International projectionist (Oct 1931-Sept 1933)

The introduction of new equipment designed to effect a substantial extension of the frequency range has precipitated a number of interesting discussions, not the least of which is the accompanying symposium which includes a letter to the editor and two replies thereto. Presentations such as this, which involve sharp differences of opinion, almost invariably result in the projection in sharp relief of the points under consideration. And so it is in this instance, with the information being presented in such interesting form as to demand the close attention of every reader. — Editor. A Letter to the Editor SIR: Regarding the August issue of International Projectionist the first subject, "Recent Improvements in Sound Systems", by "The Onlooker", reads like a catalog instead of information for the projectionist. It is evident that "The Onlooker" believes everything that erpi claims. There iiever was an amplifier, new or old, capable of reproducing sound from 100 cycles to 5,000 cycles. It is true that sound can be secured from a unit at 5,000 cycles, but the gain would be increased to such a degree that at 2,000 cycles the volume would be terrifflc; in other words, the curve of the unit would look just as crooked as a curve of almost any good amplifier. By this I mean that the curve of any amplifier used in a theatre is very poor from zero to 150 cycles, and rotten from 4,000 cycles up. Changing a transformer, or the addition or removal of a condensor or a resistance will not increase its range, that is, give it a flat curve — the kind erpi would like to have one think its amplifiers have. The little Tin Horn of ERPl's may be a help in reproducing higher frequencies, if they really have straightened the corkscrew curve of their amplifier. The little dynamic speaker in a big horn will build up the lows some. But the recording has a very poor curve at about the same frequencies as have the amplifiers. Most of this trouble is due to the "mike", and the moving coil is not much help. It would be interesting to see a curve (that is, an actual curve and not the pretty one they put in the catalogs), of a track recorded with the new erpi equipment, and also a curve of the new amplifier. Of course, the speaker units would not be included in this curve, as that is still another curve. Some months ago I had a look at the "Tin Horn" and the mike". It looks to me as though the producers are being taken for so much more of the $$$$, even if this new equipment is good or just a little bit better, if any. I can't see why the added weight of a coil to the diaphragm of a "mike" will help in getting a wider range of frequencies. "The Outlooker" REFINEMENTS AFFECTING THE FREQUENCY By ''The Onlooker'' (Aaron Nadell) IT is irrelevant, no doubt, but I cannot help wondering about the motives of your correspondent, "The Outlooker", in writing the letter that he did. I present here a few elementary refutations of some of his most glaring mis-statements. Many more similar references are available, but it seems to me that the only open question about this whole matter is the one bearing on how much time one has to spend in finding authoritative statements that 2 and 2 are 4, and that green isn't blue. It appears that by concentrating my reply on the most obvious errors in the statement by "The Outlooker" we all shall be able speedily to reach certain definite conclusions. Our first consideration shall be the statement that: "I can't see why the added weight of a coil to the diaphragm of a 'mike' will help in getting a wider range of frequencies." The moving coil microphone is discussed in the December, 1931, issue of the Journal of the S.M.P.E., page 977. et seq. The condenser and carbon types of microphones are described in the January, 1931, issue of the same Journal, page 3, et. seq. It is not necessary even to read these papers to discover that these three types of microphone are three different and distinctive devices: the photographs or line drawings showing the construction of each will make that clear to the least technical reader. The papers describe the construction of each, the principle on which each operates, and their relative advantages and disadvantages. Types of Microphones As a brief summary, which I should be glad to have checked by the text of those articles: the carbon microphone operates on the principle that pressure, when applied to a mass of carbon particles, decreases the electrical resistance of that mass. If such a. mass of particles is packed behind a diaphragm free to [14] RANGE vibrate, a current passing through them will vary in strength according to the vibrations imparted to the diaphragm. The carbon microphone is therefore not a generator; its action is in the nature of that of a valve. The condenser transmitter consists of a charged condenser, one plate of which is free to vibrate. The capacitance of any condenser depends, among other things, upon the spacing of its plates. If one plate vibrates, the condenser will charge (assuming a source of current is connected with it), when the plates move together, and will release a portion of its charge when the plates move apart. In this sense the condenser microphone acts as a generator, since it converts the direct current with which it is charged into alternating current created by its alternate charge and discharge whenever the free plate vibrates. The moving coil microphone is a pure generator, creating current, as a dynamo