Let’s talk about metal sheets used as musical sound sources.
You can use metal sheets to make musical instruments in either of two ways:
1. go for the sounds of a sheet in itself, excited directly by percussion, wobble-flexing, edge-bowing, superball friction, or other means
… or …
2. use a sheet as sound-radiator/resonator for an external source of vibration.
I have made musical instruments of both types, and we’ll discuss both in this article. To get us started I’ll dive into some general notes that apply to both.
Sheet Size
Metal sheets of different sizes behave differently. Sheets that are very large may be amazingly reverberant. If a very large sheet can be mounted in a manner that leaves it really free to vibrate without damping, then it may sustain for a very long time. Large sheets were the basis of the classic plate reverbs used in studio recording in years past, with sheets of around 8′ x 4′ suspended from cord-like coil springs to allow free vibration. A transducer in the form of a piezo or electromagnetic driver was placed at one end of the sheet, fed with an audio signal such as an Elvis voice or twangy guitar or whatever it was that was to be reverberated. At the other end of the sheet was another transducer, this one converting the physical vibrations of the sheet back into a now heavily reverberated version of the original signal. The important thing to note here is that in a plate this large, there are so many overlapping resonances throughout the musical frequency range that you can expect the sheet to be reasonably responsive to whatever frequencies you might introduce. With smaller sheets this would not be something you could depend on: the resonance peaks within the audible ranges would tend to be farther apart, especially in the lower and mid ranges where much of the pitch information resides. The resulting sound for the smaller sheet would be unevenly colored by those separate resonances. With a very large sheet you would not have such conspicuous specific frequency peaks within the musical range. (There would, however, be a characteristic plate-reverb sound arising from the biases, however subtle.)
Mounting
A central question for all sheet-metal instruments is how the sheet is held. For maximum reverberation, especially for large sheets, the ideal would be to have the sheet miraculously suspended in space, where it would be entirely free to vibrate with no damping from whatever would otherwise be holding it. Since that’s not feasible, a common method has been to suspend the sheet from cords or cord-like coil springs, as with the plate reverbs. Long coil springs which are not too stiff do noticeably better than inelastic cords. (Elastic cords, if they are strong enough to hold the weight, might do just as well, but they tend to deteriorate over time.)
The late instrument inventor Tom Nunn, a leading exponent of sheet-steel instruments, found a wonderfully clever way to create a compliant and barely-there mounting: he mounted his large sheets on latex balloons. The balloons were sized so that the bottom part of each balloon fit snugly in a small-ish bucket with the upper portion rising above the rim. The metal plate rested atop three or four of these bucketed balloons. This arrangement supports the plate yet leaves it wonderfully free to vibrate and sustain seemingly forever.
In these approaches the intent is to leave the whole sheet as free as possible to do its own thing vibration-wise, with no external encumbrance. Another approach is the opposite: make whatever it is that holds the sheet as hard and solid as possible, so that the soundwaves that are active in the sheet will reflect at the mounting point instead of damping out there. When it comes to allowing free resonance and long sustain, this approach isn’t as good as balloon-mounting, but if done well it can still allow for clear and well-defined resonances. And it has practical advantages: some sort of rigid mounting is likely to be more stable, hassle-free and compact than the free-floating approaches.
How you design the rigid mounting can make a big difference. I won’t go into details here, but I’ll briefly mention a few things. The hardness of whatever holds the sheet is crucial. Any softness or squishiness will diminish reflection and add damping. More variable is the question of how rigidly the mounting elements are held. Sometimes it’s best to have the whole mounting be as heavy and solid as a rock; that guarantees the most reflection. Yet sometimes a somewhat flexible holder can work — imagine, for instance, mounting atop an upright metal rod which has a bit of flex in it. If the holder has any resonance frequencies of its own (as most things that aren’t heavily damped do), those tendencies may work for good or ill. Finally, the location of the mounting point on the sheet can make a big difference. Since it’s difficult in most cases to analyze in advance just how things will play out acoustically within the sheet for any given mounting point, experimentation with different mounting points is the key.
