For those who've never had the experience, hearing the bullets strike is enlightening. We use water filled jugs for bullet testing and even the lowly sub-sonic 22lr makes an impressive THUMP when striking soft but resistant targets. Suppressed 9mm sub-sonic bullets striking this type of media is much louder than the shot. One of the reasons it's not possible to shoot several animals out of a group is due to the sound of the bullet hitting the first causing the rest to scatter.