Heating \(H=\varepsilon\,h\) with axisymmetric \(\varepsilon(\rho,z)=G_\perp(\rho)\,B(z)\) and temporal Gaussian \(h\) (FWHM). Field panel: 2D spectral illustration only. Detector solid: 3D Kirchhoff \(p_\delta=\partial_t(t\,\bar\varepsilon)\) with \(\mathrm{d}R=\min(\mathrm{d}\rho,\mathrm{d}z)\), then \(p=p_\delta*h\). Optional reflect z=0: perfect water–vacuum pressure release (\(r=-1\)) via odd extension of \(\varepsilon\) across the entrance (method of images); the upstream-going wave reflects and reaches an axial detector later — the delay encodes penetration depth. Dashed: far-field \(p\propto(-\hat{\mathbf{u}}\cdot\nabla\varepsilon)*h\) (with the same extension if reflecting).