US1701911A - Acoustic apparatus - Google Patents

Description

Feb. 12, 1929.

L. DE FOREST ET AL ACOUSTIC APPARATUS Filed June 30, 1926 VVENTOR Lea afar 7 BY if? L- Fz nMw s m @L 130 Wm: ATTORMEZS.

Patented Feb. 12, 1929.

UNITED- 1,701,911 PATENT OFFICE.

LEE DE FOREST, OF NEW YORK, AND JOHN LOUIS REYNOLDS, F ASTORIA, NEW YORK,

ASSIGNOBS, BY MESNE ASSIGNMENTS, T0 GENERAL TALKING- PICT'U'BES CORPORA- TION, A CORPORATION OF DELAWARE.

' ACOUSTIC APIABATUS.

Q e pplication flled .l'une 30, 1926. Serial'No. 119,777.

This invention relates to improvements soundreproducing apparatus used in con- 'Iiection with auditoriums and the like.

One of the objects of thisinvention is the improvement in the circuits used in connection with sound reproducing devices whereby any sound reproduction may be varied or adapted at will to the acoustic requirements oi an auditorium.

Another object of this invention is theprovision of a combination of apparatus for providing voice and musical productions in au ditoriums in connectionwith the projection of amotion picture film.

j A still further object of the invention is the combination of a motion picture projection machine and 'a\phonofilm attachment with means for varying the sound reproductions in accordance with the character of the sound being produced in order to adapt it for use with the auditorium.

These and many other obpects are successfully sought by this invention as will appear hereinafter.

This invention resides substantially in the construction, arrangement, combination and relative location of parts and circuit assemblies, as will'be clearly described below.

, Referring to the drawings in which the preferred embodiments of my invention are disclosed; (Figure 1 represents in diagrammatic form the general arrangement of the elements of our invention.

Figure 2 is a diagrammatic showing of the circuits used in connection .with a modification of our invention.

Our invention relates particularly to the combination of an amplifier and sound reproducing device for theatre work, where the amplifier is advantageously located the projection booth of the motionpicture theatre and the'loud speaker is located near the projection screen in the auditorium, and at some distance from the motion picture proof different sizes and arrangements, the acous;

tic requirements to give the best'reproduc-i of greater volume than the higher pitched tones produce in frequent instanees, overpowering echoes which render it dlfiicult for jection booth. We find that in auditoriums people located at distances from the horns 65 to clearly understand. In such cases, it is important to reduce the volume of the reproduction in certain passages of the sound rec- 0rd to a greater degree than at other times. For example, when an orchestral number (musical) is being reproduced the deep tones add richness and beauty to the record and it is possible under such circumstances that the low pitched tones may be reproduced to ,7, advantage in full volume; whereas, in a speaking number where it is important that every word he clearly understood, we find that it is'advantageous to reduce the sound volume during suchkpassages and particularly the volume of the low pitched sounds. m Inasmuch as it is impossible for the operator in the projection booth to clearly hear what is going on from the horns at the other end of the auditorium due to the loud noises inseparable from the operation of the projection machine, it is highly desirable that the controlling means for regulating at will the sound volume be located in the auditorium at a dista'nce from the projection booth. Such an arrangement is shown in Figure 1, where 8 1 represents the walls of the projection booth containing the projection machine, 3, equipped with the phonofilm reproducing attachment 4, such as described in the co-' pending application of Lee de Forest, Serial 5 No. 635,334, filed April 28th, 1923, connected to the amplifier 5 and projecting the picture 7 through thewindow 2 in the wall of the projection bogth, upon the screen 10 located at the other end of the auditorium. The loud 9 speaker horn 9is shown in this case located underneath the screen 10 and is connected to the output of the amplifier 5 by means of the walls 6 in one of which is included the variable resistance 8.

By means of thearrangement here shown, some competent person located the auditorium, as for example, the orchestra con-. ductor hasit within his power to readily control at will the volume of the sound reproduction to suit the acoustic requirements of that particular auditorium and to modulate the particular number which is being reproduced to give the best and most intelligible results. '105 By the arrangement shown in Figure 1, however, it is only possible to reduce all, sound frequenciesalike without discrimina- 4o tance 29' and e resistance is known as an 7.

" tion. It is. highly desirable in sadism to this toamplify at will the higher frequencies :record'is projected. This cell is energized by of a soun record or to diminish the volume of the lower frequencies without at the same tlme that of the higher frequency sound. m a An arr ment for doing this is shown in Figure. 2, in the two stage amplifier circuit.

In this figure, 20 is the photo-electric cell into which thelight from the photographic sound 1 means of the battery 21 and the two in series are coupled to the input circuit of the vacuum tube 26 by means of the input resistance .22.

.ries with the ,tube'27 is connects A condenser 23 is inserted 1n the lead to the grid and a high resistance grid 'lead2 in s'eed'across the grid and film of the vacuum tube 26. The output-of this tube is connected through the primary 31- of a transformer and a battery 34. Connected across the terminals of the primary 31 is an inductance 28 having connected in one lead thereof the variable resistance 30. The variable capacitance 29 is shunted across the'inductance 28.

The secondary 32 of the transformer is bridged by means of. the potentiometer 33.

