MECHANICAL NETWORKS 125 



The loud speaker was mounted in a 3-foot irregular baffle. The response 

 was obtained employing a velocity microphone located on the axis of the 

 speaker at a distance of two feet. 



A. Conventional Single Coil Loud Speaker. — The response frequency 

 characteristic of the conventional loud speaker, referred to above, is shown 

 in Fig. 1.1 A. The equivalent electrical circuit of the mechanical system is 

 also shown in Fig. 1 .lA. The constants have been indicated as the mass 

 of the voice coil mi, the compliance of the centering suspension, the 

 cone impedance including the cone outside suspension and the radiation 

 resistance, etc., lumped as Zmc- The response is well maintained to 12,000 

 cycles. For this reason, this loud speaker is well adapted to illustrate the 

 performance of mechanical networks for controlling the response at the 

 higher frequencies. 



B. Loud Speaker with a Compliance Shunting the Cone Impedance. — 

 One of the simplest means for attenuating the high frequency response 

 of a loud speaker is a compliance inserted between the voice coil and the 

 cone. This compliance Cm may take the simple form of a bead or cor- 

 rugation pressed into the voice coil form. The response characteristic of 

 a conventional loud speaker with a compliance between the voice coil and 

 cone is shown in Fig. 1 .IB. In the equivalent electrical circuit the com- 

 pliance Cm shunts the cone impedance Zmc- Comparing with Fig. 1 ^ A 

 it will be seen that there is some attenuation at the higher frequencies. 

 However, the attenuation is not large. This is due to the fact that the 

 impedance %mc does not increase appreciably with frequency. At the 

 higher frequencies a light cone, in particular, does not vibrate as a piston. 

 In a large diameter light cone the action changes gradually from piston 

 action to wave propagation at the higher frequencies. As a consequence, 

 the impedance does not increase directly with the frequency. In some loud 

 speakers Zmc actually decreases with frequency at the higher frequencies. 



C. Loud Speaker with a Compliance Shunting; a Compliance and Mass 

 in Parallel, Connected in Series with Cone Impedance. — In a radio receiver 

 it is desirable to attenuate the response at 10,000 cycles so that the 10,000 

 cycle adjacent channel beat note will not be reproduced. A parallel cir- 

 cuit inserted in series with a line causes high attenuation at the resonance 

 frequency. See Sec. 4.9. By inserting a parallel circuit in series with the 

 voice coil and cone the response will be reduced at the resonance frequency. 

 The amount of attenuation will depend upon the magnitude of the resis- 

 tance in the compliance. An example of this system is shown in Fig. 1 .IC. 

 The compliance and mass are m^ and Cmi- Comparing with Fig. 1 71 A the 

 attenuation at 10,000 cycles is about 25 db. This system is also easy to 



