Easy to make, wonderful sound. If you decide to make them, you can choose whether to keep it simple or take a more sophisticated approach.
Article 1: A Simple Approach
I have recently made a number of simple flat gongs out of aluminum disks. If you can get your hands on some aluminum disks of suitable size, these gongs are wonderfully quick and simple to make, and they really sound lovely. While you can make excellent-sounding gongs with a minimum of specialized skill and knowledge, it’s also possible, and quite rewarding, to take a more sophisticated approach in search of a more refined tone quality. In this first of a pair of articles to be posted here I’ll describe the simple approach. In a following article I’ll get into more sophisticated stuff. (As a teaser for the second article, I’ll tell you now that the more sophisticated approach has to do with listening for the pitches of the different modes of vibration in hopes of getting the overtones within the tone to line up in coherent relationships.)
Important disclaimer: These notes are not about the making of traditional Asian gongs. That time-honored discipline is well beyond the reach of this writing. Rather, these notes concern the making of very simple disk gongs with minimal hammering required.
THE SIMPLE APPROACH
Here’s the basic idea: a flat aluminum disk of suitable thickness and diameter, hung form the right suspension points and struck at the center with the right sort of mallet, makes a lovely tone. It has many modes of vibration and thus many different frequencies within the tone, but when played as just described one of these modes stands out as the defining tone, giving it a clear sense of pitch, while the other modes blend in to help create the overall tone quality. If you’re happy with the pitch the gong is making, then no more need be done. If you want to tune the gong to another pitch, you can do so by hammering a small nipple or boss at the center to raise the pitch, while leaving the rest of the gong flat. All this will be described as we go along.
Where to get aluminum disks:
My original supply came from a metal scrap yard. I don’t know if I was lucky to find them there, or if they turn up at such places quite regularly. If you don’t find them there, aluminum disks are available from metals suppliers online (at high prices). You can also purchase sheet aluminum from metals suppliers and cut the disks yourself using one of several power tools that are up to the task, or have them custom-cut. (Metal-working tools are best, but because aluminum is the softest of commonly used metals, woodworking tools such as a woodworking bandsaw with the right blade can manage the work.)
Suitable aluminum alloys, suitable sizes:
The most common and affordable aluminum alloy is referred to as 6061; it works fine for this purpose. Thicknesses in the range of 3/32” to 1/8” are most suitable for flat gongs in small-to-medium sizes. Suitable diameters range between about 20” to about 6”. The 1/8” thickness is best for gongs over about 11” inches in diameter, and 3/32” for smaller. These sizes are suitable for gongs for pitches from about C3 (C below middle C) to G5. It’s possible to go lower with thicker material and larger diameters, but this takes us beyond the “simple gong making” we’re talking about in this article. In short, we’re focusing on this range because disks of this size and material are typically not too difficult to find or fabricate, they’re easy to work with, they’re neither so thin as to be easily damaged with heavy use nor so thick as to require too much effort in hammering, and they sound very well.
Where to drill the suspension holes:
Typically these gongs will be suspended on cords, two cords per gong. The cords pass through small holes drilled in the gong (about 1/8” diameter) at points roughly corresponding to 10 o’clock and 2 o’clock on an imaginary clock face, each located about 1/3 of the radius from the edge. To minimize fraying of the cords, slightly bevel the edges of the holes. A countersink bit works well for this.
The reasoning behind these hole locations: the gong has several modes of vibration producing different pitches. One of these is most important for us in that it’s the one we’d like to dominate the sound and define the perceived pitch. Accordingly, we want to suspend the gong in a way that imposes minimal damping on this particular mode. It happens that this mode has node – an area of minimal vibration — in the shape of a ring at a distance of about 1/3 of the diameter in from the edge, so we place the holes within that ring for minimal damping.
Tie loops of fine string or strong thread through the holes to create a temporary way of holding the gong for pitch-testing during the tuning process. You may replace these with something stronger later, but for the moment a very fine string is better because thick strings would get in the way and create unevenness during hammering.
The mallets you use will make a huge difference in the sound of the gong. The preferred sound comes from a moderately soft mallet head, and it turns out that there’s a commonplace item that is perfect for these gongs: superballs. The 1¼” size, or anything close to that, will work well for smaller gongs, and the 1¾” size is good for larger ones. Select a wooden dowel or similar material for the handle; drill a hole very slightly smaller than the diameter of the dowel through the superball, and press-fit the ball onto the mallet. If needed, add a drop of glue such as superglue (cyanoacrylate) to keep it in place.
