In the same way that free space has inductance and capacitance (mu_0 ~= 1.257 uH/m, e_0 ~= 8.84 pF/m), and together these define the impedance of free space and the speed of light (Zo = sqrt(mu_0 / e_0), c_o = 1 / sqrt(mu_0 e_0)), so too do the density, and stiffness (elastic modulus), define the acoustic impedance and speed of sound of a bulk material.

There are different kinds of waves in solids, so you need to use the right units (shear modulus <--> shear velocity, etc.), to get the respective wave property.

The best part is, this not only tells you how much the pulses will be delayed, but also how best to connect those piezo discs so the pulse will be strong. (Namely: matched to a TL of the same impedance as the piezo disc, and placed inline, not just at the end. The reason is easy to see, if you draw a free-body diagram of the piezo with respect to the pressure and displacement applied to it. If there's no reaction mass behind it, it's just flapping in the breeze, and will only transmit what pressure is dropped across its thickness, which will not be much at most frequencies.)

Tim