Noise suppressor for vacuum sweepers and the like

A noise suppressor for vacuum cleaners is characterized by a muffler formed of a succession of tubes of different cross sectional areas and lengths. The muffler defines a low pass acoustic filter, and is connected in line with an exhaust outlet from the vacuum cleaner. Preferably, the muffler is used in place of a sleeve conventionally found in a filter bag for the vacuum cleaner, and thereby acts as a stand pipe to maintain the inlet to the filter bag above collected debris so that the inlet is not choked off with debris before the bag is filled.

BACKGROUND OF THE INVENTION 
The present invention relates to reduction of noise levels associated with 
exhausting air from devices employing blowers or fans, and in particular 
to a muffler adapted for use with vacuum sweepers, floor scrubbers, carpet 
maintenance equipment, blowers and the like. 
Appliances utilizing high speed fans to move air, such for example as 
vacuum cleaners and blowers, typically produce high frequency sounds 
emanating from the air exhaust outlet and attending the passage of air 
through the fan chamber. Ordinarily, given the same sound pressure level, 
high frequency sounds are more annoying than low frequency sounds. 
Noise levels can be reduced by installing equipment such as dampeners or 
mufflers at the exhaust outlet from the air motor. The exhaust outlet of a 
vacuum cleaner, employing a fan system through which passes debris 
entrained in air, is provided with an enclosure to capture the debris. The 
enclosure is usually made of a porous material and called a filter bag, so 
that air entering the enclosure may pass therethrough while debris is 
contained. When the filter bag is mounted in a location above the air 
motor, as for example in upright and external canister vacuum cleaners, it 
contains a tubular sleeve which attaches to the vacuum cleaner exhaust 
outlet. The function of the sleeve is to act as a stand pipe to keep the 
bag inlet above collected debris, so that debris does not choke off the 
inlet to the bag before the bag is full. 
Conventional filter bags do little to reduce high frequency noises 
attending the passage of air at high velocity. The art therefore 
contemplates various muffling techniques to reduce the level of such 
noises, and in U.S. Pat. No. 2,130,495 a muffler for a vacuum cleaner 
extends from the exhaust outlet to an upper portion of a filter bag and 
comprises inner and outer tubular cloth sleeves having a tubular felt 
sleeve therebetween. In U.S. Pat. No. 2,287,867 a muffler for a vacuum 
cleaner comprises a short tubular section of sponge rubber positioned 
between the exhaust outlet and an inlet to a filter bag, and U.S. Pat. No. 
4,015,683 teaches a somewhat similar foam material muffler, except that 
the flow path through the muffler comprises a central passageway having a 
plurality of radially extending grooves. In all of the aforementioned 
mufflers, the cellular texture of the muffler surface has the property of 
absorbing some volume of the noises in the air, and to that extent 
contributes to a greater quietness of operation. However, a disadvantage 
of such mufflers is that they tend to be rendered entirely useless or 
experience a significant decrease in efficiency within a short period of 
time as a result of the interstices in the cellular surfaces becoming 
filled with debris. 
OBJECTS OF THE INVENTION 
An object of the present invention is to provide an improved muffler for 
reducing high frequency noises attending high velocity movement of air in 
vacuum cleaners, carpet maintenance equipment, blowers and the like. 
Another object is to provide such a muffler which may readily form a 
portion of a filter bag into which is introduced a high velocity flow of 
debris carrying air. 
A further object is to provide such a muffler which does not experience 
with use decreases in noise reduction efficiency. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided in combination 
with an enclosure for connection with an exhaust outlet from an air 
blower, wherein the enclosure has means for exhausting air introduced 
therein, a sound reducing muffler comprising a generally tubular housing 
having a generally cylindrical medial section of enlarged cross sectional 
area. The housing is connectable at one end with the exhaust outlet from 
the air blower and extends at an opposite end to interior of the enclosure 
for conveying air from the blower into the enclosure, the muffler acting 
as a low pass noise filter to reduce high frequency noises attending 
passage of air through the blower. 
