160 



HORN LOUD SPEAKERS 



dition obtains, particularly with close spacing between the diaphragm and 

 the front boundary of the air chamber, the distortion may be tremendous. 

 Physically the low frequency modulates the high frequency. 



In the above discussion the air chamber is assumed to be a pure acoustic 

 capacitance. This assumption is not correct at the higher frequencies 

 where the dimensions of the air chamber are comparable to the wavelength. 

 Regardless of the form of this acoustic impedance, it is, nevertheless, a 

 function of the spacing between the diaphragm and the air chamber and is 

 therefore a nonlinear element. 



C. Distortion Due to the Diaphragm Suspension System ^^ — The outside 

 suspension is another example of a variable circuit element in an acoustic 

 system. In certain types or, as a matter of fact, for unlimited ampHtudes 

 in all types of suspension systems the stiffness is not a constant, but a 



Cmi 



EaUIVALENT CIRCUIT 



SUSPENSION HORN 

 CONE, 



CROSS-SECTIONAL VIEW 



Fig. 8.10. Mechanism having a diaphragm with a nonlinear suspension system. Equivalent 

 circuit of the vibrating system and the wave shapes indicates the effect of the nonlinear 

 element. The graph shows a typical distortion characteristic obtained on an 8-inch 

 diameter diaphragm coupled to a large throat horn and delivering an acoustic output of 

 3 watts. 



^unction of the amplitude and, in general, increases for the larger ampli- 

 tudes. 



In the case of a horn loud speaker the amplitude of the diaphragm for 

 constant sound power output is inversely proportional to the frequency. 

 Furthermore, the mechanical impedance becomes comparable to the other 

 impedances in the system at the lower frequencies. Consequently, the 

 greatest distortion due to the suspension system will occur at the low fre- 

 quency end of the working range. 



21 Olson, H. F., RCA Review, Vol. 2, No. 2, p. 265, 1937. 



