What do the videos show?
The videos show a fully functional wooden model of the hearing process, from the sound wave to the inner ear. Between the magnetically coupled pendulums representing the sound wave and the mechanically coupled pendulums representing the hair cells of the inner ear lies the middle ear with the eardrum and the three ossicles. When the sound wave is excited at different frequencies, resonances occur in different areas of the inner ear.
Please keep in mind and communicate to the students that every model has its limitations, even if it is a working model like this one. Although the model of the sound wave clearly shows the propagation of higher air density, pupils may still have problems making the connection between the physics of a sound wave and the pendulum model. Also, the inner ear pendulum chain is challenging. The anatomy of the inner ear (fluids separated by membranes) does not directly reveal why coupled pendulums of different length are a functional analogy.
Video 1: Sound Wave
The video shows the propagation of a wave in a chain of coupled pendulums which repel each other with magnets. This is a model for the sound wave, where a change in air density propagates.
The second part of the video shows that in a vacuum or in very thin air, the formation of a sound wave is hardly possible. The vibrations do no longer reach the ear drum.
Video 2: Middle Ear
The video shows the model of the three ossicles (small bones of hearing) in the middle ear. You can see how the pendulums of the sound wave move the eardrum. This movement is transmitted by the three flexibly connected ossicles from the eardrum to the inner ear.
In the second part of the video, you can see in more detail how the ossicles malleus, incus and stapes transmit the excitation.
Video 3: Inner Ear
This model shows how the movement of the eardrum through the middle ear causes the cord to oscillate. In this highly simplified model, the cord represents the fluid in the cochlea and the movements of the pendulums represent the displacement of the membranes which hold the hair cells. The length of the pendulums illustrates the mass of the fluid that is moved up to a specific place in the cochlea and the stiffness of the basilar membrane which also changes along the cochlea. In the two parts of the video it is shown that different sound frequencies cause resonances in different places along the cochlea.
Video 4: Conductive Hearing Loss
In a middle ear infection, movements of the ossicles are hampered by secretion. A blocked Eustachian tube (not shown in the model) obstructs the outflow of fluid from the middle ear. The middle ear infection is simulated here by foam chunks. You can observe that the immobilization of the ossicles damps or prevents the transmission of sound vibrations to the cochlea.
Video 5: Sensorineural Hearing Loss
With a progressive hearing loss which develops with age, sensitivity to high tones is usually affected first. To show this in the model, the shorter pendulums, which resonate at higher frequencies, are removed. Due to this loss of hair cells at the basis of the cochlea, excitation through high tones can no longer be transmitted to the brain.
Science
- The physics of sound: https://en.wikipedia.org/wiki/Sound
- The middle ear: https://en.wikipedia.org/wiki/Middle_ear
- The inner ear, also called "cochlea": https://en.wikipedia.org/wiki/Cochlea
- Barton's pendulums: https://en.wikipedia.org/wiki/Barton%27s_pendulums
- The role of the basilar membrane in sound reception: https://www.open.edu/openlearn/science-maths-technology/biology/hearing/content-section-3.3
