Making Self-Resonating Chimes

Last modified 1-14-2010

THE CHIME CAN BE ITS OWN AIR RESONATOR
Fuller Tone for Wind Chimes, Tubulons & Other Tubular Chimes
 

A few years ago Experimental Musical Instruments put out a book on making wind chimes. In that book I included a photo and a few words about a set of chimes in which the body of air enclosed in tube is tuned to resonate with the chime tone. This gives the chime a fuller and louder sound, particularly in low-pitched chimes. There’s a recording of the resonated chime set included on the audio CD that accompanies the book. I didn’t include a full description of how to make such a thing though; I was trying to keep the book simple and friendly and accessible, and I was afraid that the attempt to describe the air-tuning process would end up long and complicated and off-putting to most readers. Since the book came out, a couple of readers have gotten in touch with me to say they loved the sound of that chime on the CD, asking why I didn’t give a good explanation for how to make it.

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The big air resonated chimes from the wind chimes book’s audio CD

In this post, I’ll talk about how it can be done. First, under the heading Paint-by-Numbers Approach below, I’ll give a recipe for making a tuned set with all necessary dimensions provided. This will allow you to make a set of resonance-tuned chimes without having to get into much detail or theorizing. Second, under the heading Design-Your-Own Approach, I’ll explain the theory, allowing more ambitious readers to make resonated chimes of any desired size and tuning.


Very Brief Notes on Mounting Options

For either approach, once you’ve got the individual chimes cut and tuned, you’ll have a choice how to mount them. I won’t go into detail on this question (which can get quite involved), but here, briefly, are the two most likely options:

1) Mounting as wind chimes. Support the chimes by cords attached at a distance of 22% of overall chime length from one end. (In the Paint-by-Numbers approach, this is the “support point” indicated in the chart). Beware of a potential problem: if you run the cord through support holes drilled through the tube (as is often done with wind chimes), the small open holes may kill or detune the air resonance. Three ways to avoid this problem: a) if possible, locate the support holes in a non-resonating part of the tube, as explained in the Design-Your-Own section below. b) back-fill the holes with epoxy or other suitable filler after stringing so there’s strictly no air opening remaining. c) Find a way to support without drilling. For instance, the support cords can be taped or glued to the tube as long as the amounts of tape or adhesive are small and well enough adhered to minimize damping effects. For complete information on support systems for wind chimes, look to our book Wind Chimes: Design and Construction.

2) Mounting as a xylophone-style bar-percussion instrument. Rest the chimes on a pair of padded supports which contact the chimes at a distance of 22% of overall chime length from each end (again, the “support point” in the Paint-by-Numbers chart). Position the chimes with the holes facing upward. This way, as the chime vibrates after being struck from above, air is pushed in and out of the holes. This helps excite the air column. You’ll have many options for how to keep the chimes in place there; avoid any methods that involve drilling open holes through them. For complete information on mounting bar percussion, look to our book Making Marimbas and Other Bar Percussion Instruments.


The Paint-by-Numbers Approach

The basic idea here is that we’re going to make a set of four chimes in which the column of air inside each chime is tuned to resonate at the same pitch as the chime itself (this is explained more fully under Design-Your-Own below). The tuning of the air column will be done by drilling holes in the side of the chime, similar to the tone holes in a flute.

As you can see in the drawing below, each chime has two pairs of holes located 90 degrees apart around the circumference. The chart shows the locations of these pairs of holes, as well as the other dimensional information you’ll need to recreate the EMT chimes.

Important: For the dimensions given below to work, the chimes need to be made from a particular kind of tubing. As a matter of practicality, I’ve chosen a widely available, very affordable type of tubing, namely: 1″ electrical metal tubing (EMT), available in hardware stores everywhere. The particular make of EMT Tubing I used for the prototype is labeled “Allied E-Z Pull” in lettering along the side of the tube. If you can get this exact type, do so; that will increase the chances that both the air and metal tunings will match closely enough and your chime will work like the prototype. There is a possibility that 1″ EMT tubing from other manufacturers will have slightly different properties, with the result that the tunings may not match up properly. If you think this may be the case, read the fuller explanation in the Design-Your-Own Approach below. This will give you an idea how to check the air and metal pitches and to adjust the tuning as necessary. arc-paintbynumbers

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The Paint-by-Numbers chimes plus one
(in this clip, one additional higher-pitched chime can be heard)


The Design-Your-Own Approach

Preliminaries

We begin by reviewing the underlying concept. Think of the hollow cylindrical form of a tubular chime. That form encloses a column of air like that in a flute. One can imagine that this flute-like air column might resonate in response to the vibration of the metal walls enclosing it, and contribute to the sound. But in most cases it does not, or at least not enough to be noticeable. The reason is that the natural frequencies of the enclosed body of air usually don’t relate to the pitch of the chime. However, by deliberately tuning the air column to match the metal chiming pitch, you can bring the resonance into play for a stronger, fuller chime tone.

This added air resonance has the potential to be particularly valuable for tubular chime instruments, because cylindrical metal tubes produce prominent inharmonic overtones, which contribute a clangy sort of sound. An air column tuned to the chime’s fundamental will not respond to the inharmonic overtones; it enriches the fundamental only. The result is fuller fundamental with clangy overtones less noticeable. This is less important in small, high-pitched chimes, but in large, low-pitched chimes it can make quite a difference.

