Lately I’ve been experimenting with string instrument soundboards that are not attached the sides of the sound box all around the periphery, but are attached only at the ends.   The illustration below will give you the picture, using a rectangular sound as an easily visualized example.  I know I’m not the only person to explore this sort of design, but I can’t recall where I’ve seen it before. 

The idea is to allow the soundboard freer vibration than would be the case if it were rigidly attached all around.  This is in keeping with something I’ve long been interested in: string instrument soundboards that are unusually light and/or flexible, so that the strings can easily activate them and drive them to large amplitudes. Because the strings deliver their energy to such soundboards rapidly, the resulting sound tends to have relatively short sustain and the tone may be kind of punchy. For lower notes there may be a bit of a thump in the attack.  The tone tends to be strong in the fundamental, de-emphasizing higher overtones. In theory such soundboards should be louder than more rigid, longer-sustaining soundboards. I haven’t always found that to be the case, but greater volume isn’t the main reason I’m interested.  The main reason I’m interested is, I like the punchy, saturated tone.

A very weak soundboard is likely to distort or collapse under the pressure of the strings.  So what I’ve been doing with these semi-detached soundboards is placing supports of dense sponge rubber under the sides of the board at selected points. (This is the same dense sponge that we sell here at EMI.  It’s great stuff.)  This prevents the soundboard from collapsing under the pressure of the strings, yet it still allows high degree of flexibility. It also adds damping to the board. The damping is probably a good thing, evening out the response and reducing the likelihood of wolf tones (wolf tones = disproportionately loud notes resulting from pronounced resonances at certain frequencies). 

Soundbox with floating soundboard

So far I’ve built three box zithers this way and a guitar-like instrument, as you can see in the photos. The soundboards are redwood, about a tenth of an inch thick.  They have no strutting or ribs underneath, except for a little cross-grain strip for added strength under the bridge in the guitar. On all but one of the zithers I added magnetic pickups, custom made to accommodate the required pickup width. (I tested piezo pickups as well as little on-board microphones with these instruments, but the magnetic pickups sounded best to me.)

So how do they sound?  You can hear them in the samples below.  I totally love the sound of the zithers, both unamplified and amplified. They really do, to my ear, have the saturated, punchy sort of tone I was after.

The sounding results with the guitar suggest a rather different story. Unamplified, the guitar sounds pretty blah. The mechanics by which the strings drive the soundboard are quite different in zithers and guitars, and what worked so nicely for the zithers is noticeably less pleasing in the guitar. But when it’s played through the pickup, I like the tone more than that of a typical electrified guitar. To my ear it sounds a lot less electric and more acoustic even than an acoustic guitar with a soundhole pickup.  The main reason is the way the highly compliant soundboard interacts with the strings and colors the string sound that the pickup hears. In addition, a couple of other design features contribute to a more “acoustic” tone, namely: 1) I’ve strung the guitar with an unusual set of soft steel strings (more about this in another posting sometime in the future).  2) The pickup is positioned as far from the bridge as possible – adjacent to the 16^th fret, which is much farther from the bridge than is typical. This contributes to a warmer tone.  (If you look back at the zithers, you’ll notice that the pickups are right at mid-string, for a similarly warm sound.) 

(About the use of the word “floating:”  duclimer makers sometimes use the phrase “floating soundboard” to refer to a soundboard which isn’t affixed to the sides of the box, but held there only by string pressure.  I’m taking the floating idea a step further, with soundboards that don’t even touch the sides all around.  The term “floating bridge” is occasionally used for guitar bridges that stand on two feet rather than lying flat against the soundboard; one again I’m taking things a step further with a bridge that doesn’t touch the soundboard at all.)

A Pair of Floating Zithers. These two box zithers are made as a pair to be played together by one or two players. The ranges complement each other, providing three and a half octaves between them. With seven strings per octave, the tuning is diatonic, but sharping levers on the left allow for quick and convenient retuning to different scales. The weighted steel arm on the larger zither adds a vibrato to the tone. Give the weight a little push and it will bob up and down because of the way it's anchored in the strings. It continues to bob as you play, wavering the tension on the strings to create the vibrato. The weighted arm doesn't work as will on the smaller zither, so it has an unweighted arm that the player can flex to provide the vibrato manually.

