US2759555A

US2759555A - Acoustic method and system

Info

Publication number: US2759555A
Application number: US300113A
Authority: US
Inventors: Jordan J Baruch
Original assignee: Bolt Beranek and Newman Inc
Current assignee: Bolt Beranek and Newman Inc
Priority date: 1952-07-21
Filing date: 1952-07-21
Publication date: 1956-08-21
Anticipated expiration: 1973-08-21

Description

llg- 21, 1956 J. J. BARucH 2,759,555

ACOUSTIC METHOD AND SYSTEM Filed July 2l, 1952 TEST CHAMBER FIG. 3

2o 3o Iomeovoaosmoo cYcLEs PER SECOND FIG. 2

FIG.

SIBHIOBG Nl NOILVDNBLLV INVENTOR. JORDAN J. BARUCH ATTORNEYS United States Patent ACOUSTIC METHOD AND SYSTEM Jordan J. Baruch, Cambridge, Mass., assignor, by mesne assignments, to Bolt, Beranek and Newman Inc., Cambridge, Mass., a corporation of Massachusetts Application July 21, 1952, Serial No. 300,113

24 Claims. (Cl. 181-50) The present invention relates to acoustic methods and systems, and, more particularly, to methods of and systems for reducing the intensity of sound energy accompanying the flow of a fluid medium within a confined space.

It is frequently desired to eliminate or markedly reduce the intensity of sound energy accompanying the flow of a fluid medium, such as air, through a confined space. In test chambers, for example, for testing the performance of aircraft engines and propellers, air passing through the chambers carries with it audible noise sounds produced by the operation of the engine and propeller of sufficient intensity to bring discomfort to the human ear. Several devices have been proposed for reducing the level of intensity of these disturbing audio-frequency components of the sound energy emitted from test chambers, but these are all attendant with serious disadvantages.

Among these devices has been a terminal portion for the test chamber divided into a plurality of ducts lined with sound-absorbing material Iand resonant at certain disturbing audio sound-frequencies in order to attenuate the said frequencies passing from the test chamber through terminal-portion ducts. Such ducts are usually employed for attenuating the very low frequency audible sounds. In order to attenuate the somewhat higher audio frequencies, the test chamber is provided with further portions containing baes. For use with the above-mentioned aircraft engine and propeller test chambers, however, the duct and baffle portions must have a very considerable overall length, of from about thirty to seventy feet, in order to effect the desired result. This technique, in addition, does not lend itself to the scaling or proportioning of the dimensions of the sound-absorbing system for different frequencies, as for the purpose of attenuating somewhat different frequency bands. Such a structure, furthermore, is subject to high aerodynamic losses because of the twisting action upon the air flowing through the ducts and then through the bafiies. It is quite difficult, moreover, to install such a system, and the masonry and steel-supporting structure necessary to effect the same is quite extensive.

Another technique proposed for solving this problem has been to utilize a labyrinth in which wall sections project from opposite sides of the terminal portions of the test chamber in order to provide a crooked path for the travel of both air and sound. These wall sections are provided with sound-absorptive layers in order to attenuate the higher audio frequency components of the sound energy which tend to travel in straight lines. Again, however, considerable expense and size is required to enclose a labyrinth at the ends of a standard test cell. The aerodynamic losses attendant upon the forcing of the air through this crooked course, moreover, are high, and the turbulence introduced by the air passing through the labyrinth very often imposes severe rnechanical strains upon the propeller or other device being tested in the test chamber.

An object of the present invention is to provide a new and improved method of and system for reducing the intensity of sound energy of a band of frequencies accompanying the fiow of a fluid medium through a confined space that shall not be subject to the above-described disadvantages.

A further object of the invention is to provide an attenuator for the said band of frequencies which may occupy a relatively small space and necessitate a minimum of installation work and expense in adapting the same to standard test chambers.

Still another object of the present invention is to provide stream-lining for the flow of the air or other fluid medium while effecting the above-described attenuat1on.

Other and further objects will be described hereinafter and will be more particularly pointed out in the appended claims.

