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| Phon - Another Unit of Loudness |
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| As the threshold of audibility varies with frequency, sounds of the same intensity but different frequencies are found to differ in loudness. Therefore, another unit of loudness is defined. This unit measures the intensity of a sound relative to a reference tone of defined pressure and frequency. The reference tone usually used has a frequency of 1000Hz and a root mean square sound pressure of 2 x 10-5 Pascal. The observer listens, with both ears, to the reference tone and the sound to be measured alternately. |
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| The reference tone is then increased until the observer judges it to be of intensity equal to the sound to be measured. If the intensity of the reference tone has been increased by n decibels to achieve this, the sound being measured is said to have an intensity of n phons. The decibel and the phon scales are not identical, as the phon scale is subjective and relies on the sensitivity of the ear, to detect the changes of intensity with frequency. |
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| The two scales agree for a frequency of 1000Hz. |
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| Example, Suppose it is required to find the loudness of a tone, say of frequency 512Hz. The standard source of frequency 1000Hz is sounded and its loudness is adjusted till it is equal to the loudness of the given tone. If the intensity level of the standard source has been raised by n decibels, then the loudness of the given tone is said to be n phons. |
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