LOUD SPEAKERS AND MICROPHONES 263 



of short duration, the direct sound is of great importance; it is this 

 sound alone which enables us to localize the source. So far as this 

 ratio is concerned, a decrease in the radiating angle of a loud speaker 

 is equivalent to a reduction in the reverberation time of the hall. The 

 efifect on the music, however, is not entirely equivalent, for the rate 

 of decay of sound in the room is unaltered by a change in directivity 

 of the source, as this depends only on the reverberation time. 



As already pointed out, some of the instruments of the orchestra 

 are quite directive and others are nondirectional. In general, it may 

 be said that the instruments of lower register are less directive than 

 those of higher register. To have each instrument as reproduced by 

 the loud speaker sound just as the instrument itself would sound in 

 the same hall, the loud speaker would have to reproduce the music 

 from each instrument with a directivity corresponding to that of the 

 instrument itself. This manifestly is impossible. The best that can 

 be hoped for is a compromise. Let the loud speaking system be 

 designed so that it is nondirective for the lower frequencies, and at the 

 higher frequencies it will radiate the sound through a larger angle than 

 the most directive of the instruments and through a smaller angle than 

 the least directive. Although this compromise means that the 

 individual instruments will not sound exactly like the originals, it 

 carries with it one advantage: At all the seats in the hall included in 

 the radiating angle and at a given distance from the loud speaker the 

 music may be heard to equal advantage, whereas with the orchestra 

 itself the most desirable seats comprise only a certain portion of the 

 hall. The optimum radiating angle is largely a matter of judgment; 

 if it is too small the music will lack the spatial quality experienced at 

 indoor concerts; if it is too large there will be a loss in definition. 



There is another respect in which the directivity of the source can 

 greatly affect the tone quality. Most loud speakers radiate tones of 

 low frequency through a relatively large angle, but as the frequency 

 is increased this angle becomes smaller and smaller. Under this 

 condition the relation between the intensities of the high and low 

 frequency tones as received directly will be different for almost all 

 parts of the hall. Hence, even with equalization by electrical net- 

 works, the reproduction at best can be good only at a few places in 

 the hall. Therefore, the sound radiated not only should be contained 

 within a certain solid angle, but the radiation throughout this angle 

 should be uniform at all frequencies. 



The Loud Speaker 

 At present two kinds of loud speakers are in wide commercial use, 

 the direct radiating and the horn types. Each has its merits, but the 