In summary, the invention relates to a method of and system for causing a fluid medium containing sound energy of a band of frequencies to travel along a zig-zag path, absorbing the high sound frequencies of the said band of frequencies as the medium travels along the said zig-zag path, and fixing the crests of the zig-zag path substantially in accordance with the half wave-length of the intermediate sound frequencies of the said band of frequencies in order to resonate the said intermediate frequencies, thereby to attenuate the same. Preferred constructional details are treated hereinafter.

The invention will now be described in connection with the accompanying drawing,

Fig. 1 of which is a plan View of a test chamber embodying the present invention in preferred form, the roof of the test chamber being removed,

Fig. 2 is a graph illustrating experimentally obtained characteristics of the chamber of Fig. l, and

Fig. 3 is a perspective view, partly broken away in order to show details, of a section of the preferred soundabsorbing device utilized in the system of Fig. l.

Referring to Fig. 1, a test chamber 1 is shown centrally provided with an engine 3 for driving a propeller 5, for test purposes, mounted upon the floor of the chamber in the customary manner. A door 7 in a side wall of the test chamber 1 may be provided to permit access to the test chamber. At each end of the test chamber is disposed a terminal section 9 provided with a plurality of longitudinally extending planar sound-absorbing devices 11. These sound-absorbing devices may be formed of fibre glass, rock wool, packed felt, or any other desired sound-absorbing material that can be fabricated in sturdy sections. The devices 11 extend from the floor of the test chamber to the roof. At periodically spaced intervals, the otherwise uniform planar devices 11 are provided with baffles 13 of the same acoustically absorbent material. The baffles 13 thus form the largest transverse dimension of the sound-absorbing devices, and they are positioned so that the baffles 13 of successive sound-absorbing devices are staggered across the test chamber. Thus, for example, the baies 13 of the second sound-absorbing device 11 in from either wall, as shown in Fig. l, are disposed between the baffles 13 of the first and third sound-absorbing devices 11. While, in Fig. l, each sound-absorbing device 11 is shown provided with but two baffles 13, this is merely for purposes of illustration, it being understood that, in actual practice, two or more such baffles may be employed depending upon the degree of attenuation and other characteristics desired. Air passing from the left of Fig. 1, through the left-hand terminal section 9, the central portion of the test chamber 1 and the right-hand terminal section 9, in the direction of the arrows, is thus forced,

as a result of this construction, to travel in zig-zag paths between the adjacent sound-absorbing devices llt-13.

Since the higher audio sounds generated by the engine 3 and/or propeller S tend to travel in straight lines, they become absorbed as the air carrying the sound is forced to travel the said zig-zag paths between adjacent sound-absorbing devices 11-13. In Fig. 2, the dash-dot curve 2 represents the variation of attenuation, produced by the effect of these zig-zag paths, with frequency of the sound energy accompanying the iiow of air, attenuation being plotted as the ordinate in units of decibels, `and the frequency, as the abscissa, in units of cycles per second. While this zig-zag path satisfactorily attenuatcs the higher audio-frequency components, say in the neighborhood of 600 to 1000 cycles per second, the attenuation in the intermediate-frequency region of, for example, 105i to 500 cycles per second, is not satisfactory to produce the desired degree of elimination of sound in this frequency range. At 100 cycles, for example, only about ll decibels of attenuation are provided; at 200 cycles, about 20 decibels; while at 500 cycles, about 50 decibels of attenuation are produced. In accordance with the present invention, the intermediate sound frequencies of from about 100 to about 500 cycles, are further attenuated by proper spacing of the longitudinal separation L of the baffles i3 of each device lll13. By spacing the successive crests, namely the successive direction-changing points, of the big-zag paths, to correspond substantially to the half wave-length of these intermediate sound frequencies, such further attenuation is achieved. ln actual practice, an average or mean wavelength of the band of intermediate frequencies may be employed. The use of such periodic or tuned half-wavelength spacingot' the sound-absorbent baflles 13 of adjacent sound-absorbing devices ll of itself produces the plurality of peaked attenuation characteristics shown in the three dotted curves It, 5 and 3 of Fig. 2. rThis periodic structure, in effect, causes resonance, actually multiple resonances 4, 6 and 8, of the intermediate sound frequencies in the space between successive bales, thereby effecting further attenuation by the acoustically absorbent material of the devices l1-13. The operation of the system may be more easily understood by considering that the transversely extending baffles i3 provide constricticns to the sound energy traveling between the longitudinally extending members ll since the transverse separation between adjacent devices il becomes reduced at the baies 13. Stich constrictions present an increase in impedance to the dow of the sound energy. When such impedance increases are made to occur periodically at successive intervals corresponding substantially to the half-wavelength of particular-requencied sound energy, or multiples thereof, there is produced at such successive intervals a very high overall impedance to the flow of that sound energy, thus appreciably further attenuating such sound energy, as illustrated at 4, 6 and 8. In the tests plotted in Fig. 2, these multiple resonance characteristics are produced by this structure in the neighborhood of about 30 cycles, about 252l cycles, and about 500 cycles.

