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Maybe I've missed something here, but no-one seems to have mentioned frequency
response.
The range of an 88-note piano/keyboard starts at 27.5Hz.
Hz = cycles [or vibrations, if you like] per second.
The lowest bass note (A1) is 27.5Hz, and the top note (C88) sounds at 4186Hz.
What you find with guitar amps/speakers, for example, is that the very lowest frequencies are not catered for simply because the guitar does not play notes as low as that.
If you look at the specs of many amps the lower frequencies may only go down to 50Hz -
this means that the whole of the bottom octave on an 88-note keyboard is not covered
(the 11th note - G11 - sounds at 49Hz)
Now to the treble end.
You may think that because the top note (C88) is 4186Hz, an amp that exceeds this would be ok.
Wrong!
The thing that gives any stringed instrument it's 'character' is the presence of harmonics - see examples shown here.
Illustrated are the first three harmonics, but this goes all the way up to 16.
Beyond that the audibility is negligible.
Each harmonic has a higher frequency than the one that preceded it, so high-end frequency response is equally as important as the low-end in order to ensure sound clarity and 'tone'.
Most adults (your hearing range decreases as you get older) cannot hear frequencies above about 16-18,000Hz, with a maximum of up to about 23,000Hz (in the case of a good piano-tuner, for example!).
So an amp/speaker combo exceeding this should cover the bases. 18-20,000Hz is usual for most decent quality systems.
Incidentally, a dog hears up to about 45,000Hz, a cat 64,000Hz, a mouse 91,000Hz and a porpoise
a whopping 123,000Hz!
I hope this is of some use!
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