1. Ultrasound is a little trickier than clapping in front of a brick wall, because the body is made of bone and tissue and fat, etc. Sound travels at different speeds in each of these materials. What physical properties of a medium do you think might be important in determining the speed at which sound trave through it? In other words, for the brick wall to form an interface, sound must travel at different speeds in air and brick... what is it about brick and air t are different and that therefore causes the sound to travel at different speeds in each? 2. As we have seen with waves on a string, the "springiness" of a material (its "tension") is one property that goes toward determining the speed at which waves travel through it. We also saw last week that the mass density of the material is important. In fluids or soft tissue the "springiness" of the materia described by the bulk modulus B, defined as B = - AP / (AV/ V), where AP is a change in pressure and AV/V is the corresponding proportional change in volume. (Does a large bulk modulus correspond to more or less "springiness"?) The mass density in fluids or soft tissue is just the density p = m/ V. Working by analogy from what we know about the speed of a wave on a string, what do you think the expression might be for the speed of sound in a fluid or soft medium, expressed in terms of B and p? e following table shows some densities and sound speeds through different parts of the human body