Sound is the sensation that is produced when the longitudinal vibrations of molecules from the external environment hit the tympanic membrane. Such vibrations in the external environment are called sound waves. Sound waves are produced when vibrating molecules generate changes in pressure. The speed of sound waves in air is approximately 344m/s at 20 degrees at sea level. An increase in temperature and altitude increases the speed of sound. Other media conduct sound at different speeds. For instance, in fresh water, sound travels at a speed of 1450m/s. Sound can be referred in a perceptual response and a physical stimulus. Sound reaches the ear as mechanical waves traveling through air, however, within the ear they get transformed into nerve impulses known as action potentials. The nerve impulses move to the brain where perception of sound occurs.
The auditory system is responsible for hearing. It is divided into peripheral and central auditory systems. The peripheral auditory system consists of the outer middle and inner ear. The central auditory system is responsible for the perception of sound. It comprises of several nuclei in the brainstem and the midbrain. These include cochlear nuclei, superior olivary nucleus and the inferior colliculus. Auditory impulses reach the thalamus before they are relayed to the primary auditory complex in the temporal lobe.
The external ear channel sound waves through the external auditory meatus to the tympanic membrane, also known as the eardrum. The auditory ossicles, i.e. malleus, incus and stapes are attached to the tympanic membrane. They are all located in the middle ear. The handle of the malleus is connected to the back of the eardrum. The head of malleus is attached to the wall of the middle ear while its short process attachés to the incus. The incus is attached to the head of stapes. The foot plate of stapes is attached to the oval window by the annular ligament. Tensor tympani and stapedius are the two skeletal muscles in the middle ear. The tensor tympani pulls malleus medially to lower the vibrations of the eardrum. The contraction of the stapedius pulls the foot plate of stapes from the oval window. The inner ear is made of the bony labyrinth and the membranous labyrinth. The bony labyrinth comprises of many channels in the temporal bone. The channels are filled with perilymph, which makes up the membranous labyrinth. The cochlear occurs within the labyrinth. It is coiled in humans and makes about two and three quarter turns. It is divided into three chambers by the basilar and Reissner’s membranes. The upper chamber is the scala vestibule, the lower one is the scala tympani and the intermediate chamber is the scala media. The oval window is connected to the scala vestibule while the round window is connected to the scala tympani. The auditory receptors, i.e. the hair cells, are located on the organ of Corti, which is found on the basilar membrane. The hair cell processes are supported by the pillar cells. The cell bodies of the hair cells are housed in the spiral ganglion. The peripheral processes from the ganglion form the cochlear branch of the vestibulocochlear nerve.
The sound waves from the external environment are converted into action potentials by the ear. The waves set up vibrations on the tympanic membrane and auditory ossicles. These vibrations are transformed into movements of the foot plate of the stapes. The movements set up waves in the perilymph. The waves in the endolymph generate action potentials on the hair cells on the organ of Corti. The tympanic membrane functions as a resonator, which transmits vibrations from the source of sound. The vibrations on the membrane are transmitted to the malleus through to the foot plate of the stapes. The function of the auditory ossicles is to act as a lever system and convert vibrations of the eardrum into movements of the perilymph in the scala vestibule. The major pathway for transmission of sound is ossicular conduction. Conduction through bone occurs through the vibration of skull bones that are transmitted to the fluid of the inner ear.
Frequency is one of the properties of sound. It is defined as the number of wave cycles, which occur in every second. Low frequencies produce sounds of low pitch. For instance, the lowest frequency of a sound for which humans can hear is 20 cycles per second. Sounds of frequencies as high as 20,000 cycles per second can be tolerated by humans. Amplitude is a property of sound, which enables us to distinguish loud sound from soft sound. It is the gap between the up-peak and down-peak in a sound wave. Small peaked waves produce soft sounds, whereas large peaked waves produce large sounds. Sound amplitude is expressed using the decibel scale. The auditory threshold for the average human is 0 dB. This is the lowest intensity of sound that can be perceived by the average human. Sound intensity of 120 dB is characterized as painful and is capable of damaging the organ of Corti. Timbre refers to the quality of sound. Sound frequencies and overtones determine the quality of sound. Timbre helps to distinguish between the sounds of different musical instruments even when notes of the same pitch are played.
The human brain has the ability to sort out wavelengths of different intensities and frequencies. It has been determined that the basilar membrane is affected differently by sounds of different frequencies. The part close to the oval window is more sensitive to sounds of high frequencies. The part close to the cochlea is more sensitive to sounds of low frequencies. The frequency theory of hearing explains that the basilar membrane behaves like a microphone by vibrating in response sound. The receptors generate and transmit impulses tied to the frequencies of the sound. Therefore, high pitched sounds generated a great number of impulses, which are transmitted by the auditory nerve to the brain. Different neurons respond to different types of sounds. There are neurons within the auditory cortex that respond to specific sounds like whistles or clicks. Others respond specifically to steady tones. Because of the asymmetry of the two cerebral hemispheres, the left ear processes sound differently from the right ear. The right ear is more sensitive to speech while the left ear is more sensitive to music.
Hearing loss occurs due to malfunction of damage within the auditory system. It can be hereditary, due to infection, disease, aging, excessive exposure to noise or head injury. In case of a hearing loss, one should visit a physician for examination to determine the type of impairment, causes and options for treatment. Treatment may include fitting of hearing aids or cleaning of ears. This will help one regain hearing needed for communication with family and loved ones.
References
Barrett, K. E., Boitano, S., Barman, S. M., & Brooks, H. L. (2010). Ganong's Review of Medical Physiology. New York: McGraw-Hill.
Goldstein, E. B. (2010). Sensation and Perception. New York: Cengage Publishers.
Guyton, A. C., & Hall, J. E. (2006). Textbook of Medical Physiology. Pennsylvania: Elsevier Inc.
Yahoo Contributor Network. (2009, December 11). Loudness, Pitch, Timbre, and there relation to Physical Sound Stimulus. Retrieved from Yahoo: http://voices.yahoo.com/loudness-pitch-timbre-their-relation-physical-4973030.html?cat=72
