US3568791A

US3568791A - Air ducting

Info

Publication number: US3568791A
Application number: US802184A
Authority: US
Inventors: Russell E Luxton
Original assignee: University of Sydney
Current assignee: University of Sydney
Priority date: 1968-02-27
Filing date: 1969-02-25
Publication date: 1971-03-09
Anticipated expiration: 1988-03-09
Also published as: AU431320B2; GB1259193A; AU3422068A

Abstract

A noise attenuator for inserting into the air ducting of an airconditioning or ventilating system which consists of a section of air ducting divided into at least two channels which differ from one another in length by an odd number of halves or quarters of the wave length of the predominant sound wave which is to be suppressed or diminished.

Description

United States Patent [72] Inventor RIllell E. LIX. 2,990,906 7/ 1961 Audette 181/42 Tlrralnum, New South Wales, Australia 3,323,305 6/1967 Klees 181/59 1 pp 802,184 FOREIGN PATENTS 523 3 121,212 12/1918 Great Britain 181/48 [73]- Assi nee i 3 is 178,815 11/1906 Germany.... 181/46 3 s d w A 602,160 12/1925 France 181/48 [32] mm a g a 869,343 11/1941 France 181/46 [33] Y A 910,922 2/1946 France 181/50 31 220/68 OTHER REFERENCES A.P.C. Application of Martin Serial No. 333,231, Published 54 m D 5/18/1943 I 1 m 3 n. Primary Examiner-Robert S. Ward, Jr.

Attorney-Singer, Stern & Carlberg [52] U.S. (I 101/44, 181/46, 181/50 [51] Int. Cl F01! 1/10,

A noise attenuator for inserting into the air duct. 56, 35, ing of an air-conditioning or ventilating system which consists [56] of Cm of a section of air ducting divided into at least two channels which differ from one another in length by an odd number of UNITED STATES PATENTS halves or quarters of the wave length of the predominant 2,027,359 1/ 1936 Wood et a1. 181/33 sound wave which is to be suppressed or diminished.

"I'II".

AIR nucrmo This invention relates to air ducts such as those employed in air conditioning and ventilating installations in which a current of air is supplied through a duct or trunk to a room or other space.

Commonly, such ducts include various corners or bends between a fan which impels the air along the duct and an air delivery outlet.

Also, because the noise made by the fan is propagated through the air along the ducts, it is in most instances important to provide some means of reducing noise transmission to the rooms or other spaces.

At corners in such ducting the simplest and most economical construction is a mitre bend, which has reasonable noise attenuating properties but, unfortunately, causes a large pressure drop in the duct flow. Another arrangement commonly employed is a radiused bend which again is easy to manufacture and has a much lower pressure drop than does the mitre bend but, unfortunately, it has poor properties as a noise attenuator. Various other bends with air-turningvanes or like items incorporated in them are substantially more costly to manufacture and even then usually have poor noise attenuating properties.

With all these bends it is usually necessary to provide a package attenuator or some other type of noise filter in the ducting in an endeavour to reduce the transmission of noise from the fan. Such noise attenuators, besides being costly and requiring an additional installation operation, cause a large drop in air pressure and a consequent increase in the fan power required to achieve the desired rate of air flow.

Further, to attenuate the lower frequency components of the noise generated by a typical fan, relatively complicated designs are often necessary.

It is the object of the present invention to obviate these difficulties and to provide a relatively simple and economical construction which will result in substantial noise attenuation with an acceptable pressure drop.

in most ventilation systems the dominant frequency is the blade passage frequency of the fan, usually of the order of a few hundred Hertz. Absorbent materials are only effective for near normal incidence at frequencies above about 1000 Hz.

Hence duct linings are ineffective for removing energy at the blade passage frequency. Commercial package attenuators rely on a combination of area changes, absorber, and sometimes resonant chambers within the duct. These attenuators, by their very nature introduce a large pressure loss into the system. 1

In the present attenuating bend two channels are provided about a profiled center body. The path lengths along the center lines of the two channels differ by one-half half (or other odd number of halves) of the wave length of the blade passage frequency. Thus, if one assumes one-dimensional propagation of a discrete frequency around the two channels, the outlet plane of the filter becomes a low impedance point when viewed from upstream. The impedance mismatch between the bend and the duct therefore results in a poor power transfer to the downstream duct, that is, a high attenuation at the design frequency. A low pressure loss may be obtained by suitable profiling of the center body and the walls of the bend.

In accordance with the present invention, air ducting in an air-conditioning or ventilating system in furnished with a noise attenuator comprising at least one section divided into at least two channels of different lengths. For best results the difference in the lengths of the two channels (or between pairs of channels) should correspond as closely as possible to one-half (or other odd number of halves) of the wave length of the predominant sound waves which are to be suppressed or diminished, the said wave length being primarily a function of the speed of the fan and the number of blades therein.

Ideally the length of the shorter channel should be approximately equal to one'quarter or another odd multiple of quarters of the predominant wavelength.

