Abstract:
An active exhaust-noise attenuation muffler for an exhaust pipe includes a control unit, a sensor assembly which is operatively connected to the control unit, and a gas-tight diaphragm constructed to be resistant to exhaust gas in the exhaust pipe and acoustically coupled with a flow of exhaust gas in the exhaust pipe. The diaphragm has a surface which confronts the exhaust gas. A transducer is operated by the control unit for causing the surface of the diaphragm to vibrate in a bending vibration mode to produce a structure-borne sound in dependence on exhaust noise.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]     This application claims the priority of German Patent Application, Serial No. 10 2005 011 747.3, filed Mar. 11, 2005, pursuant to 35 U.S.C. 119(a)-(d), the content of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates, in general, to an active exhaust-noise attenuation muffler for exhaust pipes.  
         [0003]     Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.  
         [0004]     Exhaust systems of motor vehicles typically include an exhaust-noise attenuation muffler to reduce the intensity of exhaust noise generated by the motor to an acceptable level. The sound pressure level may amount up to 160 dB(A). Regulations provide hereby a legal standard for noise reduction.  
         [0005]     Passive mufflers operate on the basis of sound absorption and include fibrous or open-pored materials having large and greatly structured surfaces. In this way, exhaust noise is diverted and reflected in absorbing and sound-suppressing mazes so that the noise energy is reduced until the exhaust noise drops below a desired level. These types of passive exhaust-noise attenuation mufflers retain exhaust gas so that engine performance is adversely affected. Other types of exhaust-noise attenuation mufflers operate on the utilization of a countersound to superpose on the disturbing noise with a compensation sound having a same frequency and intensity as the disturbing noise but being phase-shifted by 180°. As a result of the interference, the disturbing noise is attenuated. Countersound may be produced passively by particularly constructed resonators and actively by loudspeakers. Resonators may be configured as λ/4 pipe and coupled to the side of the exhaust pipes. Sound is reflected phase-shifted by 180° at the end of the λ/4 pipe. Reflecting sound waves superpose on the disturbing noise to effect the noise attenuation. As a consequence of the time that is required for the sound to travel twice along the length of the λ/4 pipe and due to dynamic conditions that result in a change in frequency of the exhaust noise in the exhaust pipe, the superimposition with the reflected sound fails to realize the desired compensation. Moreover, the frequencies where complete suppression is possible are limited to a multiple of λ/4 for physical reasons.  
         [0006]     In view of these limitations, the use of active exhaust-noise attenuation mufflers has been developed which are equipped with a secondary sound source to produce a compensation sound by means of loudspeakers. For purposes of generating the compensation sound, control circuits and closed loops have been used. The control circuits include sensors to ascertain relevant parameter for the exhaust noise, like e.g. motor speed, load state of the motor, exhaust temperature. A control unit generates output signals in response to incoming input signals to operate the loudspeaker disposed on the exhaust pipe. The use of a closed loop is able to further enhance sound reduction by complementing the sensor assembly with a pressure sensor or microphone. In response to the exhaust noise ascertained by the sensor assembly, the control unit is then able to generate jointly with the loudspeaker a compensation sound which is suited to the dynamic changes of the exhaust noise.  
         [0007]     Establishing physical parameter in connection with reduction of exhaust noise is very difficult. As noted above, exhaust noise may reach a sound pressure level of up to 160 dB(A), whereby the exhaust gas pulsates. The temperature of exhaust gas in the exhaust pipe may reach up to 600° C. so that the speed of sound rises from 330 m/s to 600 m/s. Moreover, the exhaust gas is chemically extremely aggressive.  
         [0008]     Heretofore, the use of cone-type loudspeakers for exhaust noise attenuation has proven unsatisfactory because these types of loudspeakers are unable to withstand the encountered rough physical conditions. In other words, the diaphragm and magnets wear off quickly. A proposal to use diaphragms of titanium has been discarded because of the prohibitively expensive costs for mass production. In order to generate low frequencies, large-area diaphragms and heavy magnets are required which however are too bulky for installation in the available space in motor vehicles and unsuitable for large-scale production in view of their weight.  
         [0009]     It would therefore be desirable and advantageous to provide an improved active exhaust-noise attenuation muffler which obviates prior art shortcomings and which is compact in structure and reliable in operation regardless of the type of vehicle involved.  
