As you adjust stethoscope volume, you will find that you hear low pitched sounds better when the volume is higher, even if you do not change filters or diaphragm pressure. This is due to a psychoacoustic phenomenon discovered by Fletcher & Munson in 1933. 

    The chart shows that as frequency increases (increasing X axis), the ear requires less and less volume to perceive the sound at "equal loudness." Going the other way, from higher to lower frequency, in order to hear a low frequency with the same sense of loudness as a higher frequency, it is necessary to "turn up" the volume as you get lower in frequency. Looking at the lowest red curve, a sound at 1000Hz and 0dB (A), will sound just as loud as a 100Hz sound that is 20dB louder (B).

   Looking at the curves higher up on the chart, notice how they are flatter? What that means is that when sounds are louder, the ear perceives lower and higher pitched sounds more equally.

    What does this mean? Quite simply, your ears don't hear bass very well when the sound is quiet, but as you turn up the volume, your ear begins to hear MORE bass. Turn up the music and you hear more bass! Now you know why sub-woofers are so large.