Abstract:
There is disclosed a silencer for attenuating sound waves produced in a fluid that circulates through a fluid conveyer. The silencer comprises an expansion chamber that is in fluid communication with the fluid conveyer, and which carries sound waves there through; a sound wave dissipater provided with the expansion chamber and arranged to absorb sound waves traveling there through; a resonator operatively associated with the sound wave dissipater and constructed and arranged to cause attenuation and reflection of the sound waves back and forth towards the sound wave dissipater; the expansion chamber having a chamber: conveyer cross-sectional area ratio and chamber length characteristics allowing maximum transmission loss for a given frequency. The expansion chamber has an exit to allow fluid containing attenuated sound waves to escape therefrom.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to silencers. More specifically, the present invention is concerned with wide absorption spectrum compact silencers.  
       BACKGROUND OF THE INVENTION  
       [0002]     A silencer may be described as any section of a duct or pipe adapted to reduce the transmission of sound while allowing the free flow of a gas. Silencers can be broken into two fundamental groups: absorptive silencers and reactive silencers. Absorptive silencers include either fibrous or porous materials and depend on the absorptive properties of these materials to reduce noise. Absorptive silencers are most useful for noise control problems associated with high frequency spectra and their low frequency absorption increases with an increasing thickness of the absorbing material and with an increasing length of the silencer.  
         [0003]     Reactive silencers contain no absorbing material but depend on the reflection or expansion of sound waves within a chamber to attenuate the sound. Peak attenuation occurs in the lower-frequency ranges, typically below 500 Kz. To provide a wide spectrum of attenuation, several chambers may be assembled in series.  
         [0004]     Some silencers combine reactive and absorptive elements. However, these silencers typically are large and heavy and have some undesirable properties, such as a large resistance to motion or air within the silencer. Accordingly, difficulties in specifying a silencer for use in a particular situation are generally found when dealing with problems such as size, weight and aerodynamic pressure losses, among others, and not in providing a silencer with adequate acoustical performance.  
         [0005]     Against this background, there exists a need in the industry to provide a novel and compact silencer.  
       OBJECTS OF THE INVENTION  
       [0006]     An object of the present invention is therefore to provide an improved compact silencer that is capable of attenuating sound waves in a wide spectrum of frequencies.  
         [0007]     It is another object of the invention to provide a silencer that through its structural arrangement of parts and dimensions relationship provides efficient attenuation of sound waves while being inexpensive to manufacture and versatile for mounting with any arrangement of fluid circulation.  
       SUMMARY OF THE INVENTION  
       [0008]     The invention generally relates to a silencer for attenuating sound waves produced in a fluid that circulates through a conveying means. The silencer according to the invention comprises an expansion chamber and means allowing the expansion chamber to be in fluid communication with the conveying means, and to carry the sound waves through the chamber. A sound wave dissipater is provided with the expansion chamber and is arranged to absorb sound waves traveling through the expansion chamber. A resonator is operatively associated with the sound wave dissipater and is constructed and arranged to cause attenuation, and reflection of the sound waves back and forth towards the sound wave dissipater. The expansion chamber has a chamber : conveying means cross-sectional area ratio and chamber length characteristics allowing maximum transmission loss for a given frequency. Finally, means are provided to allow fluid containing attenuated sound waves to exit from the expansion chamber.  
