US2139006A - Public address system - Google Patents

Public address system Download PDF

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Publication number
US2139006A
US2139006A US82244A US8224436A US2139006A US 2139006 A US2139006 A US 2139006A US 82244 A US82244 A US 82244A US 8224436 A US8224436 A US 8224436A US 2139006 A US2139006 A US 2139006A
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Prior art keywords
neutrals
emergency
neutral
grounded
network
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US82244A
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Irving F Durst
Begun Semi Joseph
Barnett Herbert
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CARLL TUCKER
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CARLL TUCKER
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Priority to US82244A priority Critical patent/US2139006A/en
Priority to FR820535D priority patent/FR820535A/fr
Priority to GB10636/37A priority patent/GB493846A/en
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; ELECTRIC HEARING AIDS; PUBLIC ADDRESS SYSTEMS
    • H04R27/00—Public address systems

Definitions

  • This invention relates to improvements in signaling systems of the type commonly known as public address systems, with particular relation to the application of such systems to shipboard.
  • the basic object of this invention is to provide a public address system for use on board ship having circuit characteristics which greatly increase the dependability of the system both under normal and emergency conditions.
  • the invention consists in the application of a signaling or public address system to a conductive network comprising the three-wire neutrals and the normal and emer gency wiring circuits commonly employed on shipboard to provide a plurality of channels over each of which the signal energy may be transmitted to operate signal responsive devices common to all the channels.
  • a still further object of the invention is to incorporate the signal responsive devices in a system of this type whereby the grounding or short circuiting'of portions of the system or of some of the signal responsive devices does not' operate to render the remainder of the system and the remainder of the signal responsive devices inoperative.
  • This invention resides substantially in the combination, construction, arrangement, and rela- 1 tive location of parts, all as will be setforth in the necessary detail in the following specificav tion.
  • Figure 1 is a diagrammatic view of one form of system in accordance with this invention as applied on board ship;
  • Figure 2 is a similar view of a modified arrangement particularly with respect to the connection of the signal responsive devices to the multichannel network;
  • FIG 3 isa diagrammatic modification of the arrangement of Figure 1 somewhat simplifled
  • Figure 4 is a digrammatic layout of a modification of a portion of the system for employing automatic means for switching any impedselective, groups thereof.
  • Figure '7- is a diagrammatic illustration of a further modified form'of system employing the principles of this invention. 6*
  • a signaling system by meansof which the oflicers on the bridge, or at other control points on a vessel be able to 10 issue orders to the entire ship, that is to the crew and passengers, or to either selectively, or to Any such system applied on shipboard finds its maximum utility at times of emergency only if the signaling system 151 is so constructed and installed that in times of emergency it maybe depended upon to function normally. It is believed that heretofore no such system has been available.
  • the system of this invention is characterized by unusual dependability not only under normal, but also under the most adverse emergency conditions because it is characterized first bythe fact that it comprises a plurality of channels over any one ofwhich the signal energy may travel to the signal responsive'devices in spite 40 of interruption of the other channels, and secondly because undesired grounds and short circuits and defective signal responsive devices do not render the other, signal responsive devices in-
  • FIG. l a portion of a ships hulland superstructure is diagrammatically illustrated at I
  • Modern sailing vessels are commonly equipped with two power supply sources. which may be termed the normal and emergency sources for supplying electric energy for the operation of the electrical equipment carried by the vessel.
  • At 2 and 3 are diagrammatically illustrated generators which are connected to supply a three-wire system comprising the circuit wires 4 and 5 and the neutral 6.
  • the circuit wires 4 and 5 have not been shown extended to various parts of theyessel since they are notinvolved in this invention.
  • Neutral 6 is shown grounded to the hull of the vessel through a resistance l which is commonly present in the grounding connection.
  • the neutral 6 is shown running on one deck level to the various parts of the ship by the wire 1. Similarly this neutral is shown extending to the various parts of the ship on the other deck levels, as indicated at 8 and 9.
  • These neutrals are connected as shown to the common neutral wire 6.
  • the emergency wiring system comprises conductors l3 and I 4 which as before are not' shown extended to the various parts of the ship, and a common neutral l5.
  • This neutral is, provided with the branch neutrals I6, I! and 18 at the various deck levels,and connected thereto by the leads l5, l5 and I5 respectively.
  • the neutrals 6 and I5 may be connected together. This has been indicated by the wire I and the switch 20.
  • which offers little resistance to the flow of power currents between the two systems from the normal and emergency sources.
  • this impedance 2! isselected so as to have a high value with respect to the signal currents so as not to form a low impedance shunt'fo'r the source of signal energy.
