Ultrasound waves are high-frequency sound waves. These sound waves are too high for you to hear, and they are not hot in themselves. Your therapist can adjust the frequency and intensity of the ultrasound so that the sound waves create heat energy in your body. Ultrasound treatment can cause vibrations in the molecules of your tissues, which creates heat. You cannot feel these vibrations, however. Ultrasound also creates heat in tissues that absorb the sound waves, such as tendons, ligaments, and bone.
Not feeling any heat doesn’t mean that the ultrasound isn’t working. If the ultrasound is targeted to deep tissues, it could be creating heat that you can’t feel. There are no heat sensors below the surface of your skin. That means you only experience heat during an ultrasound if the warmth can travel back to the skin surface.
It is also possible that your therapist is setting the ultrasound so that it doesn’t cause a build-up of heat. Therapists usually try to avoid building up heat in tissues that are swollen or severely inflamed.