Tunebot is a music search engine developed by the Interactive Audio Lab at Northwestern University.
Since ratios between note lengths are used, the tempo of the performance does not affect the rhythmic similarity measure.
Queries and targets are then matched by a weighted string alignment algorithm between the note intervals and rhythmic ratios.
A cappella sung examples are used as search keys because it is much easier to compare one unaccompanied vocal (the sung query) to another (an example search key) than it is to compare an unaccompanied vocal to a full band recording, which may contain guitar, drums, other singers, sound effects, etc.
Tunebot then pairs that song with the user's query, analyzes the differences, and runs a genetic algorithm.