Why are long wavelength ultrasound waves more penetrating than short wavelength ultrasound waves?
Most materials have natural frequencies that are comparatively slow and low, which means that longer, slow frequency waves are more likely in tune with materials natural frequencies than shorter, high frequency waves. A natural frequency is the one you’d hear if you struck the material and let it vibrate. A tuning fork, for example, has a natural frequency usually set to middle C. Even things like bridges have natural frequencies, which are very very slow and, consequently, low. In tune has a technical term, which is called matched impedance. Impedance is a measure of resistance to sound penetration. (The same term is also used for electrical resistance to current flow.) Impedence is best matched when the frequency and certain material characteristics are the same. If frequency, material characteristics, or both are different, that results in an impedance mismatch. When impedance is matched, the sound in the air is in tune with the material natural frequency and the characteristics of