         [0009]     Advantageously, the silencer should be compact and light. Also, it should preferably attenuate sound waves having a wide spectrum of frequencies and provide only minimal resistance to a flow of gas there through. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     The invention will now be illustrated by means of the annexed drawings which are given by way of limitation and without limitation. In the drawings:  
         [0011]      FIG. 1  is a perspective view of a silencer according to the invention including a dissipater and a resonator;  
         [0012]      FIG. 2  is a side cross-sectional view of the dissipater and resonator of  FIG. 1 ;  
         [0013]      FIG. 3  is a perspective view of the resonator of  FIG. 2 ; and  
         [0014]      FIG. 4  is a front view of the resonator of  FIG. 2 . 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0015]      FIG. 1  shows a silencer  10  for attenuating sound waves. The silencer  10  includes an inlet  12 , an outlet  14 , an expansion chamber  16 , a dissipater  18  and a resonator  20 . The expansion chamber  16  is in fluid communication relationship with outlet  14 . Dissipater  18  is provided within the expansion chamber  16  as shown. Resonator  20  is in a fluid communication relationship with inlet  12  and expansion chamber  16 . Resonator  20  is further disposed within expansion chamber  16  and includes three baffles  30  (shown in  FIG. 2 ) configured and sized to direct sound waves propagating within the resonator  20  towards dissipater  18 .  
         [0016]     Silencer  10  provides attenuation of sound waves at frequencies covering a wide spectrum, in a compact and light format. The expansion chamber  16  and the resonator  20  provide attenuation mainly at high frequencies, although they are intended to also attenuate some low frequencies.  
         [0017]     The silencer  10  shown in  FIG. 1  is one that is normally adapted for a Heating, Ventilation and Air Conditioning (HVAC) system. However, the reader skilled in the art will readily appreciate that silencers similar to silencer  10  could be used in many other applications such as, for example, attenuating sound waves in gas turbines, generators, vacuum cleaners and compressors, among others. In fact, silencer  10  can provide sound wave attenuation in any system wherein a fluid passes through a duct or a pipe.  
         [0018]     The silencer  10  according to the invention is adapted for use in a ventilation system (not shown in the drawings) that is part, for example, of a HVAC system. To that effect, inlet  12  and outlet  14  can be of a diameter that is standard in the HVAC industry. Inlet  12  and outlet  14  can be soldered, or fixed through any other means, to the ventilation system. In a specific example of implementation, the silencer  10  attenuates sound waves in an air duct directing air towards one or more rooms in a building. However, it goes without saying that a silencer according to the invention may be used in conjunction with any fluid circulation system where noise is a problem.  
         [0019]     Expansion chamber  16  includes a peripheral wall  22  and first and second end walls  24  and  26 . Inlet  12  is provided in the first end wall  24  while outlet  14  is provided in the second end wall  26 . While the expansion chamber  16  shown in  FIG. 1  is substantially cylindrical, it could take any other suitable shape. For example, if a HVAC system includes pipes having a square cross-section, a silencer having a substantially square cross-section could be used advantageously.  
         [0020]     As shown in  FIG. 2 , resonator  20  includes a substantially cylindrical perforated inner wall  28  and a plurality of baffles  30 , here three, that are provided within the resonator  20 . Furthermore, the resonator  20  is surrounded at least in part by dissipater  28 , which will be described in further detail herein below.  
         [0021]     Perforated wall  28  is optionally of a diameter that is substantially equal to the diameter of inlet  12 . Also, perforated wall  28  is in the continuity of inlet  12 . The perforations  29  within wall  28  are sized to provide attenuation of sound waves within the resonator  20 , as will be described herein below, while allowing high frequency sound waves to escape at least in part from resonator  20  towards dissipater  18 .  
         [0022]     It was found that a perforated wall  28  having perforations  29  covering at least 33% of the area of the perforated wall  28  provides advantageous sound absorption characteristics to the silencer  10 . However, any other suitable type of perforations is within the scope of the invention.  
         [0023]     In a specific example of implementation, the perforated wall  28  is of a length that is equal to the length required to provide maximal destructive interferences of sound waves present within resonator  20  and expansion chamber  16 . This length is preferably equal to a fourth of a wave length of a sound wave to be attenuated. Accordingly, silencer  10 , through expansion chamber  16  and resonator  20 , operates optimally at a single frequency and at its harmonics. However, although silencer  10  provides an optimal attenuation of sound waves for only a few selected frequencies, other frequencies are also attenuated. This additional attenuation is, in part, caused by perforations  29  within perforated wall  28  and by partially destructive interferences of sound waves propagating substantially longitudinally within silencer  10 .  