       SUMMARY OF THE INVENTION  
       [0010]     According to one aspect of the present invention, an active exhaust-noise attenuation muffler for an exhaust pipe includes a control unit, a sensor assembly operatively connected to the control unit, a gas-tight diaphragm constructed to be resistant to exhaust gas in the exhaust pipe and acoustically coupled with a flow of exhaust gas in the exhaust pipe, with the diaphragm having a surface which confronts the exhaust gas, and a transducer operated by the control unit for causing the surface of the diaphragm to vibrate in a bending vibration mode to produce a structure-borne sound in dependence on exhaust noise.  
         [0011]     The present invention resolves prior art problems by superimposing the exhaust noise with the compensation sound of a loudspeaker which operates as electroacoustic transducer on the basis of bending waves. Loudspeakers of this type have a diaphragm with a surface on which bending waves and shear waves can propagate when caused to vibrate by a transducer. The wave propagation in diaphragms may be realized in many ways. In thicker diaphragms, compact waves and dilatational waves are dominant, whereas thinner media have in addition bending and shear waves. Excitation of bending waves has been shown suitable for application in loudspeakers in view of their amplitude and their propagation performance. The propagation performance of bending waves in a diaphragm is primarily impacted by the bending stiffness of the diaphragm, with the bending stiffness being frequency-dependent. In the so-called coincidence frequency, the phase velocity of the wave in the diaphragm matches the phase velocity in air. At this frequency, the wave separates from the diaphragm at an angle of about 0°. Above the coincidence frequency, the angle increases up to 90°, thereby abruptly increasing the efficiency. The coincidence frequency thus constitutes the lowest frequency at which the bending waves can be converted into air sound waves. Below this frequency, the diaphragm vibrates predominantly in a piston-like manner.  
         [0012]     Loudspeakers of this type can be made flat. The diaphragm is thin and can be made planar or slightly curved. A flat configuration of the diaphragm significantly simplifies a calculation of bending waves and the overall configuration of the diaphragm. In addition, larger diaphragms can be made more compact and may be disposed, for example, in close proximity to the exhaust pipe at the bottom of the vehicle body.  
         [0013]     According to another feature of the present invention, the transducer can be secured to the backside of the diaphragm and coupled therewith. The diaphragm may suitably held in a frame of the housing of the loudspeaker. Suitably, the housing is disposed on the exhaust-distal side of the diaphragm in surrounding relationship to the transducer.  
         [0014]     According to another feature of the present invention, the diaphragm is constructed to be able to withstand chemical and thermal impacts in the exhaust pipe. In addition, the diaphragm should be gastight.  
         [0015]     The loudspeaker is operated by the control unit which is operatively connected to the sensor assembly via signal lines. The exhaust-noise attenuation muffler can thus be constructed with a control circuit as well as a closed loop in order to actively and efficiently reduce exhaust noise.  
         [0016]     According to another feature of the present invention, the diaphragm may be disposed in an opening in a wall of the exhaust pipe. As a result, the exhaust-noise attenuation muffler is compact and the loudspeaker can be placed in close proximity of the exhaust pipe. As a result of the direct linkage of the loudspeaker with the exhaust gas flow, attenuation is significantly simplified because fewer factors need to be taken into account to generate the countersound. As an alternative, it may also possible to dispose a resonator in an opening in the wall of the exhaust pipe, wherein the diaphragm is operatively connected to the resonator at a distance to the opening. As a consequence of the distance to the pipe, the diaphragm is thermally decoupled from the flow of exhaust gas so that the diaphragm undergoes less stress and thus can be constructed simpler. Moreover, the resonator may be used to align the sound and to focus the sound toward the exhaust gas flow.  
         [0017]     According to another feature of the present invention, the diaphragm may be configured to conform to a contour of a wall of the exhaust pipe, resulting in an arched configuration of the diaphragm. This slight curving of the diaphragm allows easy integration in the exhaust pipe. Thus, the available installation space can be efficiently utilized.  
         [0018]     According to another feature of the present invention, the exhaust-confronting surface of the diaphragm may be coated with metal through a vapor deposition process. In this way, the chemical resistance and the temperature-resistance of the diaphragm can be significantly enhanced. As an alternative, a stainless steel foil may be applied upon the exhaust-confronting surface of the diaphragm.  
