How can we use sound to find tiny hidden objects?

How can we use sound to find tiny hidden objects? I'm Rachel Edwards and I'm a professor of physics at the University of Warick and I do research into how to use ultrasound to find things. So, I've got here a piece of railway track and it's got some tiny little defects on the top. We need to be able to find those defects to make sure that everything stays safe. To find these defects, we can use sound. Here's my soundwave, and I'm going to send it into the metal and listen for an echo. So, if I know how fast the soundwave has traveled, I can listen for how long it takes to come back to me and use that to find out where the defects are and how big they are. We might want to look inside something like this and find defects. So, we would like to be able to see sound coming through. Now, sound is energy and so is heat. And we can see changes in temperature by using a sheet that's got liquid crystals inside it. This has got the same holes in, but it's metal. What we can do is send the sound through it, and we see the shadow of the defects inside it. This shows us making some sound and sending it through this metal sample. You can see everything changing color when the sound reaches it. And we can analyze the video and see the shadow that the holes make. I love being able to do research and taking lots of different bits of physics and chucking them together to make something that can make it safer for you next time you're traveling on a train.