         [0024]     The dimensions of expansion chamber  16  and of resonator  20  can be determined according to the intended use of the silencer using methods that are well known in the art.  
         [0025]     Baffles  30  are fixed in known manner to perforated wall  28  and are preferably angled at an acute angle with respect to the perforated wall  28  as shown in  FIG. 2 . The baffles  30  are configured and sized to reflect sound waves that are propagated within the resonator  20 , towards the dissipater  18 . Resonator  20 , shown in  FIG. 2 , includes three baffles  30 . However, any number of baffles could be used in conjunction with the invention, as will be appreciated by one skilled in the art.  
         [0026]     In the illustrated embodiment, each baffle  30  includes a sector of a substantially frustoconical shell. However, other shapes of baffles are within the scope of the invention. As shown in  FIGS. 3 and 4 , the baffles  30  are placed, configured and sized such that when the resonator  20  is seen along a longitudinal axis, the baffles completely block to view an annular region within the resonator  20 . Accordingly, the baffles  30  appear as a cone when seen from this point of view. Optionally, and as better shown in  FIG. 3 , baffles  30  adopt a substantially helicoidal configuration when mounted in the resonator. In addition, but non-essentially, the baffles  30  are oriented such that a narrow portion of each baffle  30  is further away from the inlet  12  than a wide portion of each baffle  30 .  
         [0027]     An efficient way to manufacture baffles  30  includes providing a frustum of a cone in a suitable material and cutting the frustum in a plurality of sectors, thereby forming the baffles  30 .  
         [0028]     Each baffle  30  includes a steel plate that may include optional perforations (not shown). However, it is within the scope of the invention to have baffles made of a different material, such as aluminum, among others. Also, each baffle  30  can optionally be covered in part or totally with a sound absorbing material of a type described in more details herein below with reference to dissipater  18 . The sound absorbing material can in turn be surrounded by a perforated metal part.  
         [0029]     Dissipater  18  includes an absorptive material  19  contained within an enclosure  23 . Enclosure  23  is defined by the perforated wall  28 , a surrounding wall  32  spacedly surrounding the perforated wall  28 , an annular wall  34  and part of the first end wall  24 . The surrounding wall  32  and the annular wall  34  can be perforated so as to allow sound waves to escape from the dissipater  18  into expansion chamber  16 . In the embodiment shown in  FIG. 1 , a gap  17  is provided between the surrounding wall  32  and peripheral wall  22 .  
         [0030]     The absorptive material  19  can include felt, rock wool, fiberglass or any other suitable sound absorptive material. In a specific example of implementation, the absorptive material  19  has a density that can vary between two and four pounds per cubic foot.  
         [0031]     The absorbing material is separated from the peripheral wall  22  by gap  17 . As a result, sound waves exiting the absorptive material  19  can be reflected back into the absorptive material  19  through peripheral wall  22  after traveling in the air contained within the silencer  10 . Accordingly, both the passage of sound waves within the air and multiple journeys through the absorptive material  19  add to an attenuation of high frequencies within the silencer  10  without requiring a large quantity of absorptive material  19 , which lowers manufacturing cost and weight.  
         [0032]     For example, a gap  17  having a width of substantially  4  inches greatly improves the performance of the absorptive material  19  in the resonator. However, any other suitable width for the gap can be used, as will be appreciated by one skilled in the art.  
         [0033]     Optionally, a facing (not shown in the drawings) made of nylon, Mylar™, Tedlar™ or felt, for example, may be applied around the absorptive material  19  to provide protection against physical and/or chemical agents. Such facing can also improve the low-frequency absorption characteristics of the dissipater while reducing the possibilities that fragments of the absorptive material  19  become dislodged and are thereafter mixed with the air that circulates within silencer  10 . This characteristic is advantageous in industries wherein dust contamination is undesirable.  