As a practical example of how these mounting and resonance questions might play out in one instance, let’s consider the case of everybody’s favorite musical instrument – I’m speaking, of course, of the wobble-board! (…that is, a metal sheet that makes nice wobble-wobble sounds when held at the edges and flexed by moving it forward and backward). If you were to hold a wobble board in big, soft mitts, you’d be adding too much damping and it would sound poorly. If you hold it in your bare hands, it works fairly well but with less-than-stellar sustain and clarity due to the hand-damping. You can get a better sound – clearer, more reverberant, more sustaining — by placing C-clamps firmly on each side and using these as handles while doing the playing movement. The hard, well-defined holding points create less damping, better-defined resonances and more sustain as the waves within the sheet reflect rather than damping out at the mounting points.
Different Metals
What sort of metal to use? Different metals produce different sound qualities. The most basic distinction is between softer metals and harder metals, with harder ones generally yielding brighter tone qualities and, with proper mounting, longer sustain. The thickness of the sheet – between something like a very thin .010″ and a relatively thick .062″ or more for a larger sheet — is important too in its effect on frequency ranges favored and capacity for sustain. Many makers have worked with stainless steel, which is on the hard side and has excellent reverberant qualities. I have usually worked with spring-tempered steel, which is the hardest of generally available metals. I expect brasses and bronzes would work well but haven’t tried them or seen them being used in this way. Low-carbon steel, also known as mild steel, will generally be less impressive than the harder metals, but it has the advantages of being inexpensive and widely available. While it will be less bright and less sustaining, you can still get some good sounds out of it. Aluminum: hmm… I’ve not worked with it for this sort of thing. In other contexts it tends to have a warmish sound (as opposed to sharp or bright), which can be pleasing.
Pickups
Some sheet metal instruments benefit from the use of pickups. This is about timbre as much as it is about volume. The pickup will often reveal tone qualities beyond what the ear hears unaided, especially in smaller sheets. The two main available pickup technologies are magnetic pickups (the kind used in electric guitars) and piezo pickups (the most common form of contact pickup). Magnetic pickups will work only with sheets of steel: no bronzes or aluminums, and also, important to note, no non-magnetic stainless steels. They have the advantage that they work without touching the sheet, so they don’t inhibit vibration, and they usually yield a clear and balanced tone quality. Piezos must be attached directly to the sheet, and the weight of the piezo and its wire may interfere with the sheet’s vibration, especially with smaller, lighter sheets. And while they sometimes produce a perfectly acceptable tone, they may produce a trashy sound. For both types, where you choose to locate the pickup makes a big difference and is worth experimenting with.
Metal Sheets as Soundboards for External Vibration Sources
Various sorts of vibrating bodies can serve as the source for vibration to be fed into a metal sheet, with the sheet then serving as the sound radiator/resonator. One likely source is strings. Another very promising one is tines, like those on a kalimba. Some makers, led by the late Tom Nunn, have done well with bronze rods brazed directly to a large metal sheet, which can be played by percussion, bowing, or plucking. Another external vibration source might be a piezo or electromagnetic driver as in the old plate reverbs discussed above. Or how about a buzzing electric razor or electric toothbrush pressed against the sheet and slid about? This and other buzzing things make a most interesting and unusual sound as the prevailing resonances in the sheet shift with the movement and varying pressure.
If you’re using vibrating elements such as strings or tines that are not inherent to the sheet itself, a question arises: are the vibrating elements mounted on a separate body, such as a board with strings on it, or are the vibrating elements attached directly to the sheet? There are some challenges involved in attaching directly (which we’ll discuss in a moment), so you might decide it’s better to mount the vibrating elements on a separate board which can then be attached to or pressed against the sheet to transmit the vibration. This can work very nicely, but with one caveat: the vibration may not transmit very strongly, and the resulting sound may lack volume. To get the best transmission, try different touch points or mounting points on the sheet. Also consider the primary direction of vibration of the initial vibrating bodies. Try to orient things so that the direction of vibration is perpendicular to the plane of the sheet. (There’s also the option of adding a pickup to the sheet to increase transmission.)
