Radio Broadcast (May 1928-Apr 1929)

JANUARY, 1929 ALL ABOUT THE DYNAMIC LOUD SPEAKER 161 C. It will be noted that A and B have markedly increased response at the higher frequencies, while C has not. In Fig. 2 are shown these same loud speakers, A and B, when the filters are used. The response of a fourth loud speaker, D, is also given. The frequency-response characteristics of two other dynamic loud speakers are shown in Figs. 3 and 4. [In analyzing these response curves the many small irregularities can be neglected, since, in general, they will not be audible to the ear. A good idea of what we might term the "average" response curve of the loud speaker can be obtained by drawing a smooth curve as we have indicated in dotted lines on curve D of Fig. 2. Note that the curves are plotted in tu. With pure single-frequency tones the minimum change in response audible to the average ear is 2 tu, but FIG. 5 Circuit of a.c. Dynamic loud speaker when the tones are complexed as they are in speech or music a variation of 3 tu will not generally be audible to the untrained ear. — Editor.] EFFICIENCY THE second item to be considered is the question of efficiency. By efficiency is meant the ratio of the output power in sound to the input electrical power. It is well known that the efficiency of the average loud speaker is very low. In the better makes of horn and fixed-edge cone loud speakers, the efficiency rarely exceeds 1 per cent. In other words, 99 per cent, of the electrical out put power of the amplifier is thrown away and only 1 per cent, converted into useful sound energy. In a good dynamic loud speaker, the efficiency is somewhat greater, and may be as high as 4 or 5 per cent. [In other words, for a given electrical input a good dynamic will turn out four or five times as much sound power as an ordinary cone, or conversely with about onefifth the input the same output can be obtained from a good dynamic as from a cone. — Editor.] It is very important to have high efficiency since the higher the efficiency, the smaller the amplifier and associated apparatus necessary to produce a given volume of sound without distortion. In considering the efficiency of a dynamic loud speaker it is usual to neglect the power required to excite the electro-magnet, since this energy is readily obtained without the use of elaborate or expensive apparatus. In order to obtain high efficiency it is first necessary to have a strong magnetic field. This is obtained by using large electro-magnets wound with many turns of wire. The limits to the intensity of the field which may be produced are the allowable heat developed in the field winding, the saturation of the magnetic circuit and the size of the air-gap across which this field must exist. There are three ways in common practice of exciting these fields. Choice among them is largely a matter of convenience, the final result being very much the same with all methods. The most common method employs a 6 to 12-voIt storage battery for the excitation of the field. The second method utilizes the field as a choke coil in the filter system of the high-voltage d.c. power-supply device. This method is very economical since the energy dissipated in the field would otherwise go to waste. The third method, which is becoming more common, employs a transformer and rectifier so that 110 volts a.c. may be used as a source of field supply. The line voltage is stepped down by means of the transformer and is then rectified in order to give a pulsating direct current for the field. In some makes of loud speaker a compensating coil is used to reduce the hum which would otherwise result from this pulsating field current. If, however, the field magnets are thoroughly saturated, the hum may not be sufficient to cause trouble. FIG. 6 Construction detail of a standard Dynamic loud speaker In order to maintain a high field strength it is necessary to have as small an air-gap as possible. Since the voice coil moves along, rather than across the air-gap, it is necessary to have only sufficient space for clearance between the coil and the iron. This clearance is made as small as possible consistent with the free motion of the coil along the gap. Its value is usually about 0.005 inch. Needless to say, the lighter the weight of the entire moving structure, the greater will be the efficiency of the loud speaker, since it is desirable to use as much of the electrical energy as possible to accelerate the air in front of the cone and as little as possible to accelerate the mass of the moving structure itself. The spring suspension Mechanical and Electrical Data for Dynamic Speakers Name Cone Voice Coil Transformers Magnetic Field Diameter Thickness Angle Impedance No. of Turns Wire Size Ratio Primary Secondary Volts Watts Flux Density Ai 8" 0.008" 90° 5. 95 ohms at 100^> 105 6 d.c. 2.4 12000 lines/sq. cm. A2 8" 0. 008" 90° 5. 95 ohms at 100^ 105 110 d.c. 3.5 12000 lines/sq. cm. Ai 8" 0. 008" 90° 5 . 95 ohms at 100^ 105 110 a.c. 4.2 12000 lines/sq. cm. B, 6i" 0.008" 90= 6.4 ohms at 100^ 6.7 ohms at 500^ 26 ohms at 5000^ 100 33 i 4000 No. 35 120 No. 19 6-12 d.c. 3.915.6 B2 6i" 0.008" 90° 6.4 ohms at 100-^ 6. 7 ohms at 500^ 26 ohms at 5000^ 100 33 i" 4000 No. 35 120 No. 19 100 to 200 d.c. 4-8 C 9" 135° less than 0.001 ohm 1 4500 1 110 a.c. 10 14000 lines/sq. cm. Di 8" 13. 5 ohms at 100^ 140 34 3600 180 6 d.c. 5.5 D2 8" 13. 5 ohms at 100^ 140 34 3600 180 90 d.c. 4.2 G 6i 110° 0 . 5 ohms at 100-~ 29 6 d.c. 6 10,000 lines/sq. cm. Note: Loud speakers A\, A% and A3, have characteristics which correspond to curves A of Figs. 1 and 2. The only difference between loud speakers identified by the same letter is in the design of the field winding. — The Editor.