  • switch 20 is open or closed does not substantially affect the signal level on the neutrals.
  • At 22 is a microphone or other suitable form of signal device feeding into any suitable and well known form of amplifier which has been diagrammatically indicated at 23.
  • the output of the amplifier is fed into two or more separate transformers of which by way of examplc the secondaries 25 and 24 are shown.
  • secondaries 25 and 24 may be connected in parallel so that the two main feeders havegti e same polarity, or inseries so that the polarity of the two feeders is opposite with respect to ground.
  • Each connection has apparent advantages and disadvantages.
  • One terminal of each is shown with a common grounding connection 26 grounded on the hull. Under some conditions single transformers can be used with grounded tapped secondary. The other terminals of the secondary are connected through the condensers 21 and 28 to the neutral wires as shown.
  • condenser 21 is connected by wire I!) directly to the neutral l and condenser 28 is connected by the wire ID to the wire l0 which extends to the other neutral 6.
  • the wires i9 and III are connected to the neutral system on opposite sides of switch 20 and coil 2
  • the signal responsive devices are connected across the two neutral wire systems.
  • the neutrals l6 and 9 provide a conductive network for the signal responsive devices on that level.
  • Connected across the wires 9 and I6 at the desired points are the primaries single transformer with a mid-tap, are grounded to the hull by the wire 3 I.
  • two separate transformers as distinguished from a single transformer having a mid-tap. This is desirable in order that if one of the lines to the speaker transformers is grounded a complete short circuit of the speaker will not result. Where a single transformer with a mid-tap is used a ground on one of the lines will either short'the other circuit to the speaker or substantially reduce the volume of sound output therefrom. For this reason two separate transformers are preferably connected as illustrated so that the grounding of one of the lines will not interfere with the power volume operation of the associated speaker.
  • the outside terminals of the primary are connected to the neutrals l6 and 9 through the condensers 32 and 33.
  • the secondaries of the transformers 29 and 30 are connected in series with a signal responsive device 34, such as a loud speaker.
  • the speaker transformers 29 and 30 have together a small impedance of the order of 3 or 4 ohms so that the impedance of the speakers is matched with the impedance of the conductors, so that when'one or more transformers and/or associated condensers and/or conducting channels are short circuited none of the speakers are affected and all remain operative.
  • a short circuit directly across the secondaries of transformers 29 and 30 will render a speaker inoperative if the secondaries of transformers29 and 30 are connected in series, and then only that particular loud speaker.
  • the impedance in the circuit between any two speakers is always normally high enough so that a short circuit on one of the speakers does not render the adjacent speakers inoperative.
  • the neutral 6' of the generators 2 and 3 is connected by the various branch neutrals :1'-, 8', 9'., I0; H3 12, l3; l4 and HP.
  • the neutral l5'-of the emergency generators H and i2 is connected through branch neutrals 5
  • is connected in: a junction box Bto the normal neutral system through condenser 52 and similarly emergency neutral 53 is connected to the normal neutral system through condenser.
  • emergency 51 is connected through 58 to thenormal, and emergencyfl is connected through the condenser 59 to the normal neutral system.
  • Emergency neutral I5" is connected to: a junction box Bto the normal neutral system through condenser 52 and similarly emergency neutral 53 is connected to the normal neutral system through condenser.
  • emergency 51 is connected through 58 to thenormal
  • emergencyfl is connected through the condenser 59 to the normal neutral system.
  • Emergency neutral I5" is connected to: a junction
  • wire 46' is not one of the existing wiresof theships network, but is installed for the purpose. It is pointed out;that'a fan circuit normally differs from the ordinary lighting circuit in thatit originates in a single panelbox and exists as asingle circuit throu hout a large part of a ship. On the other hand, the individual" lighting circuitsnormally feed no more than four or five staterooms.
  • the loud speakers 24 in this case each have one terminal grounded to the hull, as ior example the partitions P as shown. The other terminal is connected between each adjacent normal" neutral and either the wire 41 or the wire48throu'gh the condensers Bland 6
  • the speakers arebconnecte'd between the various neutrals through the transformers Hand 30 and thev condensers 32 andj33 as described in con-- nection with the systernof Figure, 1.
  • One side of each of the primaries 'of' these-transformers is grounded' at 3
  • Their primaries are connected in seriesand the se condaries are connected so that one terminal of each is interconnected and provided with a common ground 85.
  • the other terminalof each is connectedthr'ough the condensers 11, 18, 19, augmthe emergency neutrals 1E and 1 5, and the normal neutrals H and 12.
  • The'butput' of the amplifier I25 respectively
  • the transformers have the.
  • Suitable impedances such as the resistances I15 and I16 'ar e connected in series acrossthefe'eclers I9 and I0 in order to supply a' direct current path for the rectifier I30. .
  • These resistances have a common connection to the groundedflcommonlead 26 through a condenser I 21.
  • These magnets control a pair of switches I3! and I32.
  • One contact of each of theseswitches respectively is connected to .thefeeders I9 and I0.
  • each of theseswitches have a common connection to the ground lead 26.
