136 



DIRECT RADIATOR LOUD SPEAKERS 



seen that the response at the high frequencies is improved. This system 

 tends to drive the cone at constant velocity for constant signal voltage 

 input. Therefore, the response will fall off below the point of ultimate 

 resistance, because the radiation resistance falls off 6 db per octave in this 

 range. See Fig. 7.1. The response may be made uniform with respect 

 to frequency by compensation of the input to the system. 



A feedback system employing an amplifier, loud speaker and microphone 

 is shown in Fig. 7.145. If a pressure operated microphone having uniform 

 sensitivity with respect to frequency is used the response characteristic of 

 the loud speaker will become more uniform as the amount of feedback is 

 increased. 



7.15. Nonlinear Distortion. — Acoustical and electrical networks are 

 assumed to be invariable; that is, the constants and connections of the 

 network or system do not vary or change with time. In one type of 

 variable-circuit element the variations which occur are controlled by out- 

 side forces which do not appear in the equations or statements of the 



SUSPENSION 



fM 



EQUIVALENT CIRCUIT 



FREQUENCY IN CYCLES PER SECOND 



Fig. 7.15. Cross-sectional view of the vibrating system of a dynamic loud speaker mounted 

 in a large baffle. In the equivalent circuit: w^c the mass of the cone and coil, w^ and rjv/A 

 the mass and the mechanical resistance of the air load. Cms the compliance of the suspen- 

 sion. Jm the mechanical driving force. The equivalent circuit indicates the effect of the 

 nonlinear element Cms- The graph shows the distortion frequency characteristics for inputs 

 to the voice coil of 2, 5 and 10 watts respectively. 



problem. In another type of variable-circuit element, the variation is 

 not an explicit time function, but a function of the current and its deriva- 

 tives which is flowing through the circuit. 



The outside diaphragm suspension is an example of the latter type of 

 variable-circuit element. It appears that practically all types of suspen- 

 sion systems are nonlinear. The stiffness is not a constant, but is a func- 

 tion of the amplitude and in general increases for the larger amplitudes. 

 A conventional dynamic loud speaker and the equivalent electrical circuit 

 are shown in Fig. 7,15. Above the fundamental resonance frequency the 



