US2520798A - Device for the stereophonic transmission of sound - Google Patents

Description

Aug. 29, 1950 K. DE BOER 2,520,798

DEVICE FOR THE STEREOPHONIC TRANSMISSION OF SOUND Filed May 2, 1947 12 2 I a 5; a:

K. DE BOER INVENTI'OR BYW M (0/ AGENT Patented Aug. 29, 1950 DEVICE. FOR THE S'TEREOPHONIC TRANSMISSION-OF SOUND K'ornelis de, Boer, Eindhoven, Netherlands,- as- Signor to Hartford National Bank and" Trust Company, Hartford, Conn, as trustee Application May 2; 1947,. Serial No..74=5-,374. Inthe Netherlands November. 27,1940

Section 1, PubIicLaw 690, August 8,'1946 Patent expires November 2-7;. 196.0

i'Claims. I

This invention; relates; to a device: for the stereophonic-transmission or sound by means of pl r lity of separate. channels, in which the sound receivers; areseparated by a sphere of at least 14 cms.. in diameter or any other object equivalent thereto: as an acoustic obstacle and which comprises separate; reproducing devices for the reproduction of the high-frequency ranges and a. single-reproducing: device for the reproduction: of. the: low-frequency ranges, the latter. device beinga arranged; intermediate the reproducing devices forthethighdrequency ranges. The requirement which is to! be: imposed in realising such: a transmission. is that a linear relation existsbetweenthe.- different valuesof the anglea at which thesound. source isarranged with-respect; tothe plane of. symmetry of the. receiving microphones and the deviation ,u of the soundimage. produced, from: the planeof symmetry. This will: be referredvtc. hereinafter as "linear projection. I

in such: a device the: microphones. are secured to an. artificial headsof. the kindwhich is 'commonlyemployed,v that. is to' say a. sphere of about 22 cms. inqdiameter; then. the disadvantage arises;. that; linear projection is obtained over: an angle qrOf. 45 only This disadvantage occurs, for example; when radio: plays: are required to-be: picked-up, of whicha linear reproduction over angleslarger than 45 is required. explanation; of. the. said: phenomenon. may be found in; the: fact that. an artificial head. 9ft. ap proximately- 2'2 ems; in: diameter; the stereo.- phonic action-0t whichcorrespcnds to that. cit-the human: head, has-sc great a shielding: action for the microphonawhich. is; remote: trom the: sound source that a disproportionately great stereo.- phonic' efiect" occurs at. the. reproducing ends, that. is to say that even: a small lateral. displacement of the original!v sound. source. results at the reproducing side in: a strong variation. of the direction from which: the soundv appears to originate.

An improvement may beobtained if use is made of; artificial heads which. are. smaller in diameter; When. using, for example,v an: artificial head of 12 cms. in: diameter, a.. linear projection obtained over. an angle oi oi 90. For this improvement protection has previously. been claimed in U. 472,778..

Consequently, the. most satisfactory stereo.- phonic. transmissionwould be obtained if for each individual case. an. artificial. head. were used of a. diameter which. ensures. therequiredlinear- S... A patent. application Ser.. No.

ity with the. freedomof action necessary in this case: within a determined. angle From. the point: of view of uniformity and practical pick up techniquethis: method is complicated since aseries of. artificial heads of different diameters would be required. In view thereof the aim will be to. realise all pick-ups occurring in practice with theuse. oi the sametype. of. artificial. head. For reasons of constructional nature one is in many cases bound. to: objects to which the micro.- phonesareto. be secured, for example a column in. thepick-up. space, which are of a size larger than that of. asphereof a diameter of 12 cms.

The. present invention provides a means. by which. a sphere. of at least 14- cms. in diameter neverthelessv permits. of: obtaining linear projection over. an. angle: q: up. to. As is. well.- known, an artificial. head. of. 22 cms. inv diameter begins toconstitutean acoustic obstacle at. afrequency of about300. cycles/sec. The differences in. intensity brought. about. at thev ears of theobserver by heads of different diameters consequently exhibit different dependences on. frequency.

