Original Microphone 



Microphone Diaphragm 



Figure 1 - Schematic of Adaptation of the Altec 21-BR-180 Microphone 

 for Spectral Density Measurements 



Wind Tunnel Wal 



Microphone with 

 Extension Tube Added 



Figure 2 — Schematic of Microphone Adapted for Cross Spectral 

 and Correlation Measurements 



THE PRESSURE TRANSDUCER 



The pressure transducer was an adaptation of the Altec 21-3R-180 open-faced 

 microphone.'^ This microphone was modified as shown in Figure 1 to produce the transducer 

 used for the spectral density measurements. A different modification was required for the 

 correlation measurements. In both cases the upper frequency limit is determined by a 

 resonance between the mass of the air in the orifice and the stiffness of the air in the chamber 

 adjacent to the diaphragm. Well above this resonance frequency the sensitivity is inversely 

 proportional to the frequency range. The resonant response was damped by a cotton plug in 

 the orifice so that some of this frequency range was salvaged. 



For the correlation measurements it was necessary to measure at two points separated 

 by a small distance. This necessitated the use of the transducer in the form shown in Figure 2. 

 Unfortunately, this arrangement resulted in a much lower resonant frequency due to increased 

 mass in the tube. Using this transducer, arrangements were made to have a series of holes 

 spaced at 0.125 inch apart in the transverse and longitudinal directions which permitted 

 correlation measurements to be made up to 1.5 inches in 0.125-inch steps. 



The sensitivity of the transducer was -61 db below 1 volt for 1 dyne/cm-^. For the 

 spectral density measurements the usable frequency range was up to 8500 cps and for the 



