Technically sound effects are called Onomatopoeias. Onomatopoeia is the formation of a word from a sound associated with what is named. For example, onomatopoeia is the hum of the bees, the pop of a balloon, or the tweet of a bird.
In this webpage you can find lots of onomatopeias grouped by tags.
Visual aspects of onomatopoeias
But it’s more than the letters, it comes to the dynamic of the form, the lines, the direction in respect to the picture to express the meaning of the sound. If the sound letters trespass the border of the picture it has an impact to the reader.
Example of Sin City by Frank Miller (Sin City: The Hard Goodbye, Episode 6, 1-3/8, 1992)
Example of Sin City by Frank Miller (Sin City: The Hard Goodbye, Episode 7)
Another aspect is readability. These words are usually at the top level of the image and have to be read easily independently of the background. Sometimes onomatopeias are outlined with a thick border or a 3d shadow.
Poem with sounds
This is an example of a dadaist poem from 1917 by Hugo Ball. The Dadaists started using typography to translate the sound’s mood to paper.