The device of the invention can be fitted in a rectilinear air duct, comprising a section of the latter divided into two or more channels of different lengths. For example, one channel may continue straight through from the inlet to the outlet of the section, while the other is slightly bulged away from the line of the first channel.

Such a device may be constructed by making one wall of a section of trunking convex and placing inside the said section a body which on one face will be parallel to the rectilinear part of the section and form a straight channel with the latter, while the other side is convex so as to form a curved channel of constant dimensions in conjunction with the convex opposite wall of the section. Y

Commonly, however, any air-conditioning or ventilating system includes various bends in the line of the air trunking and the device of the invention may usefully be incorporated in one or more of such be'nds, thus avoiding an extra pressure loss penalty.

Preferably, such bend or each such bend comprises two channels, one of which is substantially similar to a radiused bend and the other, which is somewhat shorter, comprises substantially a chord to such bend. Such chord does not need to be a straight lineand usually, if straight for part or most of its length, will be curved at each end to merge into the air inlet and outlet connections at the ends of the bend. The detailed shape of the channels is determined ultimately by the requirement that the air flow through the channel should be as smooth as possible and should not separate from the surfaces.

A device of the type just indicated can conveniently be constructed by providing a bend for an air duct made up from flat top and bottom plates separated from one another by front and rear sidewalls, the rear sidewall being substantially an arc and the front sidewall substantially a chord extending across said arc but curved at its ends. At each end of the device flanges or other means for connection to inlet and outlet air trunks are provided.

Positioned inside the bend is a specially shaped body which, in conjunction with the front and rear sidewalls, will form two separate channels, one of which will be of substantially arcuate shape in plan (or in the form of an arc with straight extensions at its ends) and the other of which, as above indicated, will be curved at each end and substantially straight in the middle and which will be shorterthan the arcuate channel, preferably by one-half or the predominant wavelength of the sound to be attenuated.

To illustrate the foregoing description reference will now be made to the accompanying drawings, the four FIGS. of which all show longitudinal sections through different noise attenuators respectively in accordance with various preferred forms of the invention.

In the said drawings,

FIGS. 1 and 2 show noise attenuators set in rectilinear air trunking and incorporating center bodies of different shapes; and

FIGS. 3 and 4 show noise attenuators set into bends in air trunking and again show center bodies of different shapes.

In all the drawings the air inlet from a fan (which is not shown) is indicated at 10 and this is preferably furnished with a sound absorbing lining or jacket 11.

The downstream air outlet in each case is indicated at 12 and these air inlets and air outlets are separated in each case by a noise attenuator secured to such air inlets and outlets by flanges l3, 14 or in any other convenient manner.

In the FIG. 1 construction it will be seen that there is a rectilinear channel 15 and a curved channel 16 separated by a hollow sheet metal body 17, the

channels

15 and 16 having the same cross-sectional dimensions throughout their lengths but differing from one another by one-half of a wave length of the predominant sound to be attenuated, or by some odd number multiple of one-half of such wave length.

In FIG. 2 the channels are depicted at 18 and 19 and these are separated from one another by a central body 20 which may be moulded from acoustically absorbent material, or which may be hollow and of sheet metal construction.

number of revolutions of such fan per second.

In FIG. 3 the shorter and longer channels are shown at 21 and 22 with the central body at 23, while in FIG. 4 the shorter and longer channels are respectively depicted at 24 and 25 and the central body at 26. i

In the constructions shown in FIGS. 2 to 4 the cross-sectionalv dimensions of one or both of the channels may vary along the length or lengths of the latter. The bends of FIG. 3 and FIG. 4, while substantially similar in their outer arcs, differ from one another on their inner arcs. The inner arc in FIG. 3 is a smooth curve while that in FIG. 4 has a fiat, which is paralleled by a flat in the surface of the central body 26.

. The lengths of the

shorter channel

15, 18, 21 or 24 may, in each case, by designated as x). where A is the wavelength of the dominant noise component to be suppressed and a is a prescribed constant. In most circumstances A is approximately equal to the velocity of sound in air divided .by the product of the number of blades in the fan impelling the air and the The length of the

longer channel

16, 19, 22 or 25 should thenbe(a+/Q)t. Y

Ideally the prescribed constant a should be equal to N/4 where N is an odd number.

The central body may be built up in various ways.

It may be hollow and be built up fromsheet metal, plywood or other convenient materials.

or it may be a solid body cast from fibrous plaster, foam rubber, polyurethane foam or other convenient material, preferably having acoustic absorbing characteristics.

Acoustic tiles or acoustic boards and perforated metal plates may also be employed to form the center body.

Or in some cases the center body may be built up from a metal frame over which cloth is stretched.

If .a hollow construction is used the center body may be. filled with rockwool, fibre glass .wool, or other sound-absorbing materials. v

One very suitable way of making suchbodies, however, is to make a template of the desired plan shape thereof and then cut out a series of sheets of fibre glass board or other soundabsorbing sheet material which can be laminated together to form a rugged sound-absorbing center body.

