414 BELL SYSTEM TECHNICAL JOURNAL 



magnetic field around the coil. In order to obtain in this instrument a 

 signal-to-noise ratio sufficiently high for all practical purposes, it was 

 not considered advisable to use a diaphragm smaller than 1 inch in 

 diameter. The loss in sensitivity resulting from this choice was partly 

 offset by making the diaphragm light in weight and of very low stiffness. 

 It is also very important that this diaphragm vibrate as a simple piston 

 throughout the entire range ; but to obtain this action over a wide range 

 of frequencies has proved in the past to be a very difficult problem. 

 For this new microphone, a diaphragm was developed which has a rigid 

 spherical center and a tangentially corrugated annulus and which has in 

 addition a high area to stiffness ratio. No evidence of vibrating in 

 other modes is shown by this structure below 15,000 cycles. The 

 diaphragm is cemented to a raised annulus on the outer pole-piece. 

 The outer and inner pole pieces are of soft iron and are welded directly 

 to the magnet which is made of high grade magnet steel. The dia- 

 phragm is damped by an acoustic resistance which is supported below 

 the coil by a brass ring. This ring is held in plac-e with rubber gaskets. 



The size and shape of the housing were selected with particular 

 reference to the requirements that had to be met. The size is such 

 that the housing fits closely over the diaphragm and thus produces little 

 more diffractive effect than would the diaphragm itself, and the spher- 

 ical form allows sufficient amount of air space behind the diaphragm, 

 which is essential to minimize the impedance to vibration. To prevent 

 resonance within the case an acoustic resistance baffle is provided to 

 divide the space into two parts. A tube with its outlet at the back of 

 the housing serves the double purpose of equalizing the inside and 

 atmospheric pressures and of increasing the response of the instrument 

 at low frequencies. 



In the non-directional microphone the resonance in the cavity in 

 front of the diaphragm is controlled by the design of the protective 

 grid. Instead of being the source of an undesirable distortion, the grid 

 and cavity have become a valuable aid in improving the response of the 

 instrument at frequencies from 8,000 to 15,000 cycles. This grid also 

 incorporates a screen which prevents dust and magnetic particles from 

 collecting on the diaphragm. 



Method of Measuring Field Response 



The method of making the frequency-response measurements is 

 similar in general details to the method outlined in a paper by \V. C. 

 Jones and L. W. Giles ^ Figure 9 shows the arrangement of the room 

 and testing apparatus. A very small, specially developed, condenser 

 microphone was used in determining the sound field pressure. The 



