A graph of equiphon curves in a free sound field, with frequency in the horizontal and sound pressure level in the vertical axis. The contours for phon units of 0 through 110 are almost equidistant, ranging from 70 to 135 decibels at 20Hz, 25 to 115 decibels at 100Hz, 0 to 110 decibels at 1 kilohertz and 15 to 120 decibels at 10 kilohertz for phon units of 0 to 110. The range is most compressed near the low end of the spectrum, shows a broad valley of about 5 decibels at 400 to 500 hertz, a heavy, sharp dip of approximately 10 decibels at 4 kilohertz, a 5 decibel increase around 8 kilohertz and a quite sharp dip of 5 to 10 decibels at 13 to 15 kilohertz. The contours wiggle and rise wildly after the 12 to 15 kilohertz mark, indicating beginning of the stopband for our ears.