Our ears don't have the same sensitivity for each frequency. The lower the frequency, the less sensitive, and the same goes for higher frequencies. The midrange, which is around 1 kHz, is the most sensitive range. This is where most of the human voice is. Scientists have measured the hearing sensitivity for different frequencies of thousands of people. The cross-section of these measurements is called the A-curve. Applying this curve is called A-weighting. The blue line in the graph below is the curve which represents the sensitivity of the human ear. As you can see, the bass frequencies (below 100 Hz) are seriously down the line; 20 dB or more. A drop of 10 dB means amplifier power needs to increase tenfold to achieve the same apparent volume. 20 dB means one-hundredfold. That's why bass guitar is so much more power hungry than non-bass instruments.
© Joris van den Heuvel 2001-2009