Should be a file for Cool Edit, which has been bought by Adobe. It might work in Acid, since I see that CE seems to understand the 'loop properties' of Acid wav files.

The Acid forum would be a good place to ask if you have more questions.

From the help file in Cool Edit Pro 2.0:

Cool Edit Loop files are essentially MP3 files with a .cel extension, but with a header that contains two important items. The first is the loop info, just as it’s saved with a .wav file and is set under View > Wave Properties > Loop Info. This includes number of beats, tempo, key, and stretch method.

The .cel file also works to avoid a problem with MP3 files. It’s in the nature of MP3 files that a very small amount of silence is added to the beginning and/or end of an MP3 file when it’s encoded. This is very short, and often only a few samples long. When working with a loop, though, it’s enough to throw the entire loop off. As it saves a .cel file, Cool Edit Pro calculates how much silence is going to be added to the MP3 file and writes this information into the .cel header. Then, when Cool Edit Pro loads a .cel file, it reads this silence information and automatically removes the silence from the file so that it’ll loop smoothly