If the vibrating elements are to be directly attached, there are practical questions as to how to make the attachments. The primary issue is, is the sheet too light and flexible to provide a solid enough base? Obviously, you can’t stretch strings at high tension on a sheet that just bends under the tension. You might be able to attach kalimba tines, since they don’t require external tension, but another important consideration follows: what is the impedance relationship between the sheet and whatever external vibrating bodies may be attached? If the attached element is too heavy and rigid for the sheet, then the sheet will not provide a solid enough foundation for the added element to vibrate well. For example, the bronze rods mentioned above, being heavy-ish, will work well only if the sheet they’re attached to is relatively heavy and rigid. Yet something lighter – think of tines made of bobby pins – might do well even on a lighter sheet. Figuring out in advance how the components will work together is difficult, and sometimes the most practical approach is educated guessing and experimentation.
For strings, there is a workable option which combines essential aspects of direct mounting and mounting on a separate board. The analogy here will be to pianos: In pianos, the cumulative tension of the strings is far greater than what a soundboard could sustain, so the strings are mounted on an external iron frame which takes the tension. They cross the soundboard at the bridge, and thus the vibration is transmitted to the board. For strings on a steel sheet, you can create a strong external frame to take the tension, and run the strings through holes in the sheet creating direct contact which enables the transmission. One way to do this is simply to have the sheet hanging from the strings. This leaves it nicely free to vibrate, but be aware that if the sheet is perpendicular to the strings, then the main direction of vibration will not be the direction that allows the sheet to propagate efficiently to surrounding air, and the result will probably be pretty quiet. For a more efficient design, see the instrument linked here [link coming soon! I haven’t yet posted an informational web page for the instrument referred to].
Another important question is how such attached elements, with their own frequencies, interact acoustically with the resonances of the sheet. This can play out in different ways. Sometimes the natural frequencies of the sheet and those of the attached element work nicely together, enhancing the overall effect. In other cases the interaction may kill or reduce what might otherwise have been effective resonances in the sheet and/or the added element. Similarly, there may be destructive interference between two of the attached elements. It’s often hard to predict how these things will play out, so experimentation, trial and error are important.
We know that soundboards color the sounds that are fed to them. They do this in part by responding more lavishly to some frequencies of frequency ranges than to others. We also know that, for traditional instruments at least, we don’t want to overdo this coloration. We’d like a board with a reasonably balanced frequency response; one that does a decent job of radiating whatever frequencies it is fed; one that does not respond in an exaggerated way to some frequencies while responding too weakly to other nearby frequencies. But soundboards made of sheet metal tend to be peaky in their resonance response curves, which is to say, they’re likely to respond very strongly to some narrowly defined frequency ranges while remaining much less responsive to others nearby. So sheet metal doesn’t seem like a good candidate for soundboard material, unless you happen to like an uneven response. Yet, often enough, the effect can be interesting or attractive. For instance, under the right circumstances a metal soundboard may bring out an interesting sizzle in the tone while, hopefully, not adding too much weirdness in the form of disproportionate biases in the lower or midrange frequencies that define the pitch.
But let’s add another wrinkle. Whatever pronounced resonance peaks may exist in the sheet, you can cause them to shift by flexing the metal. The peak frequencies change depending on how you flex. If you bend the metal while the note sounds, you get a peculiar situation in which the original vibration source (the string or whatever) injects an ongoing steady frequency, while the resonances of the soundboard sheet shift beneath it. There are many ways this can play out given the fluctuating relationships between the steady input and the roaming resonances, but often the outcome is an enchantingly evolving timbre. Another way to create the shifting-resonance effect is, starting with the sheet metal surface positioned roughly horizontal, place a small amount of water on the surface and allow the surface to tilt slightly one way and another. The resulting movement of the water on the surface gives rise to the ambient resonances. We see this in the waterphone and related water-modulated instruments such as this and this. The effect is similar to sheet metal flexing in both sound and underlying acoustics.
Metal Sheets Sounded Directly
Much of what you might want to know about metal sheets played directly (that is, not serving as soundboard for another vibrating body) you will already have gleaned from the foregoing, but one thing we haven’t addressed is the various playing techniques you can bring to them. If you’re not going to sound your sheet by means of some external or attached vibration source, then how will you sound it? Here are some of the excitation methods you can use.
To start with the most obvious: you can use percussion. The choice of mallet is essential. With heavy or light, soft or hard, large or small, you can get very different sounds. In general, lighter, harder and smaller mallets will preferentially excite high frequencies for more edgy sounds; heavier, softer and larger will bring out warmer sounds. Wherever on the sheet you strike, you’re likely to excite multiple modes of vibration, and striking in different locations will excite different blends of modes for different tone qualities. Don’t forget to explore edge-striking from the side, especially with small, hard mallets; this often produces the purest sizzle.