         [0034]     In the illustrated embodiment, expansion chamber  16 , resonator  20  and dissipater  18  include steel parts. However, the readers skilled in the art will readily appreciate that any other suitable material could be used in manufacturing expansion chamber  16 , resonator  20  and dissipater  18 .  
         [0035]     In use, an air stream enters silencer  10  through inlet  12 . The air stream in turn strikes baffles  30 . The angle at which the air stream strikes the baffles and the geometry of the baffles create a pressure differential between air upstream of resonator  20  and air downstream of resonator  20 . The disposition of the baffles  30 , which tends to push air circulating within the resonator  20  around the baffles  30 , along with the Bernoulli effect caused by the narrowing of the baffles  30  in a direction substantially identical to the general direction of the air flow within the resonator  20  help to limit the pressure differential. The air flow then exits from the resonator  20  within the expansion chamber  16 . Since the expansion chamber  16  is filled with air, air is continuously expelled from silencer  10  through outlet  14 .  
         [0036]     With respect to the acoustical properties of silencer  10 , it will be realized that the sound waves incoming at inlet  12  broadly have two different routes to travel through silencer  10  depending on their wavelength. Low frequency sound waves create standing waves within the resonator  20  and the expansion chamber  16 . Since the expansion chamber  16  and the resonator  20  are preferably sized to provide attenuation at low frequencies, the standing waves created destructively interfere and cause attenuation in sound wave intensity at these low frequencies. Low frequency sound waves are also attenuated within the resonator  20  through a transmission loss caused by the frustoconical geometry of the baffles, which provide attenuation similarly to a single-piece frustum of a cone located within a cylindrical tube.  
         [0037]     The high frequency sound waves are reflected by the baffles  30  toward dissipater  18 . Accordingly, these high frequency sound waves are absorbed by the dissipative material contained within the dissipater  18 . In addition, gap  17  between peripheral wall  22  and surrounding wall  32 , along with the expansion of sound waves within the expansion chamber  16 , further contribute to the attenuation of low and high frequencies within the silencer  10 .  
         [0038]     It has been found advantageous to provide baffles  30  having a high acoustic impedance at some of the frequencies to be attenuated by the silencer  10 . Thus, a sound wave amplitude of sound waves reflected by the baffles  30  is relatively large and only a minimal portion of high frequency sound waves reaches outlet  14 . In this case, because of the frustoconical geometry of baffles  30 , the sound waves are reflected in many directions within the silencer  10 , which creates many different apparent gap thicknesses in the reflected sound waves. As a result, low frequencies are also absorbed more efficiently than in prior art silencers.  
         [0039]     It has also been found that sound wave attenuation by the silencer  10  is not a linear function of the length of the resonator  16  as absorption is very large with only a few baffles in the resonator  16 . Accordingly, silencer  10  can be very compact while having good sound attenuation characteristics.  
         [0040]     However, it was realized that it is essential to provide the expansion chamber with critical dimension characteristics. For example, the ratio between the cross-sectional area of the expansion chamber and the cross-sectional area of the conveying means such as that at the inlet, and the length of the chamber should be such that these parameters allow a maximum transmission loss for a given frequency. More specifically, transmission loss is achieved when TL is at a maximum value. For this purpose, TL is represented by the following formula: 
        TL=10 log[1+¼(m−1/m) 2  sin 2  kl]db     wherein     TL represents transmission loss;     M=cross-sectional area of chamber/cross-sectional area of fluid conveying means;     k=wave number=2π/λ;     l=chamber length;     λ=wave length of sound at temperature of gas in the expansion chamber.        
 
         [0048]     In an alternative embodiment of silencer  10 , the resonator  20  and the dissipater  18  are located outside of and in series with the expansion chamber  16 .  
         [0049]     Although the present invention has been described hereinabove by way of preferred embodiments thereof, it is obvious that it can be modified, without departing from the spirit and scope of the invention as defined in the appended claims.