TV holograms for spatial phase stepping are formed with a small angular offset between the object and the reference beams to give a spatial frequency bias to the pattern recorded by the TV camera. It is common to set the bias so that there is a 90° or 120°phase shift between adjacent pixels and to use the irradiance of three or more adjacent pixels to evaluate the phase of the interference. We report the Fourier-transform evaluation of such recordings to obtain their phase data. We also demonstrate the direct calculation of the phase difference between successive recordings without intermediate calculation of the random phase of each hologram. This technique is proposed as an approach to pulsed TV holography.

Hologram interferometry is applied to study resonances of a violin body. A violin is made and after each major step, modes of vibration are studied. The top plate is studied in six steps: without and with f-holes, with f-holes and bass bar, with an artificial sound post, in the assembled instrument without and with strings. The back plate is studied in four steps: without and with artificial sound post, in the assembled instrument without and with strings. Interferograms of the lowest five to seven modes ranging from about 400 to 1 300 Hz are shown. The top plate is shown to be divided into plate and body resonances. The back plate is acting more as a unit together with the ribs thus mainly giving body-resonances. Deformations of the violin are studied when strings are pressed against the fingerboard, by double exposure hologram interferometry. The lowest air resonance of the sounding box and frequency responses are acoustically measured. In playing the unvarnished violin the tone is found powerful and even.

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A scanning laser Doppler vibrometer is used to record two-dimensional ultrasound fields in air. The laser light of the vibrometer traverses the sound field to and from a rigid reflector and determines the velocity field, a quantity proportional to the sound pressure rate, in each scanned point relative to the sound source. The object sound is the scattered field from objects outside the recording area. Digital reconstruction by use of phase conjugation (time reversal) of the object sound field is then performed, and the original object field intensity and phase is reconstructed.

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