Reeds: The Internal Geometry of Hard D vs Back D
Originally published in The Pipers' Review 2009

Thereís a well-known penalty for an increasingly easy hard D: a weak back D. In addition to the exterior adjustments of scrape and length trimming, thereís an interior geometric factor that can be manipulated and often with very little change to the blades. The key is the change of curvature at the tip of the blades, a curve that in traditionally made reeds gradually increases from the lips toward the staple.

If the curvature remains almost as shallow as at the lips, too far along the blade toward the staple, hard D becomes easier while back D becomes more unstable. Since reeds often flatten especially in the tip end a bit as they age, this is an important reasn that back Dís can weaken over time. This reed had aged into a sinking back D, along with a very easy hard D, and the photo of the inside of a blade shows why. The shadow shows that the entire top 1/3 of the blade, at the right side of the photo, has the same shallow curve as the lips.

The traditional remedy is to trim the lips or move the bridle higher. But with so much of the end of the blade having relaxed into the same very low curvature, adjustment from the outside may not have revived it. So even though this blade was already scraped thin, it was worth trying to create a greater interior arch just below the lips by sanding the interior on a curved cylinder. Only a tiny amount of cane needs to be removed, little enough that often the blade is not weakened too much.

A wide cylinder that is only slightly more curved than the lips is chosen, medium to fine sandpaper placed on it indicated by the dotted line in the diagram, and the upper end of the reed interior is sanded on it. Approach the lips no closer than about 3/16Ē or 4-5 mm. The photograph shows the region to be sanded. If the particular blade has more curvature around the middle of the scrape, cut your sanding length in half or so, still beginning near the lips, but reaching only a short distance toward the staple. If a cylinder is used too far toward the tails, it begins to cut into the edges of the reed and cause leaking.

If the low interior curvature has also caused the octave to go flat, you can use a 2nd or 3rd cylinder, increasingly narrow, to sand increasingly curved zones closer and closer to the position of the staple eye. This will open up more airspace inside the head, flattening the 1st octave and sharpening the 2nd octave a bit.

The change in this blade was so little that itís difficult to photograph clearly, so Iíve overlaid black lines to illustrate the area that was sanded. You may be able to see in this view that the very tip of the reed at right is a bit smoother than the area under the lines, and that there is just a little more shadow visible under the upper edge (in the photo) of the blade indicating that the blade has a small increase of curvature.

When this reed was reassembled, the back D was strong again, without any trimming of the lips, and the hard D was not compromised.

Both blades need to be examined. Itís not unusual for one blade to be more flattened than the other, and in that case only the flatter blade may need treatment.

If instead back D is solid while hard D is very difficult to sound, the upper interior is often too curved close to the lips.

If the edges and corners are reasonably thick, sanding the very tip on a slightly wider cylinder may ease the hard D a little. Otherwise the blades can be wet, and the upper ¼Ē of the tip firmly bound onto a wider cylinder, and allowed to dry for several days.

David Daye
Cuyahoga Falls, OH