Mechanical vibration, in the context of human tissue, refers to the oscillatory motion induced by an external force. This concept is particularly relevant in the field of therapeutic ultrasound and other medical imaging technologies, where mechanical vibrations in the form of sound waves are used to visualize or treat various tissues in the body.

Sound is a type of mechanical wave, meaning it propagates through a medium via the oscillation of the particles in that medium. In the case of human tissue, these particles are the cells and the interstitial fluid. When a sound wave encounters a tissue, it causes the particles in that tissue to vibrate back and forth along the direction of the wave's propagation.

The speed at which sound travels through a medium depends on the medium's density and elasticity. In general, sound travels faster in denser and more elastic media. For example, sound travels faster in bone, which is dense and elastic, than in fat, which is less dense and less elastic.

As sound waves travel through layers of human tissue, they can be attenuated, or reduced in intensity, due to three main phenomena: reflection, scattering, and absorption.

1. Reflection: This occurs when a sound wave encounters a boundary between two tissues with different acoustic impedances (a property that depends on the tissue's density and the speed of sound in the tissue). Some of the wave's energy is reflected back towards the source, while the rest is transmitted into the second tissue. The amount of reflection depends on the difference in acoustic impedance between the two tissues.
2. Scattering: This occurs when a sound wave encounters a structure that is smaller than the wavelength of the sound, such as a small air bubble or a cell. The wave is scattered in different directions, reducing the amount of energy that continues in the original direction.
3. Absorption: This is the conversion of sound energy into heat. It occurs to some extent in all tissues, but is particularly significant in tissues that have high water content or that are highly vascularized, as these tissues can more effectively convert sound energy into heat.

The combined effects of reflection, scattering, and absorption result in the attenuation of the sound wave as it travels through human tissue. This attenuation must be taken into account when using ultrasound for imaging or therapy, as it affects the intensity and quality of the ultrasound signal.

References:

1. Szabo, T. L. (2014). Diagnostic ultrasound imaging: inside out. Academic Press.
2. Duck, F. A. (1990). Physical properties of tissue: a comprehensive reference book. Academic Press.