Noise characteristics of circular nozzles for high velocity/low volume exhaust ventilation

Noise characteristics of circular exhaust nozzles were studied for average face velocities ranging 25-175 m/sec (5000-35 000 fpm). Experimental parameters were examined for nozzle design variations consisting of: (a) seven profile shapes having 2.54 cm (1.0 inch) inside diameter, and (b) five plain profiles of different size, 1.27-3.81 cm (0.50-1.50 inch) inside diameter. All nozzles exhibited a "peaking" characteristic in which the maximum noise level was reached at less than the maximum face velocity. As maximum face velocity (flowrate) was achieved, noise spectra exhibited a pronounced drop in high frequency noise, with a shift in peak frequency from 4000 Hz to 2000 Hz. Both of these characteristics indicated the onset of aerodynamic "choking", as sonic velocity was established in the nozzle flow. Variations in nozzle shape and size had significant effects upon noise levels propagated into the "workspace".