RECEIVERS AND MICROPHONES 



569 



the lower the sensitivity. In fact, it can be shown from equations 

 (1) that, if the scale of frequencies is changed by a factor, k, the 

 relative values of the ordinates will be unchanged provided r^ and ro 

 are multiplied by k, and s^ and ^o, by k^, but that the amplitude per 

 unit of force at corresponding points on the curve will be changed by a 

 factor equal to \/k\ A receiver transmitting up to 10,000 c.p.s. will 

 thus be 12 db less efficient than one transmitting equally well up to 

 only 5000 c.p.s., the same mass and size of diaphragm being assumed. 



Construction of the Receiver 

 The general construction of a receiver embodying the above prin- 

 ciples is shown in Fig. 4. The central portion of the diaphragm is 



Fig. 4 — Moving coil head receiver. 



drawn into the form of a spherical dome to increase its rigidity. The 

 receiving coil is of the self-supporting ribbon type, the construction 

 of which has been described previously.^ It is rigidly attached to the 

 base of the domed portion of the diaphragm. The radial magnetic 

 field is derived from a permanent magnet. The mass of the diaphragm 

 plus that of the coil corresponds to mo in Fig. 2, the stiffness of the 

 diaphragm to ^o and the mechanical resistance to Tq. 



A small volume of air is completely enclosed between the diaphragm 

 and pole-pieces save for a narrow slit at 0. The acoustic resistance ^ 



of a slit of this character is equal to -7^ — and the reactance, j -z — -, co, 



d^w 5 ivd 



where jx is the viscosity of air, / the radial length, d the width, w the an- 

 nular length of the slit and p the density of air. If the air in the chamber 

 were incompressible a mechanical resistance and reactance would be 

 imposed on the diaphragm by virtue of the air flow through the slit, 

 their respective values would be equal to the acoustic resistance and 



- Bell System Technical Journal, Vol. VII, p. 144, 1928. 



^ Lamb "Hydrodynamics," 4th ed., p. 577. 



