Thinklabs One User Manual page 18

Psychoacoustics and Stethoscope Volume
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.
When you auscultate, increase stethoscope volume and low pitched sounds will be
more prominent. Turn down volume and bass diminishes faster than treble. This
is psychoacoustic, not real. It's also why an acoustic stethoscope is so unsuited
for low-pitched heart sounds!
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