In this demonstration, we use a Schlieren optics system with a strobing light source to visualize ultrasonic waves emitted by a transducer driven at a frequency of 28 kHz.Images employing schlieren optics are very sensitive to changes in the density of air. Since sound waves are pressure waves, and pressure variations result in density gradients, it stands to reason that one should be able to image sound waves traveling through the air. Unfortunately, the waves move at the speed of sound (343 meters per second), which makes it difficult for us to see them. With stroboscopic animation, we can slow down the apparent motion of the sound waves.Although 28 kHz is beyond the range of human hearing, ear protection should be worn whenever attempting this experiment to avoid damaging vibrations in parts of the ear. Any high-pitched whines you hear in this video are not dangerous.