The resultant effect of the before-mentioned zigzagpath attenuation 2, and the periodic spacing attenuation il, 6, S, is plotted as the solid-line curve l0 of Fig. 2. lt will be observed that the intermediate frequencies from about 1GO cycles up to about 500 cycles are appreciably attenuated with from about 28 to about 58 decibels of attenuation, and that the high frequencies of the band above about 600 cycles are attenuated with about 58 decibels of attenuation. High attenuation is not so iniperative for the very low frequencies, below about 100 cycles, inasmuch as they are not so disturbing to the human ear.

Though the before-described resultant attenuation characteristic i0 is satisfactory insofar as the acoustic results are concerned, the baies 13 present an obstruction to the flow of air. As a result of the structure of the apparatus employed in accordance with the present invention, however, this obstruction is easily overcome by securing an acoustically transparent septum 15 between the baffles 13 and, for example, the mid-regions i7 of the sound-absorbing devices 11 disposed between adjacent baffles i3. This acoustically transparent septum may, as an illustration, assume the form of Wire screening. A streamlined zig-zag path, preferably of substantially uniform width S, is thus provided permitting air tiow without obstruction, but allowing the sound to pass t.roiigli the septum l5 to the absorbing material of the devices ll-3 in order to produce the before-described resultant attenuation effect 10, Fig. 2.

As an illustration of a typical installation in a standard aircraft engine test chamber for producing results similar to those shown in Fig. 2, the transverse spacing S defining the width of the Zig-zag paths between the longitudinally extending sound-absorbing devices 11--13 muy have a value of from about 2 to about 6 feet, and the longitudinal separation L of the successive battles 13 of each device 11-13 may have a value from about 4 to about l2 feet.

ln accordance with a further feature of the presentinvention, if it is desired to shift the resultant attenuation curve l0 to the left or to the right in order to cover lower or higher frequency bands for any particular application, it is merely necessary correspondingly to scale or proportionately reduce or increase the before-mentioned dimensions, thereby automatically to produce the desired attenuation characteristic lt) over the desired frequency band. This is extremely convenient inasmuch as a single design of an installation, such as that illustrated in Fig. l, will per-mit the adaptation of the design to other frequency bands by this simple proportionate scaling process alone.

While the invention has been described in connection with a test chamber for aircraft engines and propellers, it is to be understood that these techniques are equally applicable to other systems in which a fluid medium containing undesired sound energy is passed through a confined space, such as, for example, in air-conditioning plants or in muier systems, and the like. It is also to be understood that it is not essential that the devices 11- 13 have the strictly planar or strictly rectangular contiguration illustrated in the drawing, but that other shapes may similarly be employed to provide the desired zig-zag paths and the desired periodic resonant spacing of the sticcessively disposed balles.

Other and further modifications will occur to those skilled in the art and all such are considered to fall within the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

l. in an acoustic system in which a fluid medium is adapted to flow longitudinally through a confined space, means for reducing the intensity of sound energy of a band of frequencies accompanying the flow of the medium comprising a plurality of longitudinally extending soundabsorbing devices spaced transversely within the conlfined space and shaped to provide for the sound energ a zig-zag path or paths there-between of varying transverse dimensions, whereby the high sound frequencies of the said band of frequencies become attenuated during 'the travel of the medium along the zig-zag path or paths,

successive crests of the zig-zag path or paths being spaced longitudinally a distance corresponding substantially to the half-wavelength of intermediate sound frequencies of the said band of frequencies in order to attenuate the said intermediate frequencies.

