Why should I use a Fretless Bass?
As you are in control of ensuring that notes sound intonated on a fretless bass, you’d be forgiven for asking yourself: “why would I want to play an instrument that gives me more things to worry about? Well, with a fretless bass, you essentially have more freedom to move around the fingerboard. You can smoothly slide in between notes, which is considered as their main advantage over fretted basses and is key to their expressive, distinctive voice.
One disadvantage of a fretless bass is that you cannot apply vibrato in a conventional manner. You can certainly try, but it’s easier to do this on a standard bass as its frets can seat a string to relieve the amount of pressure required to move it up and down with your fretting finger(s). On a fretless bass, vibrato is achieved by sliding your fretting finger side-to-side instead. Some bassists actually find this more controllable though, and it also gives you greater opportunity to create a wider and more exaggerated-sounding vibrato.
A cool trick that many fretless bass players use is to pluck a natural harmonic and then slide their finger up the string to create a rising pitch sweep. This technique can be heard in the popular Pearl Jam song “Even Flow” (skip to 0:20):