Jeffrey A. Zimmer
BBN Syst. and Technol., 70 Fawcett St., Cambridge, MA 02138
Robert D. Collier
49 Eagle Ridge Dr., Lebanon, NH 03766
Pulse combustors are pulsating acoustical systems which radiate high-intensity radiated noise. The Forbes energy engineering experimental pulse combustor was operated in a ``high pressure mode'' at a fundamental frequency of 50 Hz and an output between 330 000 and 390 000 BTU/h. The noise levels of the baseline configuration, measured around the exhaust outlet, had an SPL over 110 dB at 50 Hz and a series of high intensity narrow-band components up to 2 kHz. The A-weighted broadband SPL in the range of 100 dB(A) exceeded damage/risk criterion by as much as 25 dB. The noise reduction goals corresponded to NC50 and SIL of 50 dB. The approach consisted of three parts: (1) A cowl silencer installed at the outlet vent for control of mid to high frequency broadband noise; (2) a custom designed tuned low-frequency muffler installed as part of the exhaust decoupler; and (3) thermoacoustic cladding applied to the structure. The silencer provided a reduction of 20 dB(A) in the mid-frequency range and the muffler resulted in narrow-band reductions of 25 to 40 dB at the lowest frequencies and 5 to 15 dB up to 2 kHz. The results, including sound intensity mapping, are discussed in terms of the goals, design modifications, and operational factors. [Work supported by The Gas Research Institute.]