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Floating Guitar. This guitar has a rather poor tone unamplified, but the amplified tone is, to my ear, natural sounding and appealing. For more details see the description in the main text above.

Over-Under Zither. This zither is almost identical to the smaller of the zither pair above, but it has a mid-string bridge in the form of a weighted bar. The string segments on one side of the center bridge are twice as long as on the other side, so the notes on either side are an octave apart. The weighted center bridge isn't anchored to anything but the strings themselves. It's free to wobble and bob, creating pitch-shifting and tremolo effects in the strings.Because of the unfixed bridge, the tone is a little thin and plinky, and the tuning is pretty seriously unstable, but the peculiar wobbly effects appeal to my ear nonetheless.

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 IMPORTANT NOTE: The following post has been superceded with better information in a subsequent posting.  Get the updated version here.

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I’ve been making an effort lately to get useful information on piezo films into these pages for the benefit of anyone who may want to use them.  Here’s one more posting along those lines. The question I get most frequently from customers regarding the piezo film is, is it OK to trim a piezo film pickup to a smaller size?

The quick and safe answer to this question is “Beware: cutting a piezo film pickup may ruin it.” But a more complete answer is, sometimes it’s OK to cut the film, sometimes it’s not; it depends on how you cut it. Read on for details.

The piezo film pickups we sell are made up of a very thin film of piezoelectric material protected by a layer of clear plastic lamination on both sides. It’s hard to see this unless you look quite carefully, but the piezo under the plastic is actually a double layer, in the form of a single piece of very thin piezo film folded back on itself. The two terminals for the wire connections, located alongside one another at one end of the pickup, are actually at opposite ends of this underlying piezo film, brought next to each other by the folding. (Think of a towel folded in half: the folding brings the opposite ends together.) Now imagine that you decide to shorten the pickup by snipping off the end. By cutting off the point where it folds, you are (unknowingly) cutting the piezo film into two separate sections; it’s then no longer a single sheet. The electrical continuity between the two terminals is broken, and the pickup will not function

So that’s the big negative: you can’t shorten the pickup by snipping off the end; that will destroy the pickup

However, you can trim in other ways, and as long as you do so without compromising the electrical continuity, the pickup will still function. First of all, you can always trim off the clear plastic borders; there’ll be no ill effects because there’s no piezo material in those clear portions anyway. Second of all, if you want a narrower pickup, you can usually get away with trimming the sides, as long as you make sure that the folded end isn’t snipped off

Keep in mind, too, that the piezo film pickups are quite sturdy. If it’s necessary to give the pickup a sharp bend or something of a fold to make it fit in the intended application, that’s ok.

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A couple of months ago I did a posting titled “Wiring Multiple Piezos Together.”  That post has information on how many piezos you can run to a single output before you start to lose signal strength and fidelity, followed by suggestions on how to wire such hook-ups.  The other day it occured to me that I didn’t address one important aspect of the topic.     

What I neglected to discuss was phase cancellation.  In multi-piezo installations where the piezos are on  independent sounding bodies — say, different bars on a marimba – then phase relationships aren’t an issue. But when you have two or more piezos on the same vibrating body — as, for instance, on a soundboard – it’s likely that the signals coming from the piezos will not be perfectly in phase. Then there will be some cancellation when you join them, resulting in a loss of signal strength. Depending on circumstances, the cancellation could be negligible or serious.  

The rest of this post discusses this issue.  To simplify the discussion, I’ll use the case of two piezos mounted on a soundboard as an example.   

There are two ways cancellation can come about when two piezos are mounted on a soundboard: 

1) Phase reversal.  Something about the way the piezos are positioned or wired could result in the piezos capturing opposite phases of the soundboard’s vibration.   This could happen for any of the following reasons: a) one piezo is positioned on the inside of the soundboard; the other on the outside.   b) One piezo is reverse-wired relative to the other. c) One piezo is attached upside-down relative to the other – that is, the two piezos are  mounted with opposite sides against the soundboard.  In any of  these cases, if the piezos are located reasonably close to each other, then their two signals will be close to 180 degrees out of phase, and serious cancellation will occur.

