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I've been told when playing live not to mic an acoustic instrument because of the feedback you can get, and that using pickups would be a better way to amplify the sound of an acoustic instrument. So I would like to know if:

a) pickups are better to use with acoustic instruments than a microphone, and b) could one type of pickup work for all of my acoustic instruments (guitar, ukulele, viola, and violin)

  • It's important to keep in mind that pickups do not 100% prevent feedback in acoustic instruments. Acoustic string instruments like guitars and violins have to have resonating bodies that make the string vibrations audible. Those bodies are also set into resonance by sound, so the sound from the PA system can cause the body to resonate which is then picked up by the pickup and then amplified by the PA and then you have a feedback loop. Sometimes a mic is actually a better option for feedback rejection. – Todd Wilcox Feb 5 '17 at 20:43
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For the first part. That is a solid sometimes. Sometimes pickups are better than mics. Pickups generally deal with feed back better than microphones. They also allow you to alter the your sound using post processing and what not. Microphones sometimes give a 'truer' sound of your instrument. Most violinists that I know use mics with their acoustic instruments.

If feedback is your only concern go with a pickup.

Second part. Generally no. You can't use any single pickup for all of those instruments. You might be able to use a stick on piezo pickup that claims to be for all instruments however those aren't know to be very durable. The also aren't known to sound particularly good. The shape of the instruments otherwise won't allow good pickups be used with the others. So for part b, in theory yes but it practice you probably won't be satisfied with a result.

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Xerotolerant's answer basically says it all already.

My take:

pickups are better to use with acoustic instruments than a microphone?

No, pickups are in principle almost always worse than microphones, as far as sound is concerned. They sound alternatively tinny, boomy, scratchy, clickey or undynamically-boring, depending on the instrument and particular pickup model (there are very different ones). For recording purposes, I would always try to catch every acoustic instrument with at least one microphone, with the exception of acoustic instruments that aren't really used acoustically (e.g. wah-wah violin, reverb-drowned percussion guitar, synth-bass–triggering trombone or distorted cello).

However, when you need to make an instrument loud for a live gig, you usually need to make a few compromises to ensure everyone actually hears it properly. Microphones have a whole bunch of problems:

  • Decent condenser mics are very sensitive to vibrations / touch. In the typical hurry of a live sound check & show, you're in for a lot of noise.
  • All of them don't like to be dropped, and the best ones are really expensive.
  • Quite heavy, too – a large-diaphragm condenser sounds gorgeous for many instruments, but it's really unwiedly.
  • They need to be placed carefully. The ideal position sometimes means you have an awkward mic stand in a stupid position on stage and the performer must not move more than a couple of inches in any direction.
  • Signal bleed. Any mic on stage will pick up all noise that's coming in. For brass, drums and guitar amps this is scarcely a problem because the signal you want is so loud that you can turn the gain way down on the console (thus making bleed very quiet). But most other instruments are barely loud enough to be heard at all; sometimes a mic close to the instrument will pick up more sound from the drumset on the other side of the stage! This makes it very difficult to dial in a good mix that actually brings out these instruments.
  • Feedback is of course down to the same problem: the PA and monitors also bleed into the mics, and if the mic itself is audible on those speakers then you have a catastrophic loop.

Most of these problems can be at least mitigated with small gooseneck microphones mounted right on the instrument (or even in the instrument), however it's seldom possible to place these mics in a way that would give the best sound. Close-miking string instruments gives to a lesser degree the same scratchy character that most pickups are notorious for.

Thus, for most live applications, I much prefer at least having a pickup for string instruments. In particular, I very strongly avoid putting any mic signal on monitors.

Then again, some pickups also don't give any benefit over even a cheap-ish gooseneck microphone. In particular, there are contact mics that can be sticked on most any acoustic string instrument, but in my experience they tend to give a much more boomy, indirect sound than a mic and are almost as feedback-sensitive.

I thus prefer pickups that more or less directly pick up the string vibrations – piëzo bridge pickups for bowed strings and classical guitar, and magnetic pickups for steelstring guitar. Those can be turned on relatively loud even on monitor speakers. These pickups generally sound quite harsh, but with careful EQ and reverb you can usually get result that's satisfying for live. But that requires a fixed setup for each instrument individually.

If you have so many mics and can't afford installing a high-quality pickup in each, you'll thus fare better with a single mic. If you don't play with other very loud instruments, I suggest a small-diaphragm condenser like Oktava Mk012 or AKG C1000, which you may have to move a bit on its stand. If you are playing with loud other instruments, use two Shure SM57 instead, one for guitar and ukulele (very close over the fretboard and pointing at the sound hole) and one for the violin and viola (alternatively a gooseneck clip-on that you can switch between the instruments).

If you use mics and need to hear your instruments on stage (in fact, even if you use pickups), in-ear monitoring is much better than floor monitors.

  • I feel like this answer paints a picture of microphones in general being a nightmare for live sound which is an understandable idea but is not at all the case in my experience. Some microphone, like the Shure SM-58 are very easy to use in live situations, while others are more challenging in general but perform better in the right environments. Live sound systems have to be correctly engineered to function properly and that requires a lot of study, training, practice, and experience, and dealing with pickups presents an entire other host of challenges and is not always easier than mics. – Todd Wilcox Feb 5 '17 at 20:49
  • For example, decent condenser mics that are properly eqed and gain staged are not necessarily more prone to noise than a dynamic mic or a pickup. Large diaphragm mics, condenser or dynamic, are not usually suited to live use (although sometimes they are brilliant). Gooseneck mics mounted on an instrument can have advantages, but also can have the disadvantages of both mics and pickups. – Todd Wilcox Feb 5 '17 at 20:54
  • Yyeah... the thing is, the super-troublefree SM-57/58s are seldom really satisfying for string instruments, even less than goosenecks. My point was, you'll generally want a microphone with great clarity and depth placed not too close to the instrument for best sound. Condensers without wind screen can easily fit that bill, but only with substantial gain (about the same as a close-placed dynamic mic, but that's much less sensitive) and mostly linear EQ. At those settings, you do easily get noise problems live. – leftaroundabout Feb 5 '17 at 21:19
  • Dealing with pickups doesn't present much challenge to the engineer, not if the musician brings a matching preamp, as they definitely should. – leftaroundabout Feb 5 '17 at 21:20

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