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 * 21. Outline the path of a sound wave through the external, middle and inner ear and identify the energy transformations that occur**

Sound waves travel into the external ear, where it hits the side of the head. The pinna collects and channels the vibrations to the ear canal. The sound canal then hits the tympanic membrane and causes it to vibrate. The sound energy is changed into mechanical energy of the moving tympanic membrane. The incoming sound wave with a frequency vibrates the tympanic membrane at the same frequency. The sound wave then enters the middle ear as the vibration from the tympanic membrane travels to the oval window of the middle ear by a series of three small bones. The malleus, incus, and stapes connected to each other, vibrate in time with the tympanic membrane. They act as a lever and reduce the amplitude of loud songs on the tympanic membrane and increasing the force of the vibrations on the oval window. As the oval window vibrates, it transfers the vibration to the round window. The round window acts as a pressure release valve. The pressure wave in the cochlear fluids passes along the vestibular canal and then back along the tympanic canal or through the cochlear duct. The fluid pressure waves push onto the cochlear duct and on the membranes close to the organ of Corti, first the basilar membrane and then the tectorial membrane. This movement of the membranes bends the cochlear hair cells. The hair cells release neurotransmitters into the synapses between them and the neurons that lead to the auditory nerve. The neurotransmitter molecules set up an action potential (nerve impulse) in the neurons that travel along the auditory nerve to the brain stem and from there to the cerebral cortex for interpretation. Thus sound energy travelling through air is converted into vibrational energy in the solids and liquids of the ear structures and finally into nerve/electrical energy in the neurons including those of the auditory nerve.

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This video is associated with the dot point as it outlines the direction and travel of sound waves in the process of human hearing. It gives a detailed explanantion and then a summary of how the waves are turned into nerve impulses and sent to the brain.


 * 22. Describe the relationship between the distribution of hair cells in the organ of Corti and the detection of sounds of different frequencies**

The detection of different sounds of different frequencies is enabled by the thin fibres of various lengths in the basilar membrane of the organ of Corti. The fibres in the basilar membrane are of different lengths and each length vibrates at a different frequency. At a particular frequency the cochlear duct vibrates, and in the basilar membrane the vibration is picked up by a particular fibre length, which essentially vibrates at that particular frequency. Hair cells are all across the basilar membrane and those with the vibrating fibres are bent. Diffferent sets of hair cells on the basilar membrane at differing locations will be activated through different frequencies. Different sets of hair cells can be activated and different areas of basilar membrane are activated simultaneously. The hair cells are located all along from low frequency stimulation to high frequency stimulation. The different frequencies are then able to be interpreted in the brain.

[|**http://www.youtube.com/watch?v=8wgfowbbTz0**]

This video is useful as it explains that hairs in the organ of corti are stimulated by high and low frequences thus expressing the relationship in an animated way.


 * 23. Outline the role of the sound shadow cast by the head in the location of sound.**

When sound waves come from above, behind or in front of the head, the sound and sound waves will reach equally to both ears. However when the sound comes from the side, the receptors in the ear closest to the sound will be activated slightly earlier and with more intensity. The sound coming from one side of the body is located by the brain. A sonic(sound) shadow is casted by the head from the opposite side of the body to the sound coming into an ear. Animals also use this method very effectively.