Thomas B. Gabrielson

The role of nonlinear effects in the propagation of noise from high-power jet aircraft

To address the question of the role of nonlinear effects in the propagation of noise radiated by high-power jet aircraft, extensive measurements were made of the F-22A Raptor during static engine run-ups. Data were acquired at low-, intermediate-, and high-thrust engine settings with microphones located 23-305 m from the aircraft along several angles. Comparisons between the results of a generalized-Burgers-equation-based nonlinear propagation model and the measurements yield favorable agreement, whereas application of a linear propagation model results in spectral predictions that are much too low at high frequencies. The results and analysis show that significant nonlinear propagation effects occur for even intermediate-thrust engine conditions and at angles well away from the peak radiation angle. This suggests that these effects are likely to be common in the propagation of noise radiated by high-power aircraft.

On the Perception of Crackle in High-Amplitude Jet Noise

Crackle is a phenomenon sometimes found in supersonic jet noise and can comprise an annoying and dominant part of the overall perceived noise. In the past, crackle has been commonly quantified by the skewness of the tune waveform. In this investigation, a simulated waveform with a virtually identical probability density function and power spectrum as an actual F/A-18E afterburner recording has been created by nonlinearly transforming a statistically Gaussian waveform. Although the afterburner waveform crackles noticeably, playback of the non-Gaussian simulated waveform yields no perception of crackle at all, despite its relatively high skewness

A simple neutrally buoyant sensor for direct measurement of particle velocity and intensity in water

Acoustic particle velocity is commonly inferred from measurement of pressure or pressure gradient; however, in water, direct measurement is simple. A moving‐coil sensor embedded in a neutrally buoyant package produces a voltage directly proportional to the particle velocity in the surrounding fluid for frequencies above the mass‐spring resonance. Leslie et al. [J. Acoust. Soc. Am. 28, 711–715 (1956)] built such a sensor by mounting a moving‐coil element inside a hollow brass sphere. The sensor described in this paper is identical in principle but is considerably easier to fabricate. Useful from tens of hertz to several kilohertz, this sensor consists of a glass‐ microballoon‐and‐epoxy composite cast around a small, commercial geophone. The sensor is inexpensive, rugged, and has good immunity to interference. In conjunction with a pressure hydrophone, acoustic intensity can be measured without the errors associated with subtraction of nearly equal signals (as in the two‐hydrophone method).

A miniature high-sensitivity broad-band accelerometer based on electron tunneling transducers

Abstract New high sensitivity microsensors have been developed using high-resolution position sensors based on electron tunneling. The design of miniature accelerometers having resolutions approaching 10 −9 g /√Hz is discussed. A new dual-element electron tunneling structure, which overcomes bandwidth limitations of single-element structures, allows design of high sensitivity accelerometers operating in a band from a few Hz to several kHz. A miniature accelerometer based on this structure can thus have application as a sensitive acoustic sensor. Thermal vibration of the proof mass is an extremely important constraint in miniature accelerometers, and can be the dominant limitation on the sensitivity. Thermal noise is analyzed for the suspended masses of the dual-element structure, and compared with electronic noise in the tunneling circuit. With a proof mass of 100 mg, noise analysis predicts limiting resolutions better than 10 −8 g /√Hz between 10 and 100 Hz, and 10 −7 g /√Hz at 1 kHz. Prototype accelerometers have been fabricated by silicon micromachining and tested. A noise resolution of 10 −7 g /√Hz between 4 and 70 Hz and 6 × 10 −7 g /√Hz at 400 Hz is observed in a damped device. The low-frequency responsivity of this device is 100 000 V/ g , decreasing to 1300 V/ g at 600 Hz.

Underwater acoustic intensity probe

An underwater probe for determining true acoustic intensity by the direct asurement of true acoustic velocity and true acoustic pressure in a neutrally buoyant package, utilizes a moving-coil geophone embedded in a casting of syntactic foam and a pair of hydrophones on the exterior of the casting.

