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Psychology (4,969)
PSYCH 1XX3 (1,042)
Joe Kim (961)


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McMaster University
Joe Kim

th Audition (March 11 , 2013) “if a tree that falls in the forest and no one is around to hear it, does it still make a sound?” --the thick of determining what sound is; is it strictly the product of an external stimulus. We may consider sound to be the result of our own sensory processing, in which case, you could argue that if no one was there to hear the falling tree, then the tree would make no sound at all?! Answer question logically, the falling tree would produce a sound wave. Just like the visual system can interpret light waves to collect information about stimuli in the environment, the auditory system can translate sound waves from vibrating objects into the psychological experience of audition. 1. Sound is the result of our interpretation of wave. The falling tree would certainly produce sound waves, but the sound waves themselves do not make sound unless an auditory system is present to translate those sound waves into perceptual experience of audition. The auditory mechanisms of different species Auditory mechanism vary across different species according to specific needs: whether they live in water, land or air; whether they need to communicate over long distances; whether they need to be able to receive high or low frequency sounds. Sound Frequency One way that the hearing abilities of various species differ is the range of frequencies that can be detected. (Ex: if you’re even tried blowing a dog whistle, you know that blowing the whistle doesn’t produce any audible sound to your own ears, but you will certainly have a dog’s attention! The dog whistle produces a sound at a high frequency that is beyond the range of human ears but well within the range of the dog’s auditory system.) Human can perceive sounds that lie anywhere between 20 and 20,000Hz, a respectable auditory range. Relatively speaking, whales, dolphins and dogs have a wider hearing range, while frogs and birds have a much narrower range of frequencies that they can detect. At the lower frequency detection extreme are fish, while at the higher frequency detection extreme are bats and rodents. So if a fish and a bat were able to communicate, the conversation would have to be based on the narrow range of frequency overlap available to both species – at about 2000Hz. This would presumably sound very high-pitched to the fish and very low-pitched to the bat. Environmental impacts on auditory structure Audible frequency range is determined in part by the evolution of the structures of the auditory system. One key structure is the basilar membrane which contains the hearing receptors; sounds of different frequencies are processed along different areas of the basilar membrane. The basilar membrane varies in length across species. It is shortest in amphibians and reptiles, longer in birds and longest in mammals. A longer basilar membrane allows processing of a wider range of frequencies. So, mammals can discriminate the widest range of frequencies while most other species can’t discriminate frequencies over 10,000 Hz. Sound waves Like light, sound travels in waves; although sound waves travel much slower and required some medium to travel through. Sound waves are initiated by either a vibrating object, like our vocal cords or a guitar string, a sudden burst of air like a clap. This causes the air molecules surrounding the source of the sound to move, causing a chain reaction of moving air particles. This chain reaction is much like the ripples you observe when you throw a stone into a pond. The point where the stone hits the pond produces waves that travel away in all directions, much like the alternating bands of more and less condensed air particles that travel away from the source of a sound. The eardrum responds to air pressure changes These alternating bands of more and less compressed air molecules interact with the eardrum to begin auditory processing. A band of compressed air molecules causes your eardrum to pushed slightly inwards, whereas a band of less dense air particles causes the eardrum to move outwards. Sine Wave The changes in air pressure over time that make up a sound wave can be graphed as a sine wave, as shown above. In our survey of the neurophysiology of vision, we examined 3 physical characteristics of a wave: amplitude, wavelength and purity; which translated into the 3 psychological perceptions of our visual world. In addition, the same 3 physical characteristics, where applied to sound waves, translate into the 3 psychological properties of loudness, pitch and timbre. I. Amplitude: measure of loudness. Variations in the amplitude or height of a sound wave affect the perception of loudness. Since waves of greater amplitude correspond to vibrations of greater intensity, higher waves correspond to louder sounds. Humans are sensitive to a very wide range of different sound amplitudes, and because of this, loudness is measured using a logarithmic scale of decibels(dB). In this scale, the perceived loudness of a sound doubles for every 10dB increase. (For example: a normal conversation takes place at around 60dB, a whisper at around 27dB and sitting in the front row at a rock concert means you get to hear the music at around 120dB. As enjoyable as this may be, even brief exposure to sounds this loud can cause physical pain and permanent damage. Many listeners crank up their personal music listening devices such that they are effectively listening to their music at 120dB.) II. Frequency: measure of pitch. Sound waves also vary in the distance between successive peaks called the wavelength or frequency of the sound and this property affect the perception of pitch. Pitch is measured in Hertz(Hz); which represents the number of cycles per second, or the number of times in a second that a sound wave makes one full cycle from one peak to the next. So, if many we peaks are condensed into one second, then this sound will be of a high frequency, and result in the perception of a high pitched sound. (we call the visible spectrum of light is only a small portion of the total spectrum of light waves; similarly, the audible zone of frequencies that humans can detect represents only a portion of the possible frequencies that can be produced.) III. Purity, which affects our perception of timbre. So far, simple sound waves consisting of only a single frequency of vibration; however, most of the sounds we hear everyday are complex sounds that are composed of multiple sound waves that vary in frequency. Timbre refers to the complexity of a sound. (For example, when you pluck a guitar string, it vibrates as a whole, which is the fundamental tone, but it also vibrated at shorter segments along the string, called the overtones. The final sound you hear is a mixture of the fundamental tone and all the overtones, and this combination is timbre. So a piccolo and a bassoon may both play the same note, but because each instrument produces a unique combination of the fundamental frequency and overtone, they still sound different to us, even though each instrument is producing the same frequency and amplitude. ) The Ear Ear can be divided into the external, middle, and inner ear; and each area conducts sound in a different way. Incoming changes in air pressure are channelled through the external ear, onto the middle ear, and amplified so that it can be detected as changes in fluid pressure by the inner ear. These changes in fluid pressure are then finally converted to auditory neural impulses. -External Ear: is made up of the pinna, the ear canal and the eardrum. The pinna is what you probably think of when referring to your ears; it is the folder cone that collects sound waves in the environment and directs them along the ear canal. Since the ear canal narrows as it moves towards the eardrum, t functions to amplify the incoming sound waves, muck like a horn. The eardrum is a thin membrane vibrating at the frequency of the incoming sound wave and forms the back wall of ear canal. - Middle Ear: begins on the other side of the eardrum, which connects to the ossicles, the 3 smallest bones in the body. These ossicles are named after their appearance and consist of the hammer, anvil and stirrup. The amplification of the vibrating waves continues here in the middle ear. The vibrating ossicles are about 20 times larger than the area of the oval window to which they connect to create a lever system that amplifies the vibrations even more. This additional amplification is necessary because the changes in air pressure originally detected by the external ear are about to be converted to waves in the fluid-filled inner ear. -Inner Ear: the vibrating oval window connects to the cochlea of inner ear. The cochlea is a fluid-filled tube, about 35mm long, coiled like a snail shell. The cochlea contains the neural tissue that is necessary to transfer the changes in fluid to neural impulses of audition.  The Oval Window is actually a small opening in the side of the cochlea, and when the oval window is made to vibrate, it causes the fluid inside the cochlea to become displaced.  The Round Window, located at the other end of the cochlea, accommodates for the movement of the fluid by bulging in and out accordingly.  Inside the cochlea is a flexible membrane, called the Basilar Membrane; that runs the length of the cochlea like a carpet. So, when the basilar membrane is pushed downwards, the fluid inside the cochlea causes the round window to bulge out, and when the basilar membrane is forced upwards, the round window bulges in
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