In a preferred embodiment, the enclosure comprises a filter bag, the 
muffler is substantially positioned within the filter bag, and the 
opposite end of the muffler housing extends toward an upper end of the 
filter bag to maintain an inlet thereto above any collected debris therein 
so that the inlet does not become choked off with debris. 
The foregoing and other objects, advantages and features of the invention 
will become apparent upon a consideration of the following detailed 
description, when taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION 
FIG. 1 illustrates a canister type vacuum cleaner, indicated generally at 
20, with which the teachings of the invention may advantageously be used. 
The vacuum cleaner includes a lower base or housing 22, which is provided 
with a pair of wheels 24 and handles 26 for ease of transport to points of 
use. An inlet 28 to the base connects through a hose 30 to one end of a 
tubular extension 32, the opposite end of which carries a floor and/or 
carpet cleaning attachment 34. An enclosure shown as a filter bag 38 for 
receiving debris laden air is carried atop the base, and may conveniently 
be supported in position by a pair of hooks 38 looped over the handles. 
Referring also to FIG. 2, to provide suction at the attachment 3 to remove 
dirt and debris from a surface to be cleaned, a motor 40 and blower 42 are 
mounted within the base 22. The blower has an inlet 44 in communication 
with the interior of the base and an exhaust outlet 46 for connection with 
the interior of the filter bag 36. The motor drives the blower to generate 
suction at the attachment 34 to pick up dirt and debris, which is then 
carried in a high velocity flow of air through the tubular extension 32, 
the hose 30 and the blower into the interior of the filter bag. The 
illustrated filter bag is of a porous filter material, so that air escapes 
through the walls thereof while debris is captured and contained. It is 
understood, however, that other types of debris containing enclosures may 
be used in place of a porous filter bag, for example those of a type 
having nonporous walls with other means being provided for escape of air. 
As is known, vacuum cleaners utilizing high speed fans to move air 
typically produce high frequency sounds, which sounds are more annoying 
and objectionable to a user than lower frequency sounds. Also, for an 
external canister vacuum cleaner of the type shown, as well as for an 
upright vacuum cleaner or any other type in which the filter bag is 
mounted above the exhaust from the air motor, a tubular sleeve is usually 
provided in the bag and extends between the exhaust outlet and a point 
toward the top of the bag to act as a stand pipe which prevents collected 
debris from choking off the inlet to the bag before the bag is full. The 
present invention provides a muffler for decreasing high frequency noises 
associated with movement of air through the vacuum cleaner, which at the 
same time serves the purpose of and replaces a sleeve in the filter bag to 
prevent collected debris from choking off the inlet to the bag. 
Referring in particular to FIG. 2, the combination noise muffler and filter 
bag sleeve contemplated by the invention is indicated generally at 100 and 
comprises a low pass noise filter which reduces the intensity of high 
frequency sound waves passing therethrough. The muffler is comprised of a 
tubular housing having one or more generally cylindrical medial sections 
of enlarged cross sectional area, which together with the remainder of the 
housing define a succession of tubes with different cross sectional areas, 
for example relatively large cross sectional area tubes 102 and relatively 
small cross sectional area tubes 104 arranged in series to alternate the 
tubes 102 and 104. 
As can be seen from FIG. 2, each of the larger tubes 102 is formed by a 
cylindrical wall of a first diameter, and each of the smaller tubes 104 is 
formed by a tubular member of a second diameter; with adjacent tubes 102 
and 104 being connected by planar end walls extending from the 
circumference of the tube 102 perpendicular to the axis thereof inwardly 
to openings coaxial with the axis of tube 102 equal to the second diameter 
of tube 104. An inlet to the lowermost tube 104 connects with the exhaust 
outlet 46 from the blower 42 and an outlet from the uppermost tube 104 
opens into an upper portion of the filter bag 36 and defines the inlet to 
the bag. The ratio of the cross sectional areas of the tubes to each other 
and to the length of the tubes determines the frequency range of sound 
that is passed through the muffler, and by choosing appropriate dimensions 
high frequency noises may be appreciably reduced at the exhaust outlet. At 
the same time, since a muffler of the type described operates without 
internal restriction, it may be used in place of the aforementioned filter 
bag sleeve. Thus, a functional filter bag provided with the muffler 100 
can reduce noise without the need for additional components. 