2. In an acoustic system in which a fluid medium is adapted to ow longitudinally through a coni-ined space, means for reducing the intensity of sound energy of a band of frequencies accompanying the How of the medium comprising a plurality of longitudinally extending soundabsorbing devices each of periodically varying transverse dimension spaced transversely within the confined space with the largest transverse dimension of each device disposed substantially opposite to the smallest dimension of the device or devices disposed adjacent thereto, whereby the high sound frequencies of the said band of frequencies become attenuated during the travel of the medium along the resulting zig-zag path or paths, the largest transverse dimensions of each device being separated from one another a distance corresponding substantially to the half-Wavelength of intermediate sound frequencies of the said band of frequencies in order to attenuate the said intermediate frequencies.

3. In an acoustic system in which a fiuid medium is adapted to flow longitudinally through a confined space, means for reducing the intensity of sound energy of a band of frequencies accompanying the iiow of the medium comprising a plurality of longitudinally extending soundabsorbing devices each of varying transverse dimension spaced transversely within the confined space with the largest transverse dimension of each device longitudinally displaced from the largest transverse dimension of the device or devices disposed adjacent thereto, whereby the high sound frequencies of the said band of frequencies become attenuated during the travel of the medium along the resulting Zig-zag path or paths, the largest transverse dimensions of each device being separated from one another a distance corresponding substantially to the halfwavelength of intermediate sound frequencies of the said band of frequencies in order to attenuate the said intermediate frequencies.

4. In an acoustic system in which a fluid medium is adapted to flow longitudinally through a confined space, means for reducing the intensity of sound energy of a band of frequencies accompanying the fiow of the medium comprising a plurality of longitudinally extending sound-absorbing devices each of varying transverse dimension spaced transversely within the confined space with the largest transverse dimension of each device longitudinally displaced from the largest transverse dimension of the device or devices disposed adjacent thereto, whereby the high sound frequencies of the said band of frequencies become attenuated during the travel of the medium along the resulting zig-zag path or paths, and a smooth lining extending from the largest to the smallest transverse dimensions of each device to streamline the flow of the medium along the said zig-zag path or paths.

5. In an acoustic system in which a fluid medium is adapted to flow longitudinally through a confined space, means for reducing the intensity of sound energy of a band of frequencies accompanying the fiow of the medium comprising a plurality of longitudinally extending sound-absorbing devices each of varying transverse dimension spaced transversely within the confined space with the largest transverse dimension of each device longitudinally displaced from the largest transverse dimension of the device or device disposed adjacent thereto, whereby the high sound frequencies of the said band of frequencies become attenuated during the travel of the medium along the resulting zig-zag path or paths, and a smooth lining disposed between the largest and smallest transverse dimensions of each device to stream-line the iiow of the medium along the said zig-zag path or paths, the largest transverse dimensions of each device being separated from one another a distance corresponding substantially to the halfwavelength of intermediate sound frequencies of the said band offrequencies in order to attenuate the said intermediate frequencies.

6. In an acoustic system in which air is adapted to iiow longitudinally through a confined space, means for reducing the intensity of sound energy of a band of frequencies accompanying the ow of the aircomprising a plurality of longitudinally extending sound-absorbing devices each of periodically varying transverse dimension substantially equally spaced transversely within the confined space with the largest transverse dimension of each device disposed substantially opposite to the smallest di-v mension ofthe devices disposed adjacent thereto, whereby the high sound frequencies of the said band of frequencies become attenuated during the travel of the air along the resulting zig-zag paths, the largest transverse dimensions of each device being separated from one another a distance corresponding substantially to the half wavelength of intermediate sound frequencies of the said band of frequencies in order to attenuate the said intermediate frequencies.

7. In an acoustic system in which air is adapted to flow longitudinally through a confined space, means for reducing the intensity of sound energy of a band of frequencies accompanying the fiow of the air comprising a plurality of longitudinally extending sound-absorbing devices each of periodically varying transverse dimension substantially equal-ly spaced transversely within the confined space with the largest transverse dimension of each device disposed substantially opposite to the smallest dimension of the devices disposed adjacent thereto, whereby the high sound frequencies of the said band of frequencies become attenuated during the travel of the air along the resulting zig-zag paths, and a smooth lining disposed between the largest and smallest transverse dimensions of each device to stream-line the flow of the air along the said zig-zag paths, the largest transverse dimensions of each device being separated from one another a distance corresponding substantially to the half-wavelength of intermediate sound frequencies of the said band of frequencies in order to attenuate the said intermediate frequencies.