2) Phase offsetting. If the two piezos are somewhat far apart on the soundboard, then each will, at any given instant, be picking up different phases of the waves as they travel through the soundboard.  The phase difference will most likely be much less than 180 degrees, so cancellation will not be severe.  More interestingly, different partials within the sound (with their different wavelengths in the soundboard) will have different and variable phase relationships at the pickup locations.  Keep in mind that this kind of effect occurs naturally in the acoustic sound coming off the soundboard anyway; it’s part of the sound our ears are used to hearing and not necessarily a bad thing.  In spite of being less efficient, with luck it will contribute to a sound that is warmer and more natural than what a single pickup would capture. 

Practically speaking, what can you do to manage these phasing questions in multiple piezo hookups?

First, regarding the 180 degree cancellation described in #1 above: This is normally an undesireable situation and it’s easily remedied by reconfiguring things so that the two piezos are in phase.  Do this by reversing the phase on one of the pickups. One easy way to do that is simply to turn one of the piezos over so that its other side presses on the soundboard.  Another way to invert the phase of the signal from one of the pickups is to reverse the wires coming from the pickup – take the wire that had been the hot lead and wire it to ground, and take the former ground lead and wire it to the hot terminal.  On the other hand, if you’re interested in a weird hollowed-out sort of sound and are willing to accept a serious loss in signal strength, you might enjoy exploring the possibilities of a deliberately out-of-phase hook up.

Regarding the subtler phase interactions described in #2 above:  In theory you could try to analyize oscillation patterns and their interactions as a way of determinining ideal soundboard locations for two or more piezo pickups. In practice this isn’t very realistic, given the almost infinite complexity of possible tones and all their harmonic and inharmonic partials in interaction. Instead, it’s usually worthwhile to spend a little time in trial-and-error mode, testing different locations for the piezos, including varied distances apart, in search of the most attractive sound. In addition, try reversing the phase relationship by inverting of one of the pickups. , Do this either by turning it upside-down or reversing the output wires as described in the previous paragraph.   

In most cases and for most musical tastes you’ll find that having two or more piezos on the same soundboard can yield a warmer and more satisfying sound than a single piezo would.  An exception is the case where you want a particularly bright and edgy tone with a sharp attack. That’s because the cancellation with multiple piezos happens most in the higher frequencies/shorter wavelenghts (as is the case in the natural acoustic sound as well).  A single piezo, being free of these high frequency cancellation effects, tends to sound brighter.  Also, multiple piezos do a better job of capturing the complexity, variablility and non-static nature of the natural sound. This makes for a softer attack,  and contributes to what the our musical ears hear as a warmer and more alive sort of sound. A single piezo, by contrast, will tend to have a slightly more “clinical” feel to its sound.   

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We sell piezo material in several forms for making musical instrument pickups. One form is cable, about a tenth of an inch in diameter. The piezo cable looks very much like regular shielded audio cable, but it’s got a very thin layer of piezo material inside which enables it to function as a contact pickup. It’s most often used in or under the bridge of string instruments, where it can respond very directly to the string vibrations.

For a typical guitar installation, you need a piece of cable about 5″ long to span the bridge with enough left over to make the necessary connections. But our customers have occasionally wondered: how much longer can a piece of piezo cable be, and still work well?  Imagine for example that you’re making something like a clavichord or harpsichord; can you use a single piece of piezo cable several feet long to pick up the sound from the full length of the bridge?

I recently performed some tests to see how length affects the performance of the cable. Mind you, the tests were very rudimentary. I didn’t build harpsichords to test them in; I just cut pieces of the cable to various lengths and hooked them up to an amplifier. I tested and compared their outputs by wedging them under the strings of a guitar and plucking the strings.  Here’s what I found. 

Finding number one: Length doesn’t seem to be a problem. Long cables were about as responsive as short ones.  The longest I tested was 48″, and the strength and quality of the response was not very different from that of a 5″ cable.

Finding number two: Regardless of length, the cables do not produce a very strong signal. Theoutput is less than that from the piezo films that we sell.  (This wasn’t really news, but these tests confirmed it for me.) That leads to these two pieces of advice: 1) Piezo cable is best used in applications where it can be given strong vibrations to respond to (such as in-bridge installations). 2) In installations using piezo cable it’s especially important to make sure all wiring is well shielded, to provide the best available signal-to-noise ratio.