Friction sounds on steel sheets can be wonderfully expressive: sometimes moaning, sometimes, singing, sometimes bellowing, sometimes lowing. People have found superballs (high-bounce balls with good traction) to be effective for the making of friction mallets. The trick is to mount the ball at the end of a very flexible stick or rod, and in playing to hold the handle fairly near the mallet head. Drag the mallet across the surface with varying amounts of pressure. Smaller superballs tend to yield higher-pitched sounds; larger yield lower.
Most suitably mounted metal sheets are responsive to edge-bowing. Violin bows work fine, as do larger bows. Edge-bowing on metal tends to be hard on the bow hairs, so you may consider using the inexpensive mass-produced violin bows that are available (this is easier and less expensive than re-stringing a good bow). You can also make your own bow. Horsetail hair is traditional, but you might be surprised at how readily a homemade bow of very fine monofilament nylon line will work. Don’t forget the rosin.
Edge-scraping with a serrated rod can produce a surprising range of sounds, including some wonderfully odd and bestial ones. The most likely candidates for the rods are threaded rods at various thread-pitches, from very fine, like 2-56 or 4-40, to coarser like 8-32 or 10-24 (those are standard designations for shaft thickness and thread pitch in the American system). You can also make your own serrated rods. Sand a ¼” or ½” steel or aluminum rod crossways with coarse sandpaper for very fine, shallow serrations. File more widely spaced grooves with a rattail file. Very widely spaced, shallow grooves, like 3/8” or 12” apart, can produce surprisingly nice effects. Important: Scraping straight across the edge, perpendicular to the sheet, does not work well. Instead, move the rod across the edge at a very close angle to the sheet, with a preference for the outward stroke. These sounds often work especially well with magnetic pickups.
Tap-Press-Hold-Slide: I don’t know of a pre-existing term for this technique, so that’s my coinage. Strike the sheet with the corner or edge of any heavy, hard metal object, and don’t bounce off but hold it there, pressing, and then perhaps slide around a bit on the surface. I often use the corner of a solid one-inch steel cube cut from a 1×1 steel bar, but many commonly available things will work, such as the head of a hammer or the end of a heavy wrench. The strike excites the vibration in the sheet, and the well defined pressure point, held firm, allows it to sustain clearly. Sliding the striker around on the surface causes the sheet-resonance tones to bend and helps to perpetuate the vibration. The result is an exotic bendy metallic sound. One problem with this technique is that the strike tone – the moment of attack – is often too loud relative to the sustaining tone that follows. In the recording studio, you might be tempted to apply compression.
Minimal History
Many instrument makers have worked with the sounds of metal sheets. I won’t go into any depth here, but I will mention a few notables.
Some of the most impressive work with sheet metal resonators was done by the late Tom Nunn. Tom made a variety of such instruments, the main ones being those of his Space Plates family. In a typical space plate the sheet is stainless steel, usually between about 2′ and 4′ in length and breadth. It was Tom who developed the ingenious balloon-mounting method described earlier. The initial vibrators for the space plate instruments are usually bronze rods of varying lengths brazed to the sheet, rising upright and sometimes then bending off into rococo shapes. The rods can be played by percussion or bowing. He employed plenty of other activation techniques and different initial vibrating bodies as well. The sounds of the space plates are hugely impressive, ranging from massive roars to delicate whistles. Tom often commented that these instruments seemed to have minds of their own: get them going, and they’ll take you on a sonic journey of their own volition as the plate roars to life with its evolving, sustaining resonances. The wobbling and flexing of the sheet means that the dominant resonances are forever shifting, imparting a palpable aliveness to the sound.
Other early large steel sheet instruments were developed by Robert Rutman and Constance Demby in the 1970s, variously called Steel Cello and Bow Chimes (by Robert Rutman), and Space Bass and Whale Sail (by Constance Demby). The Steel Cello was a sheet of stainless steel, about seven feet long and two feet wide, suspended from a steel frame by a single cord and anchored by another cord from below. It was pulled into a slightly bowed shape by a string running from corner to corner. This was the “cello” string, played with a bow. This mounting allowed for a lot of bending and shifting and flexing in the sheet as it was played. Like the Nunn instruments, the Steel Cello was wonderfully complex, diverse and alive in its sounds; it could at times take off and really roar, and it had the same mind-of-it-own, takes-you-for-a-ride quality.
