Military jet noise source imaging using multisource statistically optimized near-field acoustical holography.

The identification of acoustic sources is critical to targeted noise reduction efforts for jets on high-performance tactical aircraft. This paper describes the imaging of acoustic sources from a tactical jet using near-field acoustical holography techniques. The measurement consists of a series of scans over the hologram with a dense microphone array. Partial field decomposition methods are performed to generate coherent holograms. Numerical extrapolation of data beyond the measurement aperture mitigates artifacts near the aperture edges. A multisource equivalent wave model is used that includes the effects of the ground reflection on the measurement. Multisource statistically optimized near-field acoustical holography (M-SONAH) is used to reconstruct apparent source distributions between 20 and 1250 Hz at four engine powers. It is shown that M-SONAH produces accurate field reconstructions for both inward and outward propagation in the region spanned by the physical hologram measurement. Reconstructions across the set of engine powers and frequencies suggests that directivity depends mainly on estimated source location; sources farther downstream radiate at a higher angle relative to the inlet axis. At some frequencies and engine powers, reconstructed fields exhibit multiple radiation lobes originating from overlapped source regions, which is a phenomenon relatively recently reported for full-scale jets.

Near-field shock formation in noise propagation from a high-power jet aircraft.

Noise measurements near the F-35A Joint Strike Fighter at military power are analyzed via spatial maps of overall and band pressure levels and skewness. Relative constancy of the pressure waveform skewness reveals that waveform asymmetry, characteristic of supersonic jets, is a source phenomenon originating farther upstream than the maximum overall level. Conversely, growth of the skewness of the time derivative with distance indicates that acoustic shocks largely form through the course of near-field propagation and are not generated explicitly by a source mechanism. These results potentially counter previous arguments that jet "crackle" is a source phenomenon.

The impact of hearing protection on sound localization and orienting behavior.

The effect of hearing protection devices (HPDs) on sound localization was examined in the context of an auditory-cued visual search task. Participants were required to locate and identify a visual target in a field of 5, 20, or 50 visual distractors randomly distributed on the interior surface of a sphere. Four HPD conditions were examined: earplugs, earmuffs, both earplugs and earmuffs simultaneously (double hearing protection), and no hearing protection. In addition, there was a control condition in which no auditory cue was provided. A repeated measures analysis of variance revealed significant main effects of HPD for both search time and head motion data (p < .05), indicating that the degree to which localization is disrupted by HPDs varies with the type of device worn. When both earplugs and earmuffs are worn simultaneously, search times and head motion are more similar to those found when no auditory cue is provided than when either earplugs or earmuffs alone are worn, suggesting that sound localization cues are so severely disrupted by double hearing protection the listener can recover little or no information regarding the direction of sound source origin. Potential applications of this research include high-noise military, aerospace, and industrial settings in which HPDs are necessary but wearing double protection may compromise safety and/or performance.

Auditory localization in the horizontal plane with single and double hearing protection.

INTRODUCTION: Although single hearing protection devices such as earplugs or earmuffs are known to degrade sound localization, little is known about localization accuracy in double-hearing-protection conditions where both earplugs and earmuffs are worn at the same time. METHODS: Listeners wearing earplugs, earmuffs, or a combination of earplugs and earmuffs were asked to localize short (250 ms) or long (continuous) pink noise signals originating from one of 24 loudspeaker locations in the horizontal plane. RESULTS: When single hearing protection was worn, localization was reasonably accurate in the left-right dimension even when the stimuli were short in duration. When double hearing protection was worn, however, left-right localization accuracy was poor even when the stimuli were on continuously. A second experiment showed that localization accuracy with double hearing protection varied substantially across different listeners, but that it varied only slightly across refittings of the same earplugs and earmuffs on the same listener. A third experiment showed that double hearing protection impaired localization in the left-right dimension much more for narrow-band sounds at frequencies above 500 Hz than it did for narrowband sounds at frequencies at or below 250 Hz. DISCUSSION: The severe disruptions in performance that occurred when earmuffs and earplugs were worn simultaneously suggest the influence of a mechanism such as bone conduction that does not normally interfere with localization when only a single hearing protection device is used.