Phonograph Monthly Review, Vol. 5, No. 9 (1931-06)

260 The needle problem scarcely falls within the scope of the present article. It is sufficient to say that whether they be Tungston, steel, Chromic, Burmese Colour, Electrocolor, or anything else, they should be selected and employed to obtain the maximum of pleasing tonal quality with a minimum of wear on the record. The pick-up, amplifier, and loud speaker are so closely related electrically that it is very difficult for the layman to tell which, if any, is not quite up to par. Generally speaking, however, it is the loud speaker that is at once the most important and the weakest of the three—the literal voice of the modern electrical instrument and its most fertile source of acoustic shortcomings. Assuming that the other factors of the system are reasonably efficient in operation, and that the pick-up arm is properly designed to secure correct needle align- ment, the instrument is still liable to a number of weakness which cannot be understood and elimin- ated without some working knowledge of the loud speaker and its associated acoustic problems. Loud Speakers — Horn Type The function of the loud speaker is to trans- form electrical energy into sound energy, and the more nearly constant the efficiency of this trans- formation—as frequency and intensity levels are varied—the better the speaker. The large paper cones, balsa wood and earlier horn types had no particular design, but were evolved by a process of trial and error into something passably pleas- ing to the average ear. Lately came genuine re- search by the technical staffs of the Bell Labora- tories, Western Electric Company, and various others, and it is now possible to determine accur- ately the whats and whys and wherefores of loud speakers. The horn type of speaker has been vastly im- proved and a new type—the so-called dynamic speaker—has been developed and virtually per- fected. The former is still widely used in public address systems and in some theatre and phono- graphic work. It not only fulfills its primary function of changing electrical energy into sound energy, but it is also to some extent an acoustic transformer. Its diaphragm being small imparts vibration to a very small amount of air, but under relatively high pessure. As the sound wave travels down the horn the amount of air Actuated increases and the pressure per unit area decreases, due to the increasing area of cross section of the horn, until by the time the large bell mouth is reached we have a recognizable sound instead of a buzzing diaphragm which we find if we take the horn away. The limitations of this acoustic transform- er are impossible to explain without complicated mathematical computations which would be out of place here. Such computations reveal two things: the lowest note that the horn can radiate The Phonograph Monthly Review is conditioned by the size of the final opening or bell mouth, (the larger the opening the lower the frequency that can be radiated) ; fidelity of re- production at all frequencies is determined by the length of the horn. In order to get down to thirty cycles, which we should do to cover the lower pitch range of electrical records or a good broadcasting station, the bell mouth should have an area of from thirty to forty square feet, and to preserve fidelity of reproduction—especially with the low frequencies—the length of the horn must be inconveniently long, say fifteen feet over all from diaphragm to final opening. A horn of these proportions should — given a good dia- phragm and associated apparatus—provide very acceptable reproduction. (Note 4). Dynamic Type The dynamic speaker operates in quite a differ- ent way from the horn type. The current pass- ing through a coil of wire actuates the conical diaphragm to which it is attached, and which is free to vibrate as a unit nearly 3/16 of an inch either side of its neutral position unconstrained either by its own stiffness or other rigid support. That is, it is inertia-controlled, as opposed to the stiffness-controlled diaphragm actuated by an armature in the horn type of speaker. The dia- phragm of the dynamic speaker, due to its larger area and lower pressure, generates the sound waves directly without the necessity of any acous- tic transformation. But there is a limitation here, exactly similar to the low frequency limita- tion of the horn. When the diameter of the dia- phragm of the dynamic speaker is less than one- fourth the wave length of the sound to be radiat- ed, no radiation takes place: the air next to the diaphragm simply sloshes back and forth from the front around the edges of the diaphragm to the back, and back again, instead of being alter- nately compressed and rarefied as is necessary for the radiation of sound. A simple calculation in- dicates that the dynamic diaphragm must be about the same size as the final opening of the horn speaker for comparable performance. Clear- ly a diaphragm of the proper size to radiate the low frequencies is no more practicable for general use than our fifteen foot horn with an opening of thirty to forty square feet. However, in the case of the dynamic speaker an equivalent result may be obtained without en- larging the diaphragm itself. A rigid baffle board may be affixed to the frame of the loud speaker surrounding the diaphragm completely and at- tached with a substantially air-tight joint, but leaving the diaphragm free to vibrate. None of the ruinous sloshing of air around the edges of the diaphragm can take place, and it can be proved that the result is quite as efficient as if the vibrating diaphragm itself had been enlarged to the size of the non-vibrating baffle board.