[pianotech] ETD stretch vs pure (octaves)

[Scott Jackson]

Yes Duaine, I agree. But....
If you then measure the frequency of C5, it will be more than twice the 
frequency of C4. That is why the octave is said to be "stretched": 
because if you put C5 at exactly twice the frequency of C4, it won't 
seem to be pure to your ear.

And further....
If we only had to make C5 sound good with C4, it wouldn't be so hard, 
but we have C1 through C8 to consider. If we make C6 sound 'pure' to C5, 
and C7 sound pure to C6, then it is unlikely that C6 and C7 will sound 
'pure' with C4. Thus we stretch slightly the C4-C5, and C5-C6, and 
C6-C7, to give us more chance of C7 sounding 'pure' with C4.

Scott Jackson


Duaine & Laura Hechler wrote:
> To me, a pure octave is when you play C4 and C5 together, and hear no 
> beats or wave-y-ness.
> Just like tuning unisons of a tri-string and they are in tune when you
> hear no beats or waves, except they are an octave apart.
>