Sam Harris asked about the interpretation of "slightly louder".
Maybe this only muddies the water, but in the Hi-Fi/Audio world,
I recall that "slightly" louder is 3 dB in engineering measurement
terms, and that "twice as loud" is 6 dB.  I seem to recall that from
somewhere in the distant past; perhaps someone here can confirm or
correct that.  Similarly I recall that 3 dB was the minimum detectable
volume change the human ear can detect.

Remembering that the dB progression is logarithmic, maybe someone can
correlate inches of H20 to dB; then you could calibrate to an audibly
measurable scale.

Consider that the dynamic range of a CD is on the order of 90 dB or so,
and that is a reasonable approximation of totally silent to near
concert hall levels, is it unreasonable that a manually played piano
should have/be capable of an approximately similar range, and if so,
what expectation is reasonable to achieve for a reproducing instrument?
60 dB or more would somehow not surprise me at all.

Also, remember that in an audio system, to double the volume, you need
4 times the amplifier power!  So keep that in mind as you start thinking
about dynamic range!

Harvey Chao