In spite of the lesser signal strength, plenty of customers have gotten back to me to report very successful applications with the cable.

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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 a 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 accessible, and I was afraid that a description of the air-tuning process would be too long and complicated. 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.

In response I’ve now written an article explaining the process. You can find it here. 

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A few months ago I posted information on how to make a pop gun. In that post I mentioned the closely related sound of suction pops, as in the sound of a cork pulled from a bottle. Pop gun pops and suction pops both are very cool and quirky sounds, but suction pops have one advantage: unlike with pop gun pops, the pitch is predictable. If you wish, you can make suction poppers tuned to particular notes. 

In this post I’ll describe how to make suction poppers. Suction poppers and pop guns are quite similar in design, and you can use most of the same components to make either. If you happened to read the earlier pop-gun post, and especially if you made a pop gun, then most of the suction popper making procedure will be familiar to you.

Here’s a sketch showing the essentials of a suction popper.

 To make the suction popper, start with the main tube.  Plastic tubing, such as PVC conduit, is suitable, and it’s widely available and inexpensive at hardware stores. Other sorts of tubing can work as well.  ¾” diameter or thereabouts is good, if only because it’s easy to find corks to fit. The length of this tube determines the popping pitch.  I have made successful suction poppers in lengths ranging from about 10 inches for high pitches to a several feet for low pitches.*

The two corks should have the truncated cone shape, as shown. The narrow end should be narrow enough to go inside in the tube; the wide end too wide to fit. This type of cork can often be found in hobby or crafts stores, if you don’t have any already around. 

The plunger cork must be cut to fit the inside of the tube snugly.  To get a perfect fit, shove the cork into the tube as far as it can go without forcing it.  Cut off the end that’s still sticking out, and sand the cut end flush with the tube end.  This gives you your fitted cork; pop it back out by pushing from the inside with a dowel coming through the tube from the opposite end.

For the plunger stick, you can use any sort of dowel or stick fits inside the tube, about ten inches long. Use a nail or screw to attach the fitted plunger cork to one and as shown in the drawing. Pre-drill the cork to prevent splitting before putting the nail or screw through it, and if the nail or screw doesn’t have a large enough head, include a washer.

That’s it; the suction popper is now ready to play.  Insert the plunger in one end of the tube, place the stopper cork on the opposite end, and pull the plunger back out with a rapid motion. If you’re ambitious, make more suction poppers in graduated lengths to create a scale. Gather a group of friends to play suction pop compositions and improvisations in hocketing style. 

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 *With longer tube lengths/lower notes, an unexpected effect comes into play: a prominent snap sound appears in the tone. I don’t know what causes this. You can hear the effect quite prominently in the sound sample on this page. The snapping on the low notes comes across almost like a distortion in the recording, but it’s actually part of the natural sound.  

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We sell a lot of piezo films, ready for our customers to wire them up as musical instrument pickups. In most applications, a single piezo film pickup is all that’s needed to capture an instrument’s sound. But there are also a lot of situations in which it’s useful to have two or three, and sometimes much larger numbers of piezo film pickups on a single instrument. (For example of an instrument calling for many, think of a xylophone-like instrument in which a pickup is needed on each bar.) The question arises: is it OK to have multiple piezos feeding into a single output? My response to this question, up until recently, has been based on book-learning and not on direct experience. So the other day I finally sat down and did a whole lot of piezo hook-ups in different configurations in order to see first-hand how many piezos you can wire together with good results.

In this posting I’ll first tell you what I learned in terms of numbers, then I’ll describe how I did the tests, and then I’ll provide additional how-to information for hooking up multiple piezos (including ideas for what to do if the number of piezos you need is too large for a single group) .  But first let me review the basic information that was already known before I did my series of tests. The ideal situation is to have a single piezo element operating independently, sending its signal through a cable to its own amplifier input.  It’s also possible to have two or more piezos wired together and going into the same input; however, the more piezos you have in the system, the weaker the signal from each piezo will be.  The question to be answered is, how many separate piezos can you wire together before the loss becomes a problem?

I experimented with both the very small piezo films we sell (piezo “tabs,” they’re sometimes called) and the big 6″ ones.  Here’s a rough summary of what I found.

