Radio Broadcast (May 1928-Apr 1929)

190 RADIO BROADCAST AUGUST, 1928 Leads to output coil •' of power stage output transformer FIG. 3. CONSTRUCTION OF THE MOVING COIL TYPE OF LOUD SPEAKER The third classification consists of "horn" and "hornless" speakers. This classification is self-explanatory. The function of the horn is to load the diaphragm and to radiate the power which it causes the diaphragm to deliver. The greater the loading, the less the objectionable effect of diaphragm resonances. The loading of an exponential horn increases with decrease in the initial throat area. However, the throat area must not be so small as to introduce appreciable air friction. The less the rate of increase of the cross-sectional area of the horn, the more uniform the loading will be. The larger the final opening, the less pronounced will be the horn resonances. Therefore, a good exponential horn should have a small initial throat area, a slow rate of taper and a large final opening. With a very few specific exceptions, all loud speakers fall within the above system of classification. While it is evident that there are many possible combinations of the above-mentioned elements, there are at the present time five principal types which will be described as representative of most of the speakers on the market. The commonest or ordinary "horn speaker" is the combination of the permanent magnet, iron diaphragm, watch-case telephone receiver unit, with a more or less conical horn, usually about two feet in length. The characteristic of a good speaker of this type is shown in Fig. 4. This type of speaker has almost every inherent fault possible. The permanent magnet is usually so small / IRO J CO <! DIAPHRAGM vJICAL HORN I 500 1000 5000 10,000 FREQUENCY, CYCLES PER SECOND FIG. 4 that non-linear distortion is certain to be present if the speaker produces reasonable volume. Due to the fact that the diaphragm is essentially a spring and therefore has a natural period or frequency of its own, frequency distortion is present in a high degree. Furthermore the conical horn discriminates against the lower frequencies. See Fig. 7. The only advantage which can be claimed for this type of speaker is that it tends to cover up the distortion due to overloaded tubes, since it cannot reproduce the high audio frequencies. The second or "exponential horn" type of speaker employs a large watch-case telephone receiver unit with an exponential horn. If properly designed, this speaker is decidedly superior, particularly since a correctly proportioned exponential horn will radiate a wide band of frequencies with good uniformity. The combination of a permanent magnet, balanced armature, small non-magnetic diaphragm (usually mica or aluminum) with an exponential horn can give a still better frequency-response curve and can also be more efficient. The frequency-response curve of an example of this type of speaker is shown in Fig. 8. A fourth and very common speaker called the cone speaker employs a permanent magnet and a balanced armature which is connected by a rod or lever system to the apex of a large conical diaphragm, commonly made of specially prepared paper, which resists changes in shape or stiffness due to heat or moisture. In the most usual type, the base of this conical diaphragm is fastened to the base of a second conical diaphragm at the common periphery. This is a type of "fixed-edge" cone. The diameter of the cone varies from one foot to three feet, eighteen inches being a very common size. If well designed this type of speaker has a good frequency-response curve. Such a curve for a fixed-edge cone speaker of well-known make is shown in Fig. 5. It will be noticed that there are a number of peaks in this curve, partly due to the fact that the cone does not vibrate as a whole, and therefore has several modes of vibration resulting in numerous resonances. Speakers of this type in the smaller sizes have the advantage of compactness, but they must be handled with care since the cone is usually quite fragile. In most speakers of this type an adjusting pin is provided to compensate the small alterations in the tension of the paper cone due to atmospheric changes. The impedance of a well-known example of this type of speaker is 1000 ohms at 100 cycles per second and 40,000 ohms at 5000 cycles per second, and is chiefly reactive. This variation in impedance is characteristic of all types of loud speakers so far considered. The ideal speaker would have a constant impedance at all audio frequencies. It is further desirable that this impedance should be as nearly pure resistance as possible. The fifth type of speaker to be described is the dynamic speaker. It has a strong constant field which is excited usually either by a 6-volt storage battery (5 ampere), or by 90 volts d.c. (40 milliamperes). The field consumes approximately 3 watts. In the low-voltage type the field can be excited from the usual A battery. A common method of exciting the field of the highvoltage type is to use this field as one of choke coils in the filter system of the B-supply device. This second way is of course more efficient. A recent model utilizes rectified 1 10-volt a.c. to excite the field. In this model the field consumes 25 watts. This permits the use of larger air-gaps, and hence more rugged construction. The magnetic field in the air-gap is radial. The moving coil, which consists of about 150 turns of very fine wire wound on a thin cylindrical shell of fibre or like material, has a very low impedance at all audio frequencies. In one well-known make this impedance is about 5 ohms at 100 cycles and 10 ohms at 6000 cycles. Its impedance is almost pure resistance and therefore is nearly constant throughout the audio-frequency range. This is a distinct advantage, since it is desired to have the speaker respond alike to all frequencies. The coil has the smallest practicable clearance in the air-gap. A cone, usually made of paper, is attached rigidly at its apex to the moving coil. The base of the cone is fastened loosely by some soft and flexible material, such as thin leather, to an outside supporting ring such as is shown in Fig. 3. Two, or sometimes three, thin flat metal springs keep the moving coil concentric in the air-gap and act as conductors of current to the coil. It may be seen from this description that the cone is almost full floating. The lightest touch of the finger will displace the cone in some cases as much as a quarter of an inch along its conical axis. Such a cone is said to have a "free-edge." The resonant frequency of the cone, coil and spring combination is made very low, usually less than 50 cycles per second, so that the speaker is practically a non-resonant pure resistance load on the amplifier, for most of the audiofrequency range. In order to utilize this type of speaker to the best advantage it is necessary to mount it in some kind of cabinet or stiff baffle board. Since the cone moves as a whole, the waves from the front and from the back tend to interfere, particularly at the lower frequencies (under 300 cycles per second), unless the shortest path from front to back of the speaker is of the order of magnitude of the length of the low frequency waves. The distance from the front edge of the cone to the back edge by the shortest mechanical path through the air around the cabinet or baffle should be at least one quarter wavelength of the lowest note to be reproduced, 32 inches for 100 cycles, 1 10 inches for 30 cycles. It might be thought that if the speaker were completely enclosed in a small box that this problem would be solved. However, such is not the case, since disturbing resonances will be produced. Therefore, the speaker should either be mounted in a console cabinet, or, better still, in a large wall. Such a speaker properly mounted and used with a good amplifier is capable of excellent results. The frequency-response curve for a speaker of this type is shown in Fig. 6. It will be noted that the solid curve shows a peak for the band of frequencies from 2000 cycles per second to 5000 cycles per second. This is due in part to the horn effect of the small cone at the higher frequencies and in part to vibration of the cone. In a recent model, a ribbed cone is used which is claimed to minimize this vibration. Some dynamic speakers contain equalizer-filters which tend to correct this defect, as shown in the dotted curve in Fig. 6. Since the impedance of the moving coil of the usual dynamic speaker is exceedingly low in com f5 -10 V f / BALANCED Al ^ FIXED-EDGE {MATURE CONE I \ \ \ 100 500 1000 5000 10.000 FREQUENCY.CYCLES PER SECOND FIG. 5