8. In an acoustic system in which air is adapted to flow longitudinally through a conned space, means for reducing the intensity of sound energy of a band of frequencies accompanying the iiow of the air comprising a plurality of longitudinally extending soundaabsorbing devices each of periodically varying transverse dimension substantially equally spaced transversely within the confined space with the largest transverse dimension of each device disposed substantially opposite to the smallest dimension of the devices disposed adjacent thereto, Whereby the high sound frequencies of the said band of frequencies become attenuated during the travel of the air along the resulting zig-Zag paths, and a smooth acoustically transparent lining disposed between the largest and smallest transverse dimensions of each device to stream-line the ow of the air along the said zig-zag paths, the largest transverse dimensions of each device being separated from one another a distance corresponding substantially to the half-wavelength of intermediate sound frequencies of the said band of frequencies in order to attenuate the said intermediate frequencies.

9. In an acoustic system in which air is adapted to iiow longitudinally through a confined space, means for reducing the intensity -of sound energy of a band of frequencies accompanying the ow of the air comprising a plurality of longitudinally extending sound-absorbing devices each of substantially uniform transverse dimension and provided with a plurality of separated sound-absorbing baies of greater transverse dimension, the devices being substantially equally spaced transversely within the confined space with the baffles of each device disposed between the baiiies of the devices disposed adjacent thereto, whereby the high sound frequencies of the said band of frequencies become attenuated during the travel of the air along the resulting zig-Zag paths, the separation of the baflies of each device from one another corresponding substantially to the half-wavelength of intermediate sound frequencies of the said band of frequencies in order to attenuate the said intermediate frequencies.

l0. In an acoustic system in which air is adapted to flow longitudinally through a confined space, means for reducing the intensity of sound energy of a band of frequencies accompanying the flow of the air comprising a plurality of longitudinally extending sound-absorbing devices each of substantially uniform transverse dimension and provided with a plurality of separated sound-absorbing baffles of greater transverse dimension, the devices being substantially equally spaced transversely within the confined space with the bafes of each device disposed between the bafiies of the devices disposed adjacent thereto, whereby the high sound frequencies of the said band of frequencies become attenuated during the travel of the air along the resulting zig-zag paths, and an acoustically transparent lining disposed between the baffies of each device along the said zig-zag paths to stream-line the fiow of air therealong, the separation of the bafiies of each device from one another corresponding substantially to the half-Wavelength of intermediate sound frequencies of the said band of frequencies in order to attenuate the said intermediate frequencies.

ll. In an aerodynamic test chamber in which air is adapted to flow longitudinally through the chamber in response to the rotation of a propeller and the like within the chamber, means disposed at each end of the test chamber for reducing the intensity of sound energy of a band of frequencies generated by the rotation of the propeller and the like and accompanying the flow of the air comprising a plurality of longitudinally extending sound-absorbing devices each of periodically varying transverse dimension spaced transversely within the ends of the test chamber with the largest transverse dimension of each device disposed substantially opposite to the smallest dimension of the device or devices disposed adjacent thereto, whereby the high sound frequencies of the said band of frequencies become attenuated during the travel of the air along the resulting zig-zag paths, and a smooth lining disposed between the largest and smallest transverse dimensions of each device to stream-line thev iiow of the air along the said zig-zag path or paths, the largest transverse dimensions of each device being separated from one another distance corresponding substantially to the half-wavelength of intermediate sound frequencies of the said band of frequencies in order to atten uate the said intermediate frequencies.

