Here is a short paragraph describing the Fletcher-Munson 
curves, the type of answer the marker was looking for when grading the quizzes:

        "The Fletcher-Munson equal loudness curves depict the average 
sensitivity of the human ear for various frequencies.  The graph plots sound 
pressure level (in dB) against frequency (in Hz).  The graph shows that the 
ear does not perceive all frequencies at the same loudness; lower and higher 
frequencies sound much quieter than mid-range frequencies of equal sound 
pressure level.  Therefore, in order to be perceived as being equally loud as 
a midrange signal, a low signal must actually have a higher sound pressure 
level.
	The Fletcher-Munson curves are important to consider 
when recording, since 
well-balanced mixes audited at high sound pressure levels will be perceived as 
bass and treble deficient when audited at lower levels."