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Acoustic holography is the process in which sound waves are recorded and arranged into a visual pattern using a computer. The sound field can be modeled to reconstruct its structure using three-dimensional (3D) images. Sound fields that radiate from the surface of musical instruments, aircraft, submarines, and automotive interiors are measured to study how the design impacts the propagation of sound. The two basic types are near-field and far-field acoustic holography.
Data are collected using a microphone or hydrophone, but sometimes arrays of microphones can be set up to monitor the acoustic pressure surrounding its source all at the same time. A computer processes these data to reconstruct the sound field in a visual format. Time data are associated with specific frequencies to create a representative set of holograms, which are broken up into waves based on their propagation characteristics. The sound source is calculated by using an inverse calculation so the field can be analyzed using each sound wave.
Near-field acoustic holography is used to localize sound from a relatively close source. Multiple microphones are arranged into a rectangular configuration, spaced horizontally and vertically. The half-wavelengths of the maximum and minimum frequencies are measured, which depend on the spacing of the microphones and the size of the array. Sound intensity calculations are possible from the measured sound source, but wave propagation can only be assessed parallel to the surface measured. The acoustic holography technique is also limited to testing high frequency sounds.
A processing technique called near-field focalization forms beams that produce spherical sound waves around microphones. The spatial resolution of the measurements is improved when it is equal to the level of focalization, or is at a higher frequency. Near-field 3D holography can be combined with other simulation techniques on a computer and, with the appropriate focalization, can be used to measure low- to mid-frequency sounds.
In far-field acoustic holography, the hologram is generated by data gathered far away from the source. The resolution is limited to a measurement of the half-wavelength. This limitation can only be compensated for by measuring the sound from a closer range.
Acoustic holography involves measuring sound pressure. It can account for various acoustic references, surfaces known to vibrate at certain frequencies, and various characteristics of a sound signal. Data can also be acquired from different places, in a vehicle for example, and be pieced together using data acquisition software to create an even bigger picture for sound wave analysis.