Hi Nigel,  There's one very good reason why you shouldn't make trumpet
pipes using harmonium reeds: the pipes you make definitely will not
sound like trumpet pipes.

There are two kinds of reeds: free reeds and beating reeds.  Harmonium
(and accordion and harmonica) reeds are free reeds.  That is, the reed
tongue passes back and forth *through* the metal plate to which it is
attached.

Typical organ (and many other musical instrument) reeds do _not_ pass
through the reed plate.  They are slightly wider and longer than the
hole in the plate so they flex until they are almost flat against the
plate (stopping the air flow) and then spring open (allowing air to
flow again).  The increased air flow sucks the reed down again and the
cycle repeats.

Each has its characteristic sound.  Trumpet, clarinet, trombone pipes
on band organs (and church organs) use beating reeds.  The shape of the
resonator effects the harmonics to make each type sound as it should,
e.g., contrast the slightly conical shape of the trumpet pipe to the
cylindrical shape of a clarinet pipe.

That said, there are a few pipe designs that use free reeds but I've
never seen them on a band organ.  One exception is the rank of free
reed pipes on early flute clocks.  You might recall that I am currently
working on an 1840s flute organ made by Damian Dufner in Furtwangen.

While making an operable beating reed pipe is a relatively
straight-forward project, making a working free reed pipe is another
story altogether.  I have found only one company (in Germany) that is
willing to make the eight pipes I need.  Unfortunately, these eight
pipes would cost as much as a whole rank of typical pipes.  So, I have
had to make these pipes myself -- and I can tell you that it is a real
pain.  And, of course, they don't sound anything like the trumpet pipes
on a band organ.

While using existing harmonium reeds seems somewhat easier, it is not
very hard to make free reeds for trumpet pipes.  Although you have to
use the right size materials and assemble them carefully, there are
few really critical dimensions to worry about.

Regards,
Craig Smith
New York State, USA -- Where it's very cold right now.