Jon Chase challenges young opera singers in Cardiff to smash glasses with their voices. After measuring the natural frequency of the glass, can the singers match it with enough power to smash it? The results are spectacular slowed down 300 times. Smashing things with sound waves isn’t just for fun. In medicine, sound waves can smash painful, hard growths inside the body, called kidney stones. The sound waves that smashed the glasses travel through air. But the kidney stone smashing waves travel through water. To find out why, two of the singers join Jon by Cardiff Harbour for an experiment. Does sound travel faster in water or air? Teacher notes: Sound is caused by vibrations and the pitch and volume of sounds can be changed in a variety of ways. Students will need to be introduced to the term ‘frequency’, measured in Hertz (Hz), as a measure of how fast the sound vibrates. Teachers might then use an oscilloscope and a signal generator to illustrate this through sine waves, and students could investigate using different tuning forks to explore the relation between the frequency of vibrations and the pitch of the note. Students could research other applications of ultrasound such as cleaning and physiotherapy.