12. In an aerodynamic test chamber in which air is adapted to flow longitudinally through the chamber in response to the rotation of a propeller and the like within the chamber, means disposed at each end of the test chamber for reducing the intensity of sound energy of a band of frequencies generated by the rotation of the propeller and the like and accompanying the tiow of the air comprising a plurality of longitudinally extending soundabsorbing devices each of substantially uniform transverse dimension and provided with a plurality of separated sound-absorbing baffles of greater transverse dimension, the devices being substantially equally spaced transversely within the ends of the test chamber with the baies of each device disposed between the baffles of the devices disposed adjacent thereto, whereby the high sound frequencies of the said band of frequencies become attenuated during the travel of the air along the resulting zig-zag paths, and an acoustically transparent lining disposed between the bafiies of each device along the said zig-zag paths to stream-line the flow of air` therealong, the separation of the baffles of each device from one another corresponding substantially to the half-wavelength of intermediate sound frequencies of the said band of frequencies in order to attenuate the said intermediate frequencies.

13. An apparatus as claimed in claim l2 and in which the transverse spacing between the said longitudinally extending sound-absorbing devices is from about 2 to about 6 feet, and the separation of the bafiies of each device from one another is from about 4 to l2 feet.

14. A method of the character described that comprises causing sound energy of a band of frequencies to travel in a fluid medium along a zig-zag path of varying transverse dimensions, absorbing the high sound frequencies of the said band of frequencies as the sound energy travels along the said zig-zag path, and fixing the suc- 8 cessive crests of the path substantially in accordance with the half-wavelength of the intermediate sound frequencies of the said band of frequencies in order to resonate the said intermediate frequencies, thereby to attenuate the said intermediate frequencies.

15. A method of the character described that cornprises causing sound energy of a band of frequencies to travel in air along a zig-zag path of varying transverse dimensions, absorbing the high sound frequencies of the said band of frequencies as the sound energy travels along the said zig-zag path, and fixing the successive crests of the zig-zag path substantially in accordance with the half- Wavelength of the intermediate sound frequencies of the said band of frequencies in order to resonate the said intermediate frequencies, thereby to attenuate the said intermediate frequencies.

16. A method of the character described that comprises causing sound energy of a band of frequencies to travel in air along a zig-zag path of varying transverse dimension, stream-lining the air flow along the Zig-zag path, absorbing the high sound frequencies of the said band of frequencies as the sound energy travels along the said zig-zag path, and fixing the successive crests of the Zigzag path substantially in accordance with the half-wavelength of the intermediate sound frequencies of the said band of frequencies in order to resonate the said intermediate frequencies, thereby to attenuate the said intermediate frequencies.

17. A method of the character described that comprises causing sound energy of a band of frequencies to travel in air along a plurality of successively converging and diverging zig-Zag paths of varying transverse dimensions, absorbing the high sound frequencies of the said band of frequencies as the sound energy travels along the said zig-zag paths, and fixing the successive crests of each zigzag path substantially in accordance with the half-wavelength of the intermediate sound frequencies of the said band of frequencies in order to resonate the said intermediate frequencies, thereby to attenuate the said intermediate frequencies.

18. In an acoustic system in which a fluid medium is adapted to fiow longitudinally through a confined space, means for reducing the intensity of sound energy of a band of frequencies accompanying the flow of the rnedium comprising a plurality of longitudinally extending sound-absorbing devices of varying transverse dimensions spaced transversely within the confined space to provide a path or paths of successively increasing and decreasing transverse dimensions through which high sound frequencies of the said band of frequencies are forced to travel in zig-zag fashion, whereby the high sound frequencies of the said band of frequencies become attenuated during such travel, successive regions of decreased transverse dimensions of each path or paths being separated from one another by a spacing resonant to the intermediate soundfrequencies of the said band of frequencies in order attenuate the said intermediate frequencies.

19. In an acoustic system in which a fluid medium is adapted to flow longitudinally through a confined space, means for reducing the intensity of sound energy of a band of frequencies accompanying the flow of the medium comprising'a plurality of longitudinally extending soundabsorbing devices spaced transversely within the confined space and shaped to force the high sound frequencies of the said band of frequencies to travel in a zig-zag path or paths therebetween, whereby the high sound frequencies of the said band of frequencies become attenuated during such travel, and means for providing a high impedance to the flow of the sound energy along the zigzag path or paths at intervals spaced longitudinally therealong a distance corresponding substantially to the halfwavelength of intermediate sound frequencies of the said band of frequencies in order to attenuate the said intermediate frequencies.

