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About sound levels and feedback

  1. Jan 4, 2014 #1
    FYI, I'm not very knowledgeable in this topic however I have noticed something intriguing whenever dealing with sound levels and feedback. My question is.. Why is it that if I were to say turn my headphones to 100% volume and my PC volume is 50% the sound will sound a little "distorted or fuzzy" (is this because of the white noise produced?). One more related question.. It seems that when I have my headphones at 75% volume and PC volume at 50% the sound is not symmetric to that sound of when my headphones are at 100% and PC volume at 25%, anyone?
     
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  3. Jan 4, 2014 #2

    Simon Bridge

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    Welcome to PF;
    The sound has a crackly quality?
    That is due to the sound-reproduction limitations of your equipment.

    The music, whatever, arrives at your headphones as a varying voltage ... you could just feed that directly into a speaker and get sound out, but your headphones probably include an amplifier that multiplies the voltage and feeds that into a speaker - so it's louder (or quieter) than if you just used the speaker by itself.

    But there is a maximum voltage that the headphones can produce - so if the signal, say, varies between -1 and +1 V in the complicated jiggly pattern for music, and the amplifier, at 50%, multiplies that by 5, and at 100% multiplies by 10 ... but the maximum voltage that can be supplied is 6V?

    What happens is that all the bits of the wave that would be above 6V after amplification get chopped off at 6V.
    A solid +6V line would make a "clunk-clunk" noise in the speaker.
    The effect is called "attenuation".

    The solution is to reduce the amplification - which you do by adjusting the volume control.

    You can also get crackly noises due to electrical faults, bad grounding etc.

    What do you mean by "symmetric"?
    Do you mean you don't get the same volume out of both sides of the headphones?
     
  4. Jan 5, 2014 #3
    qorz,
    Have you forgotten how to calcullate percentages?
    What is 75% of 50%?
    What is 100% of 25%?
     
  5. Jan 5, 2014 #4

    sophiecentaur

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    Are you 110% sure about those figures. :wink:
     
  6. Jan 5, 2014 #5
    No. There is a misunderstanding (I did not do a good job on creating a clear message)..
    What I mean is both scenarios SHOULD produce the same "level/amount" of sound correct? However, this does not seem to be true.

    P.S. Like I stated before in my first post (literally signed up JUST to ask this question because it was bugging me) I am not very knowledgeable in Physics, maybe I am thinking about this totally wrong.
     
  7. Jan 5, 2014 #6

    Simon Bridge

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    There's no reason to think that the percentages on each volume control refer to the same scale.
     
  8. Jan 5, 2014 #7

    rcgldr

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  9. Jan 5, 2014 #8

    Simon Bridge

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    I just realized that my question was getting answered:
    No, it won't be ... there's no reason to believe that the two percentages are of the same scale.

    The volume control links to an amplifier that just tries to take the input signal and multiply it it by some [strike]constant[/strike] number.

    If we interpret the % value as the multiplier, then 50% would play the music at half volume ... that is what you are thinking right?... so:

    The PC takes the signal and multiplies it by the percentage, then the headphones take that number and m,ultiplies is by it's percentage - that's the volume at your ear.

    For PC volume = 50% and headphones at 100% would play the music at 1/2 volume at your ear.
    For PC volume = 75% and headphones at 50% would play music at (3/4)x(1/2)=3/8 volume at your ear.
    For PC volume = 25% and headphone volume at 25% would play music at (1)x(1/4)=1/4 volume at your ear.

    However: for PC volume = 50% and headphone volume at 75% should give the same 3/8 volume at your ear right?

    Does it?

    I wouldn't expect it to (although it might) because those percentages refer to the positions on the dial or on a slider bar. 0-100% on the PC output likely produces a different range of volumes, from the same signal, to the 0-100% on the headphones.

    It's easy to see if you think about different computers outputting different volumes of sound at the same volume level. The numerical value of the control does not mean anything much.

    It's OK - I'm factoring that into the description.
     
    Last edited: Jan 6, 2014
  10. Jan 6, 2014 #9

    sophiecentaur

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    Absolutely. Good audio attenuators have (or aim at) a logarithmic (quasi dB scale) rather than a linear one. A dB scale gives equal perceived steps in loudness for equal 'distances' along the scale. Cheap and cheerful analogue volume controls often approximate a log scale by a suitably loaded linear potentiometer. At one time you could buy 'log pots' for volume control. Dunno if you still can.

    If you could actually measure the levels from your headphones and look at the waveforms, you could, perhaps resolve the question but most people don't have the necessary equipment.

    It might not be hard to find what the sound card does with the loudness number it is presented with.

    In any case, it's always dodgy to do actual sums where consumer audio is involved. Loudspeaker frequency and dynamic response is a case in point, when you sit in your front room and think you are getting all the £££worth from those expensive speakers.
     
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