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the unit of loudness level. Since sounds of differing intensity—that is, of differing sound pressure—may have the same loudness at different frequencies, the loudness of a sound is evaluated by comparing it with the loudness of a pure reference tone, usually a tone with a frequency of 1,000 hertz (Hz). One phon is the difference between the loudness levels of two sounds of a given frequency whose intensities, or sound levels, at a frequency of 1,000 Hz differ by 1 decibel (dB). For pure tones with a frequency of 1,000 Hz, a difference of 1 phon in loudness level is equal to a difference of 1 dB in sound level.