Abstract:
A resonator and method of designing a resonator including a cavity having an effective length that exceeds λ/8 such that a standing wave having an anti-node will form in the cavity. First and second necks each having first ends are attached to and in fluid communication with the cavity, the first neck being positioned adjacent the anti-node and thereby operable to interfere with said standing wave.

Description:
TECHNICAL FIELD 
   The present invention relates to resonators used to reduce noise emanating from the induction system of an engine, and more particularly relates to a resonator having an unconventionally long cavity and two necks for coupling to an air intake of an automobile engine induction system. 
   BACKGROUND OF THE INVENTION 
   Side branch resonators have been used for many years to reduce radiated induction noise in automobile engine compartments. In one common application, a resonator having a cavity includes a neck interconnecting the cavity to the air intake duct of the engine induction system. The compartment and hence cavity are typically tube or rectangular shaped and there may be one or more necks (see, for example, U.S. Pat. No. 6,609,489). The parameters of cavity, neck length and neck diameter dictate at what frequency the resonator will resonate. The resonating frequency is chosen to match the frequency of the induction noise. Thus, when designing a resonator, the engineer will choose the cavity and neck diameter and length to achieve a resonating frequency that will match and cancel the frequency of the induction noise it is desired to attenuate. 
   The strength of the resonator is proportional to the square root of the cavity volume for a constant neck size. Thus, strong resonators require a large resonator cavity, however, large cavity size may not be feasible due to space constraints in the engine compartment. In other words, the available space dictates how large the engineer can make the cavity in terms of length, width and depth. Another potential problem is that making one dimension much larger than the other two will cause the resonator cavity to exhibit plane wave behavior and the resonator will thus not resonate at the predicted frequency. The engineer is thus forced to reduce this dimension size until the plane wave behavior ceases, however, this reduces the resonator strength as well. It will thus be appreciated that resonator design has been limited by space availability and attempts to increase resonator strength through an increase in linear dimensions of the cavity are typically futile. 
   There therefore exists a need for an improved resonator and method for reducing induction noise emanating from an engine that provides strong attenuation while occupying a small space in the engine compartment. 
   SUMMARY OF THE INVENTION 
   The present invention addresses the above need by providing a resonator and method for attenuating induction noise in an engine that is both strong in attenuation and relatively small in size, particularly when compared to conventional resonators of similar strength. 
   In one aspect, the invention comprises a resonator having a cavity defined by a compartment that has a characteristic length that is longer than the characteristic length in conventional resonators. In conventional resonator theory, any linear dimension or characteristic length cannot exceed the maximum allowable length which is equal to the wavelength “λ” divided by 8. Knowing that wavelength equals the speed of sound “c” divided by the tuning frequency “Fr”, no linear dimension can exceed the speed of sound divided by the product of 8 times the frequency (maximum allowable length &lt;c/(8×Fr). For example, in a conventional resonator, an engineer designing a 200 Hz resonator would know that the cavity will have a maximum allowable length of 0.7 ft. or 8.4 inches where c=1125 ft/sec. 
   In a second aspect, the invention comprises a resonator having first and second necks that interconnect the cavity to the air intake duct. Since the inventive resonator exceeds the maximum allowable length of conventional side branch resonators, a standing wave is formed in the cavity. This standing wave has an anti-node or high pressure zone that forms at an end of the cavity. One neck is positioned adjacent the anti-node and acts to eliminate the standing wave. The position of the other neck may be almost anywhere along the length of the cavity but preferably is no greater than the wavelength divided by 16 along the length of the induction system, or a quarter the length of the cavity, from the other neck. Importantly, since the inventive resonator cavity exceeds the maximum allowable length of conventional resonators and corresponding theory, the resonance frequency (f r ) of the inventive resonator is not predictable using the conventional resonator equation, which is as follows:
 
 f   r =180√( A   o ÷( L   e   V ))  (Eq. 1)
 
where:
 
   A o  is total neck area 
   V is compartment cavity 
   L e  is effective neck length 
   One method of predicting the resonance frequency of the inventive resonator is using three-dimensional finite elements which are used to describe the resonator and transmission loss is calculated with a finite element code. Three dimensional acoustic theory may be performed using computational vibro-acoustic software such as SYSNOISE by LMS Corporation). Another method is to use one-dimensional acoustic waves to calculate transmission loss. Characteristic dimensions such as tube length, tube area, neck length, neck area, neck separation distance and neck location are modeled with acoustic waves according to the acoustic wave equations explained more fully below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
       FIG. 1  is a perspective view of a mesh form of a prior art side branch resonator used to reduce induction noise of an engine; 
       FIG. 2  is a top plan view of a mesh form of one possible embodiment of the inventive resonator; 
       FIG. 3  is an acoustic wave schematic of the resonator of  FIG. 2 ; and 
       FIG. 4  is a pipe area/length schematic of the resonator of  FIG. 2 . 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , a prior art conventional side branch resonator  10  is shown which is used for attenuating induction noise from an automobile engine (not shown). Prior art resonator  10  has two basic components: a compartment defining an internal cavity  12  and a neck  14  in fluid communication with cavity  12 . The opposite end of the neck  14  connects to the intake duct  16  which leads to the induction system of the engine. Noise generated by the engine can travel and escape through the intake duct  16 . Resonator  10  is operable to reduce this undesirable “induction noise” by resonating at the frequency of the induction noise (the “induction frequency”), thereby creating pressure waves which, through the principle of interference, cancel the majority of the induction pressure waves at the resonance frequency. The strength (i.e., noise attenuation ability) of resonator  10  is limited by its size which, in turn, is limited by the available space in the engine compartment (not shown) as well as the diameter of the intake duct  16  to which neck  14  attaches. 
   The basic configuration of cavity  12  is a rectangular box-like structure and the resonance frequency is predicted according to the known equation:
 
