Music professor Anthony Maiello at George Mason University said he enjoys the freedom to choose from the instruments, difficulty levels, key signatures and time signatures available on the site.

The Sight Reading Factory website may soon have even more for users.

Crafton and Rabung are working on expanding the difficulty levels that allow users to fully customize their own music, including range, rhythm and articulation. Rabung said he aims to add the new feature by the end of the next school year.

Crafton eventually wants the website to run in other languages. As of May, 23 percent of Sight Reading Factory’s users are from overseas.

The two founders said they did not do much marketing other than paying Google and Bing to display advertisements, but “music is an international language,” Crafton said.

Crafton and Rabung said they have a long list of potential features to add to site, which has already been updated to the second version. But the said it takes time to develop them, especially since Rabung is the site’s only programmer.