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The photoacoustic effect is governed by a wave equation with a source term proportional to the time derivative of the optical heat deposition per unit volume and time. Although the typical configuration for generation of the photoacoustic effect makes use of pulsed or amplitude modulated optical beams, the form of the source term in the wave equation indicates that a continuous optical source moving in an absorbing medium is capable of sound generation as well. Here, the properties of simple sources moving in one, two, and three space dimensions are reviewed. The salient feature of sources moving in one-dimension at sound speed is that the amplitude of the acoustic wave increases with time without bound according to linear acoustics. Two schemes, one in the time-domain and the other in the frequency-domain, that take advantage of this principle for increasing the sensitivity of trace gas detection are reviewed. [ABSTRACT FROM AUTHOR]

Although the photoacoustic effect is typically generated by amplitude-modulated continuous or pulsed radiation, the form of the wave equation for pressure that governs the generation of sound indicates that optical sources moving in an absorbing fluid can produce sound as well. Here, the characteristics of the acoustic wave produced by a radially symmetric Gaussian source expanding outwardly from the origin are found. The unique feature of the photoacoustic effect from the spherical source is a trailing compressive wave that arises from reflection of an inwardly propagating component of the wave. Similar to the one-dimensional geometry, an unbounded amplification effect is found for the Gaussian source expanding at the sound speed. [ABSTRACT FROM AUTHOR]