Unison coupling

[Richard Brekne]

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Kent .. you might find Harold Conklins article "Design and tone in the
mechanoacoustic piano"... parts 1,2, and 3 available from  JASA.. interesting.
Also the articles in "5 Lectures on the Acoustics of Pianos" edited by Askenfelt
has a whole bunch on this. This second is tough to get ahold of tho. The first
can be ordered from

sosmail@carl.org

Richard Brekne
I.C.P.T.G.  N.P.T.F.
Bergen, Norway


Kent Swafford wrote:

> Ron Nossaman wrote:
>
> > Not that I'd do any better gathering data myself, but at least I
> >could determine first hand why I was still confused. I played around with
> >an old Strobo-Con in a killer octave a while back to see what I could see.
> >With one string, the pitch went up on the attack and stayed high for about
> >1.5 seconds, then drifted down a bit. Adding the second string, the attack
> >pitch height was the same, and the decal pitch was the same (as nearly as I
> >could tell), but the dwell time at the higher pitch went to about a second.
> >Adding the third string left the pitches the same again, but the high dwell
> >fell to a little over a half second. No real measurement on the time
> >durations, just tuner's relative time sense. This was with one of my
> >boards, and I haven't gotten to try it on anything else, so there's nothing
> >remotely informative here, though I was surprised at what I saw.
>
> I have no information to offer as to _why_ "unison coupling" exists, but
> I thought I would chime in here that my observations match yours, Ron,
> fairly closely. The phenomenon exists on many pianos.
>
> I describe it like this:
>
> The frequency of vibrating piano strings is not stable, but tends to
> lower as the string continues to sound.
>
> In electronic music, the graphic representation of the changing
> amplitude, frequency, or other measurable character of a tone is known as
> the "envelope."
>
> My observation then is that strings sounding together have little or no
> frequency that is not present when the strings are sounding singly. It
> appears rather that the envelope, that is, the change in frequency of the
> vibrating strings over time, is accelerated for some reason when the
> strings sound together. In other words, the flatness observed when
> strings sound together may just be because the strings get to the flat
> part of the envelope faster, fast enough that the sharper part of the
> envelope goes by fast enough that it can be missed.
>
> For what it is worth, an explanation for why this phenomenon was not
> observed and described until just a few years ago might be that, if the
> phenomenon occurs throughout the scale, as I believe it does, then, if
> the temperament throughout the scale of the piano is tuned with single
> strings, then the temperament of the piano with all strings of the
> unisons sounding will simply be "shifted" slightly flat with few ill
> effects to the temperament.
>
> It also is possible that the apparent frequency shift caused by unison
> coupling is as small or smaller than the normal frequency shift caused by
> loud and soft playing, perhaps making the effect negligible in practice.
>
> Kent Swafford

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