Radio Broadcast (May 1928-Apr 1929)

110 RADIO BROADCAST DECEMBER, 1928 then a 7-inch cone will require 0.062 inch down to the same frequency. If, however, a dynamic speaker could reach only 100 cycles, it would not be satisfactory; and to reach 48 cycles, a stroke of plus or minus \ inch or \ inch is necessary. It is, of course, theoretically possible to make a dynamic unit with this stroke, but at the present time there are only two commercial speakers of this character, and both are sold complete with amplifier and power plant. The very best dynamic unit for 7-inch cones has a possible stroke of plus or minus 3% inch. Others have a possible stroke of tV inch. One type examined in our laboratory had as little as a 3Vinch stroke." The engineer: "This depends upon how much energy must be radiated. Under any reasonable conditions it will be superior to the Western Electric type of cone in radiation (area x amplitude) at frequencies below 300 cycles because of the baffleboard's effect." ELECTRICAL TROUBLES TP HE release: "Electrical troubles are caused * by the following factors: To begin with, dynamic units are not provided with a spring resilience as are the electromagnetic units. Pushpull actuation is therefore more essential. Again, in all cases an output transformer of special design is required, and unless made of special alloy cores, which are quite costly, it can introduce quite as much distortion as a third stage of audio amplification." The engineer, replying on the spring resilience of the cone-type speaker: "That is the reason dynamic units are so very much superior to balanced-armature speakers from the standpoint of harmonics generated. Since they are always actuated by a.c. theoretically no restoring force should be necessary. The current itself will always return the moving system to the starting position. Push-pull actuation is obtained by the ?.o, not by any push-pull feature of the amplifier system. The currents fed to a speaker by a push-pull amplifier can in no possible manner be told from those fed by a single-tube amplifier as long as neither is overloaded and the d.c. component is eliminated by a transformer or blocking condenser." The release: "Many makes of dynamic speakers now on the market are provided with cheap transformers, so that the distortion caused by combinations of capacities, chokes and resistances— including cut-off filters intended to cover up poor design — tends to cut down the efficiency. The engineer: "This is equally true of other types." The release: "The greatest electrical loss, how . Permanent Magnet FIG. 2. CONSTRUCTION DETAIL OF A BALANCED-ARMATURE MAGNETIC UNIT ever, occurs in the movable coil itself. The impedance of the movable coil is not matched with the secondary of the output transformer; and even when it is nearly matched, the I2R loss of the coil is enormous. The reader can obtain a really practical picture of this loss if he stops to consider that in a correctly designed step-down transformer whose ratio is 50 to 1, the secondary wire is No. 18 while the dynamic coil is wound with No. 30 or 32! This means that the dynamic coil is being overloaded from 300 to 400 per cent." The engineer; "The losses (I2R) in the coil are fairly large compared to those in certain other parts in the system, and account largely for the 30 to 50 per cent, efficiencies instead of 100 per cent. It is still better than the 3 to 7 per cent, efficiency obtained by other types. The moving coil cannot be considered as "overloaded" unless it is mechanically or electrically likely to be destroyed." FIELD COIL EFFICIENCY HpHE release: "The electrical efficiencies dis*■ cussed above are those of the signal system only. In addition, we must consider the efficiency of the field, which is from 10 to 25 per cent. When a field for the signal of a 1 12-type tube was required, this was quite immaterial. To obtain a field for the signal from the 2 10 or250-type tubes in push-pull, however, requires enormous power. "Dynamics of the kind having a signal efficiency of about 20 per cent., with the field rated at 1 ampere at 6 volts were found to take 6 watts at 6 volts, while those having a signal efficiency of 30 per cent., required 16 watts. Those with a signal efficiency of 50 per cent, demanded a 50watt field. The latter field requirements are far above the usual output of eliminators. Hence the field cannot be used as the eliminator filter choke, but must be fed with a separate rectifier." The engineer: "Proper design of the magnetic circuit can reduce greatly this loss and apparent inefficiency." In this connection, we understand the Vitaphone loud speakers in factory production have an average efficiency of 35 per cent., and that the field of these speakers consumes about 10 watts. The release: " In conclusion, it should be noted that the deep bass notes developed by many dynamics are additive resonance, and if one likes this effect one can readily obtain it by much cheaper and simpler means. The deep resonance is caused by the fact that most dynamic cones are fastened to a metal frame which, in turn, is bolted to a large wooden baffle acting in the capacity of a diaphragm. This serves to accentuate certain low frequencies. To prove this, let the reader actuate a dynamic unit thus mounted, with an organ record of low pitch, and he will hear clearly a bass drum accompaniment. If this effect is desired, the same results can be produced with an impregnated cloth cone (of curved angle) glued to the same kind of baffle and actuated by a high-power electromagnetic unit." The engineer, "No loud speaker is quite so free from the effects of resonance as the dynamic with a piston-type paper cone or properly designed moving system and horn. The reproduction of low notes is not due to resonance. The wooden baffle does not act as a diaphragm. In fact, the less it moves ,nure effective it becomes." OUR DECISION \ A/E HAVE enjoyed this little argument. * ^ The cone desigr.ji is correct in stating that good cone-type speakers can deliver excellent quality; the engineer, whose feeling in favor of "dynamic" speakers is so evident, is correct FIG. 3. CONSTRUCTION DETAIL OF A MOVING-COIL SPEAKER UNIT too — for the best possible reproduction, a good moving-coil speaker is superior to a speaker of the Western Electric type. There is only one more statement we hope everyone will understand — all speakers are dynamic speakers in that some of their parts move. All dynamic speakers, however, are not "moving-coil loud speakers" which are the type the engineer favors, and what nearly everyone thinks of when the word dynamic is mentioned. Up to the present time there is no type of speaker generally available which is the equal of the better grades of moving-coil loud speakers More Data on Loud Speakers HpHE above argument will be followed by addi* tional data on dynamic speakers, as the Editors have made arrangements with Joseph Morgan, of the International Resistance Company, for the preparation of another of his articles on loud Speakers — the first appeared in August Radio Broadcast and was called "All About Loud Speakers." The second of Mr. Morgan's articles will be "All About Dynamic Loud Speakers," and he has been instructed to take every type of dynamic speaker now easily obtainable and put them through their paces in his laboratory. His article will contain curves showing how the various speakers respond to the necessary audio frequencies, what field currents they require, how much audio power is necessary, what their advantages and disadvantages are, and where the magnetic type of speaker stands in the path of progress toward better and better radio reception. The advantage the second of Mr. Morgan's articles will have over other dynamic-speaker descriptions is that his remarks will be based upon laboratory measurements and actual tests, not upon matters of opinion. Together with the present article made up of the remarks of two engineers, it ought to equip any serious radio thinker with sufficient data to decide for himself and advise his friends on the question, "Is it worth while to invest in a dynamic speaker?" — The Editok.