Room Acoustic and Human Ear The human inner ear (cochlear) performs a continuous
ID: 2305503 • Letter: R
Question
Room Acoustic and Human Ear The human inner ear (cochlear) performs a continuous frequency analysis on any signal spectral amplitude to local frequency mapping. In that process, phase information between various frequency components appears to be lost. a) Two-tone signals presented separately through a single speaker may look quite different when measured with a microphone and displayed on an oscilloscope, but sound exactly the same to a human being. Explain the phenomenon. b) Two different signals from a single source transmitted to a listener over two acoustic paths of different lengths may be perceived as separate sounds if one signal is sufficiently delayed to the other signal. In the case the delay is sufficiently short, only one single sound will be heard. The cross over time from one type of perception to the other is about 0.05 seconds. Describe how this is related to the lowest frequency that human beings can hear as on tone ratherthan as a sequence of two auditory events. What needs to be considered when echoes should be avoided? What will be the maximum difference between direct and reflected acoustic path in an auditorium considering the information given in b)? c)Explanation / Answer
a) This occurs due to the phenomenon of sound wave interference. As long as the two tones have frequencies below the critical bandwidth, the two signals tend to interfere with each other either constructively or destructively such that they sound as one tone to the human ear. However, on an oscilloscope, the difference in frequencies can be observed visibly even for signals below the critical bandwidth.
b) The inner ear works as a frequency filter. When two signals of different paths and a cross over time of 0.05 seconds, i.e. of slightly different frequencies, are exposed to human ears, the hair cells are affected. Due to nonuniform distribution, these hair cells result in being excited together (as critical bandwidth) because of both the incident tone signals. Thus, it is perceived as one tone however with changing intensity. This occurs upto the point where the difference in the frequencies of the two signals is greater than the critical bandwidth, which is the lowest frequency that human beings can hear as one tone than as a sequence of two auditory events.
c) Echo essentially are the reflection of sound signals from the walls of an auditorium, in this case, that reach the human ear with a delay after the direct sound. Two ways in which echo can be avoided is:
1. By manipulating the signals such that the time delay between the direct sound and reflected signal is less than 0.1 seconds.
2. By inserting absorptive elements in the room such that the sound signals get damped and are not reflected from the walls.
Human ear cannot distinguish echo from direct sound signal if the difference between them is less than 0.1 seconds. This is the maximum difference between direct and reflected acoustic path in an auditorium before echo starts appearing. Therefore, for a difference of 0.05 second, the human ear will hear it as one sound.
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