When we see images of sound waves (usually in DAWs like Audacity or Pro-Tools) they are usually visualized as transverse waves but in reality they are longitudinal waves caused by the displacement of air molecules resulting in compression and rarefactions in the air. (Check out the previous link for some helpful images to see the difference).
Of course, sound waves are usually invisible, so we usually rely on these visualizations to picture what is happening in reality. This NPR story shows us that by using high-speed photography combined with an photographic technique known as Schlieren Flow Visualization, we can literally capture sound waves, propagating outwards from the source – such as a hand clap, or book hitting the desk.
Check out the video below for a great explanation and real examples of what sound actually looks.