# siDecompressionParam atom ('wave') An atom that stores data specific to a given audio decompressor. ## Overview The `siDecompressionParam` atom provides the ability to store data specific to a given audio decompressor in the `SoundDescription` record. As example, some audio decompression algorithms, such as Microsoft’s ADPCM, require a set of out-of-band values to configure the decompressor. These are stored in an atom of this type. This atom contains other atoms with audio decompressor settings and is a required extension to the sound sample description for MPEG-4 audio. A `'wave'` chunk for `'mp4a'` typically contains (in order) at least a `'frma'` atom, an `'mp4a'` atom, an `'esds'` atom, and a [`Terminator atom`](/documentation/quicktime-file-format/Terminator_atom). The contents of other `siDecompressionParam` atoms are dependent on the audio decompressor. ## Topics ### Data fields [`Size`](/documentation/quicktime-file-format/siDecompressionParam_atom/Size) An unsigned 32-bit integer holding the size of the decompression parameters atom. [`Type`](/documentation/quicktime-file-format/siDecompressionParam_atom/Type) An unsigned 32-bit field. [`Extension atoms`](/documentation/quicktime-file-format/siDecompressionParam_atom/Extension_atoms) Atoms containing the necessary out-of-band decompression parameters for the sound decompressor. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)