In tubular metal chimes, it generally turns out that the resonance frequency of the air column is well below the metal chiming frequency. The job, then, is to raise the air frequency to match the chime frequency. I, and others, have tried a couple of different methods for doing this. The most effective seems to be the one that involves drilling holes along the side of the tube. The holes function like the tone holes in a flute, allowing the tube to function, air-resonance-wise, as if it were shorter than its actual length. The trick is to get the hole locations and sizes just right to bring the air tone up to where it matches the metal chiming pitch.

But before drilling a bunch of holes in any tuned chime we should ask, will these holes alter the metal’s chiming pitch? They do. The effect is fairly small as long as the holes are small and there aren’t too many of them. The detuning will almost always be in the direction of lowering the chiming pitch, correctable if need be by very slightly shortening the tube (cutting or grinding a bit off of one or both tube ends).

And there’s another, more irregular effect: A hole drilled in the side of a tube will very slightly weaken the tube for one direction of vibration and not another direction. This gives the tube a dual pitch effect, depending on the direction it’s struck from. To get around this problem, instead of drilling single holes along one side of the tube, we’ll drill smaller holes in pairs located about 90 degrees apart around the tube’s circumference. Although it’s not intuitively obvious, this spacing distributes any weakening roughly evenly around the tube for different directions of vibration, minimizing dual pitch effects. (Dual pitch effects, which aren’t always undesirable, are discussed more fully in both our wind chimes book and our bar percussion book.)


More Preliminaries: Principles for Tone Hole Sizing and Location

To understand how hole location and sizing works, look at the drawing below. arc-designyourown


In the drawing you can see that the chime has portion of its length that is free of toneholes. It is in this area that the resonance takes place. As for the remaining portion of the tube toward the other end, we don’t want potentially conflicting resonance effects arising there, so the drawing shows extra pairs of holes is drilled in that section to break it up and separate it acoustically from the main resonating section.

The natural frequency of the resonating section is controlled by two things: its length, and the size of the holes, especially the pair of holes that mark the end of the resonating section (I’m simplifying a bit here to keep the explanation simple). The rule is:

The shorter the resonating section and/or the larger the holes, the higher the resonance pitch.

To do the tuning process, you need to be able to check both the chiming metal pitch and the air column’s pitch. To hear the metal chiming pitch, hold the chime loosely between thumb and forefinger anywhere around 22% of overall length from one end, and strike near the center. To hear the air pitch, blow flute-style over the edge of the end of the tube, or over the edge of one of the holes. Unless you have a wonderful flute embouchure, this air-tone will be a little vague; it may even produce a dual pitch. Hearing it clearly is a challenge, but it’s good listening practice.


Making and Tuning the Chimes

Start by tuning the chime to the intended metal pitch. Do this by length: cut it a little longer than needed so that the pitch is too low, and bring it up to pitch by shortening. (For a fuller description of this, including calculating methods that can save a lot of ear-work, once again see either the Making Marimbas book or the Wind Chimes book.)  Deliberately tune the chime a tiny bit high – maybe a fifth of a semitone — to allow for the slight drop that comes with hole drilling.

Now it’s time to decide where to drill the toneholes to get the correct air column pitch.  Unfortunately, the answer varies widely depending on the type and diameter of chime tubing you’re using, and the overall pitch range. It may be that the resonating section needs to extend over most of the length of the tube (this is more likely to be the case with relatively low-pitched chimes). This would mean that the holes should be drilled near one end, leaving most of the tube unobstructed by tone holes. In other cases the resonating section may need to be quite short, extending over less than half the tube length (this is more likely with higher pitches). In that case, only this shorter section well be left hole-free, while the remainder of the tube is broken up by suitably spaced pairs of tone holes.

So how do you know where to drill for your particular chime? Until you get experience with your chosen tubing material, trial and error will be the key. A sensible approach is to start by drilling a pair of fairly small holes near one end, leaving most of the tube hole-free to function as the resonating section. Using the pitch-checking methods described above, check the air pitch against the metal pitch by tapping and then blowing. Most likely the air pitch will still be too low, so drill another pair of small holes a little farther along the tube to shorten the resonating section and get a higher air pitch. Repeat until the air pitch is just a little below the chime pitch. Then see if you can bring the air up to the desired pitch by enlarging this last pair of holes. Enlarging the previous pair of holes as well will also raise the pitch a bit.

This all sounds reasonably systematic, but in practice the procedure is often problematic. The air pitches may be vague and hard to identify and match with the chiming pitch. There may also be problems with the slight flattening of the chiming pitch as the metal is weakened with each additional tone hole drilled. If the chiming pitch has dropped too low, you can bring it back up by shortening the overall chime length, cutting or grinding a tiny bit off at the non-air-resonance end. Lowering the chiming pitch if it remains too high is more difficult, but you may be able to bring it down a tiny bit by very carefully and evenly cutting a shallow saw kerf all the way around the circumference of the chime at the middle (don’t cut all the way through the metal wall!). After you’ve tuned a few chimes this way, your sense of the variables at play will improve.

If you want to double-check, you can tell when the air resonance is in tune and doing its job by placing adhesive tape stretched tight over the pair of holes at the end of the resonating section.  The chime should sound weaker with the tape on; fuller and stronger with it off.

An additional note: For chimes in which the air resonating section is less than half the tube length, you can give the chime a tuned resonating section at each end. An advantage of this is that you can get away with fewer holes (less need to drill multiple holes to break up the long non-resonating section of the tube, because with a resonating section on each end, there is less non-resonating section). This is how the higher-pitched chimes in the Paint-by-Numbers Approach earlier in this posting are designed, and you can get an idea how it might look by looking at the drawing there. Note that this approach leaves a non-resonating section in the middle o the chime. If this section is more than a few inches long, it may be helpful to add one or more a pairs of tone holes in there to break it up.  


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