It is advantageous if the body has some sound-absorbing properties of its own but this is not essential. The main sound attenuating effect is achieved through interference of sound waves with one another and is produced 'even if all surfaces are hard and fully sound reflecting. The attenuation in the bend depends to some extent on the configuration of the upstream and downstream ducting.

The passages through the bend interact in such a manner that the bend presents a very high impedance to noise of the characteristic frequency and acts as an acoustic filter for the latter. The sound power is reflected upstream towards the fan from the bend and thus it is desirable to precede the bend by a length of trunking lined with a material which is capable of absorbing sound energy.

The shape of the center body and the duct walls should be adjusted as far as possible to minimize pressure losses and reductions in the rate of air flow.

The aerodynamic design principles which have been developed in connection with the design of turbine blades may be found convenient in designing the curves of the center body and duct walls for any particular installation.

The invention makes it possible to eliminate package attenuators in most installations, while achieving an improved attenuation of fan noise and good air flow comparable with that obtainable by normal radiused bends and substantially better than that obtainable with mitre bends.

Further noise suppression may sometimes by achieved by having a change in the cross-sectional areas of the air inlet and outlet trunking. That is to say the openings at the two ends of the bend may be of different sizes.

lclaim:

1. A noise attenuator for insertion in the air ducting of an air-conditioning or ventilation system, comprising a section of air ducting divided into at least two channels which differ from one anot er in length by an odd number of halves of the wavelength of the predominant sound wave which is to be attenuated, and a profiled center body positioned within the section and which divides the section into the two channels.

2. A noise attenuator according to claim. 1, in which the length of the shorter channel is approximately one-quarter of the predominant wavelength.

3. A noise attenuator according to claim 1, in which the length of the shorter channel is an odd number of quarters of the predominant wavelength.

4. A noise attenuator as claimed in claim 1 in which the profiled center body is a hollow structure made from rigid material.

5. A noise attenuator as claimed in claim 4, in which the hollow body is filled with sound absorbent material.

6. A noise attenuator as claimed in claim'l in which the profiled center body is cast from a material selected from'the group consisting of fibrous plaster, foam rubber, polyurethane foam and other castable material having acoustic-absorbing characteristics.

' 7. A noise attenuator as claimed in claim 1 in which the profiled center body is cast from a material selected from the group consisting of fibrous plaster, foam rubber, polyurethane foam and other castable material having acoustic-absorbing Characteristics.

8. A noise attenuator as claimed in claim 1 in which the profiled center body is formed from a material selected from the group consisting of acoustic tiles, acoustic board and perforated material.

9. A noise attenuator as claimed in claim 1 in which the center body consists of a metal frame over which cloth is stretched.

10. A noise attenuator as claimed in claim 1 in which the center body consists of a metal frame-over which cloth is stretched.

l1..A noise attenuator as claimed in claim 1 in which the profiled center body is cut from a series of sheets selected from the group consisting of sheets of fibre glass board and sheets of other sound absorbing sheet material, which sheets i are laminated together.

(SEAL Attes Patent No.

Inventor(s) UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 3.668391 Dated March 9. 1971 RUSSELL ESICDURT LUXTON It is certified that error appears in the above-identified pate Column 1, line 60, after "frequency.

Column 3,

and that said Letters Patent are hereby corrected as shown below:

" insert: ---The sound power associated with this frequency is therefo reflected upstream of the filter where it must absorbed in a suitable length of duct (i.e., in duct having a length greater than half of the w length of the frequency) lined with an absorbin medium.--.

cancel line 53.

line 5%, cancel "length of trunking lined with and substitute therefor --from the bend and thu the bend must be preceded by a trunking of leng greater than half the wave length of the charac teristic frequency and which is lined with a--.

Signed and sealed this 25th day of January 1972.

EDWARD M,FLETCHER, JR. Attesting Officer ROBERT GOTTSCHALK Commissioner of Patent

Claims

1. A noise attenuator for insertion in the air ducting of an air-conditioning or ventilation system, comprising a section of air ducting divided into at least two channels which differ from one another in length by an odd number of halves of the wavelength of the predominant sound wave which is to be attenuated, and a profiled center body positioned within the section and which divides the section into the two channels.

2. A noise attenuator according to claim 1, in which the length of the shorter channel is approximately one-quarter of the predominant wavelength.

6. A noise attenuator as claimed in claim 1 in which the profiled center body is cast from a material selected from the group consisting of fibrous plaster, foam rubber, polyurethane foam and other castable material having acoustic-absorbing characteristics.

7. A noise attenuator as claimed in claim 1 in which the profiled center body is cast from a material selected from the group consisting of fibrous plaster, foam rubber, polyurethane foam and other castable material having acoustic-absorbing characteristics.

10. A noise attenuator as claimed in claim 1 in which the center body consists of a metal frame over which cloth is stretched.

11. A noise attenuator as claimed in claim 1 in which the profiled center body is cut from a series of sheets selected from the group consisting of sheets of fibre glass board and sheets of other sound absorbing sheet material, which sheets are laminated together.