subchunk

A subchunk is a subdivision within a chunk in RIFF- and IFF-based file formats. Each subchunk has a four-character ASCII ID, a four-byte unsigned integer size, and a data payload of that size. Subchunks allow a file to store multiple types of information in a flexible, extensible way. In WAV audio files, which use the RIFF container, subchunks organize format metadata and data.

In WAV, the file starts with a RIFF chunk that declares the container type (such as "WAVE") and contains several subchunks. The most common are the format subchunk "fmt " and the data subchunk "data". The fmt subchunk encodes audio format, number of channels, sample rate, byte rate, block align, and bits per sample. The data subchunk contains the raw audio samples and its size equals the number of bytes of sample data. Other subchunks, such as "LIST" for metadata or "fact" for certain compressed formats, may appear. Subchunks are arranged sequentially within the parent chunk and may be padded to align on even-byte boundaries.

When reading, software processes each subchunk by reading its 4-byte ID and the 4-byte size, then consuming size bytes of payload before continuing to the next subchunk. Because some formats use odd lengths, a single padding byte may be added after an odd-sized subchunk data to preserve alignment. The concept of subchunks is common in RIFF-family formats, enabling extensibility without changing the overall container structure.