US 2790164 A
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April 23, 1957 E. o. OBERG 2,790,164
SOUND SIGNAL TRANSMITTER DEVICE Filed Dec. 2, 1954 2 Sheets-Sheet 1 INVENTOR BY IZQBQM;
ATTORNEY United StatesPatentfO ce 2',790-,1-64 SOUNDLSIGNAL TRANSMITTERDEVTCE mEI'ik Ot'filberg; laidingo," Sweden; assignor :to Svehska Aktieboiaget Gasaceumulator, Lidingo, Sweden, acorporation of Sweden -Appiication 'December 2," 1954 Serial'No 472;7 50 Claims priority; application Sweden-April's, 1 954- scams. or. amazes r Assoundsignal transmitters especially adapted for beazc'onsiand similarnavigatory'means of assistance are nowadays chiefly used electromagnetic sound 'sigrial transmiters 2 according to the telephone principle? These usu'ally .7 on-tain a weak iron-core provid'ed witha windi-ngy 'said ufwinding influencing a diaphragmor eventu'ally a diaplaces within the transmitter. It has been proposed to meltz thesaid ice bark or show i bark away by meansof electricheater eo'ils', but the exp'eriences" hitherto made have had no success in this respect.
The present invention is based upon-an'investigation Labout the currents'of air present in 'the exponential horn "of a sound transmitter of theabove mentioned kind; and
of the' elimaticoccurrences in the sound signah transmitt'er; respectively; determining theplacebf'forniation of the'ice bark or'thesnow bark; 'andtheinvent ion' refers '---to an arrangement=based-onthese theoreticaltinvestigations, by which one can effectively prevent theformation of detrimental snowbarker icebark. I J
The inventionisfurther described. in connectionf'with I Tithe attached drawings, which show a sound signal trai'nsmitten'of the presen't kind at two difierent occasions,
Fig; 1 3 shows a sound signal" transmitterwithou tthe ater eoil of the present'invention. Fig? 2" shows a soundsignal transmitter utilizihg the heater coil and 'cor're'sponding'electricahnetworks.
n the drawingstheweakiron core isrindicated 10, and thewihdingsurrounding said-went iron'core, which is fed by alternating ciirrent ofaudio-"frequency is indicated-11. The weakironcore iSwinI'thiS ca'setassumed alto be linear; and consequentlyonechas appliedaneupper" diaphragm 12 and a lower diaphragm 13 opposite to each end of the core 16. The magnetic circuit is completed through a casing 14, which is arranged between the diaphragms. The diaphragms are clamped in place by means of clamping rings 15, and outside these clamping rings lids 16 are provided, said lids being combined with the exponential horns 17.
As snow bark and ice bark have substantially the same influence in the respects here in question they will be 'indicated below as bark without any difierence being made between them. The experience has now proved that bark appears in the opening of the two exponential horns, however usually to a greater extent in the upper exponential as indicated at 18, and to a less extent at the lower exponential horn, as indicated at 19. Furthermore, bark usually also forms on the upper diaphragm 12, as indi-r .rphragm applied at each'end 'of' the rod=formedcore; he'
30 v *Investigations fof 50 "independently thereof the" ibrationsof the 2,790,164 Paeented Axpri zli, 1 957 earl i a t, t. It has proved, however; that h s is usually no he i'case. T-he' opening=21*wliichforrn y thesour i sound' transmitter wt a ra we i $i bfi neat we median g'ger; so*smtill;;liowever, thatanessential tlecrease'otthe power 0 the sound ansmitter occurs just ori that"occasion whenfthetran' m ssion from the souhd transinitter is of the"greatest"in v The formation 29 of bark causes further anurre a vload s0und"transmittei" is'put out'i'of funtionxf It'is therefore fbark fis1ffornied, but'these'trials have hitherto 1 given no "goodd'e'sults. The freason forthis is assumed to bethat m-has; "not"tak'en' sufficient Yregard'to the aircirculation '"conditionsfemanating' in fan exponential'horn".oftheilkind "There in question i v pg the basis tithepate -i sen- *ftion, as ai'matteriofifact have provedthe followingrelav I of'coinpressihility of the iairif that meansthefcharacter stic'stating the compression pressure 1'iof the *air as a'function of 'foriinstancelthe' moveme t of These two circulations .ofi air are 'superposed'on :eaeh ibthergf and they-:f'o'llow substantially "F the same. lawsffwh h" also exist in radio Itechnic' rectificatibn,in which a directlcur reht is created the rectification, but in whichth'reiisf'siiperposedon,this direct ourrent r a "re'st ent offth'e: high frequency currency.
