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Thread: The Human Ear and its limitations.

  1. #1 The Human Ear and its limitations. 
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    Hi everyone, I have a question...


    The human ear suffers from a number of limitations and anomalies. These being:

    - Frequency response (20hz-20khz or so)
    - SPL (0dB - 153dB)
    - Fletcher-Munson curve which is the shift in the preceived frequency content according to the intensity of sound.

    What about changes in polarity of sound? For example, if you are listening to your stereo (ie two speakers - left and right) and then you reverse the polarity of just the left speaker you definately hear the change in sound. But I don't think you are actually hearing the change in phase of sound, rather the result of the sound coming from the left speaker destructively interferring with the sound coming out of the right speaker to form a very different complex waveform.

    If you isolate this change in polarity (IE. phase shifting a waveform 180degree which effectively turns a positive pressure wave into a negative one) does the human ear preceive it as something different? The changes in pressure that the ear detects will still be identically proportioned since changing the polarity doesn't alter the amplitude or frequency content of a waveform.

    Does the ear detect something different if there is only an inversion of phase and nothing else (IE, only a single mono sound source being inverted)? Or do we hear something that sounds exactly the same, thus the only time a change in polarity is heard is when it interferes with other waveforms to form more complex sounds?

    I hope that made sense.


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  3. #2  
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    You can consider a phase change as merely a lapse of time if listening to a single tone, it has no reference so phase changing may produce 'clicks' or if gradual, a temporary perception of frequency change but no more.

    The ears are positively tuned to detect phase changes which gives you a sense of direction, very definitely an advantage if you are 'prey' - you need to know in which direction to take flight!

    Not sure why you say the human ear suffer from limitations, it's a bit like saying a bicycle is limited as transport because it cannot fly,the ear is 'tuned' to a range of frequencies suitable for it. Remember that if it were not for a limitaion in frequency range and a non-linear response in amplitude you would reduce it's ability to interpret phase changes as well as it does.


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  4. #3  
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    Quote Originally Posted by Megabrain
    You can consider a phase change as merely a lapse of time if listening to a single tone, it has no reference so phase changing may produce 'clicks' or if gradual, a temporary perception of frequency change but no more.

    The ears are positively tuned to detect phase changes which gives you a sense of direction, very definitely an advantage if you are 'prey' - you need to know in which direction to take flight!

    Not sure why you say the human ear suffer from limitations, it's a bit like saying a bicycle is limited as transport because it cannot fly,the ear is 'tuned' to a range of frequencies suitable for it. Remember that if it were not for a limitaion in frequency range and a non-linear response in amplitude you would reduce it's ability to interpret phase changes as well as it does.
    You will have to forgiven me. I am an audio engineer by trade and every bit of discussion (argument) related to audio equipment always makes the human ear seem somewhat redundant in its ability (I know, its a contradiction... but we must record audio up to 96khz :? :wink: )

    The initial question was sparked when I spent some time messing with the phase of recorded mono complex waveforms, a single guitar sound for example. In this situation I have the ability to replay the same waveform whilst sweeping the phase through a full 360degrees as well as instantly inverting it by 180degrees. Its a situation where you can rapidly switch the phase of a waveform... but for the life of me I cannot hear a difference between the two. Granted, these are rather controlled situations that don't really exist anywhere else.

    As far as the ear's ability to determine direction, do you think this is the ear detecting phase or rather determining the difference in phase between the sound picking up by the right ear and the sound picked up by the left ear?
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  5. #4  
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    I believe it is phase detection, you know sound travels at around 300m/s so a 1khz wave is about 30cm long, imagine it from a single source, if you are facing it then the sound reaches both ears together, as a single tone you cannot tell though whether it is from in front or behind, only when other frequencies are mixed in can you tell that. Back to the 1K tone, turn your head to the side and the phase change can be up to 270 degrees (assuming your head is around 20cm wide!).

    Also I have noticed that sometimes you can turn your head to a particular direction and tones of a certain frequency will 'dissappear', I'm sure you will have found that playing with sound.

    Stereo sound you can using speakers make it appear as if a truck/train etc is travelling from one side of the room to the other by adjusting the level between the speakers AND making subtle changes to the phase.

    If you do not adjust the phasing correctly it just sounds like two trucks from two different speakers. My conclusion is that phasing is therefore the key factor.
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  6. #5  
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    I've always considered what you are referring to as an anomalie related to the environment in which you are doing the listening. Within a room the ear isn't hearing just a single sound source, but the sound source combined with many reflections. By moving your head you are hearing the result of the source sound and those reflections combining in different ways. It sounds different because it is different. When you move your head it can enter a constructive or deconstructive node where the reflected sound combines with the source sound and either becomes louder (both the reflected and source sounds are in phase) or becomes quieter (the reflected and source sounds are out of phase). The reason why only certian frequecies either become emphasized or disappear altogether is of course because each frequency has a different wavelengt, thus a comb filtering effect.
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  7. #6  
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    I was ignoring reflections and giving an opinion based upon memories of working in an anechoic chamber, as I indicated: From a pure single tone the brain derives very little in the way of directional informaton.

    You are welcome to play the 1K tone game and let me know what happens...
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