  • switch I34 When an open circuit occurs on one of the feedthe voltageacross the associated magnet I29 or I28 .builds'up to close the associated switch. The closing of this entire sy'stern in operation.
  • switch I34 would control the microphone 22 and amplifier 23 and would close and. I32.
  • Switch I34 controls the circuit comprising the wires I36 and I31 having a suitable current source I35.
  • magnet I28' would be energized to close switch I32 from opening thereby maintaining the short circuit acrossthe feeder I0 of the common lead 25. The net result of all this is that all the signal energy frorn'the amplifier will then go out over the circuit including. transformer I24 and will not be absorbed and lost in transformer I25.
  • the coupling condensers H0 and these neutral systems may be interconnected by means of thefswitch' 20 through the inductance 2l.
  • the signal responsive devices or speakers. are connected in series with the secondaries of a pair of transformers 29 and 30. In this case the primaries of these transformers are wound 1 in opposition and are connected across the main and emergency neutrals.
  • condenser 3I as distinguished from the pair of condensers 32 and 33; as. employed in the direct and emergency neutrals with respect to the power supply, it is not necessary to provide a pair -'of condensers in the leads to each of the primaries of the transformers 29 and 30.
  • a. single condenser in the groundqlead With the arrangement illustrated the supply of current from the amplifiers into the two channels is in parallel.
  • the primaries of the speaker transformers are in parallel with respect to the feeders but series.
  • a combined modulatorvand amplifier which is fed by means of the microphone 200 or other equivalent transmitter.
  • 202, 203 and 204 are diagrammatically illustrated control switches for the modulator amplifier, or transformers connected to the output terminals in the amplifier as previously described.
  • I Y a The primaries are connected in series as are the secondaries which. are provided with the common grounding connection 201.
  • These transformers feed into the lines 208 and 209 through 2II- as before.
  • Feeder 208 is connected to' the emergency neutral EN of the emergency power system, and feeder 209 is connected to the mainneutral MN of the-main power supply system.
  • the inductance 2I2 and switch 2I3 are employed.
  • the main and emergency neutrals extend to the panel boxes from which the branch neutrals. radiate.
  • An emergencybranch neutral is indicated at EI and amain branch neutral is shown at MI.
  • the reproducing devices TM are connected between pairs of'main and emergency neutrals the secondaries are in source.
  • the first reproducing device isconnected across the output terminals of a rectifier bridge 22]] the input terminals of which are fed by the secondaries of the transformers 2 and2l5.
  • the terminals of'these secondaries are bridged by a tuned circuit ZI'S responsive to one of the carrier frequencies of the source.
  • the primaries of these transformers are connected in series between, E! and Ml through the, condensers 2 l6 and 2
  • Theprimaries have a common grounded connection 2H3.v
  • the arrangement for the second reproducing device only differs from the first in that the tuned circuit 219' is tuned to respondto another of 'the carrier fre quencies'.
  • the third reproducingdevic'e' is shown connected to a rectifier bridge and supplied by anuntuned circuit so as to be operable by all of the carrierirequencies.
  • the rectifiers employed in the: rectifying bridges may be of any suitable form available for the purpose, such as copper oxide rectifiers and the like, vacuum tube rectifiers, etc.
  • the system of Figure 7 illustrates a rather simplified form ofthe invention in which the basic principle of supplying energy from a common signal source over a plurality of channels to each of' a group of loud speakers is illustrated.
  • the main generatorsZ and 3 are shown grounded to the hull of the ship by meansfoffla connection which includes the impedance J. It is noted that in somecases theimpedancerepree sented by'l may be nothing more than the impedance of the grounding wire tothe hull. It is here emphasized, asis true of the other systems disclosed, that the principlesof the invention may be appliedfto networks which are not grounded.
  • the emergency generators are'shown'byll and I2 in the superstructure of the ship.
  • the main neutral feeder. 6 is shown provided with the branch neutrals 6, 6 and 6 running to the various panel boxes 1B.
  • the emergency neutral i5 is shown provided with thebranch neutrals I5 I5 and 'lii likewise'running to separate normal and emergency circuit l, panel boxes B.
  • the switch 20 isprovidedas before to interconnect the main and branch neutralsin the normal operation of the main and emergency power systems.
  • a microphone or its equivalent 22 feeds into asuitable form of amplifier 23, one terminal of :which is grounded by the, wire 26 on the metal hull of theship.
  • the other terminal is connectedfby the wire 2V to the main and emergency neutrals through the condensers 21 and 21 respectively.
  • the main branch neutrals 6*, B and 6 are shownrunning from the panel boxes to which the branch neutrals 6 and 6 extend.
  • the emergency branch neutrals iii and IS are shown extending from the'panel-boxesto which'the emergency neutra-ls 15 and 15 extend.
  • the main branch neutrals B), B and V, and the emergency branch neutrals l5? and l5 represent a fan or lightingjcircuit extending from the panel boxes.
  • A'feature of this form of the invention involves the interconnection of main branch neutrals to main branchfneutrals, emergency branch neutrals to emergency branch neutrals, and main brancnne'utrals to emergency branch neutrals.
  • condenser C connects the emergency branch neutral l5 to the mainbranch neutral G