The endlessly inventive François and Bernard Baschet made extensive use of sheet steel sound radiators. Their most highly developed and well known instruments are those of the Cristal family, in which the steel sound radiators were driven by tuned steel rods which were in turn activated by moistened finger friction against on attached glass rods. Look them up if you are not already familiar with them.
For exquisite water-modulated instruments, look to the famous Waterphones of Richard Waters (now made by Brooks Hubbert as well as off-brand makers); see also Jacques lovely Dudón’s Aqua Vina.
For the most refined investigations of sheet-metal forms as speaker-like sound radiators, look once again to the instruments of the Baschet brothers and their followers, as well as the classic resophonic instruments of which Dobros are the most famous example.
… And of course, these are just a few — in our sprawling internet-accessible world, a bit of searching will reveal many more explorations of sheet metal sound.
Sheet Metal from the Hopkin Instrumentarium
For my own recent sheet-sound explorations, I would have loved to get into large-sheet instruments. The main reason I didn’t was space. The issue of storage has become a major consideration around here. But as discussed above, with the right sorts of mountings it’s possible to find a lot of interesting sounds with smaller sheets, and so I’ve done a number of more manageably sized pieces. Many of these are described in the Instrumentarium section of the BartHopkin.com website with photos and audio clips, and I’ve included links here. For those that have not I hope to add them soon. The descriptions following now are minimal but I encourage you to click the links for fuller descriptions (or check back later for those that don’t have links yet).
SSS-K: a lamellaphone (kalimba-like instrument) with small spring-steel tines attached directly to a 12″ x 24″ steel sheet. It was recently made and is not yet posted to the Instrumentarium at www. BartHopkin.com; check back soon for the link.
Rotisserie Friction: A cylinder of sheet steel (repurposed waste paper basket) is rotated by an electric motor. Hovering over it are superballs mounted on springy wires. The player depresses the wires to press the superballs against the cylinder surface to produce moaning friction sounds.
Icicles: Multi-string instruments with steel plates hanging directly from the strings to serve as sound radiators.
Rocking Stainless: A table-top guitar in which a bowed steel plates rests on the strings and modulates the sound by rocking. LINK
SSS – Small Plates: Under this heading I’m including three separate instruments, each involving small-ish steel plates that can be activated in various ways. They were recently made and are not yet posted to the Instrumentarium at BartHopkin.com; check back soon for the link.
Wobble-Steel Guitar: A guitar-like instrument in which the strings drive a flexible steel soundboard. By flexing the board the player can create bends and timbral shifts.
SSS-Z: Eleven strings held by a rigid frame. The strings pass through a steel sheet which serves as the soundboard. The sheet divides each string two, so there are 22 playable segments. The way the strings pass through the plate ensures maximum transmission from strings to plate, and the plate’s mounting system leaves it with plenty of freedom to resonate. This instrument was recently made and an information page has not yet been posted to the Instrumentarium at BartHopkin.com; check back soon for the link.
Cassidy Roaster: the signal from an inexpensive electronic keyboard goes to a driver which is pressed against a sheet metal roasting pan and moved about. This instrument is also more fully described here.
Aquaalt: A zither-like instrument, played by plucking or bowing, in which the string vibrations pass through the bridge to the flat bottom of a stainless steel pan with a bit of water on the bottom. The water moves as the instrument rocks.
Psalt Water: another water-modulated instrument, this one in the form of a bowed psaltery.
Aluminum Flat Gongs: I’ve made several of these tuned gong sets of very clear pitch. Gongs of this sort are also extensively discussed in this article and this article.
SSSSS (Spring Steel Sheet Sound Station): A single sheet-steel soundboard set up so that it can serve as a soundboard any of four separate instrument types that have been made /to work with it, including a zither, a bowed psaltery, a lamellaphone, and a guitar. There’s a system by which each of the instruments can be temporarily attached to the plate for vibration transmission. SSSSS was recently made and an information page for it has not yet been posted to the Instrumentarium at BartHopkin.com; check back soon for the link.