  Explicit time-marching schemes are popular for solving time-dependent partial
differential equations; one of the biggest challenges these methods suffer is
increasing the critical time-marching step size that guarantees numerical
stability. In general, there are two ways to increase the critical step size.
One is to reduce the stiffness of the spatially discretized system, while the
other is to design time-marching schemes with larger stability regions. In this
paper, we focus on the recently proposed explicit generalized-$\alpha$ method
for second-order hyperbolic equations and increase the critical step size by
reducing the stiffness of the isogeometric-discretized system. In particular,
we apply boundary penalization to lessen the system's stiffness. For $p$-th
order $C^{p-1}$ isogeometric elements, we show numerically that the critical
step size increases by a factor of $\sqrt{\frac{p^2-3p+6}{4}}$, which indicates
the advantages of using the proposed method, especially for high-order
elements. Various examples in one, two, and three dimensions validate the
performance of the proposed technique.
