US2580439A - Directional acoustic system - Google Patents

Description

Jan. 1, W. E DIRECTIONAL ACOUSTIC SYSTEM Filed Sept. 7, 1949 'LANCING IMPEDANCE F/GJ 2/ LOUD SPEAKER HORN AC OUS TIC LENS INVENTOR WEKOCK ATTORNEY Patented Jan. 1, 1 952 Wires sr res 7 Claims.

This invention relates to directional acoustic systems and more particularly to directional transmitters, receivers, microphone-receivers, and

combinations thereof for distant talking telepursue It has long been recognized as desirable to pro- Vide means for concentrating or focussing compres'sional waves, "for example in directional microphones, receivers, and distant talking tele-- phones, to avoid the production and reception of waves outside 'of the area of interest and to utiliae the energy of the waves in that area to a maximum. Various devices have been suggested in attaining these results including devices for 56th reflecting and retracting the Waves in com- J biiiation with wave transducers. The reflecting evices have taken several forms including single horns and horns with various reflecting devices mounted therein. The major difliculties with or disadvantages of these devices are their bulk and L their'pcor transmission 'efliciency due to the spfeading of the waves from a single horn, and in the case of a horn employing supplementary reflecting elements, due to the internal reflectioi'is'set up by the reflectors and their supports.

The wave'refracting devices or lenses disclosed in my application Serial No. 52,350, filed October 1, 1948, have high efiiciences in focusingcompressionai waves. However, when these devices "are used with'present types of wave transducers,

spreading of the wave energy occurs between the lens and the instrument with a resultant loss in over-an efii'ci'ency and, in the case of distant talking telephones employing separate transmitters and receivers cross-talk may be so great as to iiiake'the system inoperative.

one object of this invention is to prevent the less of wave energy between wave transducers and associated'wave refractors.

Another object is to prevent audio feedback between the transmitter and receiver of a distant talking telephone. 1

Further objects are to reduce the size and increase the efiiciency of distant talking telephones.

In accordance with one feature, of this invention, a shield is provided between a compressional one another and occupying substantially the whole volume of a space having the shape of an optical lens, a compressional wave transducer, and a closed horn shield extending between the refrac tor and the transducer. A pair of these assemblies mounted so that they have a common focus and associated with a telephone circuit by suit-'- able couplings and amplifiers provides a distant talking telephone which permits the subscriber to effectively employ the instrument while at a distance therefrom Within a limited area of focus without the signal from the loudspeaker assembly being transmitted to the sensitive microphone assembly with sufficient intensityto cause the telephone circuit to sing or howl. This combina tion tends to concentrate the sound emanating from the loudspeaker in a space occupied by the subscriber and likewise amplifies the'sound em'a hating from his lips by focussng it onthe micro-- phone transmitter, thus effecting 'a two-fold increase in loudness which permits a reduction in the amount of amplification necessary in the microphone and receiver circuits.

Other and'further objects and features of this invention will be more clearly and fully under stood from the following description of an illus trative embodiment of the invention taken in connection with the appended drawing in which:

Fig. 1 is a perspective view of a distant talking telepho'ne constructed in accordance with one vention is disclosed in Fig. 1. It comprises a housing ll containingthe elements of a distant talking telephone as represented schematically in "Fig. 3 and including a hybrid c0i1l2 with an' associated balancing impedance [3 connected to the telephone line iii and atransmitting and receiving amplifier I4 and I5 each of which is connected to an individualfocussing assembly.-

Each iocussing assembly, as disclosed more clearly in Fig. 12, comprises an electroac'o'us'ti'c transducer ll, a preferred embodiment utilizing the ring armature type of transducer disclosed in application Serial No. 704,485, filed October 19, 1946, now Patent No. 2,566,850, granted se tember 4, "1951, of Edward E. Mott, exhibiting characteristics which permit its utilization as either a microphone or loudspeaker, a shield I8 extending from the transducer to an acoustic lens l9 made, in accordance with my abovenoted application, of a plurality of rigid members mounted in spaced relationship and occupying substantially the whole volume of a space having the shape of an optical lens. This assembly when employed in a distant talking telephone causes the sound waves emanating from the loudspeaker 2| to converge and meet at a focus F (not shown) and acoustic waves originating at the focus F to be condensed by the receiver lens 25 and focussed on the microphone 24.

