TOP SPEAKER MOUNTING FOR ACTIVE NOISE CANCELLATION

Technical Field of the Invention

The invention relates to the control of the sound level in a vehicle operator station and in particular to the positioning of the cancelling speaker in an active noise cancellation zone.

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 taking 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 usually 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 (DSP) and fabricated in the form of a 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 under conditions where the noise sources is localized and the operations of sensing, cancelling and monitoring can be positioned serially as in the case 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 dimensional 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 an arrangement of sensing, cancelling, and monitoring operations in serial order may not be readily achievable. In implementing the active noise cancellation principle in vehicle operator stations, further considerations require attention. The operator station is frequently in an enclosure with a relatively small volume so that there is minimum sound attenuation in the air between the walls and the occupant. The noise enters the enclosure both by the surrounding air and by vibration. There are many different types of noise associated with a vehicle such as engine, fan, hydraulic, gear drive as well as noise produced from motion of the vehicle, the sources of all of which are close to the operator station. The material out of which the vehicle is made is usually a good transmitter of vibration.

There has been some effort in the art toward reducing the noise in a three dimensional enclosure in a vehicle. In U.S. Patent 4,506,380 a system is shown for reducing sound throughout a multi-occupant enclosure wherein a possible noise condition, such as a particular engine speed, that has been measured with a tachometer is countered with a single speaker cancelling signal in the vehicle enclosure. In the system of the patent 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 for an entire multi-occupant enclosures being provided through a plurality of peripherally mounted speakers, and with the monitoring being through a distributed plurality of microphone pairs positioned at each seat.

In U.S. Patent 4,977,600 the principle of active noise cancellation is applied to an individual vehicle seat. In that patent cancelling speakers are positioned in wrap around portions of the seat back near the head of the occupant and flexibly mounted monitoring microphones are positioned in the vicinity of the ears of the occupant.

Heretofore in the art, the considerations in positioning the elements required for active noise cancellation in an operator station enclosure has not received much attention.

Summary of the Invention The invention provides a large and focusable, localized reduced sound zone, in the workplace of an operator in a vehicle. The zone can be focused about the head and shoulders of an operator as well as on a working surface. The invention involves the placing of one or several active noise cancellation speakers in the top portion of an operator enclosure in a vehicle. The invention provides a cancelling speaker mounting position that improves the focus and intensity of the cancelling signal, provides a larger reduced sound zone and simplifies the processing of the signals.

Brief Description of the Drawings

Fig. 1 is a perspective view of the mounting of active noise cancellation speakers at the top of an operator enclosure of a vehicle.

Description of the Invention

The invention involves the placing of the speaker or speakers used in active noise cancellation in the workplace of an operator in an enclosure of a vehicle, in the top of the enclosure. The positioning of the speaker or speakers in the top of the enclosure improves the focus and extent of a zone with a reduced sound level formed in the workplace of an operator by active noise cancellation. The positioning of the sound cancellation signal transducer or speaker in the top of the operator enclosure further increases the intensity and extent of the zone of reduced sound level. Where the cancelling speaker or speakers is directly above the workplace, there is permitted an extension of the reduced sound zone throughout more of the volume of the enclosure giving the operator more space to move and places to work.

Referring to Figure 1 a perspective view is shown of the mounting of the sound cancellation speaker or speakers in the top of the enclosure. In

Fig. 1 an example utility vehicle enclosure 1 is shown in which the sound cancellation speaker 2 is mounted in the top 3. The mounting in the top surface provides greater headroom clearance for the operator and the ability to position speakers in several other locations to be used individually, in pairs or in other multiples. Example locations for other speakers are shown dotted in the top surface of the enclosure 1. The enclosure 1 has a layer of passive sound attenuating material 4 on essentially all inside

surfaces. The enclosure 1 surrounds a volume of space. In that space an operator can sit such as in the seat 5, stand and or work at the surface 6. A portion of one wall of the enclosure may have visual properties such as a glass windshield 7 that has high sound reflecting properties but on which the passive attenuating material 4, being opaque, cannot be used. The passive sound attenuating material has low efficiency at the low 80 to 500 Hz frequencies and the material of which vehicles are made is a good transmitter of vibration from the many noise sources. The background sound conditions inside the enclosure are very high and often detrimental to workplace comfort and attention but there is little flexibility in construction to alleviate the situation. The positioning of the speakers in the top permits getting the most benefit from the use of active noise cancellation. Monitoring microphones 8 and 9 are positioned in the sound field projected from the overhead speakers. Selected sources of sound such as sounds from the engine or fan, not shown, are sensed at the sources for use as input to the active noise cancellation system. Such sound sources have a low probability of containing needed information such as emergency indications essential to the role of the operator. An input microphone 10 on the rear of the enclosure shown dotted may also serve as a remote source of input sound.

In operation the sound level in the zone in the enclosure is lowered by a cancellation signal that is developed from the input signals that have been acoustically sensed at remote locations and from signals from the monitor microphones that are mathematically compared in an algorithm using incremental steps until the monitor signal is

inimized. The combination of remote sensing and the passive attenuation material reduce the possibility of the undesirable occurrence of feedback of cancellation signal into the input. The top positioning of the cancellation speaker or speakers also contributes to increased simplification of the algorithm. The top positioning permits larger speakers and in many instances a single speaker. A single speaker reduces the number of branches in the model of an adaptive algorithm to a single one. In an adaptive algorithm model a branch is necessary for each source of input to the monitoring microphone. The space for the multiple speakers shown dotted in the top 3 permits a construction where a cancelling signal can be projected throughout the enclosure 1.

In vehicles where movement is slow or where aerodynamics is not a predominant consideration the speakers can be mounted to extend above the top 3 surface. Where it is undesirable to have the speakers extend above the surface of the top 3 they can be placed inside the enclosure on the under surface of the top 3 by giving up some headroom or selecting minimal height speakers. A satisfactory cancelling speaker is the

Rockford Fosgate PR0128 12" Sub Woofer. A satisfactory microphone for use as a monitor microphone is the SM98A by the SHURE Co. A satisfactory microphone for use as an input microphone is a model PZM distributed by the Radio Shack Co.

What has been described is the mounting of the cancelling sound speaker or speakers in the top of an operator workplace enclosure in a vehicle in order to get the most benefit out of the application of the active noise cancellation principle in the enclosure.