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sympathetic strings

U

uethanian

Mar 11, 2007
1,070
4
#1
i made a post in the electronics section but its not seeing a lot of action...i'm not sure if this is more of an electronic or lutherie question. anyways:

imagine removing all the sympathetic strings off a sitar and moving them 'elsewhere,' where they can be excited remotely by electrical means. that's what i'm trying to make for bass...a board strung with sympathetics that reacts to the signal coming from the instrument. the best thing i can think of at the moment is to put a piezo element under the bride of the sympathetic strings, which 'play' the signal from the instrument and consequently vibrate the sympathetics.

1st: how would i wire such a thing (providing that it's possible). what would i need to 'drive' the piezo? i don't want any sort of compression or equalization of the sound, because i want as legitimate an acoustical response as possible. the only control i see the board needing is a volume.

2nd: how would i maximize the response of the board? i was going to use the small bridge of a sitar to get the buzz, and just use 13 strings because that's what its slotted for. the bridge sits on two low feet...piezo under one foot or both?

i forgot to mention, the motion of the sympathetic strings would be picked up by a regular bar pickup, and sent to a secondary amplifier.
 
#3
Remotely mounting sympathetic strings is possible but here's another solution that could be fun. You'll need a dulcimer to try it out.

Take a small cab and place it face up. Place a dulcimer on top of the grill, making sure it won't vibrate off. Tune the dulcimer's open strings accordingly. Either mic the dulcimer or for use a dulcimer shaped contact pickup. Send the dulcimer's feed (highpassed) into a mixer/second amp/speaker.
 
#4
ah, thats exactly what i had thought initially, i was going to have the thing touching my cab to take in the physical vibration...but after some inspection, i really don't think my cab vibrates enough...at moderate volume, my bass was vibrating more than my cabinet.

i tried putting an acoustic guitar in front of my cab, and i could hear a very soft response...but especially if i'm using a sitar bridge, i think the vibration needs some force behind it, to get that special 'swell' that sitar sympathetics have.

so does anyone know how to do this with a piezo (i mean those cheap circular ones from radio shack maybe)?
 
#5
Yeah, you could try driving a piezo as you suggested. Do a separate bridge for the sympathetic strings, acoustic guitar-style with piezo undle a single saddle. Use the piezo in "speaker" mode, driving it with a simple circuit from a Radio Shack idea/project book, or there might even be a driver circuit printed on the packaging for the piezo. Take the signal from the pup from your normal strings.
 
#6
with the way the bridge is, a piezo would either have to go under one of the legs or glued underneath (inbetween the two legs).
stsb.jpg


yea, i'll have to look at the piezo, i really don't want to buy an instruction book for something this small
 
#8
I've actually designed a circuit that does what you need: it's got a power amp (to drive the physical media, in this case strings), then a preamp to take the sound out of the sympathetic strings, a mixer to mix the original instrument with the symp. strings, and a tone control. I've got a bunch of PCBs made for it, but haven't really had time to play around with all the possibilities.

I would try using a magnetic pickup to drive the strings rather than a piezo, but I can't say for sure that it would work better since I haven't done it yet.

When I'm an old guy, I plan to lock myself away from humans and build the all-acoustic modular synthesizer I've been designing, using techniques like the one you describe. I think mechanical and acoustic effects are an unexplored world of awesome possibilities.
 
#9
conical johnson said:
I've actually designed a circuit that does what you need: it's got a power amp (to drive the physical media, in this case strings), then a preamp to take the sound out of the sympathetic strings, a mixer to mix the original instrument with the symp. strings, and a tone control. I've got a bunch of PCBs made for it, but haven't really had time to play around with all the possibilities.

I would try using a magnetic pickup to drive the strings rather than a piezo, but I can't say for sure that it would work better since I haven't done it yet.

When I'm an old guy, I plan to lock myself away from humans and build the all-acoustic modular synthesizer I've been designing, using techniques like the one you describe. I think mechanical and acoustic effects are an unexplored world of awesome possibilities.
Click to expand...

could you possibly draw up a quick circuit for me? i want it to be
instrument>driver>piezo...maybe with a volume control in there somewhere...

then the pickup>tone pot>output can be a separate circuit. what i want to know is, can you drive a piezo with something as simple as an 8V battery, without any fancy circuitry or level controls?
 
#10
Well, it's going to be a product in the future, so I'd kind of be screwing myself and my design partner by posting a schem. ;)

You can't make a driver with just an input jack, battery, and piezo. You need an amplifier. This can be pretty simple or quite complex depending on how much amplification you want, how "safe" you want the circuit to be, etc. I don't know what constitutes a "fancy" circuit to you, but you need to have some knowledge of electronics to design something like this.
 
#11
yea all i need is a basic level control for the piezo...

idk, i was screwing around with a broken loudspeaker the other day, putting an 8v to the terminals and seeing it jump...but it couldn't be that simple...:hiding:

obviously i don't need a lot of power behind the amplification, because i just want to vibrate the bridge the amount that it would be vibrated on an acoustic instrument, i don't need to hear the thing buzz. i guess i'll have to look through one of those books...
 
#13
just got back from radioshack, picked up 3 things

12V piezo buzzer
.10W 8Ohm mini speaker
400mW audio amplifier

the audio amplifier is an 8-pin that has the form

Gain Gain
-Input Bypass
+Input Vs
Ground Vout

can someone explain how to use this with the piezo? so far i'm guessing that the positive signal from the instrument goes to +input, the ground to ground, and the -input is left open. between the two gains there'd be a pot(?), bypass would be left open for now...