A lead from "one terminal of. the secondary is connectedthroughthe condenser 35 to one of the film leads. The

tiometer 33 b means'of the adjustable connection40. T e output or plate circuit of the the sound reproducing device 10 andthe plate battery 38. The filament battery 37 supplies thgznergy for lighting thefilamen-ts of the tu The'circuit terposed between the two tubes inclu the inductance 28, the capaciequalizer circuit. The function of the equalizer circuit is to shunt'off from the input circuit of the three-electrode tube 27 to a greater or less degree the lower. frequency currents from the output of the three electrode tube 26.- The relative amount of low frequency as high "frequency currents; which 1s thus shunted can be regulated by varying either one ofthe three elements in the equalizer :circuit, but preferably by varying the capacitance 29 or the resistance 30. By

for the operator at will to by-pass practically means .of such an arrangement, it is possible or-biasing battery 25 is shunt-.

grid circuit of the vacuum tube 27 is connected to the potenthrough the coils 36 of where the echoes are less pronounced, it is possible to introducevmore of thelow fre Euency tones without harmful interference. onsequently, we find that such a circuit arran ement as just described is highly desira le in equipping the sound reproducing apparatus in various types of auditoriums.

Inasmuch as it is highly desirablethat the sound equalizer system of the amplifier be actually embodied in the same, and that ordinaril this amplifier is 100 ted in the projection oth where it is ve 'flicult or imprac- 'tical for the operator to clearly hearor judge the sound reproduction in the auditorium, we prefer to adjust the equalizer circuit once for all to best adapt the tonal qualities of the sound to that particular auditorium and not to vary the elements of the equalizer from time to time as this or that sound record is I reproduced. However, wedo not wish to be limited in this respect, for it is perfectly feasible to install the amplifier or part of it down in the auditorium where the musical conductor or other expert can at will vary the control elements and thus emphasize .to a greater or less degree the high or tonal qualities of the sound from the loud speaker.

' Instead of the equalizer system as shown in Figure 2, other equivalent arrangements well known in the art may be used. For example, an equalizer system may be inserted in series between the photo-electric cell and the input of the first audion' amplifier, or it may be inserted in series rather than in shunt-between any two audion tubes of the amplifier series, or the equalizer may be inserted directly in the output circuit of the last audion tube of the amplifier series. In this latter case, it is,

of course, easy to locate the same in theauditorium in v shown in Figure 1. We find, however, that lace of the simple rheostat as there are many advantages in inserting the equalizer system interstage in the amplifier as shown in Figure 2.

We have found in actual practise that with any type of photo electric cell containing gas there is a certain time lag or delay in opera- .tion to full sensitivity of the cell, which lag operates to reduce the relative strength of response to highfrequencylight variations.

compared to that oflow frequency light variations. For example, when the film record of a sound of 3,000 to4=,000 vibrations per second passes in front of theslit, thus cutting ofi or reducing the light which falls up on the photo electric cell, 3000 to 4000 times per second the relhtive apparent changes in resistance of the cell are far less pronounced, than when this same film is run in front of the slit at a much slower speed, so that the lightfluctuations fall upon the cell at the rate of say 500 to 1500 vibrations per second. This apparent loss in sensitivity of the 'photo-electrlc cell for high frequency light fluctuations may be a phenomena peculiar to the small intensities of light obtained when we employ such a short or narrow slit as is necessary with the phonofilm device, but without going further into the theory or exact explanation of this observed phenomena, the fact remains that there is an apparent lag in gaseous photoelectric cells to high frequency lightlfii'lctuations.

We have discovered the above effect, which was not recognized heretofore by authorities on the photoelectric cell, and we have discovered a practical method of neutralizing this apparent cell lag or of compensating for same, by utilizing the equalizer circuit as described herein. By this method, the amplifier is made to over-amplify the higher frequencies or under amplify the lower frequencies to a degree approximatelyinversely proportioned to the relative loss in sensitivity of the photo-electric cell with increased frequencies, as above described.

We, therefore, desire and intend to cover the function of compensating for the time lag of the photo-electric cell, as well as adapting the sound reproducing device to the acoustic requirements of any given auditorium. We are'well aware of the fact that many changes in the details of construction and relative ar rangeinent of parts may readily occur to those who are skilled in this art, and we do not, therefore, desire to be strictly limited to the particular illustrative embodiment shown and described by us, but rather do we desire to be limited only by the appended claims.

What we seek to secure by United States Letters Patent is: I

1. In a sound reproducing system, the combination of an amplifier, a loud-speaker connected therewith, an auditorium containing said loud-speaker, and means controllable at will for adapting the sound output from said loud-speaker to the acoustic requirements of said auditorium, said means including an equalizer vcircuit in said amplifier circuit, adapted to vary at will the relative intensities of low and high frequency currents amplified in said amplifier.

2. In a sound reproducing system, the com- U adapted to vary at will the relative intensities of low and high frequency currents amplified in said amplifier, for the purpose of adapting the sound output from the loud-speaker to the acoustic requirements of said auditorium.

3. In a sound reproducing system, the combination of a photographic sound record, a photo-electric cell influenced by said sound record, a vacuum tube amplifier having a loud speaker connected thereto, and an auditorium containing said loud speaker, said amplifier circuit including as an element thereof, an equalizer circuit adapted to amplify certain of the frequencies of a sound reproduction.-

4. In a sound reproducing system, the combination of a photographic sound record, a photo-electric cell influenced by said sound record, a vacuum tube amplifier having. a loud speaker connected thereto and an auditorium containing said loud-speaker, said amplifier circuit including as an element thereof, an equalizer circuit adapted to vary the volume of the high frequency sounds without correspondingly varying the lower frequency sounds of any sound reproduction.

5. The method of adapting a sound reproduction to the acoustic requirements of a given space which comprises changing light vibrations including a range of frequencies to sound Vibrations and controlling the sound vibrations at will by increasing the volume of the high frequency sounds without correspondingly varying the low frequency sounds.

In testimony whereof we have hereunto set our hands on this 1st day of June, A. D. 1926.

LEE DE ronns'r. JOHN Lo IsREYNoLDs.

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