Sound from Explosive Chemical Reactions

Acoustic intensity measurements of the F-22A Raptor are analyzed as part of ongoing efforts to characterize the noise radiation from military jet aircraft.  Data were recorded from a rig of microphones and an attached tetrahedral intensity probe at various locations to the sideline and aft of the aircraft. Numerical analysis of the intensity at one-third octave band center frequencies along various measurement planes and at a 23 m radius arc reveals the magnitude and directionality of the vector acoustic intensity.  Differences in the trends for low-frequency and high-frequency data are discussed and, via a simple ray tracing back toward the source, interpreted in terms of source location and extent. [Work supported by ONR.]

Near-field acoustical holography has been shown to be a useful tool for visualizing jet noise fields. It has been applied to a full-scale jet on an installed military aircraft with promising results, but the source characteristics in the extreme near field have been difficult to characterize because of the interference of acoustic reflections off the rigid reflecting plane beneath the jet. To provide accurate sound field reconstructions, a modified approach to statistically optimized near-field acoustical holography (SONAH) is implemented. In conventional SONAH, the sound field is represented by a matrix of elementary wave functions at all desired spatial locations. In this modified approach, advantage is taken of the property that arbitrary, user-defined functions can be selected for this matrix. Here, two sets of cylindrical wave functions, one centered on the jet centerline and one on the image source centerline, are used to obtain an accurate near-field reconstruction.

As part of a sound system evaluation at Brigham Young University's football stadium, measurements were made before and during games by an upper-level undergraduate acoustics class. The measurement experience provided significant training opportunities for the students. Teams of students used sound level meters to make recordings at numerous locations both inside and outside the stadium. These measurements were then correlated with data from stationary microphones placed near the field. From the data, the predicted slow, A-weighted equivalent levels in and around the stadium were calculated relative to a sideline location. Straightforward outdoor sound propagation prediction methods involving geometric spreading, atmospheric absorption, barriers, etc. were successfully used to validate the measured data within 1-2 dB at many locations, including one in the foothills to the southeast of the stadium at a distance of approximately 2.7 km. The students appreciated the hands-on experiences gained by participation in the measurements and analysis.

The radiation of finite-amplitude waves  from the open end of a baffled, circular pipe is considered as a direct continuation of work begun by Kuhn, Blackstock, and Wright more than three decades ago [Kuhn et al., J. Acoust.  Soc. Am. 63, S1, S84 (1978)]. In this paper, a 1 kHz sinusoidal pulse with initial peak pressure amplitude of nearly 1.2 kPa has been propagated down a 6.1 m pipe, whose open end (5.1 cm inner diameter) has been placed off-center in a large rectangular baffle. As the steepened or shock-like waves exit the pipe, the measured waveforms are comprised of sharp impulses that are delta function-like in nature, particularly on axis. Although linear piston theory predicts similar waveform shapes, there is also evidence that nonlinear propagation of these impulses, which exceed a peak pressure amplitude of 1.5 kPa near the pipe opening, is occurring.

Recent research efforts on nonlinear propagation from high performance jet aircraft have revealed an interesting challenge to predicting community response. This challenge focuses on receiver perception of these unique acoustical signals, which contain acoustical shocks that appear to increase their relative loudness and/or noisiness. This current finding suggests a need for an improved description of a receiver perception of the loudness of these signals in order to improve the assessment of noise impacts from these aircraft. Looking backwards, an interesting question emerges: did the earlier low bypass jet engines on commercial and transport aircraft also include these acoustical shocks? If they did contain these features, then the perceptual differences observed between aircraft and other transportation noise sources may be partially explained.

Jet noise research has seen increased use of autocorrelation analyses to glean physical insight about the source and its radiation properties. Length scales and other features have been identified in support of models incorporating large-scale (LSS) and fine-scale (FSS) turbulent structures. In this paper, the meaningful use of autocorrelation in jet noise analysis is further examined. A key finding is that the effect of the peak frequency on autocorrelation width needs to be removed prior to making conclusions about the relative LSS and FSS contributions. In addition, the Hilbert transform is applied to create an envelope of the autocorrelation function to more consistently define a characteristic time scale. These methods are first applied to the analytical LSS and FSS similarity spectra, previously developed by Tam et al. [AIAA 96-1716, 1996]. It is found that the envelope of the FSS similarity autocorrelation function is more similar to that of a delta function than the LSS envelope. These curves are used to more effectively quantify FSS and LSS features in noise spectra from the F-22A Raptor. [Work supported by ONR.]