Compression

[Eppingr@xxxxxxx]

Just as compression is a key factor in the power and performance of a car
engine, so is it also in a harmonica's power.  Compression, or air-tightness,
will affect the level of response, volume, clarity of tone and even ease in
bending and over-blowing.  Anywhere the various parts parts of a harmonica
are joined together there can be slight surface irregularities which will
allow
some small amount of air to escape.  Taken each by itself, these leaks 
may have little or no noticable effect on the harmonica's performance- many
leaks, in fact, fix themselves as saliva deposits build up and seal them off.
Usually it is only after one has worked over all potential leakage sites that
a noticable improvement may be achieved.  There may be gaps between 
the comb and reedplates (either from a reed cell to the outside or from cell
to cell), between the reedplates and covers, or in the slide assembly of a
chromatic.  Most of the air that escapes from a reed cell, however, is 
around the reed itself, and it is here that one can do the most to improve
compression.

Making sure that screws are tight and parts correctly assembled will help
insure good compression.  Old, beaten covers may leak at the mouthpiece
and need to be re-bent into shape; a slight bow in them will help keep the
midsection in contact with the plates and the plates against the comb.  
Some folks sand the comb (flush-mounted types only) and draw reedplate,
or cement the plates to the comb.  Cement will provide an absolute seal
and, if it is not too flexible, can improve the overall resonance of the
instru-
ment.  The drawback, of course, is that servicing is complicated.  I have
tested various gasket materials but found that, while air leaks were elimin-
ated, the vibration of the reedplate (as opposed to that of the reed tongue)
was dampened, leaving the harmonica dull sounding.

The gap between the reed and its slot is of vital importance in the develop-
ment of compression.  A reed must be offset from the reedplate in order
to start swinging when air is sent by it, but the further away from the plate
it
is adjusted, the more it will leak air when it is at rest and its opposing
reed
is playing.  The free-swinging end of the reed should be adjusted as close
to the reedplate as possible without the reed choking up at one's loudest
volume.  A reed set too far from the plate will be slow to respond at soft
volume and will sound breathy, while a reed set too close to the plate 
will play only at low volume, or not at all.  In addition to this, a reed
should
not be completely straight, but should have a very slight curve to it so that
for the first half or two-thirds of its length from the rivet, its bottom
surface
is flush with the top surface of the reedplate.  The offset, then, is only 
toward the free end of the reed.  Adjusting the reed this way will cut down
on much of the air loss along the sides of the reed.  Any re-shaping of the
reed curvature or major adjustment of the offset is likely to affect the
tuning,
but I would not attempt to retune the reed for a few days, until it has a 
chance to de-tension.

There is an old technique that accordion reed makers sometimes used to
improve the tolerance between the reed tongue and slot.  The top edge of
the slot can be burnished, drawing it in closer to the reed tongue and
thereby providing a tighter seal for the reed.  This is a fairly simple
procedure
if the reeds are left in place, though one can only work about halfway back 
without affecting the reed's offset.  I have occasionally rotated the reeds
as
far to the side as possible, which allows most of the slot to be burnished, 
but this is quite time consuming, especially when the rivets need to be
re-tightened.  Sometimes a reed will be left binding after the slot is
burnished,
but more often than not, a careful centering of the reed will free it.
 Correctly
done, this technique will make a small but noticable improvement in com-
pression.

Any work done on reeds requires care and practice, and without live demon-
strations or, at least, graphic illustrations the techniques described here
may
be somewhat difficult to grasp.  Anyone wishing to try them for the first
while
would do well to work on old, worn out harps, as mishaps are bound to 
occur.

Next time: Tuning.