arXiv:2610.24704v1 Announce Type: new Abstract: This paper presents the first $L^2$-norm error analysis for a nonlinear thermoelastic system with acoustic boundary conditions. The model involves three coupled variables: the acoustic potential $u$, the temperature $\theta$, and the normal displacement to the boundary $\delta$, with nonlinearities $|u|^d u$ in the domain and $g(\cdot,u')$ on the boundary, in addition to a nonlocal term $\beta(\int_\Omega\theta dx)\Delta\theta$ in the heat equation. We establish error estimates for the fully discrete problem, using the Crank-Nicolson method for time discretization and finite element method with basis of degree $r \geqslant 1$ for space discretization. We demonstrate that the domain-boundary coupling induces a half-power loss in the spatial convergence order for the variable $u$, and this effect propagates to the remaining variables due to the thermoelastic and boundary couplings, resulting in the final error estimate $\mathcal{O}(h^{r+\frac{1}{2}} + \tau^2)$. A bootstrap argument is employed to ensure the uniform boundedness of $\|U_h^n\|_{L^\infty}$, which is essential for handling the nonlinearity. Numerical results using Lagrange finite element basis of degrees $ r = 1, 2, 3 $ (linear, quadratic, and cubic) corroborate the theoretical estimates and illustrate the transition from suboptimal to optimal convergence orders when the exact solution possesses additional $L^\infty$ regularity.