![]() The physics, physiology, and anatomy do not define the perceptualĮven though we have an understanding of the physics of sound, the Nerve fibers (those at position 2 in the figure) will be recruited when In the auditory nerve, this means that additional nearby auditory ![]() But there is also a region nearby that did not respond to the weak stimulus, but that does respond if the intensity is increased. This is particularly true for the part of the basilar membrane that is most sensitive to the tone we are playing. Increase the sound intensity and the membrane is displaced by a larger amount at each point. When the sound is weak, displacements are generally quite small and only a small region of the basilar membrane moves sufficiently to evoke any responses. As a traveling wave passes down the basilar membrane, each point of the membrane oscillates at the frequency of the tone. Were this hypothesis correct, we could (using White's cochlear implant apparatus) control the perceived loudness simply by injecting patterns of current into the neuron that cause it to respond at a more rapid firing rate. Low amplitude sounds evoke few spikes while high amplitude sounds evoke more spikes. The amplitude of the sound determines the number of spikes per burst. The rising (high pressure) phase of each cycle of the sound signal evokes bursts of spikes in a collection of auditory nerve fibers. For weak tones, the basilar membrane is displaced little, hair cells are not pushed very far, and there are few spikes in the auditory nerve fibers. The rate of firing in the auditory nerve might determine perceived loudness. The bottom curve the the threshold of hearing each point along that curve represents the physical amplitude and frequency of a barely audible pure tone.įiring rate hypothesis. Each of the curves in this graph represent auditory stimuli that sound equally loud (as determined in a loudness matching experiment). Two sounds that have the same physical sound pressure levels (but different frequencies) are often perceived to have different loudnesses. When we measure the range in terms of psychological units (the sones scale, established by Stevens and others via magnitude estimation), we find a rock band sounds about 1000 times louder than rustling leaves. Loudness, like pitch, is a perceptual (not a physical) quantity. Note that the scale on the x-axis is in decibels (db) and recall that 20 dB is a factor of 10, so a rock band (120 dB) is 100,000 times greater in amplitude than rustling leaves (20 dB). The range of sound intensity is enormous. Perception Lecture Notes: Loudness Perception and Criticalīands Perception Lecture Notes: Loudness Perception and Critical Bands Professor David Heeger What you should know from this lecture
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