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Units of loudness of sound, being a subjective quantity

  1. Mar 21, 2014 #1
    I am learning sound. My teachers and textbook say loudness of sound is a subjective quantity, it is different for every person, while intensity is the objective quantity. The first section says intensity is a measurable quantity, WHILE LOUDNESS IS JUST A SENSATION. On the very next page they have given the units of loudness of sound, decibel and phon. Don't the two contradict each other? If loudness of sound is just a sensation, how does it have units to measure it? This has completely boggled my mind. Please help
     
  2. jcsd
  3. Mar 21, 2014 #2

    phinds

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    Decibels are an absolute measure of a characteristics of a sound. How your particular set of ears perceive a given decibel level is objective and can differ from mine.
     
  4. Mar 21, 2014 #3

    Drakkith

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    While it may primarily be a subjective quantity, you can still take people and have them rate how loud a sound is and then take those results and assign a value to the measurements.

    This is identical to how the magnitude system was initially developed to measure the brightness of different stars. For example, a magnitude 0.0 star is 2.5 times brighter than a magnitude 1.0 star, but the early magnitude system claimed it was only 2.0 times brighter since it looks only twice as bright to the eye.
     
  5. Mar 21, 2014 #4

    AlephZero

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    Sound intensity (in decibels) is just a different scale for measuring the change in air pressure in a sound wave.

    The scale for sound loudness is different for each person, and changes as the person gets older. The scale for loudness (in phons) was made by doing experiments with people, asking them to adjust to amplitude of sounds so that two sounds of different frequencies had the same loudness, or to adjust sounds at the same frequency so one was "twice as loud" as the other. The standard scale for loudness in phons is the average of those experiments on many people, and is defined as a set of curves relating phons to decibels.

    eqlou.gif
    (from http://hyperphysics.phy-astr.gsu.edu/hbase/sound/eqloud.html)

    Note that the shape of the curves varies with the sound level, and the intensity (in dB) for equal loudness changes with the frequency of the sound.

    FWIW there is a similar effect for the pitch of a sound as judged by humans, compared with the frequency as measured by the number of vibrations per second. Low pitched sounds appear to get lower as they get louder, high pitched sounds appear to get higher.

    pitchloud2.jpg
    (from http://acousticslab.org/psychoacoustics/PMFiles/Module05.htm)
     
  6. Mar 21, 2014 #5
    Thank you
     
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