US1804759A - Acoustic device - Google Patents

Description

May 12, 1931- P. B. FLANDERS 1,804,759

ACOUSTIC DEVICE Filed April 28, 192s IUI VEN 70H ffm/L H ANoL-Hs Y A 7` TURA/EY lbodiments, and from the y Patented May 12,.-'1931 UNITED STATES PAUL B. FLANDERS, OF EAST ORANGE,

'YORK PATENT oFFicE l NEW JERSEY, ASSIGNOR T BELL TELEIfI-IONE LABORATORIES, INCORPORATED, OF NEW YORK, N.

Y., A. CORPORATION OF NEW .ACOUSTIC DEVICE Application filed April l28, 1928. Serial No. 273,524.

This invention relates to acoustic devices and particularly to an acoustic attenuator comprising series and shunt impedance elements having a negligible reactance component at speech frequencies.

' In accordance with the present invention y there is provided an attenuator comprising series and shunt impedances which have a negligible reactance component at speech fremquencies.` Resistance elements suitable for use in such an attenuating network are disclosed and claimed in one of my copending applications, Serial No. 273,523, filed of even date herewith; In accordance with o-ne of the several specific embodiments of the invention herein shown and described the resistances forming the acoustic attenuator comprise sound passages in the .form of parallel walled slots less than 0.003 inch in width in a direction substantially perpendicular to the direction. in which acoustic waves are transmitted therethrough. `The sound passage comprising a series resistance is so posij tioned that acoustic waves pass therethrough in going from one portion to another ofthe acoustic transmission line of which the at-v tenuator forms a part, and the sound passage comprising a shunt resistance extends from the acoustic vtransmission line to4 the free atmosphere. By the use of very small separations of t-he order mentioned above, between the walls of the passages, a purely resistive character of the impedance is ensured for a range of frequencies wide enough to include all the essential frequencies of speech or music. The attenuation produced by the device is therefore substantially uniform at all frequencies in this range, and the device is o therefore suited for the accurate control of sound volume in an acoustic system, without causing any distortion of the sound intensityfrequency characteristic.

The nature of the invention will be more fully understood from the following detailed description of a selected number of its emaccompanying drawings of which:

Figs. 1 and 2 are cross-sectional views of one embodiment of the invention;

Fig. 3 is an impedance diagram corresponding to Fig. l;

Fig. 4 illustrates a invention; and

Figs. 5 and 6 illustrate the adaptation of the device of Fig. 4 to an acoustic transmission system.

In Fig. l the main sound conduit is formed preferred*` form of the bythe tubular elements l, and the slots by -pending upon the degree of attenuation desired. The plugs may be held in concentric relationship by means of axial pins. vShunt leakage paths 9 are provided at the ends of block 3 in the form of narrow radial crevices between the block and the tubular element-s.

The width of these crevices is determined by the height of a plurality of pins, such as 5, inserted in the end of the elements 1. Screw caps 6, engaging a thread on the outside of block 3 serve to clamp the tubes l and the center block together as a unit. Holes 7 in the ends ofthe screw caps permit the air leaking through the crevices 9 to escape to the outer atmosphere. A pin 8 insertedv in block 3 and engaging holes in the ends of elements vl aids in keeping the assembly in alignment. The cross section of `the assembly along the line 2 2 is shown in Fig. 2.

Fig. 3 illustrates schematically ,the impedances of the device of Fig. l, the usual electrical conventions for the representation of a wave transmission system being employed. Conductors l0 and l1 correspond' to the mai-n acoustic channel formed by tubes 1. Shunt impedances 2ZkJ correspond to the impedances of leakage paths 9 and series impedance Za represents that of the concentric slots 4. The acoustic impedance of an acoustic path is defined as the ratio of the excess pressure intensity, i. e. the wave pressure applied to the path, to the volumetric rate of displacement of airin the path. For a very narrow parallel slot the impedance is resistive and has substantially the-value given by the equation y TFE Where Z is the impedance, l

d p. is the vcoeiiicient of viscosity of the med isv the width of the slot,l

Zthe len -h in the direction of wave propagation, an v A the cross-sectional area.

Inc. g. s. units p. has the value 1.86 10 for air. Th'e above formula enables the impedances Za and 2Zb to be computed from the physical dimensions of the slots, but in applying the formula to the radial paths 9,.v it is necessary to make an approximation tothe cross-sectional area, since the area in-V creases from the inner end of the path out-.

' wards. It is suilicient to take the area at the mid-section between the inner and the outer end of the path and to assume that the path has this uniform area throughoutits whole length. f.