As is apparent, the muffler 100 is, in its most primitive form, a simple 
expansion chamber which acts as a low pass filter. The effectiveness of 
the muffler is determined by its noise transmission loss (L) in decibels, 
which is defined as 10 times the logarithm to the base 10 of the ratio and 
sound power incident on the muffler to sound power transmitted by the 
muffler. Considering only a single one of the relatively large cross 
sectional area tubes 102, which may be considered as an expansion chamber, 
along with the two relatively small cross sectional tubes 104 on its 
opposite sides, the noise transmission loss or decrease in the level of 
sound passing through the three tubes may be determined by the following 
equation: 
##EQU1## 
In essence, the performance of the muffler 100 may be interpreted in terms 
of the sound wave system existing inside the muffler. Reflected sound 
waves inside the muffler interfere destructively with incident waves at 
the inlet to the muffler, and thus provide a mismatch of impedance between 
the interior of the muffler and the inlet thereto, leading to reflection 
of sound energy back along the inlet toward the source of the sound. 
Although the foregoing discussion was limited to a single expansion chamber 
or relatively large cross sectional area tube 102 connected in series with 
two relatively small cross sectional area tubes 104, it may be extended to 
include two or more expansion chambers in series, for example the two 
tubes 102 and the three tubes 104 as shown. As the number of expansion 
chambers increases, the transmission loss L and the effectiveness of the 
muffler increases. The transmission loss L also increases at the lengths 
of the tubes separating the expansion chambers increase. Thus, by an 
appropriate selection of the number of tubes 102 and 104, along with a 
selection of the ratios of the cross sectional areas of the tubes and 
their lengths, the transmission loss of the muffler and the frequency 
range of sound passed therethrough may be determined and controlled. It is 
appreciated, however, that additional expansion chambers 102 and long 
separating tubes 104 tend to increase the size of the muffler, so that for 
practical purposes some compromises in effectiveness may be necessary. 
The invention thus provides an improved noise filter or muffler for vacuum 
cleaners. By virtue of the muffler serving in place of a sleeve otherwise 
required in a filter bag for a vacuum cleaner, the muffler may be 
efficiently incorporated into the bag without need for additional 
components. Although the muffler may be separate from the filter bag and 
the bag positioned therearound in use, it is advantageously formed as part 
of the bag itself, just as a sleeve would be formed as part of the bag, so 
that no special care need be taken in mounting the bag on the vacuum 
cleaner. Of course, if it were desired to do so, the muffler could be 
separate from the filter bag and positioned between the bag and vacuum 
cleaner exhaust outlet, although that would necessitate provision of a 
separate sleeve in the bag where the bag is to be positioned above the 
vacuum cleaner exhaust outlet. 
It is also to be appreciated that unlike prior noise mufflers of the 
cellular or porous surface type, which tend to become less efficient or 
completely useless with time as a result of the interstices in their 
surfaces becoming filled with debris, the noise filter of the invention 
remains fully effective with use and does not decrease in efficiency, 
inasmuch as it does not rely upon any cellular surface areas for its 
operation. In addition, while the muffler has been described for use in 
connection with a vacuum cleaner, it is understood that it would readily 
find application with other appliances utilizing high speed fans to move 
air, for example other types of floor care equipment and blowers. 
While embodiments of the invention have been described in detail, various 
modifications and other embodiments thereof may be devised by one skilled 
in the art without departing from the spirit and scope of the invention, 
as defined by the appended claims.