The wave focussing assembly of Fig. 2 is bidirectional permitting identical units to be used as the transmitter and receiver assemblies, the transmitter assembly comprising a loudspeaker 2|, a shield or horn 22 and a transmitting lens 23, while the receiver assembly comprises a microphone 24, a shield 25, and a receiving lens 26. Preferably, the shields extending between the transducers and the lens are completely closed to eliminate the leakage of undesirable compressional or audio waves from or to the transducer, the cone form of shield being entirely satisfactory in this combination,

. The shield, for example, may be a cone having a fifteen-inch diameter mouth and a fifteeninch depth and having the transducer mounted at its throat 30 and the lens mounted at the mouth 29. Such a construction has been found to radiate a narrow beam of sound, the beam having the following dimensions at various frequencies. At 11,000 cycles, the signal radiated by this fifteen-inch combination is down twenty decibels in power at angles of 5.2 degrees off the axis, while at 1,100 cycles, this same twenty-decibel'reduction occurs at 52 degrees off the axis. Thefifteen-inch lens has been made exemplary for its convenient size; however, if a sharper focus is desired, such may be obtained by increasing the dimensions of the lens and the horn; thusa thirty-inch lens will have a signal down twenty decibels in power at angles of 2.6 degrees off the axis at 11,000 cycles and at 26 degrees off the axis at 1,100 cycles. At lower frequencies, the beam from the focussing combination becomes proportionally broader but since the response of telephone instruments and also the importance in intelligibility falls off at lower frequencies, tendency to howl at these still lower frequencies is less likely. Most telephone instruments are designed with a maximum transmission capability in the vicinity of 1,000 cycles and the directivity of the wave transmitted by this fOCUSSiDg combination at that frequenc is suficient to substantially eliminate any tendencies for audio feedback, since the sound passing through the lens, is directed and focussed and very little emerges sideways and enters the receiver assembly. The shield extending from the transducers to the lenses inhibits the radiation of the minor side lobes from transmitter 21 to microphone 24 and thus prevents direct cross-talk in that manner.

The acoustic efficiency attainable with this telephone set permits the use of greater amplification of the signal without the danger of crosstalk and howling occurring while requiring a lesser amount of amplification to obtain the same signal intensity at a given point than instruments of the prior art. Thus, the effective range of such instruments can be substantially increased without any detrimental effects.

While the embodiment disclosed in the specifi- I cation is for a distant talking telephone, it is 4 to be understood that the combination of a single acoustic transducer and a rigid acoustic lens and a shield extending between the two elements, may also be effectively employed in highly directional microphones and loudspeakers as individual units.

What is claimed is:

1. A directive acoustic system comprising an acoustic refractor, an acoustic transducer positioned at one focus of said refractor, and a closed continuous shield extending from said transducer to said refractor.

2. A directive compressional wave system comprising a rigid refractor for compressional waves, a compressional wave transducer positioned at one focus of said refractor, and a closed continuous shield extending from said transducer to said refractor.

3. A directional compressional wave system comprising a horn, a compressional wave trans-{ ducer in the throat of said horn, and a rigid refractor for compressional waves positioned in the mouth of said horn so that its axis coincides with the principal energy axis of said transducer, said horn being closed and continuous between said transducer and said refractor.

4. A directive compressional wave system comf' prising a plurality of rigid members mounted in spaced relationship to one another forming a compressional Wave refractor, a compressional wave transducer positioned at one focus of said refractor, and a closed continuous shield extending from said transducer to said plurality of rigid members.

5. A directive compressional wave system comprising a compressional wave transducer, aplu: rality of rigid elements uniformly spaced and occupying substantially the whole of a volume of space having the shape of an optical lens and forming a refractor for compressional waves, and a shield extending from said transducer to said refractor.

6. A distant talking telephone comprising a transmitter, a receiver, an acoustic refractor having one focus at said transmitter, a second acoustic refractor having one focus at said re ceiver, and shields extending from said first re fractor to said transmitter and from said second REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,033,337 Harmer Mar. 10, 1936 2,177,769 Erickson Oct. 31, 1939 2,224,698 Roseby Dec. 10, 1940 2,411,004 Sanial NOV. 12, 1946 2,423,459 Mason July 8, 1947 2,483,231

Peterson Sept. 27,1949

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