Projection engineering (Sept 1929-Nov 1930)

Page 14 Projection Engineering, June, 1930 quency. There is no frequency that an ultra high quality amplifier can afford in its output that this condenser reproducer will not respond to, from the highest to the lowest. Only in the smallest and miniature sized acoustic condenser is it necessary to resort to the assistance of auxiliary baffling to obtain satisfactory radiation of bass sounds. Except for minor peaks and valleys caused by suppressed resonance effects inherent in the diaphragms, however lightly and flexibly constructed to minimize such effects, the major response is surprisingly uniform over a very wide range, covering all of the necessary fundamental frequencies and their harmonic overtones. Parasite harmonics and exaggerated peak resonances common to other types of reproducers are entirely avoided in the improved acoustic condenser. There is a noticeable dropping off in db. response below 100 cycles in the smaller and medium sized acoustic condensers but all will respond satisfactorily to the deepest bass desired. It is a great satisfaction to hear the smallest acoustic condensers of this type clearly emitting the deep bass of the contra-bassoon, the kettle drum, or the tuba, unmarred by barrel nasal type of resonance. Listeners who have acquired "dynamic ears" will be equally astonished to hear the true repro duction accompanied by full rich overtones in the case of high notes of the violin, piccolo, oboe, etc., all of which are distinctly and unmistakably characterized to the musically trained ears. But it is in the renditions of child voices or soprano voices of artistic qualities evidenced in practised technique that the full capabilities of this acoustic condenser are fully realized by even untrained ears. This is because the reproduction is in fact lifelike. The thin flexible diaphragms are unweighted by attachment to other heavy members which must carry exaggerated vibrations as in the case of other types of reproducers. A noted composer stated that in listening to a rendition of a violin by this condenser speaker the reproduced notes were true pitched. On a similar test, a leader of a popular dance orchestra stated that the various instruments of the orchestra in the selection reproduced were in correct volume perspective. Such expressions are considered of more practical significance than any set of mere singlefrequency response curves, because a diaphragm behaves differently to a multiplicity of mixed frequencies than it does to a single frequency at one time. How Bass Response Is Attained It has been made evident that no reliance on large baffles is required to obtain highly satisfactory bass response from this speaker. Nor has it been found necessary to resort to exaggerated or weighted dimensions for the moving parts. Rather, the answer was slowly developed by proper selection of the flexible thin metal strips combined with the air permeable diaphragm electrode. These afford all the necessary air damping and air gripping needed for reproduction of the deepest bass desired. Let it be noted too that this deep bass is not lost in a maze of muffled sounds lacking in characterizing overtones, but that each individual bass instrument reproduced is clearly intelligible and distinguishable from other bass instruments of similar low fundamental frequency range. In connection with suitable operating circuits for this speaker, it is necessary to have an output amplifier capable of supplying the desired undistorted bass frequencies and an output coupler capable of transmitting such frequencies to the speaker. Otherwise the same speaker which responds correctly to the proper output will give poor results on a poor or distorted amplifier output. How Overtone Response Is Obtained Attaining high-frequency response in the audible range has in general been less difficult than the low-frequency SPEAKER ' TERMINALS FLEXIBLE ENVELOPE SPEAKER FRAME wr " GASKET SEAL" WEATHER t PROOFED FABRIC ENVELOPE FIG.5 WAX SEAL range in the case of condenser speakers. But to retain the desired overtones in correct proportion, without loss of the fundamental by exaggeration, was not so simple. In the acoustic condenser the metal cloth used is normally at a greater distance away from the impermeable electrode than are the flexible strips supported thereon, but at the same time such metal cloth itself affords a multiplicity of air pores permitting efficient response to the highest audible frequencies the amplifier output can supply. See Figs. 1 and 2. Precautions With High Gain Amplifiers From the foregoing considerations, since the acoustic condenser will respond to extremely high audio frequencies, it follows that in the use of a high gain amplifier to feed same, precaution must be taken, first, to fully shield the amplifier as is customary in amplifiers built on a metal chassis, and second, to ground the pickup circuit in the case of a phonograph needle pickup, to avoid capacity feedback effects through the amplifier. Even high gain direct-coupled amplifiers using a 224 type (space charge) input stage feeding a 250 output stage can be operated with this precaution. In some cases it is further desirable to employ more than ordinary spacing hetween the primary and secondary windings of the output transformer to minimize capacity coupling between the windings, or to use an electrostatic shield. Ordinarily the only precaution required is that the chassis frame of the amplifier be grounded. Location of Condenser Speakers Since this type of acoustic condenser propagates sound waves from its back as well as its front side, best operation may be had when at least six inches of clearance is allowed between the speaker and the wall. In mounting a speaker in a radio cabinet of conventional style, sufficient air space must be left to permit free travel of the propagated sound waves. It is permissible to interpose part of the chassis or power pack of a radio set in the path of the condenser speaker wave propagation provided that such parts are securely fastened so as not to impart false resonances by self vibration. There is ordinarily no trouble in this respect and even the self vibration of thin veneered cabinet walls usually introduces no harmful effect. In the case of small sized condenser speakers mounted in a radio cabinet or a baffle box, the cabinet or box need not exceed twelve inches in depth. While the speaker may be conveniently mounted at the front of the cabinet or box, more pleasing results will sometimes be had by recessing the position of the speaker. For example, (see Fig. 3) in the case of a box compartment 12 inches deep, the speaker may be located seven inches back from the front end of the baffle compartment. The use of baffles of any sort is not required on the larger sizes of speakers, and satisfactory results are had from such speakers mounted in thin walled picture frames hung from walls in the customary manner. In the case of large rooms or auditoriums, it is preferable, where possible, to mount the speaker with clearances from both the floor and the ceiling, with its midpoint at about a height of six to eight feet above the floor. Very large audience coverage (up to 1200 persons) may be had with correct disposition of even a single moderate sized acoustic condenser of this kind. For very large audience coverage (i. e. 1500 to 10,000 persons) the use of multiple grouped acoustic condenser speakers is the best practice. Large audience coverage, not practical because of high cost in the case of other types of reproducers, is highly satisfactory (Concluded on page 32)