I'm at an internship and I saw a container labeled ultrasonic gel, which is used to prevent air from coming between a scanner and the human body. This is necessary because apparently ultrasound doesnt travel through air well. Why is this? (don't say because the density is low, because I will reply by asking why low density means ultrasound waves can't travel far) Also, I'm not sure if it's that the waves spread out in air, or just that they don't propagate as far in air. The website were fairly obscure since they were targeting their customers, not curious high schoolers who want to learn physics..