         [0019]     The nature of the exhaust noise is substantially influenced by the motor, i.e. speed level and load state as well as by the temperature of the exhaust gas flow. Thus, it may be suitable to operatively connect the control unit with an electronic motor control system. The direct link enables a continuous transmission of motor data during operation to the control unit, without requiring complex signal conversion processing with resultant decrease in efficiency and loss in time. Suitably, respective interfaces are provided at the output of the electronic motor control system and at the input of the control unit.  
         [0020]     According to another feature of the present invention, the sensor assembly may include a temperature sensor for measuring an exhaust temperature in the exhaust pipe. As the speed of sound is dependent especially on the temperature of the exhaust, consideration of the exhaust temperature significantly enhances the efficiency of the exhaust-noise attenuation muffler.  
         [0021]     According to another feature of the present invention, the sensor assembly may include a throttle sensor for determining a throttle position. The arrangement of a throttle sensor provides inference about the load state of the motor. Sensors of this type are typically used in current motor vehicles and their structure and operation are generally known to the artisan.  
         [0022]     According to another feature of the present invention, the sensor assembly may include a speed sensor for determining a motor speed.  
         [0023]     According to another feature of the present invention, the sensor assembly may include a pressure sensor, e.g. a microphone, for determining the exhaust noise in the exhaust pipe. Provision of a closed loop for producing the compensation sound is especially effective to attenuate exhaust noise. The sensor assembly is hereby resistant to the exhaust gas like the loudspeaker.  
         [0024]     According to another feature of the present invention, a microprocessor may be provided for control of the control unit. This affords flexibility to adapt the exhaust-noise attenuation muffler to various situations at hand. The control performance of the control unit is program-controlled. The programs may be modified or exchanged via a respective interface on the control unit. In this way, exhaust noise can be configured like a sound design. A microprocessor-controlled control unit further simplifies the installation of the exhaust-noise attenuation muffler independent from the motor vehicle because it is only required to suit the software while the hardware can remain the same.  
         [0025]     According to another feature of the present invention, the control unit may be constructed to allow adjustment of the control performance. As a result, the operator is able at any time to directly influence the noise of the vehicle by actuating switches or variable transformers. In other words, the noise can be adjusted by the operator to sound especially racy or gentle.  
         [0026]     According to another feature of the present invention, the transducer may be constructed as an oscillation coil. Use of an oscillation coil results in a compact configuration and arrangement of few moving parts. As an alternative, the transducer may include an electric motor having a driveshaft, with an eccentric secured to the driveshaft and coupled to the diaphragm via a connecting rod. In this way, the frequency of the vibrating diaphragm can be adjusted in an especially easy and robust manner. The electric motor may be placed separate and away from the exhaust gas flow, thereby decreasing the exposure of the transducer to thermal stress. In the event, the transducer is intended for securement directly to the diaphragm, it is suitable to construct the transducer heat-resistant.  
         [0027]     According to another feature of the present invention, a housing may be provided for accommodating the diaphragm and the transducer. The provision of a housing serves two purposes, namely as protection from the environment, and facilitation of the assembly because the transducer together with the diaphragm and the housing can be constructed as a prefabricated unitary structure that can be shipped as a unit for assembly. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0028]     Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which:  
         [0029]      FIG. 1  is a partial longitudinal section of a first embodiment of an active exhaust-noise attenuation muffler according to the present invention;  
         [0030]      FIG. 2  is a partial longitudinal section of a second embodiment of an active exhaust-noise attenuation muffler according to the present invention;  
         [0031]      FIG. 3  is an enlarged detailed view of a loudspeaker of the active exhaust-noise attenuation muffler of  FIGS. 1 and 2 ; and  
         [0032]      FIG. 4  is an enlarged detailed view of a variation of a loudspeaker of the active exhaust-noise attenuation muffler according to the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0033]     Throughout all the Figures, same or corresponding elements are generally indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the drawings are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted.  
         [0034]     Turning now to the drawing, and in particular to  FIG. 1 , there is shown a partial longitudinal section of a first embodiment of an active exhaust-noise attenuation muffler according to the present invention, generally designated by reference numeral  1 , for an exhaust pipe  2  that may find application for example in a motor vehicle. The active exhaust-noise attenuation muffler  1  is provided to superpose the noise of the exhaust with a 180° phase-shifted compensation sound of a loudspeaker  3  so as to reduce or suppress the exhaust noise.  
