| Popis: |
A probe microphone has been developed suitable for measuring sound fields within such structures as altitude wind tunnels and jet engine test cells. This paper describes the methods of testing and analyzing the instrument. The microphone consists of a 38‐in. inside diameter probe tube with a porous metal tip, a condenser microphone, and a spiral resistive termination. It operates at sound‐pressure levels up to 170 db with 2% distortion, at ambient pressures down to 0.2 atmosphere, and with probe tip temperatures up to 900°F. The normal incidence response is flat within ±3 db from 10 to 10 000 cps.An electrical analog of the acoustical system is given as a basis for explaining and predicting the performance of the microphone under varying ambient conditions. It appears that the microphone can be used satisfactorily as a cavity terminated probe microphone for frequencies below the frequency at which the spiral termination ceases to be “anechoic.” The required length of the spiral termination is then determi... |
