Hearing is the brain’s interpretation of vibrations. Sound moves through the environment as pressure waves, and the ear is a biological instrument built to capture and translate these waves. The process begins with the mechanical transfer of energy from the air to a membrane, initiating a chain of events that the brain ultimately perceives as sound.
The Path of Sound Waves
The journey of a sound wave into the ear starts with the visible outer structure, the pinna. It functions like a satellite dish, collecting fluctuations in air pressure and directing them into the ear canal. This canal is a tube approximately 2.5 cm long. As the pressure waves travel down this channel, they are funneled toward a structure at the canal’s end. The sound wave travels until it reaches the boundary separating the outer ear from the middle ear.
The Eardrum’s Response
This boundary is a thin, translucent, and cone-shaped piece of tissue called the tympanic membrane, or eardrum. With a diameter of about 8-10 millimeters and a thickness of only 0.1 millimeters, it is highly sensitive to pressure changes. When the incoming sound waves strike the eardrum, the force of the compressed air pushes it inward, and the subsequent rarefied air pulls it outward, causing it to vibrate.
The vibration’s characteristics directly mirror the sound wave’s properties. A sound’s frequency determines how fast the membrane vibrates; high-pitched sounds cause rapid vibrations, while low-pitched sounds result in slower ones. The wave’s amplitude, or intensity, dictates the membrane’s displacement. Louder sounds create larger movements, while the softest audible sounds move the eardrum a distance smaller than a single hydrogen atom’s diameter.
Amplification in the Middle Ear
The vibrations of the eardrum are immediately transferred to the middle ear, an air-filled chamber containing the three smallest bones in the human body. These bones, the malleus, incus, and stapes, are connected in a chain that links the eardrum to the inner ear.
This chain of bones, known as the ossicles, functions as a lever system to amplify the vibrations. The malleus and incus act as a lever with a mechanical advantage, where the malleus is longer than the corresponding part of the incus, achieving an amplification factor of about 1.3. This amplification is necessary to effectively transmit the sound energy from the air of the middle ear into the fluid-filled inner ear. Without this mechanical boost, most of the sound energy would be reflected off the fluid’s surface.