Ultrasound is the name given to frequencies above the human range of hearing, which is about 20000 {rm Hz}. Waves above this frequency can be used to penetrate the body and to produce images by reflecting from surfaces. In a typical ultrasound scan, the waves travel with a speed of 1500 {rm m/s}. For a good detailed image, the wavelength should be no more than 1.0 {rm mm}. What frequency is required?