Prediction of sound field from recoilless rifles in terms of source decomposition
Introduction
Muzzle blast, a type of impulse sound, is a phenomenon associated with the rapid discharge of compressed air and propellant gas, which displace the ambient air and form blast waves at the front of the spherical volume of the gases [1]. These shock fronts contribute to huge variations in the local acoustic pressure within a very short time during propagation. This is a major characteristic that describes the acoustical similarity of muzzle blast and is called impulsiveness, which is revealed by the presence of several acoustical features such as short duration time, nonlinearity near the source, and broadband noise [2].
Most studies of muzzle blast [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15] are based on this similarity of the impulsiveness. Weber [3], [4] developed a blast source model to describe the strength and spectrum of the blast noise for an explosion in air depending on the radius of a spherical volume of compressed gas, and the International Organization for Standardization (ISO) [5], [6] has recommended procedures to estimate the source data with respect to the energy, direction, and frequency contents using the Weber radius and provides guidance for calculating the sound propagation from shooting ranges. In addition, Fansler et al. [7], [8] proposed a scaling law to predict the blast wave overpressure levels from an arbitrary gun muzzle according to dimensionless parameters such as the scaling length, Mach disc location, and blow-down parameter in a perspective on damage risk to the human body. Likewise, an optical measurement of the muzzle flow field by Schmidt and Shear [9], numerical simulations using a remarkable source model by Bin et al. [10] and Jiang [11], [12], [13], [14], and a theoretical approach by Erdos and Del Guidice [15] also investigated its physical mechanism and characteristics based on these similarities.
In this study, the characteristics of shooting range from M40A1 recoilless rifle were investigated by acoustic pressure signals based on the principle of these similarities [16]. Typically, the recoil force is induced as a reaction to the propelling force that accelerates the projectile when a weapon is firing, and it has an impact on an operator. However, M40A1 becomes recoilless by cancellation of the recoil force, so this force rarely affects the user. Hence, there might be significant changes in the acoustic features that can enable “recoil” and “recoilless” rifles to be distinguished by means of the similarities in impulsiveness.
Accordingly, the sound field was measured to investigate the similarities and dissimilarities of M40A1, and a prediction model that can include the effects of the dissimilarities is proposed. This article first presents an ISO standard procedure, which is one of prediction methods for shooting range based on the similarity, and introduces the measurement conditions in Section 2. Subsequently, the acoustic properties of muzzle blast from M40A1 are examined using measured signals, and the remarkable differences from recoil weapons are analyzed in Section 3. Then, a hybrid prediction model is suggested, and its accuracy and validity are examined in Section 4.
Section snippets
ISO 17201
Parts 2 and 3 of the international standard series ISO 17201 cover the prediction of source data and propagation of shooting noise, and they ultimately specify methods for the calculation of the sound exposure level (SEL) at a certain receiver point from muzzle blast. These procedures are briefly described in Fig. 1.
In order for the source data to be used as input variables for further calculation, the data must be frequency and angle dependent because the acoustic energy and spectrum of muzzle
Measurement
Fig. 3 shows the acoustic pressure signals that were experimentally measured at positions of 0°, 90°, and 180° from the line of fire. In terms of the expected characteristics, these results seem to be unusual. Acoustic signals resulting from muzzle blast generally show similar waveforms regardless of the measurement position. Overpressure across a shock front is first observed and followed by relatively small rapid changes in the acoustic pressure that last for a certain period of time. This is
Hybrid prediction model
On the basis of the results acquired from several analyses, a hybrid prediction model is proposed to predict the sound field from M40A1 recoilless rifle. Blast noise as predicted by the ISO standard procedure is combined with jet noise predictions from the empirical formula established by NASA [23]. Finally, to verify the applicability of the hybrid model, the sound field from M67, another recoilless rifle with a 90 mm caliber, was estimated and compared with experimental results. The
Conclusion
In the present study, the characteristics of the sound sources of M40A1 recoilless rifle were investigated by analyzing the acoustic pressure signals measured when it was firing. The waveforms of the measured signals differ significantly with the measurement location, and this seems to be caused by additional source(s) that exists in addition to the blast noise at the muzzle. Based on this assumption, the measured results were examined from temporal, spectral, and directional perspectives, and
Acknowledgements
This work was supported by the Human Resources Development program (No. 20124030200030) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy. In addition, this research was also supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF – 2013R1A1A2011462).
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