To avoid'wave reflection effects it is .desirf able that the resistance combination shouldbe designed with-respect to the acoustic line iny which it vis inserted. The combination represented by the three impedances of Fig. 3 may be regarded as a symmetrical section carved out of an infinite line of recurrent structure, comprising series impedancesZ1 and shunt im edances Zh.' The impedance Z., of such a line, if measured at the middle Ifvnow the values of VZ.,L and Z1, are so chosen that ythe impedance Z.. is the same as the Za? (2)A I characteristic impedance of the main acoustic line in which the device is inserted, then there will be no wave reiection inthe system. t

If the device is tofbe inserted in an acoustic conduit'of cross-sectional area A0, the acoustic impedance of which is given by c. g. s. (3)

Then to avoid wave reflection the following Further there. are two vcomplete sections, and .the attentuation will'therefore be equal to e'", where T is the attentuation as defined for a single section.

design condition is imposed tion velocity inthe ratio 6", where e is. the

of a shunt branch, i. e., terminated by a shunt' base of lthe natural logarithms, the second design condition is imposed, that Z i Y 4 Z-b sinh 2 From equations 4 and 5 the simplified design relations follow:

Za=Z sinh T AZa Zb coth lFor the development and further consideration of the foregoing equation reference is `made to the article by Gr. A. Campbell, The pghysical theory of the electric wave filter, ellvSystem Technical Journal vol. 1, No. 2, November, 1922.

The preferred form of the attenuating device shown in Fig. 4 comprises two sections made up of three series impedance elements -r512', 13, and 14, and 2 shunt impedance .elements, 15 and 16, assembled together in a carrier tube 17. The. series elements may each consist of a close coiled spiral of metallic ribbon, wound vwith a uniform pitch ,to give the effect of a very large slot of narrow width. This type of element and the manner of constructing it are fully described in my aforementioned copending application. The ribbon units Yare mounted on sup orting frames 18, 19 and 20, which are assemg carrier tube 17. The shunt resistance eleled within the ments comprise close spaced helices of metallic ribbon wound edgewise on open cylindrical frames 21 and 22. This type is also described in m above mentioned copending application. o ,permit the escape of the air traversing the shunt resistances the carrier tube 17 is provided with a series of peripheral holes 23. The several elements are held inV ositionl in the carrier tube 17 by means o screwed clampin rings 24.

The formulae given for t e computation of the resistances in the device of Fig. 1 may be used in this case also, but the ollowin differences between the two structures ould be noted'. The device of Fig. 3 ends in series lmpedances and therefore Acorresponds to a a f ortioin-ofan infinite recurrent line formed y cutting atthe middle points of two series branches, instead of by splitting the'slmnt branches. The end branches should there'- fore have impedances of the reproduced sound. The attenuating Ydo- 13 already Vice is here arranged to be inserted in the sound conduit between the tone arm 25, and

" the horn 27 of the phonograph. For this purpose one er more complete units are as-v sembled in a tube 29, forming transverse passages therethrough, this tube being slidably mounted in a cylindrical casting 28, forming a union between the tone arm and the horn. Fig. 5 is a transverse section or" the assembly and illustrates how the various parts may be mounted together on the turntable platform 20. Fig. o is an external end view of the assembly as seen in the section 6 6. ln the assemblyY illustrated a single attenuating unit is shown, the second transverse passage in tube being simply an open tube. By sliding the tube 29 in the casting 28 either the open. tube 30 or the attenuating device may be inserted in the sound conduit, giving a choice ci' two degrees of loudness of the reproduced sounds. Additional degrees of loudness .may obviously be secured by the use of additional attenuatine units.

What is claimed is:

l. An acoustic attenuator comprising se ries and shunt impendances at least one of which is in the form of a sound passage less than 0.003 inch wide in a direction substantially perpendicular to the direction in which sound wavesare transmitted therethrough.

2. .an acoustic attenuator comprising series and shunt resistances each oit which comprises a sound passage ot the order of 0.001 inch wide in a direction substantially per? pendicular to the direction in which sound waves are transmitted therethrough.

3. ln an acoustic transmission line, a network of series and shunt dissipative elements, said series element comprising an opening through which sound waves pass in going from one part of the transmission line to another, said shunt element comprising an opening through which sound waves pass out ot the transmission line, these openings being less than 0.003 inch wide in a direction substantially perpendicular to the direction in which sound waves pass therethrough.

4. lln an acoustic transmission line, a network of series and shunt dissipative elements, said series element comprising a plurality of openings through which sound waves pass 1n going from one part of the transmission line to another, said shunt element comprising a plurality of openings through which sound Waves pass out of the transmission line, each of these openings being of the order of 0.001 inch wide in a direction substantially perpendicular to the -direction in which sound waves pass therethrough. Y

5. In an acoustic transmission line, a network of series and shunt impedances, at least one of said impedances comprising a member formed of a thin ribbon wound in the Jform of a spiral, the adjacent turns of which are so closely spaced that the air passage between them has an impedance to the ilow of air substantially free from reactance atspeech frequencies.

6. ln an acoustic. transmission line, a network of series and shunt impedances, at least one of said impedances comprising atmember formed of a thin ribbon wound in the form of a helix having the broad surface of the ribbon normal to the axis of the helix, the adjacent turns of which are so closely spaced that the air passage between. them has an impedance to the flow of air substantially free from reactance at speech frequencies.

ln witness whereof,I l hereunto subscribe my namethis 27th day of April, i928.

.PAUL B. FLANDERS.

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