The AHM File-Format assumed its name from the "AHM" acronym derived from Acoustic Head Map.  AHM's are maps of HRTF's directionally arrayed around a listener's head.  A typical dataset of HRTF's for a single subject is represented by a large number of transfer functions each representing a particular direction.  Typically regular grid spacings may be 12 to 72 azimuths against 6 to 36 elevations.  AHM's can be three-dimensional with varying range as well. 

Generalized Support
The AHM File-Format has been generalized beyond binaural HRTF datasets to support:

• Multi-sink AHM's (not binaural)
• Filters represented as weights of components
• Acoustic Filter Maps (AFM), arbitrary arrays of filters
• Equalization Filters (EQF), compensation and cross-overs
• Complex infinite impulse response (IIR) filters, which are useful for both component filters and equalization

Structure

	Section	Type	Size	Description
	Title Header	ASCII	192 Bytes	Type, subject, comments
	Data Header	Binary	208 Bytes	Fixed structure
	Data	Binary	Stated in header data	Dynamic structure

Background & Origins 1991
The AHM File-Format initially formulated in 1991 with a consortium of 3D audio researchers discussing ways to share data, keeping intact sample-rate, subject information, matrix structure, spherical grid configuration, etc. The principal participants included:

• Doris J. Kistler, Waisman Center, Univ. of Wisconsin
• Dr. Elizabeth M. Wenzel, NASA Ames
• Scott H. Foster, Crystal River Engineering

William Chapin made a proposal for version 1.0 of the AHM specification, and circulated it for approval.  There was a lot of disagreement and many additional suggestions.  The consortium fragmented and languished.  Crystal River Engineering needed to include a file-format with its product and abandoned AHM v1.0, in favor of a proprietary "Tron Format" with a TRN extension. 

Subsequently in 1992 and 1993, CRE proposed v1.1 and v1.2, respectively; each successively addressing objective feedback, but no version was adopted. 

Aureal & A3D 1996
After MediaVision and CRE merged to form Aureal in 1996, Aureal decided that the HRTF datasets were the most-precious intellectual property in the 3D audio skirmish between Microsoft (DirectSound3D), Intel (RSX), Creative Labs (SoundBlaster 3D), and others, and thus elected to encrypt all HRTF data.  Aureal thus created a proprietary file-format called Acoustic Head Map and versioned to 1.3.  AHM v1.3 was never disclosed and had no similarity to any previous AHM format, except the leading signature (first 40 bytes) which has been the same in all AHM versions.  The data block in AHM v1.3 files must be unencrypted through Aureal's proprietary decryptor. 

AuSIM & AuSIM3D 1998
With the founding of AuSIM in 1998, the AHM File-Format was revived, brought forward to version 1.4, and published on AuSIM's website and to all AuSIM customers.  AuSIM invites any interested party to participate in its ongoing development.