20. In anacoustic system in which a iiuid medium is adapted to llow longitudinally through a confined space, means for reducing the intensity of sound energy of a band of frequencies accompanying the flow of the medium comprising a plurality of longitudinally extending soundabsorbing devices spaced transversely within the confined space and of periodically varying transverse-dimensional shape to force the high sound frequencies of the said band of frequencies to travel in a zig-zag path or paths therebetween, whereby the high sound frequencies of the said band of frequencies become attenuated during such travel, the successive direction-changing points of the zig-zag path or paths being spaced longitudinally a distance corresponding substantially to the half-wavelength of intermediate sound frequencies of the said band of frequencies in order to provide a high impedance to the flow of the sound energy at intervals spaced longitudinally a distance corresponding to the said half-wavelength, thereby to attenuate the said intermediate frequencies.

21. In an acoustic system in which air is adapted to flow longitudinally through a confined space, means for reducing the intensity of sound energy of a band of frequencies accompanying the ow of the air comprising a plurality of longitudinally extending sound-absorbing devices each provided with a plurality of separated soundabsorbing balfles of greater transverse dimension than other portions of the devices, the devices being substantially equally spaced transversely within the confined space with the baffles of each device disposed between the ballles of the devices disposed adjacent thereto, whereby the high sound frequencies of the said band of frequencies become attenuated during the travel of the air along the resulting zig-zag paths, the separation of the bacs of each device from one another corresponding substantially to the half-wavelength of intermediate sound frequencies of the said band of frequencies in order to attenuate the said intermediate frequencies.

22. In an acoustic system in which air is adapted to ilow longitudinally through a conned space, means for reducing the intensity of sound energy of a band of frequencies accompanying the ow of the air comprising a plurality o-f longitudinally extending sound-absorbing devices each provided with a plurality of separated soundabsorbing baffles of greater transverse dimension than other portions of the devices, the devices being substantially equally spaced transversely within the confined space with the battles of each device disposed between the bales of the devices disposed adjacent thereto, whereby the high sound frequencies of the said band of frequencies become attenuated during the travel of the air along the resulting zig-zag paths, and an acoustically transparent lining disposed between the bafes of each device along the said zig-zag paths to stream-line the ow of air therealong, the separation of the balles of each device from one another corresponding substantially to the half-wavelength of intermediate sound frequencies of the said band of frequencies in order ot attenuate the said intermediate frequencies.

23. In an aerodynamic test chamber in which air is adapted to ilow longitudinally through the chamber in response to the rotation of a propeller and the like within the chamber, means disposed at each end of the test chamber for reducing the intensity of sound energy of a band of frequencies generated by the rotation of the propeller and the like and accompanying the ow of the air comprising a plurality of longitudinally extending sound-absorbing devices each provided with a plurality of separated sound-absorbing baflles of greater transverse dimension than other portions of the devices, the devices being substantially equally spaced transversely within the ends of the test chamber with the baflles of each device disposed between the baffles of the devices disposed adjacent thereto, whereby the high sound frequencies of the said band of frequencies become attenuated during the travel of the air along the resulting zig-zag paths, and an acoustically transparent lining disposed between the baffles of each device along the said zig-zag paths to stream-line the flow of air therealong, the separation of the baffles of each device from one another corresponding substantially to the half-wavelength of intermediate sound frequencies of the said band of frequencies in order to attenuate the said intermediate frequencies.

24. An apparatus as claimed in claim 23 and in which the transverse spacing between the said longitudinally extending sound-absorbing devices is from about 2 to about 6 feet, and the separation of the baflles of each device from one another is from labout 4 to 12 feet.

References Cited in the le of this patent UNITED STATES PATENTS 701,496 McKinnie lune 3, 1902 1,658,402 Warth Feb. 7, 1928 1,821,688 Bourne Sept. 1, 1931 1,963,341 Weber June 19, 1934 1,998,386 Powell Apr. 16, 1935 2,233,966 Tominaga Mar. 4, 1941 2,270,825 Parkinson et al. l an. 20, 1942 2,271,892 Bourne Feb. 3, 1942 2,297,046 Bourne Sept. 29, 1942 2,519,161 Tucker Aug. 15, 1950 2,704,504 Wilkening Mar. 22, 1955 FOREIGN PATENTS 285,604 Great Britain Feb. 23, 1928 746,961 France June 9, 1933