 RESULTS

 1″ piezo tabs

With the tabs, I was happy to find that you can get away with more pickups in a group than I expected: the signal strength deteriorates as you add more piezos, but not as badly as I had feared. In the chart below, the numbers in the left column are the number of piezos wired together in a group; the comment on the right describes the resulting signal strength and quality.

1          Excellent

2-3       Almost no discernable loss

4-5      Very little loss

6-10    Increasingly noticeable loss, but still functional

10-20   Increasingly serious loss

 Recommendation for multiple piezo tabs: The fewer piezo tabs wired together in a group the better, but anything less than five will be OK in most applications. Depending on your requirements you may be able to work with as many as nine or ten. Use more in a group only if you can accept compromised signal strength and sound quality.

6″ piezo films

With the larger 6″ piezos, the signal strength deteriorated more rapidly as the numbers increased.

            1          Excellent

            2-3       Very little loss

            4-5       Increasingly noticeable loss, but still functional

            5-10     Increasingly serious loss

Recommendation for larger piezo films: The fewer piezos wired together the better, but two or three of the large piezos is probably OK, and you may be able to get away with up to five.  Use more in a group only if you can accept compromised signal strength and sound quality.

2.5″ piezo films

I didn’t test these in-between-sized films, but if you’re working with them you can assume the results will fall somewhere between the larger and smaller ones described above.

TEST SET-UP

Managing such large numbers of temporary hook-ups (up to 20 for the piezo tabs) was slightly chaotic in a fun sort of way. The test set-up was pretty informal. I did the hook-ups using lengths of hook-up wire with alligator clips at the ends. I’ve got a large supply of these convenient little connectors for just this sort of purpose.  I started by hooking up a single piezo film, running its output to an amplifier, and testing it simply by flicking the end, noting the signal strength and tone quality.  I then added a second piezo, and a third, and so forth, testing by flicking after each new one was added.  After I had hooked up and tested twenty of the small piezo tabs, I then went back and tested a single piezo once again, in order to directly compare the full multiple set-up with the single piezo. I did the same for a total of ten of the 6″ piezos.

Something worth noting about this set-up: the hook-up wire I used isn’t shielded.  This means that as I was adding more and more hook-ups, it was to be expected that increasing noise would appear in the system from stray electromagnetic frequencies in the air.  This did occur, especially when I switched on certain lights nearby, but it wasn’t as bad as I feared. In a real installation, of course, you’d use shielded wires.

HOW-TO INFORMATION FOR MULTIPLE-PIEZO INSTALLATIONS

 You can find full information for hooking up single piezo films here.  This is the sheet that comes with the piezo films when you buy them from us.  The basic idea is that you run two wires, called the hot wire and the ground wire, from two terminals on the piezo, through a cable and to the preamplifier. (The preamp is often incorporated into a regular amplifier input.) The shorter the length of cable from the piezo to the preamp, the better. 

In hooking up multiple piezos, you have a choice of whether to join them in series or in parallel. Because of the electrical nature of piezos, series connection yields poor results; parallel is the way to go.  In practice this means: connect the hot lead from each of the piezos in the group to a common wire for the hot side of the output, and connect the ground from each peizo to a common ground wire.

What if the number of piezos you need for an instrument is larger than the number that can work well wired together in a single group? Example: imagine you’re putting pickups on a home-made xylophone with 12 bars, but to prevent signal loss you want to keep the number of piezos grouped together to five or less? The answer is to wire the piezos in two or more smaller groups, and keep the groups electrically “buffered” from one another. In this case, for the twelve piezos needed you might create three groups of four, or perhaps two groups of six.  To buffer the groups from one another, they need to go to separate preamps before their signals are mixed.  If you’re an electrical whiz, you can build miniature op-amps into each circuit before mixing them.  If you’re not an electrical whiz, the easy solution is to send them to separate mixer inputs. This is quite feasible because nowadays there are very compact and affordable mixers on the market with as few as four inputs – you may even be able to affix a mini-mixer to the body of the instrument somehow.  This also has the advantage of giving you separate volume and tone controls for each of the groups. 

Addendum (March 26, 2010): This later post has information on another consideration in multiple piezo installations, phase cancellation.  

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