130 DIRECT RADIATOR LOUD SPEAKERS 



namely: to provide a baffle for the loud speaker. In the case of the 

 midget cabinets the sound path from the front to the back is very small and 

 the low frequency sounds are not reproduced. In the case of the large 

 console cabinets the acoustic path length is sufficiently large to insure 

 good reproduction of low frequencies. One of the most troublesome acous- 

 tical factors in conventional cabinets is the resonance in the enclosure back 

 of the cone. This resonance is termed cabinet resonance. The system 

 may be considered from the standpoint of lumped or distributed constants. 

 In the case of most systems, the latter viewpoint seems to yield better 

 agreement with experiment. The cabinet enclosing the back of the cone 

 may be considered to be a pipe with distributed constants. The termina- 

 tion at the back may be considered to be the same as that of a piston. 

 The first resonance occurs when the velocity at the back is high and the 

 pressure at the cone is high. At this frequency the efficiency of the entire 

 system is relatively high and the response is accentuated. The response 

 characteristic on a typical console cabinet is shown in Fig. 7.9. It will 

 be seen that the response is accentuated in the range between 100 and 200 

 cycles. In order to maintain the response below 100 cycles the funda- 

 mental resonance of the cone and its suspension system is usually placed 

 somewhere in the neighborhood of 80 cycles. It will be seen that satis- 

 factory response is maintained down to 70 cycles. The accentuation of 

 the response due to cabinet resonance is most noticeable on speech in the 

 accentuation of low frequency response which reduces the articulation and 

 destroys the naturalness. 



It is possible to reduce the response in the region of cabinet resonance 

 by means of electrical compensation in the amplifier feeding the loud 

 speaker. 



Helmholtz resonators tuned to the frequency of cabinet resonance may 

 be placed within the cabinet at the point of high pressure. The Helmholtz 

 resonator shunts the cabinet network and, therefore, reduces the response 

 at the resonant frequency. See Sec. 4.9. 



For wide range reproduction acoustical networks are incorporated in 

 the cabinet to improve the coupling between the cone and the air at the 

 low frequencies. It is the purpose of the sections which follow to describe 

 some of these systems. 



7.10. Back Enclosed Cabinet Loud Speaker. — A reproducer with the 

 back of the cabinet completely enclosed is shown in Fig. 7.10. At the low 

 frequencies the system is a simple source. See Sec. 2.2. The output will 

 be independent of the frequency if the system is mass controlled, that is, 

 if the velocity of the cone is inversely proportional to the frequency. This 



