Floating Soundboards

by Bart on April 30, 2010

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

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 16th 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 either 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.

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-guit72

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.


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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.

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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