The motion picture projectionist (Nov 1931-Jan 1933)

Record Details:

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November, 1931 Motion Picture Projectionist 21 ing from the diaphragm, strikes the upper surface of the tone chamber and is reflected back to the diaphragm. This period of reflection is definite in time and when the note is struck which corresponds in period to the natural period of the tone chamber, resonance occurs which builds the note of this particular frequency up to many times the volume of other notes, thus causing blasting and distortion. A third disadvantage of this type of tone chamber is due to the fact that the enlarged air cushion above the diaphragm absorbs higher frequencies, causing them to reach the throat of the horn with such diminished amplitude that they are inaudible. A fourth disadvantage lies in the fact that due to the ready movement in the air of the tone chamber, the air does not resist the movement of the diaphragm or load it, the diaphragm energy thus being extended in its own motion rather than in being transmitted into motion of the air. As previously stated, the purpose of this article is to examine the work of past investigators to ascertain the various methods adopted to solve this problem. Tone Chamber Modifications As early as 1878, Eickemeyer inserted a plug in his tone chamber slightly above the diaphragm, causing the air paths to travel out from the center of the diaphragm and in from the periphery to a common passageway formed between the plug and the inner shell of the tone chamber. Eickemeyer's construction is shown in Pig. 2. In his studies of the phonograph, Tainter gave careful consideration to the form of air chamber above the diaphragm. By placing a plug within his tone chamber, he was able to increase the efficiency of his recording device on high frequencies, and to record higher frequencies than he was able to attain without this improvement. In a patent granted to him in 1887, Tainter states, "As heretofore made the recorder did not act as efficiently in recording sounds of high pitch as in recording sounds of lower pitch. This is believed to be due to the fact that when the sound waves impinge directly against the center of the diaphragm the latter does not act as a unit, for if the sounds are sufficiently high, two or more waves in opposite phase may, owing to the length between the center and circumference of the diaphragm, be acting upon it at the same time, and a differential effect is the result instead of the full force of the wave. This difficulty may be overcome by constructing the sound conveying passage to the diaphragm in such manner that the atmospheric vibrations will act simultaneously upon the whole surface of the diaphragm. "The best embodiment of this prin ciple would be to construct a large air chamber of conical form in front of the diaphragm, communicating at its apex with the mouth-piece; but this plan is not feasible, because the vibratory air-space in front of the diaphragm must be made very thin, or otherwise a disagreeable hollow quality is manifested in the reproduced sound. The object sought, however, may be practically attained without dispensing with the thin airspace, which is found to be advantageous, by dividing the sound-conveying passage as it approaches the Fig. 4 diaphragm into a number of branches equal in length and communicating with different points on the surface of the diaphragm, whereby the latter is caused to vibrate with practical uniformity in every part, even with sound of high pitch." Tainter's device is shown in Fig. 3. The Hill "Air-Hammer" In 1901, Hill while working on the phonograph employed a cylindrical plug in the throat of the tone chamber. This plug he termed an "airhammer." Speaking of his device he says, "I find that by the employment of an air-hammer secured to the diaphragm as explained, the vibration of the diaphragm causes the air-hammer to vibrate within the passage, setting up air vibrations in the passage in exact accordance with the soundrecord, resulting in very perfect reproductions and eliminating to a large extent extraneous and disagreeable sounds." In all probability, had Hill removed his plug or "air-hammer" from the diaphragm, spacing it at an appropriate distance therefrom, he would have obtained in addition to those benefits which he describes, a very greatly increased efficiency over that with which he appears to have been content. Pritzsch in 1917, also employed a plug attached to a diaphragm. The Fritzsch plug being conical in shape is described by him as follows: "The conical boss (or plug) is secured centrally to the inner face of the diaphragm with the cone concentrically projecting into the throat so as to constrict but not close the passageway between the sound generating chamber and sound arm. Apparently this cone acts as a governor in this sound generating chamber to prevent over-vibration, the effect being to prevent blasting or sounds produced by improper diaphragm operations." In 1907, Smith employed a somewhat similar conical plug and arranged a small baffle above the diaphragm to cause the lengths of travel from various parts of the diaphragm to the throat to be equal. The Hunter Device Figure 4, shows a Hunter device of 1907. Speaking of his invention Hunter said, "My invention relates to devices for taking and receiving the sound waves developed by a vibrating diaphragm, whether vibrated directly or by mechanical action, to collect the waves and deliver the vibrations into a tubular passageway, and the object of the invention is to provide a chamber for the reception of the wave vibrations, with a plurality of passageways therefrom into the conveying tube, whereby the vibrations may be collected, condensed, and delivered more perfectly than with the sound recorders heretofore in use. "Heretofore it has been customary for talking machines and the like to provide a cup-shaped chamber in the rear of the diaphragm, which receives the sound vibrations, and to deliver these vibrations from the recorder through a single central passageway. I have ascertained by repeated experiments, however, that if an annular chamber is provided and the sound waves conveyed into the tubular passageway through a plurality of openings from this annular chamber, a very much smoother and better volume of sound is produced, or conveyed by the apparatus." Tigerstedt's Contributions In 1918, Tigerstedt produced a number of loudspeakers, recording machines and microphones, in all of which he employed a tone chamber with a heart shaped core which spread the sound waves from the center of the diaphragm outwardly toward the periphery and thence into free air via a space formed by the inner surface of the tone chamber and the outer surface of the heart shaped plug. Tigerstedt's device is shown in Figure 5. Tigerstedt discovered that by placing a surface adjacent to the diaphragm and compelling the sound