In this research, Laser-Induced Ultrasound (LIU) longitudinal waves are studied for a 1D three-layer boundary condition problem, considering the heat flux propagation time ( \(\tau _q\) ). A photothermoacoustic boundary value problem is defined via the Maxwell–Cattaneo–Vernotte equation for temperature, coupled with the photoacoustic wave equation for pressure. The temperature scalar field was then analytically calculated in the frequency domain and included in the wave equation as the source of the longitudinal pressure waves. The corresponding solutions suggest that the main LIU source is the heat flux. Furthermore, the characteristic heat lag time is compared with the laser pulse time \(\tau _p\) . In that case, the acoustic pressure remains constant in the interval where \(\tau _q\le \tau _p\) , a similar behavior previously observed for \(\tau _q\rightarrow 0\) , when the Fourier heat diffusion model is considered. However, if \(\tau _q>\tau _p\) , photoacoustic longitudinal waves are strongly modified, implying that the laser pulse time acts as a band pass filter, modulating the acoustic signal.
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