On. the ground. thereof. the separation. between the high-frequency and low-frequency ranges. in the. device. according. to. the invention is chosen to be not.- higher than 1200. cycles/sec. and so much. higher than. 600. cycles/sec. that the excessive. stereophonic action of. the pick-up device is neutralised by. the absence of the stereophonic action of. they low portion. of. the high-frequency range exhibiting. stereophonic action. The. disadvantage inherent in the: use ofv artificial heads of. atleast 14 cms..in.diameter. is thus suppressed by admitting adecrease of the. differences in intensity of. the sound at. the ears of the. observer over a determined. frequency range comprising the range of low frequencies and a portion of the range. of. high frequencies. up. to a determined limit, according to. the diameter of, the head.

In the device. according to American patent specification 2,126,929 the differences in. intensity atthe, ears. of the listener are likewise reduced but in this. case-either. for the whole of the frequency range to an. equal extent, or for the range. of high. tones only,. since a third loudspea-ker arranged. intermediate the two hightone. loudspeakers. reproduces either the whole of-the frequency range, or the range of the high frequencies only. Since in this case the reducticns in intensity are always required to comprise. the. range of. the high. frequencies, variationof. the maximum. angle q within which the artificial. headis still capable of linear projecby means of filters 2| and 22.

tion, does not take place here when the device would cooperate with an artificial head, in contradistinction to the operation of the device according to the invention. v

In order that the invention may be more clearly understood and readily carried into effect, it will now be described more fully by reference to the accompanying drawing. I I

In the figure reference numeral l2 indicates an acoustic obstacle, for example a desk or. a pillar of the hall, to which microphone l3 and I4 respectively are secured on each side. The microphones are connected toamplifiers l5 and 16 via separate wires. The output ends of the amplifiers are connected to loudspeakers l9 and 20 respectively through filters l1 and I8 which only allow passage of the high-frequency range. The lower limit of the filters is comprised between 1000 and 1200 cycles/sec. The amplifiers also serve to feed a loudspeaker 23, intended for the reproduction of the low-frequency ranges of both microphones, the frequencies of which above a limit of 1000 to 1200 cycles/sec. are suppressed V The energies of the two microphones are mixed behind the filters.

In the example as shown this reproducing unit comprises a single loudspeaker 23 having a baffle board 24, which is 7 arranged centrally intermediate the loudspeakers l9 and 20. More particularly in large installations the reproduction of the low-frequency range is generally divided over a plurality of loudspeakers which are jointly arranged on one bafile board. In this case all of the loudspeakers may be connected in par- 'allel' and fed via the filters 2i and 22, or each of the filters isconnected to a number of 'the total quantity of low-frequency loudspeakers. :The reproduction of the high-frequency range may, if .desired, likewise be divided over a plujrality of parallel-connected loudspeakers arranged side by side, instead of over a single loudspeaker l9 or 20.

The dimensions of the object l2 are, for constructional reasons, such that the minimum dimension is equal to at least the width of the human head, hence at least 14 cms. Consequently, the shielding action for sound waves originating from the sound source I I is such that for oscillations. to which a directional effect is to be attributed, that is to say for those above 300 cycles/sec, the microphone l3 receives a sound intensity which is considerably smaller than that of the microphone l4. I

Displacement of the sound source 10, symmetlrically with respect to the microphones l3 and I4, towards the position II will consequently have the effect that the sound, instead of coming through the loudspeakers, l9 and 20 strength, seems to originate substantially from the loudspeaker 20. Further displacement of the sound source in the lateral direction does not bring about further variation in the intensity ratio of the sound transmitted by the loudspeakers !9 and. 20, so that the auditor will not notice further displacement.

