If you're a member of a ukulele group or ever play in noisy environments, a ukulele tuner is an essential bit of kit. It'll enable you to tune even if you can't hear your ukulele. It's also useful for newbies who want to check that they are in tune - although it's well worth learning to tune by ear as well. The main things to be looking for are:
Dedicated Ukulele Tuners: These tuners are designed specifically for ukuleles. For each string, they will show you whether you are high, low or dead on. Most do this by a series of lights.
They are limited to ukuleles; they only show the standard ukulele tuning. Most will work for either C or D tuning but nothing else.
Chromatic Tuners: These tuners allow you to tune to any note at all. They can be used to tune any stringed instrument and can be used for any tuning. If you play guitar and ukulele, a chromatic tuner will suit both.
Chromatic tuners are usually much more fancy than dedicated ukulele tuners with flashing dials and numbers. That does make them a little more complicated to use and you do have to know which note you are aiming for (G4,C4,E4,A4 for standard C-tuning). But it won't take you too long figure that stuff out.
Clip On Tuners: These tuners don't use a microphone. You clip them to the head of your uke and they pick up on the vibrations of the ukulele. This is very handy if you're tuning up in a noisy environment such as at a gig or in a uke group. This is the sort I use. My favourite is the Snark tuner.
Just remember to take them off before you start playing.
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