Bone Conduction Hearing Demonstration

Materials

You will need a hanger, string (90 cm), and a table or hard surface with an edge.

Procedure for Volunteers

Cut a length of string (approximately 90 cm).

Tie the string around the hanger so that there are two long strands of the string of about the same length. Place these two strands between your index finger and thumb on each hand. Swing the hanger so that it taps the side of your table.

What do you notice: 

Can you feel the vibrations?
Can you hear any sounds? If so, how would you describe them?

You will repeat the swinging of the hanger, but this time, before swinging the hanger, bring the two strands towards your ears, and then, as you remove your thumb, compress the string against each tragus. The tragi (plural for tragus) are the triangular flaps of skin posterior to your cheeks and superior to your ear lobes. (Note: Your fingers should not be in your ears.) Now, swing the hanger such that it taps the side of your table. Try to swing with the same force you used the previous time.

What do you notice: 

Can you feel the vibrations?
Can you hear any sounds? If so, how would you describe them?

What’s happening?

In class, we learned about how sound waves travel through air until they reach the tympanic membrane. Then, they travel through the bones of the middle ear to the oval window, which vibrates the endolymph which surrounds the organ of Corti (in the cochlea). When the string is pressed up directly against your skull, the sound waves travel through the bone, bypassing the tympanic membrane and the bones of the middle ear. This is called bone conduction. See a diagram of the path the sound waves are taking.

Because of this, different frequencies are able to arrive at the organ of Corti. Remember: stiffer materials will vibrate at a higher frequency compared to softer materials. Is that explanation consistent with your observations of the sound the hanger makes when the string is in your hands versus pressed against your head? Why or why not?

Bone conduction is used clinically in bone conduction hearing aids. These devices can be worn as a headband, attached to the skin with adhesive, or attached surgically. Read more here.

Thermal Grill Illusion

Materials

You will need two containers for water of varied temperatures, the thermal grill apparatus, a thermometer, a kettle to boil water, towels, ice, and a source of water.

Procedure for Volunteers

Tubes will connect the containers of water to the thermal grill. These tubes alternate their water source. The odd numbered grills have water arriving from one container, and the even numbered grills have water arriving from the other container. One of the containers will hold cool water around 18-20 °C. The other will contain warm (not hot) water around 40-41 °C.

Let the water circulate through the thermal grill for a few minutes so that the metal parts take on a temperature close to the temperatures of the two sources of water.

Volunteers can try a couple of things:

1. Put your hand in the water of each container. This demonstrates that the water being pumped through the grill is neither extremely cold nor hot.

1. Place a single finger on an individual pipe of the grill. Does it feel cool or warm? Is it a painful sensation?

1. Try the neighbouring pipe. Does it feel cool or warm? Is it a painful sensation?

1. Place your entire hand down on the grill, palm down. What do you perceive?

1. Try the illusion with your forearm or back of your hand. How does what you feel with your forearm compare to what you felt with the palm side of your hand?

Read more about the thermal grill illusion here.

Thermoreceptors are the cutaneous receptors that sense temperature. There are two classes of thermoreceptors. The low-threshold thermoreceptors detect innocuous, non-harmful temperatures in the range of 15 to 45 °C. High-threshold thermoreceptors detect painful and potentially damaging temperatures hotter than 45 °C or colder than 15 ° C (this paragraph copied verbatim from Open Neuroscience Initiative, CC-BY-NC 4.0).

Based on the differences you experienced between your palm side, back of your hand, and forearm, which areas would you predict have the most low-threshold thermoreceptors?