You might need a couple placed on either side of the bridge plate to get even string response, and you'll probably also want one placed one the soundboard somewhere to round out the sound and keep it sounding more "acoustic", maybe behind the bridge. I haven't actually put piezo disks in an acoustic bass, so I'd really only be guessing, but my intuition would be to start by putting them where there's the most movement in the top, but also a decent amount of mass to help stop feedback (low frequencies being the usual problem for piezo amplified acoustic instruments), so directly under the bridge would probably be the best place to start. A good place to start is, like you said, right under the bridge But you might not need more than one or two sensors, anyway.Ī useful method for placement is to use double stick tape- not the foam stuff- to place them in different locations, and listen to the result ( I recommend recording and using headphones.like wine tasting, it can get hard to keep track of what tasted best) When you've found spots that sound good, replace the tape with something stronger, like epoxy. Also, the cable between the sensor and the buffer is notoriously microphonic, which most guitarists notice after they step on the cable to their preamp, and having few of them, and short ones, is good. Funny sounds happened, and though I never tried to analyze it ( and would love to see what really was happening) the impression I got was that there was phase cancellation, maybe because one piezo could be driven by another piezo, becoming a transducer. I have not myself had good luck with running several piezo pickups in parallel. Once the input impedance is high enough, piezos become decent voltage sources, and it also happens that the high input impedance also serves to retain a reasonable frequency response. The real reason we need such a high input impedance is because piezos are such terrible current sources. Basically, the output impedance of a piezo is not the reason it needs such a high input impedance, as its output impedance is actually pretty low, especially around the resonant peak, but this is not part of the useful range, we want to be using the bandwidth below the resonant peak, but above the high pass filter. Regarding your thinking on wiring the sensors in parallel, yes it will add the static capacitance, lowering the resonant frequency, but it does not lower the output impedance as simply as you might hope, as the " dynamic" capacitance of the voltage source (and remember that piezo's are a voltage, NOT a current source), does not simply add like a true capacitor. I've tried foam double stick tape and that does alter the response (in a bad way), and I've tried harder mounting like epoxy without seeing any change that made it worth the extra effort, so I've been sticking to what works. Looks like you've gathered some interesting data, although I'm not sure how meaningful it really is.Īnyway, I've always found that so long as it is attached with something reasonably solid, it works well.
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