Radio today (Jan-Mar 1939)

WHAT SIZE AMPLIFIER TO USE Effect of room size and noise level upon power required for sound installations The amount of acoustical sound energy for a public address or sound installation depends upon the size of the room, the acoustical conditions in the room, and the noise level against which the sound system must compete. While sound level measurements in the room are desirable, it is possible, to obtain satisfactory results in most cases by using charts based on average sound installations. The first step in using a chart for determining the sound energy required is to calculate the volume of the room based on dimensions. Next the normal background noise level should be estimated. Background noise levels For the sake of simplicity, the noise level can be compared to levels with which almost everyone is familiar. The most quiet place where a sound system would be used is a church. The second step on the list is a quiet theater,' third a moderately-noisy theater, and finally a very noisy room or auditorium". These four classifications are suitable for practically every possible installation, and are shown on the accompanying chart of acoustical watts vs. room volume. For high-fidelity reception it has been found that 40 decibles above the noise level is desirable. It is on this basis that the curves have been developed. So after calculating the volume of the room, the acoustical power for any stated noise level can be found from the chart. Values for a moderatelynoisy room are read from the chart, using the line labeled ' 'moderatelynoisy theatre," etc. The values given are for high-fidelity reception. If only speech reproduction is needed and the low frequencies are filtered out, it is possible to use an acoustical power of approximately onehalf of that shown by the chart. Most sound men, however, prefer to install the full amount of power required, for later on high-fidelity music reproduction may be desirable. Speaker efficiency To convert the acoustical power to electrical requires a knowledge of the loudspeaker efficiency. In other words, it is necessary for the amplifier to supply the losses in the speaker unit or units as well as the acoustical energy. AVERAGE SPEAKER EFFICIENCIES Low-cost cone 3% Good cone unit 5% High efficiency cone 5-8% Horn if used increases efficiency of cone speaker from 2 to 3 times Exponential type horns . . 25-45% Loudspeakers, depending upon the baffle, the unit, and directional characteristics, have efficiencies that vary from 3 per cent up to 55 per cent. Assuming identical installations, the choice of speakers would have more tnan a 18 to 1 effect on the electrical power needed to provide the same acoustical energy. If the speaker efficiency is 3 per cent, then the electrical power must be 33 times the acoustical sound power. Or if a 50 per cent speaker efficiency is achieved, the electrical energy must be twice the acoustical energy needed. 3 to 5% efficiency usual "The average electrodynamic speaker," according to Mr. Weibler of Operadio Mfg. Co., "delivers 3 to 5 per cent of its input when mounted on the usual-size flat baffle. If a horn is used, this is increased to as high as 40 to 45 per cent; the efficiency increasing as the horn approaches the exponential type, creating a more effective coupling to the air with a greater air column loading. "The true exponential horn uses an electrodynamic unit differing from the usual paper cone reproducer. "The figure of 5 per cent efficiency may reach as high as 8 per cent when a reproducer is a high-quality permanent magnet assembly. The higher figures of 40 to 45 per cent do not change materially, however, as the field input has much less bearing on the efficiency, inasmuch as the increased efficiency is obtained through better coupling to the media of sound." Mr. Weibler adds, "Generally the efficiency ratings are only fairly reliable to one thousand or fifteen hundred cycles per second. The lower frequency efficiency is greatly determined by the effectiveness of the baffle used. Obviously, no figures can be given at the lower frequencies without considering specific cases. "The enclosure type of baffle will increase the efficiency at the lower frequencies. The efficiency at the fundamental frequency of the enclosure will approach that of the true exponential horn. This condition will exist to a gradually decreasing amount at each multiple of the fundamental reinforcement frequency." Horns more efficient Concluding, Mr. Weibler states, "The efficiency of horn type assemblies is generally based on figures obtained within the useful coverage angle of the horn and does not refer to the amount of sound received at the side or back of the horn. The horn type of unit, however, is most useful where sound is to be directed as in a theater. It enables the operator to somewhat control the distribution so as to obtain an equalization of intensity throughout a given area." In order to help the sound man quickly' determine the efficiency of different types of speakers, the accompanying table has been compiled. Because of numerous variables and different characteristics of speakers of various manufacturers, the figures are only average. Wide variations may be expected. Fortunately these variations in efficiencies are not too serious if the acoustical requirements are first obtained from the accompanying chart. Special installations such as roller skating rinks, steel mills and the like have not been included on the chart. But since high-fidelity reception is never expected in such places, the sound man will be quite safe in using the figures given for an extremely noisy room. In a roller skating rink it is only necessary to have sufficient sound level so as to hear the recordings or orchestra. Likewise, in factories .speech intelligibility is the only requirement. Church chimes Outdoor installations offer special problems that cannot be discussed in this article. But in passing, a word about chimes is not amiss. Major Ranger of Rangertone states that 30 to 50 watts in an amplifier is desirable in churches. One or more amplifiers should be used depending on the number of directions that need to be covered. Using a 35 to 50-watt amplifier with a single speaker, the chimes can be normally heard some half mile away 14 Radio Todav