Why are long wavelength ultrasound waves more penetrating than short wavelength ultrasound waves?

Answer 1

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 the air and material are not too dissimilar. Thus, the sound penetrates (travels through) the material easier because of the matched impedance. If the impedance is not matched (the frequency in the air is way different from the natural frequency of the material or the air and material characteristcs are too dissimilar), the impinging sound tends to bounce off the material rather than penetrate it...although some of the energy may be absorbed, depends on how far off the natural frequency the sound is and how different the material is from air (or other media, like water).

Answer 2

Speed of light in a vacuum (or air) equals the wavelength in a vacuum (dependent on medium!) times the frequency (which is not medium dependent) Therefore 3e8 m/s = Wavelength in air * 76 Hz Wavelength in air = 3.95e6 meters

Answer 3

Because the wavelength suffers less loss over distance.