  • Condenser C connects the main branch neutral 6? to the main branch neutral 6*.
  • Condenser C connectsthe 'main branch neutral 6 to the emergency branch neutral 15.
  • Condenser 0 connects the main branch neutral'i to the emergencybranch neutral 15
  • Condenser C interconnects the emergency branchneutrals 15?? and I5Y'.
  • the branch neutrals are notnormallyfinterconnected but" the above connections through condensers are made to produce a'systern in accordance withthe invention.
  • a series of signal responsive'dgvices, as for example loud speakers 34, 34', 34 34 34 34 34" and-34" areconnectedbetween'the various branch neutrals, main and emergency, to'ground on the metal walls or decks P of the ship through the condensers 33 'Circuits maybe trac'ed'to the reproducing devices 34*,*3 4 'and 34 over two channelsi" Energy is supplied from'the amplifier 23 to these speakers overthe circuit comprising the wires 15, and 6*. Energy is also fed from the amplifier over wire 6, wire 6 and wire 6 to whet ⁇ .
  • the ground ing or'opening of either of these separate circuits will not render more than one of these three speakers inoperative even then may a ground fromnear oneof the speakers,'and then onlyfthe speaker nearest that ground,
  • thecircuit arrangement is such that the volume level 'of the signals at the speakers is automatically distributed throughout the ent lre network. This is an exceedinglyimportant feaa direct short circuit of a 'speaker'can' affect only one of these speakers.
  • a signaling system the combination com prising a pair of grounded networks, a grounded signal source coupled to said network, for impressing a common signahcurrentflupon both of said networks, a plurality'of signal responsive devices each connected between said networks and each having a pair of grounded coupling d9? vices connectedbetween saidnetworks ior sup plying energy from the'network to said signal responsive devices simultaneously over two paths or over either path during an 'open circuit. or a short circuit on .the other. p r
  • a signaling system of the type described comprising a pair of grounded networks, a grounded, signal device coupled to the networks for supplying a common signal cursrent to both, a plurality of signal responsive devices, and means connected between each-of said networks and ground for supplying energy to said devices simultaneously over. two paths or over either. path during an opencircuit or a shortoir-I cuit on the other.
  • a signaling system comprising a pair of grounded networks, a grounded signal device connected to said networks for supplying a common signal current to both, a plurality of signal responsive devices, and low impedance means connected -be tween each of said networks and ground for supplying energy to said devices simultaneously over two paths or over either path during an open circuit or-a short circuit on the other.
  • a signaling system of thetype described comprising at least two grounded conductors, a grounded signal source connected to said conductors for applying a common signaling current to said conductors, and a plurality of signal responsive devices each connected between said conductors and ground whereby the signal current is supplied thereto over both conductors or ,over either during short circuit or open circuit on the other.
  • a signaling system of the type described the combination comprising a grounded network, a grounded source of signal energy for applying a signal current to said'network at spaced points, a plurality of signal responsive devices, a pair of impedance means connected between spaced points on the network, means providing a common ground for said impedance devices, and means for connecting the signal responsive devices respectively to. said impedance devices.
  • a signaling system comprising a grounded conductive network, means for applying a signaling current to said network at spaced points, a plurality 'of signal responsive devices, and grounded coupling devices for con.- necting said signal responsive devices to the network at spaced points, whereby signals reach each for grounding the'other, terminal signal responsive device over two different portionsoffsaid network.
  • a signaling system comprising a grounded conductive network, means for applying a signaling current to said networkat spaced points, a. plurality of signal responsive devices,'andmeans for' connecting said signal responsive devices to the network each comprising a transformer having its primary connected through acondenserto the network,'a common ground forthe primaries, and means for connecting each signai'fresponsive device in series with the secondaries of a pair of primaries having a common ground.
  • the combination com-' prising a grounded conductive network, means for applying a signaling current to said' network at'spaced points, a plurality of signal responsive devices, means for. connecting'each signal responsive device'to the network" each comprising two device, being connected between ground and the mid-point of 'a' pair of said condensers,
  • a signaling system the combination com: prising a grounded 'network, a source of signal energy having two output transformers, 'a coinmon ground for the secondaries of the'transformers, means for connecting th'e'other terminals of the secondaries to spaced points on the network, a plurality of signal responsive devices, and a pair of low impedance devices having a common ground and connected between spaced points on thejn'et'work forconnecting each signal responsive device to the network.
  • a signaling, system on shipboard comprising a network composed oi! thegrounded neutrals of the main and emergency power'systems, grounded signal means forimpressinga common signal on both of said neutrals, a plurality of signal responsive devices, and low impedance means having a common ground and connected between the said neutrals for operating" therespective signal responsive devices.