If loudspeaker 23 transmits the low-frequency range with the upperfrequency limit of 300 to 500 cycles/sec. the direction from which the sound at the reproducing side apparently originates does not vary in practice since these low frequencies have no directional effect. The auditor cannot determine the direction of sound having such low frequencies. By extension of the frequency range, if those higher frequencies are emitted by the sound source, energy from both microphones will always be transmitted also to loudspeaker 23, which consequently reproduces sound waves which do-h'ave a'directional effect. The auditor hears sound from three different directions, that is to say from the loudspeakers I 0, 23 and 20. The human ear combines this sound into a single apparent direction of the sound source. Since the loudspeaker 23 is arranged midway intermediate the two others, the apparent direction of the sound is closer to the loudspeaker 23 than it would be the case if no sound waves with directional effect were emitted. The apparent lateral displacement is therefore decreased and since the maximum stereophonic effect remains substantially unchanged, comparatively large lateral displacements of the sound source are still perceivable. This phenomenon is stronger as the loudspeaker 23 reproduces a larger proportion of the frequencies above 300 cycles/sec, in other words as the separation between the high-frequency and low-frequency ranges is shifted to higher frequencies. It has been found in practice thatwith the use of a sphere of 22 cms, in diameter, which is commonly employed, separation in the range between 1000 and 1200 cycles/sec. yields satisfactory results.

What I claim is:

1. A device for the stereophonic transmission of sound comprising a plurality of sound receivers, an acoustic obstacle separating said receivers, said obstacle'having a diameter of at least 14 cms., a plurality of separate high-frequency range reproducing devices, a low-frequency range reproducing device arranged between at least two of said plurality of devices, means connecting one of said receivers with one of said plurality of devices and with said lowfrequency range reproducing device, means connecting another of said receivers with another of said plurality of devices and with said lowfrequency range reproducing device, and means in said connections for separating the high-frequency and low-frequency ranges at a point not higher than 1200 cycles/sec. and above 600 cycles/sec, whereby an excessive stereophonic action of the device as a whole is compensated for by the absence Of stereophonic action for the low portion of the high-frequency range which shows stereophonic action. 4

2. A device for the stereophonic transmission of sound comprising a plurality of sound receivers, an acoustic obstacle separating said receivers, said obstacle having a diameter of approximately 22 cms., a plurality of separate highfrequency range reproducing devices, a low-frequency range reproducing device arranged between at least two of said plurality of devices, means connecting one of said receivers with one of said plurality of devices and with said lowfrequency range reproducing device, 'means connecting another of said receivers with another of said plurality of devices and with said low-frequency range reproducing device, and electrical filter devices in said connecting means for transmitting high frequencies to said plurality of reproducing devices and low frequencies to said low-frequency reproduction device, said filter devices having a frequency separation value of between 1000 and 1200 cycles/sec.

3. A device for the stereophonic transmission of sound comprising a pair of sound receivers, an acoustic obstacle separating said receivers, said obstacle having a diameter of at least 14 cms., a pair of separate high-frequency range reproducing devices, a low-frequency reproducing device arranged between said pair of devices, means connecting one of said receivers with one of said pair of devices and with said low-frequency range reproducing device, means connecting the other of said receivers with the other of said pair of devices and with said low-frequency range reproducing device, and means in said connections for separating the high-frequency and low-frequency ranges at a point not higher than 1200 cycles/sec. and above 600 cycles/sec, whereby any excessive stereophonic action of the device as a whole is compensated for by the absence of stereophonic action for the low portion of the high-frequency range which shows stereophonic action.

4. A device for the stereophonic transmission of sound comprising a pair of sound receivers, an acoustic obstacle separating said receivers. said obstacle having a diameter of approximately 22 cm's., a pair of separate high-frequency range reproducing devices, a low-frequency range reproducing device arranged between said two devices, means connecting one of said receivers 5 with one of said pair of devices a ,yvith said low-frequency range reproducing means. connecting the other of said receivers; with the other 01' said pair of devices and with said lowfrequency range reproducing device, and electrical filter devices in said correcting means fortransmitting high-frequencies to said pair 01' reproducing devices and low-frequencies to said low-frequency reproducing device, said filter devices having a frequency separation value 01 between 1000 and 1200 cycles/sec.

Kommm on Bonn. REFERENCES cum n The following references are of: (word in the file of this patent:

UNITED STATES PA'riru rs

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