How did enslaved African people in North America use material culture to create and signal their own identity? In a paper that has much significance for many other periods and places, the authors draw on archaeological and documentary evidence to show how African spiritual spaces were created in houses and gardens in the form of coded landscapes that were often hidden – though in view.
Abstract.Initial results of broadband (1 to 10 kHz) spatial coherence measurements taken during the June 2003 shallow-water (8 m) propagation experiments will be presented. The results will show spatial coherence estimates over a 12 m long horizontal array and over a 6 m vertical array. The data was taken over a range of sea states and at ranges of approximately 70 and 150 m.
It is well known that bubbles, even in small numbers, cause significant changes in the acoustic properties of seawater. In shallow water bubbles enter the sea, either by wind-driven breaking waves or by surf-induced rip currents. Both have been observed in acoustic transmission experiments [IEEE, JOE 25, 507–515 (2000); J. Acoust. Soc. Am. 106, 617–625 (1999)]. Wind-driven bubbles were observed, during a period of high winds, to reduce sound speed from the undisturbed sea condition of 1520 m/s to about 1494 m/s. From bubble theory, it is estimated that the average bubble void fraction was approximately 2.6×10−6. For a distribution of bubble radius size, n(a), there will be a frequency-dependent sound speed and absorption related by the Kramers-Kronig relation, from which the change in a pulsed signal can be determined. By using a bubble size distribution of n(a)=Na×10−4, for radii between values of 200 and 10 microns and the void fraction estimated above, the dispersion and absorption are calculated. The impulse response can be calculated as a function of range. [Work supported by the Office of Naval Research.]
The active reduction of sound reflected off an object is of much interest to researchers in acoustics. The approach often used is to control the sound in a given region (sensed by a series of point microphones) with destructive interference from a set of localized (point) sources whose strengths, phases, and locations can be adjusted. The work described herein involves extended surfaces which react to an incident wave in a prescribed manner. If this reaction is such that the surface radiates a wave 180 ø out of phase with and of the same amplitude as the reflected wave, the reflected wave is canceled. This paper reports on the construction and testing of a planar active surface capable of canceling the reflections of obliquely incident acoustic plane waves. Theoretical analysis is given, and the viscosity and thermal conductivity of the medium in which the surface operates are found to have negligible effect on the surface's performance. Since the phase of an obliquely incident sound wave varies over an extended surface, an array of transducers is required to accomplish the desired control. The active surface described herein has the unique feature that the wave number and frequency of surface vibrations may be independently controlled. The surface was shown to perform well for 30-to 50-kHz sound. This technique also should be applicable to lower frequency sound, which is not easily controlled passively.
Measurements of the resonant reverberation of sound in a tube containing N2/H 2, N2/He, N2/ CH4, and N2/H20 mixtures have shown an amplification of the sound following a rapid excitation of the gas by an electric discharge [F. D. Shields and L. D. Lafleur, J. Acoust. Soc. Am. 83, 2186-2189 (1988) ]. This effect has been named SACER (sound amplification from controlled excitation reactions). This paper reports similar measurements in N2/CO and H2/ He/CO mixtures. It has been possible in the past to determine vibrational relaxation times and relaxation times for conduction of translational and vibrational energy to the tube walls from the changes in the observed translational temperature of the gas following the electric discharge. In the work reported here, these rates have been confirmed by direct measurement of the decay times of the CO infrared emission.
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