  • a signaling system onshipboard comprising a network composed of the grounded neutrals of themain and emergency power systems, grounded signal means for 1m pressing a common signal on both of said neutrals, a pairof transformersfor each signal responsivedevice, means for connecting the prima-' ries of each pair'of transformers in series across the normal and emergency neutrals through condensers, a common g'roundfor the primaries, and
  • a signaling system for use on shipboard, the combination comprising a network composed of the normal a'ndfemergency grounded power circuit neutrals, a source of energy having a pair of output transformers, a common ground for one terminal of each of the secondaries of the output transformers, means for connecting the other terminals of the secondaries to the net- 1 to said low impedance coupling meansfrespectively.
  • a grounded network composed of the neutral wires of the normal and emergency power circuits, means for generating a signal current having a plurality of output transformers, means for providing a common ground for the secondaries of all of said transformers, means for connecting the other terminals of the secondaries to the network at spaced points, a plurality of signal responsive devices, and means comprising a pair of grounded low impedance devices connected between the normal and emergency neutrals for coupling the signal responsive devices respectively to the network.
  • a signaling system for use on shipboard, the combination comprising a network composed of the grounded neutrals of the main and emergency power systems, a grounded signal source, a pair of coupling transformers fed from said source, said transformers having a common ground for one terminal of each, means for connecting the other terminals of the transformers to the main and emergency neutral systems respectively, a plurality of grounded signal responsive devices each connected between the main and emergency neutral systems, and means for automatically short circuiting the secondaries of either of said coupling transformers upon the development of an open circuit in the means for connecting them to the main and emergency neutrals.
  • a signaling system for use on shipboard, the combination comprising a conductive network composed of main andemergency power circuit neutrals, means for impressing signal energy upon the network thus formed, a plurality of signal. responsive devices, a pair of transformers for each of said devices,-the secondaries of each pair being connected in series with the associated device, the primaries of each pair of transformers being oppositely wound and connected in series across the main and emergency neutrals, and a common ground for the primaries having an impedance therein.
  • a signaling system for use on shipboard and the like, the combination comprising a conductive network composed of main and emergency power neutrals, atransmitter for impressing signal energy upon the network thus formed, a plurality of signal responsive devices, a pair of transformers for each of said devices, the primaries of the transformers being connected in series between the main and emergencyneutrals and having a common ground, and connections between the secondaries of the associated pair of transformers and a signal responsive device.
  • a signaling system comprising a pair of separate conducting networks, means for interconnecting the-networks to .efiect signal energy transfer therebetween, a common source of signal energy connected to both networks to energize them, and a plurality of signal responsive devices connected to both of said networks whereby energy is supplied to each signal responsive device from both networks or from either upon failure of the other by short circuit.
  • a signaling system comprising a conductive network, an impedance connected into said network to isolate it substantially. into two portions with respect to the currents to be transmitted thereover; a grounded source of signal energy connected to the portions of the network on opposite sides'of said impedance, and a plurality of signal responsive devices connected between said portions of the network on opposite sides of saidimpedance whereby a common signal is transmitted to eachofsaid' devices over said a portions of the network.”
  • the combination comprising a conductive network, an' impedance connected into said network to isolate it substantially into two portions with respect to thecurrents to be transmitted thereover, a grounded source of signal energy connected to the portions of the network on opposite sides of said impedance, a plurality of signal responsive devices, and grounded coupling means for each of the signal responsive devices for individually connecting them to said portions of the network.
  • saidgrounded cou pling means comprising a pair of condensers connected'in series across said portions of the network and having their common points grounded.
  • a signaling system comprising a conductive network, an impedance connected into said network to isolate it substantially into two portions with respect to the'currents to be transmitted thereover, a grounded source of signal energy connected to the portions of the network on oppositesides of said impedance, a plucoupling means for each of the signal responsive devices for individually connecting them to said portions of the network, said means comprising a pair of transformers, the primaries of which are connected in series across the said portions of the network and havinga common ground, the signal responsive devices being respectively connected to the secondaries of said pairs of transformers in series therewith.
  • a signaling system comprising a conductive network, an impedance cone nected into said network to isolate it substantially into two portions with-respect to the currents to be-transmitted thereover, a groundedsource of signal energy connected to the portions of the network on opposite sides of said impedance, a