 f   r =180√( A   o ÷( L   e   V ))  (Eq. 1)
 
where:
 
   A o  is total neck area; 
   V is compartment cavity; and 
   L e  is effective neck length. 
   The space constraints imposed on a resonator designer as explained above means that the resonator  10  will inevitably have a limited strength, i.e., resonator  10  may not be able to attenuate the entire induction noise being targeted. There frequently is limited space on top of the engine to attach a conventional shaped resonator volume due to low hood lines on modern cars. Smaller resonator volumes are less effective at attenuating noise, as is having to relocate the resonator further from the engine air intake due to space constraints. 
   The present invention provides a uniquely configured resonator which is stronger and more adaptable to fit into the available space than that possible with the design provided by prior art resonator  10 . More particularly, as seen in  FIG. 2 , one possible embodiment of the invention is seen to comprise a resonator  20  for attaching to an air intake duct  30 . Resonator  20  includes a cavity  22  defined by a compartment that has a characteristic length L that is longer than the characteristic length L in conventional resonators. Resonator  20  further includes first and second necks  24  and  26  that fluidly connect the cavity  22  at first ends  24   a ,  26   a  thereof, and to the air intake duct  30  at the opposite second ends  24   b ,  26   b  thereof, respectively. 
   The overall shape of the cavity  22  is unimportant and the designer thus has a large degree of freedom in shaping the cavity as needed or as dictated by the space constraints of the area where the resonator is required. Thus, in the embodiment of  FIG. 2 , the cavity is curved into a hook shape such that it extends around the air intake duct  30 . This particular cavity shape effectively uses available space while at the same time having a relatively long cavity. 
   Since resonator  20  has a length L which exceeds the maximum allowable length of conventional side branch resonators, a standing wave is formed in the cavity  22 . This standing wave has a high pressure zone or anti-node  28  that forms adjacent an end  22   a  of the cavity. One neck  24  is thus positioned adjacent the anti-node  28  and acts to eliminate the standing wave. The position of the other neck  26  may be almost anywhere along the length of the cavity as long as the neck ends  24   b ,  26   b  join the air intake duct along the same flow path plane. If neck ends  24   b ,  26   b  will not be positioned along the same flow path plane, they preferably are no greater than the wavelength λ divided by 16 along the length of the induction system, or about a quarter the length of the cavity, from each other. 
   As explained above, since the inventive resonator cavity  22  exceeds the maximum allowable length of conventional resonators and corresponding theory, the resonance frequency (f r ) of the inventive resonator is not predictable using the conventional resonator equation. There are two methods that can be used for calculating transmission loss (attenuation) of the resonator and both require computational analysis as is well understood to those skilled in the art. 
   One method of predicting the resonance frequency of the inventive resonator is using three-dimensional finite elements which are used to describe the resonator and transmission loss is calculated with a finite element code. In this method, three dimensional acoustic analysis is performed using well known computational vibro-acoustic software such as SYSNOISE by LMS International. 
   Another method is to use one-dimensional acoustic wave analysis to calculate transmission loss. Characteristic dimensions such as tube length, tube area, neck length, neck area, neck separation distance and neck location are modeled with acoustic waves according to the following acoustic wave equation:
 
 p ( x, t )= Ae   i(wt+kx)   +Be   i(wt+kx)   (Eq. 2)
 
where the resonator is modeled with acoustic wave coefficients as shown in  FIG. 3 . The transmission loss is calculated according to the equation:
 
Attenuation=20×LOG 10 ( P   A1   /P   A8 )  (Eq. 3)
 
where the resonator is modeled with the pipe area and pipe length as seen in  FIG. 4 .
 
   In this method, one dimensional acoustic analysis is performed, again, using well known computational vibro-acoustic software such as SYSNOISE by LMS International. It is noted that the three dimensional analysis method described above will generally give more accurate and reliable results due to the complex three dimensional configurations that are possible according to the present invention. 
   Using either of the above methods for calculating transmission loss, an iterative process is used to tune the resonator to the desired resonance frequency as understood by those skilled in the art.