LOUD SPEAKERS AND MICROPHONES 275 



are introduced. The generation of harmonics in a horn when trans- 

 mitting waves of large amplitude already has been discussed. Let it 

 suffice here to say that, for a given percentage harmonic distortion, 

 the power that can be transmitted through the horn is proportional 

 to the area of the throat and inversely proportional to the square of 

 the ratio of the frequency to the cut-off frequency. 



Inasmuch as the moving coil microphones used for" the transmission 

 of music in acoustic perspective have been described previously ^ they 

 will not be discussed here at length. Their frequency response char- 

 acteristic as measured in an open sound field for several different 

 angles of incidence of the sound wave on the diaphragm are shown in 

 Fig. 10 where it is seen that the response at the higher frequencies 

 becomes less as the angle of incidence is increased. In general, this 

 is not a desirable property, but with the instruments as used in this 

 experiment the sound observed as coming from each loud speaker is 

 mainly that which is picked up directly in front of each microphone; 

 sound waves incident at a large angle do not contribute much. 



At certain times the sound delivered by the orchestra is of very low 

 intensity. Therefore it is important that the microphones have a 

 sensitivity as great as possible, so that the resistance and amplifier 

 noises may readily be kept down to a relatively low value. At 1,000 

 c.p.s. these microphones, without an amplifier, will deliver to a trans- 

 mission line 0.05 microwatt when actuated by a sound wave having 

 an intensity of 1 microwatt per square centimeter. This sensitivity is 

 believed to be greater than that of microphones of other types hav- 

 ing comparable frequency response characteristics, with the possible 

 exception of the carbon microphone. 



APPENDIX 



Load Impedance of a Diaphragm Near a Parallel Wall with 



Slot Openings 



First assume a diaphragm and a parallel wall of infinite extent 

 separated by a distance h, and that the wall is slotted by a series of 

 equally spaced openings as shown in Fig. 11. From symmetry it is 

 known that when the diaphragm vibrates there will be no flow per- 

 pendicular to the plane of the paper or across the planes indicated by 

 the dotted lines. Therefore only one portion of unit width, such as 

 abcdef need be considered. Let the x and y reference axes be located 

 as shown. If the general field equation 



d~(p d-(p 

 ox^ dy 



. + 7^ + ^V = (8) 



