A fact chunk in the Wave file format contains the following information.
|Byte sequence description||Length in bytes||Starts at byte in the chunk||Value|
|chunk ID||4||0x00||The ASCII character string "fact"|
|size||4||0x04||The size of the fact chunk (number of bytes) less 8 (less the "chunk ID" and the "size")|
|data||4||0x08||Various information about the contents of the file, depending on the compression code|
Fact chunks exist in all wave files that are compressed or that have a wave list chunk. A fact chunk is not required in an uncompressed PCM file that does not have a wave list chunk (see Format chunk (of a Wave file) for a list of compression codes).
According to the fact chunk's initial specification, the data portion of the fact chunk will contain only one 4-byte number that specifies the number of samples in the data chunk of the Wave file. This number, when combined with the samples per second value in the format chunk of the Wave file, can be used to compute the length of the audio data in seconds.
Wave file format