The Phon
Figure 2.17 shows free-field equal-loudness contours for pure tones (observer facing source), determined by Robinson and Dadson at the National Physical Laboratory, Teddington, England, in 1956 (ISO/R226-1961). The phon scale is of equal-loudness level contours. At 1000 Hz every decibel is the equivalent loudness of a phon unit.
For two different sounds within a critical band (for most practical purposes, using 1⁄3 octave bands suffices) they are added in the same manner as decibel readings.
An interesting experiment in this regard is to start with two equal level signals 10 Hz apart at 1000 Hz and gradually separate them in frequency while maintaining their phon level.
They will increase in apparent loudness as they separate. This is one of the reasons a distorted system sounds louder than an undistorted system at equal power levels. One final factor worthy of storage in your own mental “read-only memory” is that in the 1000- Hz region most listeners judge a change in level of 10 dB as twice or half the loudness of the original tone.
Figure 2.18 is a chart of frequency and dynamic range for various musical instruments and the upper and lower frequency range of the average young adult.