A miniature, high-sensitivity, electron tunneling accelerometer

Abstract Prototype low-noise miniature accelerometers have been fabricated with electron-tunneling transducers. The electron-tunneling transducer permits detection of small displacements of the proof mass with high electrical response; such a transducer is essential for a high-performance miniature accelerometer. Prototype accelerometers have shown self-noise of approximately 10 −7 g ( Hz ) − 1 2 or less between 10 and 200 Hz, and close to 10 −8 g ( Hz ) − 1 2 near the resonance frequency of 100 Hz. Directivity measurements give nulls at least 50 dB below the maximum. A dual-axis prototype designed for underwater acoustic applications is packaged in an 8 cm3 volume with a mass of 8 g.

Development of an accelerometer-based underwater acoustic intensity sensor

An underwater acoustic intensity sensor is described. This sensor derives acoustic intensity from simultaneous, co-located measurement of the acoustic pressure and one component of the acoustic particle acceleration vector. The sensor consists of a pressure transducer in the form of a hollow piezoceramic cylinder and a pair of miniature accelerometers mounted inside the cylinder. Since this sensor derives acoustic intensity from measurement of acoustic pressure and acoustic particle acceleration, it is called a p-a intensity probe. The sensor is ballasted to be nearly neutrally buoyant. It is desirable for the accelerometers to measure only the rigid body motion of the assembled probe and for the effective centers of the pressure sensor and accelerometer to be coincident. This is achieved by symmetric disposition of a pair of accelerometers inside the ceramic cylinder. The response of the intensity probe is determined by comparison with a reference hydrophone in a predominantly reactive acoustic field.

St. Lawrence blue whale vocalizations revisited: Characterization of calls detected from 1998 to 2001

From 1998 to 2001, 115 h of acoustic recordings were made in the presence of the well-studied St. Lawrence population of blue whales, using a calibrated omnidirectional hydrophone [flat (+/- 3 dB) response from 5 to 800 Hz] suspended at 50 m depth from a surface isolation buoy. The primary field site for this study was the estuary region of the St. Lawrence River (Québec, Canada), with most recordings made between mid-August and late October. During the recordings, detailed field notes were taken on all cetaceans within sight. Characterization of the more than 1000 blue whale calls detected during this study revealed that the St. Lawrence repertoire is much more extensive than previously reported. Three infrasonic (<20 Hz) and three audible range (30-200 Hz) call types were detected, with much time/frequency variation seen within each type. Further variation is seen in the form of call segmentation, which appears (through examination of Lloyds Mirror interference effects) to be controlled at least partially by the whales. Although St. Lawrence blue whale call characteristics are similar to those of the North Atlantic, comparisons of phrase composition and spacing among studies suggest the possibility of population dialects within the North Atlantic.

Frequency constants for transverse vibration of annular disks

Frequency constants for the transverse vibration of thin, annular disks are presented. All nine combinations of the boundary conditions, free, simply supported, and clamped, are included for Poisson’s ratios of 0.0, 0.3, and 0.5. Many of these cases have been treated earlier in the literature but with occasional, significant error. Consequently, several tests of the validity of the present solutions are described.

Measurement and Prediction of Noise Propagation from a High-Power Jet Aircraft

Static engine run-up noise measurements have been made on the F-22 Raptor at low and high power settings. At afterburner, the propagation measurements reveal significant evidence of nonlinearity in that there is much greater high-frequency energy than is predicted by linear theory. The measurements have been compared against the results of a nonlinear numerical model based on the generalized Mendousse-Burgers equation. Although the model simplifies the propagation environment in that it neglects ground effects and atmospheric variability, agreement between the measured and nonlinearly predicted spectra is quite favorable. This comparison demonstrates that nonlinear effects can play a significant role in the propagation of high-amplitude noise and that prediction of these effects is possible with this type of numerical model.