Increased density slows down propagation. It is easy to come up with false explanations of why, but here is one I think is probably true: If you imagine a bar

Increased density slows down propagation. It is easy to come up with false explanations of why, but here is one I think is probably true: If you imagine a bar that has been struck by an xylophone mallet, it makes sense that increasing its inertia would slow down the periods of its alternating motion. You can build traveling waves from sums of its modes of oscillation, and that carries the effect of higher density to wave speed.