acoustical holography

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Acoustical holography

The recording of sound waves in a two-dimensional pattern (the hologram) and the use of the hologram to reconstruct the entire sound field throughout a three-dimensional region of space. Acoustical holography is an outgrowth of optical holography, invented by Dennis Gabor in 1948. The wave nature of both light and sound make holography possible. Acoustical holography involves reconstruction of the sound field that arises due to radiation of sound at a boundary, such as the vibrating body of a violin, the fuselage of an aircraft, or the surface of a submarine. Both acoustical holography and optical holography rely on the acquisition of an interferogram, a two-dimensional recording at a single frequency of the phase and amplitude of an acoustic or electromagnetic field, usually in a plane. Gabor called this interferogram a hologram. See Holography, Inverse scattering theory

Two distinct forms of acoustical holography exist. In farfield acoustical holography (FAH), the hologram is recorded far removed from the source. This form of acoustical holography is characterized by the fact that the resolution of the reconstruction is limited to a half-wavelength. This resolution restriction is removed, however, when the hologram is recorded in the acoustic nearfield, an important characteristic of nearfield acoustical holography (NAH), invented by E. G. Williams and J. D. Maynard in 1980.

Nearfield acoustical holography has been used in the automotive industry to study interior noise and tire noise, in musical acoustics to study vibration and radiation of violin-family instruments, and in the aircraft industry to study interior cabin noise and fuselage vibrations. Applications are also found in underwater acoustics, especially in studies of vibration, radiation, and scattering from ships and submarines. See Acoustic noise, Musical acoustics, Underwater sound

Typically, temporal acoustic data are acquired by measurement of the acoustic pressure with a single microphone or hydrophone, which scans an imaginary two-dimensional surface. In some cases, an array of microphones is used and the pressure is measured instantaneously by the array.The measured data are processed in a computer to reconstruct the pressure at the surface of the object as well as the vibration of the surface. The measured time data are Fourier-transformed into the frequency domain, creating a set of holograms, one for each frequency bin in the transform. In the inversion process, each hologram is broken up into a set of waves or modes whose propagation characteristics are known from basic principles. Each wave or mode is then back-propagated to the source surface by multiplication by the known inverse propagator, and the field is then recomposed by addition of all these waves or modes.

McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.

acoustical holography

[ə′küs·tə·kəl hō′läg·rə·fē]
A technique for using sound to form visible images, in which acoustic beams form an interference pattern of an object and a beam of light interacts with this pattern and is focused to form an optical image.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
Compared with beamforming, acoustic holography method can achieve a quantitative measure of moving sound source, so in recent years it has been widely studied and applied.
However, a research team headed by Peer Fischer, Research Group Leader at the Max Planck Institute for Intelligent Systems and Professor at the University of Stuttgart, has found a way of using acoustic holography to move large numbers of micro-particles simultaneously so that they can be assembled into larger more complex structures.
Peer Fischer, who usually works on tiny robots, was "looking for a way to move large numbers of microparticles simultaneously so that we could assemble them into larger more complex structures" when his team came up with acoustic holography, he said in a statement.
Then, for even more information we continue with the use of wideband holography and near-field acoustic holography." The microphones in the array are irregularly spaced, to allow the same array to be used for these different disciplines.
aACAoMasters in Physics with specialization in Quantum Biology and Acoustic Holography from IIT, Delhi - 1975.
A new feature, near-field acoustic holography, is used to process close-to-the-source measurements significantly improving spatial resolution with accurate results in the low frequency range.
Automated techniques for capturing needed information for iBIMs could include laser scanning and photogrammetry technologies for determining building geometries and lighting characteristics, a combination of infrared and thermal measurements to estimate building envelope characteristics, acoustical measurements and inverse modeling (acoustic holography) to allow spatial and time-varying visualization of acoustical performance, and reduced-order computational fluid dynamic (CFD) models for fast simulation of the indoor air environments.
In our days holography is used for data storage [12], as a holographic memory, protection of documents [13], as well as for art, holographic interferometry [14-18], interferometric microscopes [19], electron holography, acoustic holography, and etc.
GN ReSound's current test equipment includes a laser vibrometer, a 3-D acoustic holography robot, general electro-acoustic measuring equipment, and a shaker/exciter--to analyze different velocity/sound pressure stresses.