...i'm a bit confused how to attach the 8V battery (the supply range is 4V-12V). the Vout must go to one of the piezo ends, so the negative piezo end goes to ground? and the +terminal of the battery goes to Vs, so the -terminal also goes to ground? :help:
 
#14
What you've got there sounds plausible to me. But didn't it come with an info sheet, or a basic circuit printed on the card? What's the RS p/n, or, what's the p/n printed on the chip - with that you may be able to look up the original manufacturer's lit. Like what range of resistance is be expected between the gain leads.
 
#15
using the audio amplifier on one of those 'learn to make circuits' boards, i was able to play an electric guitar through the built in speaker...i also tried the built in buzzer, which seemed to sputter for a second (but only when playing the lower strings) at low voltages, and just screech at higher voltages. so i'm not sure what that means...too much power? too little power? mismatched resistances? i don't know the stats on that buzzer. i tried switching the wires from the buzzer to a piezo disc, but nothing happened. interesting thing, the buzzer i got actually had three connections (on the disk) and a small chip inside...touched to an 9V, it emitted a set frequency...the ones i bought a few years ago had two connections and no chip. i basically broke it opening the case because i didn't realize that it had a chip...so i'm wondering if these piezo's can be used for what i'm doing, or if they can only emit one frequency no matter what...*rubs chin in contemplation*...
 
#16
well unfortunately, someone by the name of leswatts responded to an old thread and i completely missed it...now i can't contact him. in short he said that it's possible but difficult with piezo's, and not really that hard with a speaker voice coil...but what i need to know is what size/wattage/impedance of the speaker (if i were to remove the cone and use it for vibration), and how to drive it...can someone give me an idea of how to wire an amplifier for a small/medium speaker? i do not think i need an EQ, just an input control.




Hmmm...
I'll try to answer... my engineering firm designs the things, among other activities.

The buzzers use PZT-5/brass unimorphs that have a depoling voltage of about 15 volts/mil thickness.

These will appear as an almost pure capacitance in their compliance operated mode, so you can't really apply any power to them with something like an audio amp.

An electroacoustic or electromechanical impedance matching network must be used to make their electrical impedance resistive so they can accept and transform power. This is done in buzzer/beeper assemblies, but only at one frequency.
They only sound at one note...usually designed at 2 to 6 kHz.

So, in short, it can be done. But it's a bit of engineering.
An electromagnetic transducer operated in mass controlled mode coupled to a spider bridge would be easier.

Here are some basics:
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This includes the Mason equivalent circuits we use to design the matching networks.

Long ago I also made a sympathetic string sounder as Hoover mentioned. I used a speaker voice coil. The strings had a jawari bridge, so they buzzed as in a sitar. They had to be tuned very precisely to sound...within a few cents. Worked well though!

So...voice coil and spider bridge...If you really really want a piezo one...well, I can design one for you!

By all means play with the buzzer unimorph disks though. Great fun. An audio amp will not hurt them, since they they cannot accept any power (unless they are still in the plastic impedance matcher, at one note.)

Oh, and I listen to some justly intoned stuff....old Beatles vocals!
 
#17
nothing? ok. well other than designing an amplifier (which i have a general, but limited knowledge of), i'm sketching out some designs for the board.

my first issue is how to attach the speaker (after the cone has been removed).
1) make a cut-out and lay the speaker in facing upwards, possibly with the bridge sitting directly over it
2) glue the rim of the speaker assembly face downwards

i'm trying vibrate the board the most with the least amount of noise produced or power required. then i'm looking at different speakers...size and wattage. 3-5" 12W speakers are cheap enough, but i don't know if they will shake enough...then again, i'm not ready to drop $40-80 on a guitar speaker for a project that might not work.

http://electronics-diy.com/schematics/712/LM386amp.gif

here's a schematic i found for the LM386 audio amp (the only one i have or have otherwise found).
 
#18
That schematic should work fine, but a 386 isn't going to put out 12 watts. It puts out more like 1/4 of one watt, but since you don't care about distortion, because you're not going to hear the speaker's output, you can drive it harder and maybe get 1/3 to a 1/2 watt. In any case, there wouldn't be much point of a nice guitar speaker.

The measurement of whether a speaker will "shake enough" as you put it is called Xmax. It's a measurement of how far back and forth the speaker is capable of moving. Cheap speakers often don't have available specs like Xmax, but look for it anyway if you want a speaker that moves a lot. I honestly don't know if you do or not in this application.

Edit: Actually, I need to clarify. are you cutting out the cone of this speaker or leaving it? If you leave it, there will be some issues. First, you will be hearing the output of the amp, which means you can't drive it really hard as I suggested before. Second, the amount of power needed to vibrate the strings might be significant, and the more you drive the strings, the louder the clean signal of your bass will be, so the strings themselves will likely be drowned out. This is not a huge problem if you intend to use use a pickup on the sympathetic strings and amplify them, but if you're going for an acoustic thing here (which I thought you were, but am unsure), this will be a problem.
 
#20
ah mark deutsch, cool guy

so to revive an old thread (or otherwise beat a dead horse) i think i may have something in mind. i found these today online:

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which seem to work really well at being loud, but that must mean they are effective enough at transferring vibration through solid objects. since they come in pairs, i could put one on either side of the bridge

then there's the more complicated issue of driving them...
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so the 'exciters' are rated 8ohm, and the amp is 4ohm...but the amp has two channels. quote "2x15W 10%RMS at 4-Ohm." i have no idea what that means. besides that, would it work to send an instrument signal directly to this amp (if i have a 1/4 jack-to-RCA)?
 

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