[en] Two sodium-immersed detectors, a microphone and an acoustic waveguide, have been developed, installed, and used for monitoring acoustic background of Experimental Breeder Reactor No. II (EBR-II). The microphone signal is free of structural resonances which appear in the signals from accelerometers mounted on the waveguide and various reactor components. Sound pressure level decreased by an order of magnitude when the primary sodium coolant flow was reduced from 100 percent to 50 percent. At full flow and independent of reactor power, the 1-millibar r.m.s. sound pressure as sensed by the sodium-immersed microphone had the characteristics of wideband noise over the frequency range 0.6 to 40 kHz. 7 references. (U.S.)