I wrote that the Special 20 was tuned:
A440, "compromised" tuning, same as the Marine Band. In fact the SP20
uses the MB reed plates; hence the cover plate etching: "Special 20
Marine Band".
The compromised tuning means that it is tuned somewhere between "Just"
Intonation and "Equal Temperament".
The term "compromised" has certain implications which I feel should not
be attached to it. In his lengthy and detailed articles covering various
tunings, Pat Missin uses the term "light temperament" to describe the
Marine Band and Special 20 tuning scheme. This may be his own term,
coined to better illustrate the point that the harp is tuned closer to
Just Intonation than to Equal Temperament. To me, "light temperament"
implies a JI harp which has been brought a little closer to 12TET for
the sake of musical flexibility.
However, the general rule of thumb is that JI tuning gives rich chords,
but the single-note playing may suffer some intonation variances.
Conversely, an Equal Temperament harp is said to provide excellent
single-note and melody playing at the expense of some tonal beauty in
the chording. By implication, any harp which claims to do both well
(i.e. MB and SP20) would have to have "compromised" its tuning to some
degree.
Either way, compromised or light, the intended meaning is the same, at
least from my standpoint.
John Balding