Tuning (hammering the nipple):
Your original flat gong, without any hammering, may produce a pitch and tone quality that you are happy with. If that’s the case, no hammering is needed and you can skip to the mounting section below. In my experience, however, a bit of center-hammering improves the tone quality, so even if you’re not concerned about the pitch of the gong, I recommend hammering a small nipple. And of course if you are concerned about pitch, then hammering is the process by which you tune the gong to the intended pitch. You can only tune upward, so the original unhammered gong must be below the desired pitch, not above it. If you want a lower pitch, you’ll need to start with a larger disk. By hammering, you can raise the gong pitch as much as a major third; more is possible but that tends to compromise tone quality and also requires more work. The upshot is: the ideal situation is to start with a flat, untuned gong which is a little below your intended pitch. Ideally this would be about a major second or minor third below, but in any case not more than about a major third below.
Here now is a description of the hammering process. Remember once again that the procedure described here is not a traditional gong-making process at all; it’s just an approach that allows making these very simple aluminum disk gongs without too much work.
To hammer the nipple, you need a flat surface that you can rest the disk on, which has a hollow that you can hammer the nipple into. Since the rest of the gong surface will remain flat, the hole diameter can be much smaller than the gong diameter – typically between about 15% and 18% of gong diameter. For this surface you can use a piece of plywood, minimum 1/2” thick and about 18” square or larger with a circular hole cut out. You can use a circle cutter for this, and because you might at times wish to hammer nipples of different sizes, you may choose to cut not one but several holes of different sizes in your plywood sheet ranging from a little over an inch for very small gongs up to about three inches in diameter for large ones. Before making each hole cut, use a compass or pencil and string to draw a series of concentric rings around the hole-to-be, spaced not more than an inch apart, up to the diameter of the largest gong you expect to make. These will allow easy centering disks of different sizes over the hole when hammering time comes.
You’ll also need to locate the center of the flat disk, if it’s not already somehow marked. There are various ways to do this; one simple method is to take half the diameter and measure that distance from several points along the side to locate the point in the center at which the measurements agree.
Rather than hammering directly on the aluminum, you may find it preferable to use a thick hardwood dowel of about 8” long with one end rounded. (A section of broomstick works well.) Place the rounded end of the dowel at the disk’s center point and hammer the end of the dowel. For smaller gongs calling for a smaller nipple, you can substitute something like a 1/2” or 3/8” steel rod with rounded end.
Hearing protection in the form of ear plugs or ear muffs is recommended during the hammering process. With the flat disk centered over the hole and the dowel positioned at the center point, begin hammering. With 3/32” aluminum, make your strokes moderately firm but not hard. With thicker material, you can strike a bit harder. After each stroke or two, lift the gong by its suspension cords and strike at the center with a suitable mallet to test the pitch of the gong. The pitch rises quite fast with the first two or three strokes, then slower as you continue. Repeat hammering and testing until the gong has risen to the desired pitch. Try not to overshoot and bring the gong up too high in pitch: while it’s possible to turn the gong over and carefully hammer back the other way to bring the pitch back down, too much of this will tend to create irregularities in the nipple shape which lead to beating effects (a slight waffling in pitch or volume) in the finished gong.
At this point you may need to replace the fine string used for suspension during with something more durable. Make a loop of cord through each hole large enough to accommodate whatever you’ll be suspending from. You can suspend the gongs from whatever you find suitable for the purpose: hang them from a convenient tree branch, use microphone boom stands, or design a nice frame. But here’s an important consideration: when the gong is suspended, the suspension cords should not lie flat against the surface of the gong. If they do, they will rattle when the gong is played. To avoid this, the cords should angle upward and out from the suspension hole. Typically this means that whatever they’re hanging from – whatever the loop is looped over – should not be too skinny; it should be wide enough to spread the top of the looped cord out a couple of inches.
That’s it. Once mounted, the gong is done. And you’ll find that while a single gong is nice, a set of gongs tuned to several different is that much more delightful still.
Following this essay I’ll be posting one which describes a more sophisticated approach to the tuning of disk gongs, taking into account the relationships between the overtones which affect the tone quality in subtle but profound ways. If that interests you, stay tuned.