Subtracted from the total. If the difference between the total and the source to be subtracted is more than three, one selects the difference from the bottom section of the graphic and follows the appropriate line upward to the curve to determine the value to be substracted from the total sound pressure level. Example: Find the resulting sound pressure level when an 85 dB machine is removed from a 90dB environment. The difference between the two is 5dB, so one starts at the bottom of the graph and moves upward to the curved section to obtain a value of 1.6 dB. The total value is then 90 – 1.6 or 88.4 dB.