' v ,raphragms 312 andflgi isthe'higjgest'in"'the middle pa r-'tfa'ndj consequently you "will'obcesr'reetificatieaiefiect ndthe strongesti presusing 2;: rec ified current of air indirection Qua/ads n the centre of the diaphragms,
. ,r i i i fi a "finia sound transmittefot j the type here in question are v l1'aiiges3wliich e'asily eatrse a rather "st iongheatingof the ai r." abe -provided: direetrenar pntentialanus coop'erates at least in the upper exponential horn 17 with the produced heat potential for providing current of ventilation. This current of ventilation will for the reasons mentioned above take place in the direction of the arrow 23 when there will be a created ventilation current also in the lower 7 exp'onential'horn 17, even if this current isweaker, as v indicated by the shorter arrow 24. In both cases the ventilation current follows the laws, which are wellknown as far as regards directional air currents superposed" 0 on sound waves. These laws mean that the current of air is to a very high degree ofexactness directed perpendicularly to the isobars representing the sound wave The purpose of the exponential horns is now that the sound wave should be propagated as far as possible along isobars running perpendicularly to the centre line 25 of the exponential horn. The consequence thereof is also that the outward directed ventilation current will rather exactly follow the centre line, however, spreading itself out across the isonode planes, and consequently the inward directed ventilation current for replacing the outward directed current will be forced to accept a current way, which closely follows the wall of the exponential horn as a surface current.
The invention is now based on two ditferent conditions, viz that one shall by causing a suitable resistance to the ventilation current decrease this current as' far as possible, and further, that one shall by the provision of heater elements in the way of the ventilation current provide for the air, fed to the interior of the exponential horn, as far as possible being so hot, that formation of bark is avoided. V
The provision of resistors in the way of the ventilation current will of course also mean that one provides a resistor in the way of the sound wave, and this is unfavourable for the function of the sound transmitter. However, practical tests have proved that the sound wave is due to the friction resistance against the wall of the sound horn propagating only to a very small extent through the surface layer, where the inward directed component of the ventilation current is passing, as is stated above. If the heating means'are nowplaced very close to the wall of the sound horn, they will create unincreased resistance to the ventilation current without thereby appreciably damping the sound waves. Thus one will in this way obtain a resistance, eventually initiated by a whirl formation at the heater means and thereby essentially improved, as far as regards the ventilation current, without said resistance alfecting the sound-carrying air current. It is thus evident from the above that the heater means should be placed close to the wall of the exponential horn, so that they heat and brake the inward directedreplacement ventilation current eventually under formation of whirls. They partly decrease the ventilation current, and, thereby also cause decreased cooling of the interior of the exponential horn, partly also heat the replacementair, which is slipping inwards along the walls of the exponential horn, and in this way one isforcing a rather high temperature of the air in the interior of the exponential horn.
' Placing now the heater. coil substantially vertically or 'in'any case substantially perpendicularly to the direction line 25 rather close to the opening of the exponential horn, one will further gain that the outward directed radiation from the heat coil will be sutlicient for melting down the bark 18, at the same time as transferring heat 4 of the heater coil 28. Thereby the resistance against inwardly directed air current will be further increased as compared with the direct current resistance provided by the heater coil 28.
Practical tests have proved that one can with a rather low power on the heater coil 28 achieve a complete debarking of sound signal transmitters of the type here in question also during the most severe climatic conditions. The power, as a matter of fact, must not be greater than about the half of the power fed to the coil 11 for creating the sound wave. For decreasing the total maximal power in a system of the present kind one therefore advantageously provides the heater coil 28 and the winding 11 to operate alternatively. This will take place according to the invention in such a way, that power is fed from the network 30 to a coupling arrangement 31, controlled by a pulse mechanism 32. The pulse mechanism 32, in time with the sound pulse which should be transmitted from the sound transmitter, combines the network with conductors running to an audio-frequency converter 33, converting the input electrical power into audio frequency power, said power being fed through the conductors 34 to the winding 11 in the sound transmitter. During the periods, when the electrical power is not transferred to the frequency converter 33, it is instead over conductors 35 transferred to the heater coils 28. The heater coils 28 are thus intermittently heated, but they should suitably have so great heat capacity, that they are'not f-ully cooled during the time, when the sound transmission is taking place. The heat present in the heater coil 28 will thus perhaps vary in temperature, but it will anyhow under all circumstances be sufficient for maintaining the abovementioned function regarding the ventilation current, by which formation of bark in the sound transmitter is efiectively prevented.
The invention, of course, is not limited to the arrangement above described in detail, but difierent modifications m-ay be made within the frame of the invention.
What is claimed is: 1. An outdoor sound transmitter comprising an exponentially formed horn open at one end, a diaphragm I at the other end of said horn, means for vibrating the diaphragm, and an electrically heated toroidal heater coil disposed immediately inside the open end of said horn, said heater coil preventing the formation of ice and snow deposits within the horn by heating the cold input air. r
2."An outdoor sound transmitter according to claim 1 wherein said vibrating means and 'the heater coil are connected to a single source of electric current and are intermittently energized'from said single current source 7 through an intermittently acting switching means.
by convection to the air moving inwards, saidheat being I suflicient for melting down the bark part 20. In the cases, when the inward moving current of air is forced into a whirl formation by the heat coil, as indicated by the dotted line 26, this whirl 27 will of course have radial deepness from the surface of the wallet the exponential horn, which is esesntially greater than the radial deepness wherein the heater coil :is mounted in lar to the center line through said he 3; An outdoor sound transmitter according to claim 1 a plane perpendicustreams Cited in the file ofthis patent UNITED STATES PATENTS I Wm "ran. 18, 1927 Soderberg Oct. 9, 1945