Radio Broadcast (May 1928-Apr 1929)

162 RADIO BROADCAST JANUARY, 1929 previously referred to must be made light and flexible so as to expend a minimum of energy in the flexure of these springs. The ohmic resistance of the voice coil should be kept low in order to reduce the heat loss in the copper to a minimum, for a given voice-coil current. LOAD CAPACITY THE third item is the load capacity of the loud speaker. This is limited by several factors. First, the tendency of the paper cone to buckle and rattle at the higher frequencies if too much energy is supplied to the speaker. Second, the tendency of the loud speaker to be thrown into violent motion at its low resonant-frequency point. The third limitation is that due to what is called "non-linear distortion." This means the introduction of frequencies into the sound radiated which were not present in the electrical input to the loud speaker. This may be due to a number of causes and this type of distortion distinctly limits the load capacity of the speaker, since such distortion increases greatly with the quantity of sound energy radiated. The major cause of this distortion is due to the inequality between the propelling and restraining forces acting on the moving structure. A well-constructed dynamic loud speaker, however, is capable of fairly large sound output without such distortion becoming apparent to the ear, and the writer has found that with most of the dynamic loud speakers on the market, the sound output capacity is limited by paper and spring rattles at the higher frequencies. The fourth limitation, which is seldom reached in the ordinary dynamic loud speaker, is that of the production of heat in the moving coil. In other words, if all three of the previous capacity limiting factors were absent, the capacity of the loud speaker would still be limited by the amount of heat which can be safely radiated from the voice coil. In order to obtain a maximum of energy in a minimum of space with the greatest allowable temperature rise, one manufacturer uses a singleturn voice coil which permits a very small ratio of insulation to conductor thereby increasing the capacity of the loud speaker. It will be seen from the above description that the design, construction, and application of the dynamic loud speaker is a problem involving many factors, consequently a great variation in the products of different manufacturers is to be expected. The writer has found this to be true, and has examined some which were excellent, some which were fair, and some which were very Ceiling Floor I K 4L ->i FIG. 7 Method of mounting six dynamic units on a large baffle poor. It is not sufficient to have a good theoretical principle. The design, construction, and application based upon this principle determine the quality of the actual product. Many of the manufacturers issue complete specifications together with frequency-response curves, and much can be learned from these data. The reader is warned against believing that a loud speaker must be good because it is a dynamic loud speaker. It is also important to remind him that a loud speaker cannot be judged unless it is used in the proper manner, and with adequate associated apparatus. The larger the capacity of the amplifier, up to a reasonable limit, which feeds the loud speaker, the better will be the quality of the reproduction with a good loud speaker. If it is possible to use a welldesigned amplifier with two 250-type tubes in push-pull in the last stage, an excellent result may be obtained. It is undoubtedly true that a sacrifice will be made if a smaller output is employed. This is particularly undesirable for the production of low tones which are so necessary for natural reproduction of speech and music. MULTIPLE LOUD SPEAKERS \ A /HERE feasible, splendid results may be " * obtained by using two or more dynamic loud speakers in the same bafffeboard. In one such design, built by the writer, six dynamic loud speakers of well-known make were placed in a single baffleboard as shown dimensionally in Fig. 7. These six loud speakers had their voice coils connected in series and were supplied by a special Amertran transformer feeding from two 250-type tubes in push-pull. At least four worthwhile advantages are obtained by the use of multiple loud speakers. First, the inequalities of the individual frequency-response curves are smoothed out. Second, for a given sound energy radiated, each loud speaker supplies less load and therefore distorts less. Third, the sound comes from a large area instead of from a small one. This last factor greatly improves the naturalness and the sense of three dimensionality. Fourth, the radiation of the low-frequency and the high-frequency tones are markedly improved and a better tonal balance is obtained. Book Reviews Practical Radio. By James A. Moyer and John F. Wostrel, Third Edition, 1928, McGraw-Hill Book Co., New York, 378 pages, $2.50. James A. Moyer, S. B., A. M., one of the authors of Practical Radio, bears after his name on the title page the following list of affiliations: Director of University Extension, Massachusetts Department of Education, Fellow of the American Associaticm for the Advancement of Science; Fellow of the Royal Society of Arts; Mitglied des Vereines Deutscher Ingenieure; Membre Titulaire Association Internationale du Froid; Member of the Franklin Institute; American Society of Mechanical Engineers; Society of Automotive Engineers; Institute of Electrical Engineers, etc. Nevertheless, the book is not without merit. Practical Radio was first issued in 1924 and has been kept abreast of developments in subsequent editions. It is one of those nonmathematical, copiously illustrated and charted expositions written for experimenters, service men, boy scientists in the more advanced grades, and other fauna produced in quantities by the spread of broadcasting. All the emphasis, as one would expect, is on broadcast technology, es pecially in reception. This bias the authors reveal on page 2 with the bald statement: "The most important use of radio is for broadcasting the human voice." I certainly do not wish Messrs. Moyer and Wostrel any hard luck, but if they are ever on a vessel which catches fire 1000 miles from land I shall seize the opportunity to debate this opinion with them at greater length. After a preliminary discussion on "What is Radio?" the book contains chapters on antennas, "radio electricity," crystal receiving sets and telephone receivers, vacuum-tube receivers, power sources for tubes, audio and radio-frequency amplification, the selection, operation, and care of receiving apparatus (Chapter X), and radio transmission by telegraph and telephone. The inclusion of material is often haphazard; in Chapter X, for example, there is a discussion of the vagaries of distribution of field strength in cities, fading, etc. Following Chapter XII, on construction and testing of receiving instruments, the trend of the discussions is highly practical; the reader is told about machinists' drill gauges, panel templets, the testing of neutrodynes, and the characteristics of various types of battery eliminators. Chapter XV is devoted to "Common Troubles and their Remedies." Clarity is a prime requisite in books of this type and Practical Radio succeeds in explaining lucidly, to the degree required by semitechnical readers, the numerous points which the development of broadcasting has raised. There are some loose statements, as when we are told, on page 51, that "The operation of the vacuum tube as used in radio sets was discovered by Edison," without any mention of Fleming and de Forest, and on page 195 where it is categorically set forth that in the Heising system of modulation "the two vacuum tubes (oscillator and modulator) should be the same type and as nearly electrically identical as possible." On the contrary, the modulator should be larger and of lower impedance. In practice this is accomplished by using a higher-powered tube as the modulator, or paralleling a number of tubes of the type used for the oscillator. Other such deviations from accuracy can be found in Practical Radio without the use of a microscope, in spite of the senior author's international feats in joinery, but to the students for whom this book has been written such details are inconsequential and the knowledge they want is certainly to be found within its covers. Carl Dreher.