Elementary Mechanics of Reeds
by Paul Bickerstaff
Reeds! To the piper they bring both joy and frustration. Joy, when in expert hands their primitive tones evoke passions and bring the deeds of yesteryear to life. Frustration, when their stubborn and sensitive personalities insist on not conforming.
In this article I will try and explain the basic physical principles governing reed behavior. I will direct my attentions to the double-bladed reeds used in the pipe chanter, which are the most complicated. In truth, because of their complexity, the details of reeds are not well understood. However, some elementary principles can be formulated and can serve to guide the piper in his quest for that elusive sound that satisfies.
A reed creates sound by vibrating. As it vibrates it causes rhythmic compressions in the air. These travel away from the reed in the manner of a wave and this wave of successive compressions is sound. The faster the vibrations the higher the pitch, the slower the vibrations the lower the pitch.
To understand the vibrations of a reed, let us consider first a simpler problem. Take a ruler, or perhaps a blunt knife, and hold one end firmly on the edge of a table so that the other extends out into the air. Now give the free end a twang. The ruler will vibrate and you will hear a sound. (Listen for a hum, not rattles occurring because the ruler is bouncing about on the table by not being held firm enough or too far back from the edge. If the vibrations are too slow the sound may be so low in pitch that it is below the audible range. Don't worry; watch how fast it vibrates and continue.) Now withdraw the ruler a little so that it extends less. Twang it again. The sound will have a higher pitch from faster vibrations. Experiment a little; you will find that the shorter the free length the higher the pitch. Why is this so? To further cast light on the matter you might try fastening a coin on the end with a rubber band. To have a noticeable effect the coin will have to be a fairly large one in comparison with the mass of the ruler. Any heavy compact object will do. Now give the ruler a twang again. You will notice that the pitch of the sound will have decreased. (Don't be confused by rattles due to loose binding of the coin. Listen to the fundamental tone.) So the important thing is not so much the length but the distribution of mass and how far it is from the pivotal point. This is important when dealing with a non-uniform thing such as the blade of a reed. The more mass that is located far from the pivotal point the harder it is to make the end move, and so the slower it moves under a given force. It is just like trying to twirl a stick around one's head and then trying to do the same thing with a sledgehammer. The sledgehammer has a large weight at the end, so is harder to swing and we end up not being able to swing it as fast --- or more to the point it takes us longer to pick up speed. Thus a reed blade with a heavy tip vibrates slower and has a lower pitch than one with a thinner, lighter tip. Another factor governing the pitch is the amount of spring in the cane. The more resilient it is, the higher the pitch. This is easy enough to understand; a stiff spring will bounce back faster than a pliable one. In the case of a reed though it is not so much the quality of the cane, (though that is variable from reed to reed and its natural resilience is affected by things like humidity) but the thickness of the cane at the base of the reed (where it is tied onto the staple). Just as a thick piece of wood is harder to bend than a thin piece and will spring back into place faster, so a reed with a thick base will vibrate faster than one with a thinner base. Thus the amount of wood in the base of the reed is affecting both the spring in the blade, and the overall mass distribution of wood in the blade. But both factors will tend to lower the reed pitch as wood from the base is scraped away.
A double-bladed reed, as in the uilleann pipe, is more complicated, though, than a simple ruler. The blades are tied at one end to a staple making them curve open and they face each other, meeting at their edges, which restricts the motion at those edges. The main motion of the blades is therefore in the center of the tips and some flexing occurs as they vibrate.
Any object set vibrating will exhibit overtones as well as the fundamental pitch. In the case of simple shapes with one dimension much longer than the others, such as a stretched string (as in a piano or violin) or long narrow tube, the overtones are closely harmonic, but with drumskins, bells and plates they are not. Much of the art of making bells is to get the main overtones so that they correspond to musical intervals and are not discordant. With a drum we don't bother as they are used for rhythm rather than melody.
