Radio Broadcast (May 1928-Apr 1929)

AUGUST, 1928 ALL ABOUT LOUD orLAKLKo 189 For example, a loud speaker whose response is poor at the higher audio frequencies will fail to reproduce much of the unpleasantness of a nasal voice or scratchy violin playing. It requires little stretch of the imagination to believe it possible that distortions of various types may, in the future, be intentionally introduced into the system in order to produce new musical effects. As an illustration of the physiological side of the problem let us consider the question of volume. It is a fact, unfortunately not sufficiently well observed, that even accurate sound reproduction will produce serious distortion in the ears of the listener if the volume of the reproduction is too great. This distortion is an "overloading" of the ear of a similar nature to the overloading of a vacuum tube. We see from this brief discussion that the system does not end with the loud speaker, but with the listener; and further, that the problems which arise after the loud speaker has completed its task are not by any means unimportant. For the purposes of the following discussion, however, we shall assume that it is desirable for the loud speaker to produce an exact replica of the original performance, and leave the special considerations of the listener, and the listener's room, for a later time. THEORETICAL REQUIREMENTS BASED on this simplified view of the problem, what should an ideal loud speaker do, if used in an ideal system? Theoretically such a loud speaker should reproduce all the frequencies from 8 cycles per second to 12,000 cycles per second — nearly eleven musical octaves — without omission of any frequencies, and it should reproduce this entire band of frequencies without discrimination; in other words, without frequency distortion. It should not produce any frequencies not present in the original sound; in other words, it should not introduce non-linear distortion. It should be capable of delivering its output at sufficient volume without the introduction of distortion due to overload or mechanical striking of any of its parts. It should be efficient; that is, the ratio of the sound output power to the electrical input power should approach as nearly as possible to unity. Its performance should be independent of ordinary atmospheric conditions. It should be rugged and durable. It should be either inconspicuous or decorative — or both. Needless to say, an instrument possessing all of the desirable qualities stipulated above cannot be found this side of paradise. Fortunately, however, we may fall short of this perfection and still obtain excellent results. We shall examine in detail each of the above qualifications to see how much variation is consistent with a practical loud speaker, and we shall then compare these practical considerations with the actual limitations of present-day loud speakers. First, let us consider the item of frequency range. It has been demonstrated that a frequency range of 30 cycles per second to 10,000 cycles per second is consistent with practically perfect reproduction of both speech and music. In fact, a system which reproduces 30 to 6000 cycles is distinguishable only by direct comparison from one which reproduces the entire range. As to amplitude, it has been determined that a variation throughout the frequency range of less than 5 tu is almost negligible, and of more than 1 5 tu is serious. As to non-linear distortion, it has been estimated that the presence of frequencies in excess of 5 per cent, (based on the energy of the original sound) is disturbing. The efficiencies of most loud speakers are low, a value of one half of one per cent, being average. In other words, the power tube of the last audio stage delivers 200 units of electrical power to the loud speaker for each unit of sound power delivered by the loud speaker to the air. Of loud speakers built for home use, there are few which have an efficiency of as much as 2 per cent. The losses occuring in loud speakers are: (1) The heat losses in the coils; (2) The hysteresis and eddycurrent losses in the magnetic circuit; (3) The work done in bending the diaphragm; (4) Losses due to air friction which do not contribute to the production of sound waves. If it were not for the very low efficiencies due to these losses it would not be necessary to use large power tubes in the last audio stage, to deliver the necessary power without overloading this stage. An example may make this more clear. Suppose that a given good speaker, having an average efficiency of § per cent, is used in an average living room, and that it is necessary to use a type 210 tube in the last stage of a two-stage audio amplifier to obtain reasonable volume without appreciable distortion. If this same reproducer had an efficiency of 50 per cent., the same volume could be obtained with no increase in distortion by using a type 199 tube in the last stage. In other words the smallest of the standard dry-cell tubes operating with a plate voltage of only 90 volts would be quite as effective as the highvoltage power tube. The desirability of increased efficiency is obvious, since a large part of the complication and cost of a good modern receiver is brought about by the necessity of using power tubes. Power tubes are more expensive and require, in general, high plate voltages and high plate currents. This in turn means power supply devices with expensive transformers, rectifiers, and filters. It is safe to say that a large proportion of the total cost of a good broadcast receiver is chargeable to the low efficiency of the usual loud speaker. PRINCIPLES OF DESIGN TELEPHONE receivers and loud speakers have been designed upon a number of basically different principles. The electrostatic attraction between two charged metal plates; the reaction between the current in a coil and eddycurrents set up in a large metal disc; the expansion and contraction of crystals under the influence of an alternating electric field; the expansion and contraction due to the generation of a gas from a chemical placed between two plates, resulting from the passage of the audiofrequency current through the chemical; the variation of the surface tension of a liquid with , Permanent Magnet FIG. 2. HOW THE BALANCED ARMATURE LOUD SPEAKER IS CONSTRUCTED Thin iron disc CASEOF ALUMINUM OR BAKELITE FIG. I. CONSTRUCTION OF THE IRON DIAPHRAGM TYPE OF LOUD SPEAKER UNIT electromotive force; thermal expansion and contraction of a wire with variation in current through the wire; the "talking" arc — all these and many other schemes have been used with more or less success. However, practically all speakers in use today depend upon the variation in pull of a fixed magnet or electromagnet on an iron diaphragm, iron bar (armature), or a coil carrying current. The essential parts of a speaker working on this principle, no matter of what type, is a constant magnetic field upon which is superimposed a second magnetic field which varies in accordance with the audio-frequency current, thereby causing the motion of a diaphragm, an armature, or a moving coil in accordance with audio-frequency current. CLASSIFICATION OF LOUD SPEAKERS THE reproducers obtainable at present may be divided into several classes. The first classification refers to the method of exciting the constant field. Those which use a permanent magnet are called magnetic. Those in which the field is excited by means of a current-carrying coil wound on a soft iron core are called electromagnetic. A second classification divides speakers into iron diaphragm, balanced armature, and moving coil types. The iron diaphragm unit has the same general construction as an ordinary watch-case telephone receiver. In this type the iron diaphragm is magnetically attracted to the pole pieces of the magnet, against its own spring tension. The diaphragm is the emitter of sound. Fig. 1 illustrates the construction of this type. The balanced armature type consists of a short iron bar or armature which acts as the core of a small coil carrying the audio-frequency current. This bar is pivoted at its center and is free to move in a small arc about its center. It is usually restrained by a light spring. Each end of the bar is near one of the pole pieces of a strong permanent horse-shoe magnet. This bar imparts its motion to a non-magnetic diaphragm (to be discussed later) through a simple rod or in some cases through a more or less complicated system of levers. Fig. 2 shows the construction. In the moving-coil type a very small, exceedingly light cylindrical coil, carrying the voice and music current, moves back and forth in the magnetic field. In this case the coil is attached (usually directly) to a non-magnetic diaphragm. See Fig. 3.