Our ears do not perceive all sounds equally at the various frequencies or sound intensities. The chart below (Figure A) is an equal loudness contour chart. The sound levels for a particular sound as defined by the level at 1000 Hz will find the same for any given frequency along the curve.

For example a 20 decibel sound at 1000 Hz would be perceived as the same sound level of 50 decibels at 100 Hz. This indicates that our ears are less sensitive to low frequency sounds than mid to high frequencies. Very important when it comes to noise control.

Figure A