) .

6. The noise control system of claim 4 wherein said processor (8) is an integrated circuit of a multiple channel filtered X least means squares algorithm modified for multiple sound cancellation speakers (4,5).

7. Sound control apparatus for forming a three dimensional quiet zone (3) comprising in combination: cancellation sound means (4,5) operable to project cancellation sound into a zone (3) ; sound monitoring means (6,7) positioned in said zone (3) ; means (29) for acoustically sensing a primary noise at a primary noise source; means (9,10,11) for acoustically sensing an input background sound (2) at each of a plurality of separate background sound sources (12,18,19,21) each at a location away from said zone (3) ; and signal processing means (8) operable to normalize signals from each said input sound source (12,18,19,21) and said primary noise sensing means (29) and further operable to deliver to said cancelling sound means (4,5) an iteratively corrected cancellation signal responsive to signals from said monitoring means (6,7) and said normalized signals.

8. A noise control system comprising in combination: a signal processor (8) ; at least one input microphone (9,10,11), each said input microphone (9,10,11) being positioned proximate for acoustic sensing of a noise at the source thereof, each said noise being a component of said background noise (2) , each said input microphone (9,10,11) being connected to said processor (8); a primary microphone (30) being positioned proximate a source of primary noise for acoustically sensing a primary noise, said primary microphone (30) being connected to said processor (8) ; at least one sound cancelling speaker (4,5), each said sound cancelling speaker (4,5) being positioned to project a sound cancelling signal into a three dimensional zone (3) , said zone (3) being separated from each said source location of a sensed component noise, each said speaker (4,5) being connected to said processor (8) ; at least one monitor microphone (6,7), each said monitor microphone (6,7) being positioned within said zone (3), each said monitor microphone (6,7) being connected to said processor (8) ; processing means (8) for supplying to each said at least one speaker (4,5) a signal that destructively interferes with at least a portion of said background noise (2) ; and said processing means (8) destructive interference signal being operable in response to signals from said monitoring microphones (6,7) signals from said primary microphone (30) and signals from said at least one input microphone (9,10,11), to adjust any difference between said cancelling signal and at least the component noises of said background noise (2) .

9. The background noise control system of claim 8 wherein all signals from input microphone

(9,10,11) are normalized to a single signal.

10. The background noise control system of claim 9 wherein there are three input microphones (9,10,11) and two monitoring microphones (6,7).

11. The background noise control system of claim 10 wherein said processor (8) is an integrated circuit of a multiple channel filtered X least mean squares algorithm modified for multiple sound cancellation speakers (4,5).

12. The process of controlling the noise level in a region surrounding at least a portion of a person comprising the steps of: positioning at least one speaker (4,5) for projecting sound into a zone (3) of a size capable of surrounding at least a portion of a person; providing at least one monitoring microphone (6,7) in said zone (3); providing a primary microphone (30) positioned adjacent a primary noise source (12,18,19,21); providing at least one input microphone (9,10,11) each positioned adjacent to an individual source of a component noise of said background noise (2), each said input microphone (9,10,11) acoustically sensing the adjacent individual noise; and processing and delivering to said speaker (4,5) a noise cancellation signal responsive to signals from all said input microphones (9,10,11), said primary microphone (6,7) and signals from said monitor microphones (6,7) said processing producing a correcting signal for any change in said cancellation signal.

13. The process of claim 12 including the step of normalizing all of said input microphone

(9,10,11) signals to a single signal.

14. The process of claim 12 including in said processing step the summing of signals produced from all signal paths between each of said speaker (4,5) and each of said monitoring microphones (6,7).