Persian music scales incorporate quarter tone type of microtones (See "Persian Modes: Reading Between The Lines Of Western Music" from the same author here). Performing such intervals on electric guitar could be tricky; but in course of composition and recording my 2012 album, "namoWoman", due considerable use of such quarter tones, I devised practical methods for performing them on electric guitar that I have put forward in this article.
Performing quarter tones on an electric guitar could be done in two general ways:
1. Pre-bending notes into quarter tones (not recommended)
Practical on all types of guitar
Preventing unwanted sounds in between played notes is almost impractical. When you want to play a note in consequence of a bended note, the bended note may make sounds in its way of bend release.
Not doable on open strings
2. Raising the pitch by pressing the right hand on the bridge (recommended)
More controllable than pre-bending technique.
Makes the performer able to vibrate or bend from a quarter tone to another tone.
Only doable on Floyd Rose tremolo system.
In both methods it is possible that between shifting notes, some unwanted sounds are produced; but such chances would considerably diminish if note shifting is done at the right moment. Here I have described the recommended method of "raising the pitch by pressing the right hand on the bridge":
Suppose we want to play (G-Ap)[*] on D string. G is on 5th fret and Ap will be resulted by raising the 6th fret’s pitch but 50% of semitone or 50 cents (note that the interval between G and Ap is ¾ tone). Normally play the 5th fret and then immediately before playing the 6th fret gently put your right hand on the bridge so that you raise the pitch by 50%. Please note that at the moment of playing a quarter tone by this method, the player is performing 3 actions approximately at the same time:
1) Putting the left hand finger on the fret 2) striking the string by right hand's pick or finger 3) pressing the right hand on the bridge.
Two important considerations:
1. Note that pressing the right hand should be done immediately before other actions, so that at the moment of striking the note the pitch should have been raised to the desired quarter tone.
2. Due difference in string tensions, the required amount of pressure on the bridge to raise a note's pitch by 50% of semitone differs among strings. E.g. high E string requires more pressure in comparison to G string.
You may do the following to help your ears and technique to improve regarding playing Persian microtones on electric guitar:
1. Listen to quarter tone music will help you build the skill of finding the right quarter tone pitch. (Check out Persian Traditional Music on YouTube here and here)
2. Practice with a tuner will help a lot in finding the right amount of pressure required to make a pitch 50% of semitone (AP Guitar Tuner is highly recommended. It is the only precise and fast responsive tuner I have found on internet).
As an example, I wrote and performed the following piece. The motives are related to one of the songs in my 2012 album, "namoWoman":