E1 Propagation of Sound in Biological Media

Topic

As a sound source vibrates, it alternately pushes and releases the particles of the adjacent medium; upon receiving this push, the particles do not permanently move away from their original positions but instead oscillate back and forth, following the same direction as the push received from the sound source. This mode of oscillation—occurring in the same direction as the propagation of the disturbance rather than perpendicular to it—defines a longitudinal wave, and sound always travels as a longitudinal wave in any biological medium.

When particles of the medium adjacent to the sound source are forced closer together by the push they receive, they form a zone of compression—a region where the pressure is momentarily higher than the medium's equilibrium pressure. Conversely, when these same particles move apart as the sound source releases them, they form a zone of rarefaction—a region where the pressure is momentarily lower than the medium's equilibrium pressure. Each compression zone pushes the particles immediately ahead of it, causing those more distant particles to form a new compression zone in turn, while the original rarefaction zone takes their place behind them; in this way, the alternation between compression and rarefaction zones propagates from one group of particles to the next, and this propagating alternation is known as acoustic pressure.

The speed at which acoustic pressure propagates from one group of particles to the next depends on the rigidity of the bonds between the medium's particles and their mass: the more rigidly bonded and lighter the particles are, the faster they push against one another and the faster the acoustic pressure travels through the medium. That is why the speed of sound differs between air, the lymph filling certain cavities of the inner ear, and the bone tissue forming part of the skull: bone tissue particles are bound together much more rigidly than lymph particles, and lymph particles are in turn bound more rigidly than air particles; consequently, acoustic pressure travels slowest in air, faster in lymph, and fastest of all in bone tissue.

The speed of sound in a given medium, combined with the frequency at which the sound source pushes and releases the medium's particles, determines the wavelength—the distance separating one zone of compression from the next within that same medium. The higher the speed of sound in a medium, the longer the wavelength for a given sound source frequency, because the acoustic pressure travels further before the source pushes the medium's particles in the same direction again. In addition to wavelength, acoustic pressure also has intensity: the more forcefully the sound source pushes the adjacent particles of the medium, the greater the pressure difference between each zone of compression and rarefaction, and the greater the intensity of the acoustic pressure ultimately traveling through the medium.

This entire chain of compressions and rarefactions, transmitted from one group of particles in the medium to the next, is essentially how acoustic pressure travels from the sound source to the auricle—the visible part of the ear that receives this pressure and directs it toward the rest of the auditory system. The speed of sound, wavelength, and intensity described by this model are not fixed properties of acoustic pressure in the abstract; rather, they are properties that depend on the medium the acoustic pressure is traversing at each stage of its path—whether air, lymph, or bone tissue. Consequently, any subsequent calculation regarding how sound reaches the auricle and proceeds to the rest of the ear must begin by identifying the specific medium the acoustic pressure is passing through at each stage.

ID:2350