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  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Acoustics & Sound (AREA)
  • Signal Processing (AREA)
  • Cable Transmission Systems, Equalization Of Radio And Reduction Of Echo (AREA)
  • Transmitters (AREA)
US82244A 1936-05-28 1936-05-28 Public address system Expired - Lifetime US2139006A (en)

Priority Applications (3)

Application Number Priority Date Filing Date Title
US82244A US2139006A (en) 1936-05-28 1936-05-28 Public address system
FR820535D FR820535A (fr) 1936-05-28 1937-04-12 Système de diffusion par haut-parleurs
GB10636/37A GB493846A (en) 1936-05-28 1937-04-14 Signaling systems such as public address systems or the like

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US82244A US2139006A (en) 1936-05-28 1936-05-28 Public address system

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US2139006A true US2139006A (en) 1938-12-06

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2433413A (en) * 1944-02-21 1947-12-30 Internat Marine Radio Company Combined radio transmitter and loud-speaker installation
US5345510A (en) * 1992-07-13 1994-09-06 Rauland-Borg Corporation Integrated speaker supervision and alarm system

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2433413A (en) * 1944-02-21 1947-12-30 Internat Marine Radio Company Combined radio transmitter and loud-speaker installation
US5345510A (en) * 1992-07-13 1994-09-06 Rauland-Borg Corporation Integrated speaker supervision and alarm system

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FR820535A (fr) 1937-11-13
GB493846A (en) 1938-10-14

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