         [0035]     The loudspeaker  3  has a flat configuration and is paced in a lateral opening  4  in the wall  5  of the exhaust pipe  2  of an unillustrated motor. The loudspeaker  3  has a diaphragm  7  and a transducer which is implemented by way of example in the form of an oscillation coil  8 . Both, the diaphragm  7  and the oscillation coil  8  are arranged in a housing  6 . The diaphragm  7  is thin and made of several material layers so as to exhibit a particularly low coincidence frequency and a broad frequency spectrum within which the diaphragm  7  is able to vibrate so as to generate a structure-borne sound. Resistance to the exhaust gas and tightness of the diaphragm  7  are ensured by vapor-depositing metal M on an exhaust-confronting side of the diaphragm  7 , as also shown on an enlarged scale in  FIG. 3 . As an alternative to the vapor-deposition of metal, the application of a stainless steel foil  29  upon the exhaust-confronting surface of the diaphragm  7  is also conceivable, as shown in  FIG. 4 .  
         [0036]     The diaphragm  7  is aligned in the opening  4  such that its surface  9  contacts the exhaust gas flow AS. Secured to the backside of the diaphragm  7  is the oscillation coil  8  which, when excited, causes the diaphragm  7  to vibrate. As a result, bending waves are able to propagate on the exhaust-confronting surface  9  of the diaphragm  7 . The oscillation coil  8  is also made of heat-resistant material. The loudspeaker  3  has enough potential to produce a compensation sound of necessary intensity.  
         [0037]     The loudspeaker  3  is operated by a microprocessor-controlled control unit  10  which is mounted in a vehicle body of the motor vehicle at a separate location. The control unit  10  has various interfaces  11 - 16 , with reference numeral  11  relating to an interface for an electronic motor control system  20  forming part of a sensor assembly, reference numeral  12  relating to an interface for data transfer, reference numeral  13  relating to an interface for input of a microphone  22  forming another part of the sensor assembly, reference numeral  14  relating to an interface for a temperature sensor  21  forming yet another part of the sensor assembly, reference numeral  15  relating to an interface for a voltage supply, and reference numeral  16  relating to an interface for a control panel  17 . In response to the signals transmitted by the sensor assembly via signal lines  27 , the control unit  10  computes a compensation vibration which is converted by a digital-to-analog converter  18  into an electric oscillation and boosted by an amplifier  19  of the control unit  10  before being delivered to the loudspeaker  3 . Computation of the compensation vibration is program-controlled, with the programs being exchangeable via the data transfer interface  12 . Each vehicle type has its own particular program. The control performance of the control unit  10  may be modified by an operator using the control panel  17  in order to give the vehicle a racy or gentle sound or to make the exhaust noise quieter or louder.  
         [0038]     The control unit  10  is directly linked to the sensor assembly comprised of the electronic motor control system  20 , temperature sensor  21 , and microphone  22 , whereby the temperature sensor  21  and the microphone  22  are mounted to the exhaust pipe  2 . Signal transfer takes place via the signal lines  27 . The electronic motor control system  20  transmits information about the speed and load state of the motor from a particular output interface  23  to the control unit  10 . The temperature sensor  21  ascertains a temperature of the exhaust gas flow AS in the exhaust pipe  2  in close proximity of the loudspeaker  3  and is constructed resistant to the exhaust gas. The microphone  22  is also constructed resistant to the exhaust gas and disposed upstream of the loudspeaker  3  in an opening  24  in the wall  5  of the exhaust pipe  2 .  
         [0039]     In order to modify the sound of the vehicle, the driver uses the operating panel  17  which is placed within easy reach of the driver during travel. The operating panel  17  includes switches  25  and a variable transformer  26  and is linked to the control unit  10  via a signal line  27 .  
         [0040]     Referring now to  FIG. 2 , there is shown a partial longitudinal section of a second embodiment of an active exhaust-noise attenuation muffler  1  according to the present invention. Parts corresponding with those in  FIG. 1  are denoted by identical reference numerals and not explained again. The description below will center on the differences between the embodiments. In this embodiment, the loudspeaker  3  is secured to a resonator  28  which is placed in the lateral opening  4  in the wall  5  of the exhaust pipe  2 . The resonator  28  has a funnel-shaped configuration and is made of sheet metal. The diaphragm  7  is disposed here at a distance A to the opening  4 . A structure-borne sound produced by the diaphragm  7  propagates in the resonator  28  and superposes on the exhaust noise in the exhaust pipe  2 , after traveling through the opening  4 .  
         [0041]     While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.  
         [0042]     What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims and includes equivalents of the elements recited therein: