Today Tonara announced that it has raised a $4 million Series A round of funding led by Carmel Ventures, and participated in by the investors of its seed round, in which the company raised $750,000 in 2011.

Tonara is an iOS application that displays sheet music, and follows users as they play the tune. Essentially, your iPad will ‘listen’ to you play, and thus change the pages for you, track your position in real time, and provide a metronome, if you wish. For practicing, it could be a real boon.

It’s also free. The company behind the app makes money by selling pieces of music inside of the app. Its business model is a textbook implementation of in-app payments. Top selling tunes cost a dollar or two. However, not everyone wants to bang out piano sonatas by the hour (however lovely they may be), so Tonara has teamed up with the Hal Leonard Corporation to provide music from a number of popular, current artists to its selection, such as Coldplay.

Tonara launched last September.

It’s interesting to see Tonara raise funds, as its business revolves entirely around a single app for a single device, the iPad. That the iPad install base is large enough to support such a company is interesting. Of course, Tonara isn’t like most apps that are purchased once and then generate no further funds for their creators – its in-app sales system means that the app can generate recurring lifetime revenues that greatly exceed the price of normal applications.

However, Tonara has a total of 60 reviews in the iOS App Store, indicating that the product may not yet have reached a mass market of users. Having $4 million in fresh cash, some of which is all but certainly earmarked for promotion, won’t hurt.

Tonara currently supports five instruments (counting voice), and plans to add more in the future.

Top Image Credit: Frédéric BISSON