Acoustic pressure fields generated by large-caliber rapid-fire weapons are sufficiently intense to constitute sources of concern to airplane designers. The operation of such weapons could result in greatly increased equipment and structure vibration and danger of hearing damage to flight personnel. Acoustic pressures from both fuselage and wing-mounted M-61-A1 guns have been measured on the fuselage and wing of an A-7 airplane. The measurements covered an area extending several feet fore and aft of the gun muzzles. Both static and flight firings were measured. Recordings of acoustic pressure were analyzed for waveform, instantaneous amplitude distributions, spectra, and spacial distribution. The waveform for a burst of fire is approximately a series of decaying sinusoids at the gunfire repetition rate. Peak-to-rms ratios as high as 5 to 6 are not uncommon. On the fuselage near the gun muzzle, peak pressures can reach 2 to 3 psi (177–181 dB SPL). The possible effect of these high-pressure fields on airplane design and operation is discussed in relation to structure and equipment vibration and personnel safety.
Systems for simulation of aircraft-propeller noise in the laboratory have been developed at LTV Vought Aeronautics. The basic unit of these systems is an electronic propeller-noise generator that was designed and built as a Company-sponsored R and D program. This unit produces a voltage signal proportional to the rotational and vortex sound pressures generated by aircraft propellers. The rotational noise is made up of the fundamental 4P blade-passage frequency plus several significant harmonics. Adjustments are possible on frequency, amplitude, and relative phase of these discrete-frequency components. Vortex noise is random in character and adjustments of over-all level, spectrum shape, and frequency-band limits are possible. In a final stage, the rotational- and vortex-noise components are mixed to produce simulation of the complete propeller-noise signal. This voltage signal is used as an input to a high-intensity progressive-wave sound system. In this system, sound-pressure levels of 150–170 dB are generated for testing of structural panels. Another use for this signal is as an input to a high-fidelity speaker system that operates at SPL's of 70 to 125 dB. This system provides simulated propeller noise for interior-fuselage acoustic investigations and human-factors studies. Equalization of either system operating into various impedances of different panels and enclosures can be accomplished by appropriate adjustment of noise components produced by the propeller-noise generator. Typical results are shown in slides illustrating measured waveforms and spectra. These results are compared to propeller noise measured from aircraft propellers.
Four versions of 20-mm guns have been utilized on the A-7 aircraft, encompassing firing rates from 960 rounds per minute to 6000, and both fuselage and pod mountings. Muzzle blast pressures rather than recoil loads have proved to be most severe for nearby equipment operation. These pressures, with peak to rms ratios in excess of 6, and resulting vibrations are described in terms of waveforms and statistical properties. Studies of methods to control both noise and vibrations and the limitations encountered are briefly discussed. A technique developed for laboratory simulation employing a series of shock pulses for equipment response testing is discussed.
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