A reed resembles a plate more than a string. Its overtones are far from harmonic and a mouth-blown reed can exhibit quite raucous sounds. Basically, the overtones correspond to vibrations which pivot about both the base of the reed and the reed edges. Thus narrow reeds are sharper in pitch than wider reeds just as shorter reeds are sharper than longer reeds. (Hence also the need for a good seal at the reed edges.) The relative strength of the overtones can be altered by scraping various parts of the reed, since with each overtone the parts that vibrate the most are different. Greater vibration occurs where the blades are thinner because there is less mass there to be moved. By scraping principally in the area of the tip and center one favors the fundamental mode of vibration which is a simple opening and closing of the tip without buckling of the blades.
The reed alone though does not create the musical tones in your pipe. It is inserted in the end of the chanter, which is basically a column of air which can itself be set in oscillation. This can be demonstrated by partially filling a glass with water and gently striking the rim. The more water in the glass (and hence the shorter the column of air) the higher the pitch of the ringing sound heard. This ringing, or resonating, is a common phenomenon and can, for example, be found in wind chimes made out of hollow tubes. The finger holes in the chanter allow one to vary its effective length.
We can skip the explanation of why the pitch is higher in shorter tubes because it doesn't have much to do with reeds. The essential thing is that the vibrating reed sets the air inside the chanter bore oscillating back and forth. As a compression pulse is generated by the reed, travels up the bore, and is echoed back, an interesting thing happens. The returning pulse of air can affect the motion of the reed blades! (This is because the pulse of high pressure forces the blades to open slightly.) Thus the resulting sound is due to a complicated coupling of the reed and the air in the bores; it is neither the sound of the reed, nor the ringing tube, but a product of the two. Sometimes one will be more dominant than the other. A stiffer reed will be less affected by the air vibrations than a weak reed, and louder echoes will be harder for the reed to force to keep in step than quieter echoes. There will also be multiple echoes. The overall strength of the echoes depends on many things including how well the reed (with its overtones) is matched to the natural sound of the chanter. This coupling of reed and air column produces a more musical tone. Poorly matched overtones of the reed are damped, as the reed responds to the air column it is forcing to oscillate. By the above discussion, though, we see that reed overtones can still be an influence and reed vagaries will be most apparent when the reed is not tuned well to the chanter.
An additional complication in the uilleann pipe reed is the use of a bridle. Its effect is to both shorten the free length of the blades, thus raising the reed pitch, and to squeeze the blades open causing them to curve and gape more. The same effects can be accomplished in other ways by altering the staple design and shape of the reed.
To understand reeds further we would need to delve deeper into their precise workings --- aspects that really require some research. However, the above should provide a few insights. Let me wind up then with a summary of results that work on a wide class of double- bladed reed instruments, including the great Highland bagpipe and the oboe.
Pitch too flat:
Cut a little off the tip of the reed or very slightly narrow the sides. (Narrowing the sides is risky because it can ruin the air-tight seal at the edges of the blades.) Alternatively, push the reed deeper into the chanter socket or untie the blades and shorten the length of the staple.
Pitch too sharp:
Lengthen the reed scrape or thin the existing scrape at the bottom. Alternatively, raise the reed in the chanter socket or retie the blades using a longer staple.
Top notes very dull and thick:
The reed is unresponsive at high frequency. Thin the tip slightly—on certain types of scrape just the very tip of the tip. Narrowing the sides will also brighten and harden the sound as well as sharpen it.
Top notes too bright and inclined to be sharp:
The reed is not well balanced between high and low notes. Make the V of the scrape more U shaped, or even W shaped.
Low notes are difficult and "sticky":
Reed is unresponsive at low frequency. Thin around the bottom of the scrape, particularly at the sides of it, on the shoulders of the reed. On some types of scrape, thin the shoulders a little below the tip. Check for a poorly made reed with one shoulder thicker than the other.
And remember when scraping your reeds not to scrape too much at once. The cane is a natural substance that takes time to fully adjust to the new stresses and strains that result after scraping. Wait overnight and check the reed again the next day before doing that final extra little bit that will make it "just right."
Hopefully, by understanding your reeds the frustration will diminish and the respect increase. Happy piping!