Acoustics – Sound-modifying means – Muffler – fluid conducting type
Reexamination Certificate
2001-12-20
2003-06-03
Nappi, Robert E. (Department: 2837)
Acoustics
Sound-modifying means
Muffler, fluid conducting type
C181S280000, C181S249000, C181S268000
Reexamination Certificate
active
06571910
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention (Technical Field)
The present invention relates generally to internal combustion engine (ICE) exhaust noise mufflers, specifically a dissipative muffler with improved maintenance, noise attenuation, durability features and reduced impact on engine efficiency.
2. Background Art
Prior art shows dissipative mufflers, which are commonly composed of an inlet port fluidically connected to an outlet port by a duct that also forms the inner wall of an annular chamber containing acoustically absorptive fill. Currently, dissipative mufflers often use a perforated metal liner defining a duct that provides a boundary between the flow of gas and the surrounding volume of acoustically absorbent fill. In typical mufflers, the absorbent fill initially is contained between the inner duct and an outer casing. In some mufflers, a perforated metal duct serves as a backing or facing for a liner made from another material, e.g., fiberglass cloth.
Some muffler apparatuses known in the art include those disclosed in the following U.S. Pat. Nos.:
4,786,256;
3,827,531;
5,565,124;
5,611,409;
4,570,322;
5,139,107;
4,905,791;
4,880,078;
5,912,441;
5,831,223;
5,773,770;
5,739,485;
5,739,484;
5,440,083;
5,340,952;
5,246,473;
4,901,816;
4,760,894;
4,712,643;
4,693,338;
4,577,724;
4,467,887;
4,413,705;
4,332,307;
4,317,502;
4,296,832.
Also, U.S. Pat. No. 5,162,620 to Ross provides particularly helpful background to the present invention.
According to Schultz, perforated metal has a “self flow resistance” (Schultz,
Acoustical Uses For Perforated Metals
, p. 56) and a “transparency index” (Schultz, p. 14) which can be calculated from the following:
Self Flow Resistance=
R
self
=R
0
+2
&Dgr;R
0
; where
R
0
is the greater of
where
R
01
=4.24(
b
2
t/d
3
)
f
0.5
and
R
02
=2.88(
b
2
t/d
4
)
and 2&Dgr;
R
0
=4.19(
b
2
/d
2
)
f
0.5
×10
−3
cgs rayls.
Also,
Transparency Index=
TI
=(
nd
2
/ta
2
)=0.04
P
/(3.14
ta
2
)
With the above variables defined as follows:
a=shortest distance between holes (a=b−d)
b=on-center hole spacing
d=perforation diameter
f=frequency
n=number of perforations per unit area
P=percentage open area
t=thickness of sheet
Thus, muffler ducts fashioned from ordinary perforated metal are considered reasonably “transparent” to sound; but, due to their modest flow resistance, they also permit diversion of conveyed gas flow into the chamber containing the acoustically absorbent media. Not only does this diversion create turbulence and static pressure loss, it can actually entrain or “blow out” fill media through the perforations and through unsealed muffler casing-to-endcap connections. This “blow out” problem is commonly encountered and well-known by users of conventional dissipative mufflers.
Ingard, (
Sound Absorption Technology,
1994, p. 4-25) shows the normalized flow resistance of most perforated metals, i.e., the ratio of the flow resistance of the perforated metal sheet over the acoustical impedance of the gas flow, is near zero for most internal combustion (ICE) muffler applications and thus, when studied in combination with the fill it is lining, is excellent for preserving virtually ideal acoustical absorption at mid to high frequencies. However, effective absorption coefficient drops dramatically in the low frequency end of the overall spectrum, with absorption worsening with increasing wavelength. The resulting poor low frequency attenuation plagues all dissipative prior art designs utilizing perforated metal as a fill liner.
Thus, for ICE and other gas flow applications that have significant low frequency sound characteristics, reactive-type mufflers incorporating single or multiple chambers and tuned Helmholz resonators are usually preferred over dissipative muffler designs when low frequency noise reduction is a primary objective. Reactive mufflers, because they do not contain acoustically absorptive fill in their design, are also perceived as offering “consistent” performance—i.e., they don't degrade or “blow out,” and require frequent replacement or re-packing of dissipative media like fiberglass fill. In today's marketplace, dissipative mufflers are usually regarded as “race pipes” that have far less backpressure than tortuous path reactive muffler designs, and thus have a reduced adverse impact upon engine horsepower, but at the expense of less low frequency noise reduction. In many instances, these “glass-packs” are desired for that purpose, and are installed to preserve deep and powerful-sounding low frequency engine exhaust tones.
When broad-band acoustic attenuation is required, a muffler can feature both reactive and dissipative elements either in series or parallel, with performance anticipated much in the same way one would design an electrical circuit. Such mufflers, however, can become quite complicated and heavy, as certain portions contain fill, while other portions have solid partitions. Additionally, due to the reliance on reactive methods for low frequency attenuation, even the combination muffler designs suffer high pressure losses and reduce the engine's overall performance.
Another sound attenuation technique known in the art, primarily for aerospace and industrial applications, is the use of components crafted from fibrous sintered metal (a.k.a. fiber metal) as a high flow resistance facing for empty cavities that resemble Helmholz resonators. The understood purpose of the cavity is to provide, like a Helmholz resonator, a quarter-wavelength distance which enables the facing material to intercept specific waveforms at their maximum amplitudes and thus yield highest attenuation for a narrow band of frequencies. The published literature (Clark, “Turning Down the Volume”,
Machine Design,
Sep. 24, 1993) summarizes the function of the fiber metal as an alternative form of dissipative attenuation which can replace traditional fill. Sales collateral from one manufacturer of fiber metal carries this theme further by noting disadvantages of fiberglass media when compared to the fiber metal faced cavity attenuation technique. Nowhere is suggestion made, however, that the cavities might be occupied with acoustically absorbent fill, or that the fiber metal element serves only as a liner or container for another material.
Two of Clark's U.S. Pat. Nos., 3,955,643 and 3,920,095, reiterate the use of fiber metal as a facing for empty Helmholz-like cavities. In the former, fiber metal is used in conjunction with other flow-resistive materials to furnish a cavity liner with “continually increasing” flow resistance. In the latter, fiber metal faced cavities are part of a combination muffler device designed to produce low and high frequency attenuation.
Yet another technique for improving sound attenuation in a muffler is to use linear occlusion of the gas flow path. In such a technique, what would otherwise be a clear line-of-sight between the inlet and outlet ports of a muffling device is blocked or obscured by obstructions, offsets, turns, or some other means. Prior art shows many ways linear occlusion can be provided, as exhibited by the following reference list of U.S. Pat. Nos.:
2,707,525;
1,236,987;
6,089,347;
5,824,972;
5,444,197;
4,809,812;
4,735,283;
3,590,947;
2,971,599;
1,772,589.
But while such means for linear occlusion may provide desirable improvements in sound reduction, there is usually a dramatic performance cost manifested by increased backpressure in the muffler. Therefore, it may be desirable to implement the least flow resistive means of linear occlusion while gaining as much noise attenuation as possible. For example, as some of the above references disclose, helical or spiral flow passages avoid the use of highly restrictive ninety-degree or reverse-turning elbows, yet still provide linear occlusion. A study of the prior art featuring such flow passage geometries resulted in the following findings: Itani (U.S. Pat. No. 4,635,753) suggests a dissipat
Baker Rod D.
Nappi Robert E.
Peacock Myers & Adams PC
Quiet Storm, LLC
Warren David
LandOfFree
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