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Introduction to the Technology
In order to clarify what particular features are needed from 3D audio, it’s important to clearly define it. Although there is a plethora of technology commonly referred to as “3D audio,” much of this technology does not fit a simple definition. 3D audio should be simply defined as the sound heard in a realistic 3D environment. For example, you can distinguish between sounds heard in front of you, behind you, above you and below you.

This simple definition sets a standard for what people should expect from 3D audio technology. However, there are features not contained in this simple definition that are critical to delivering sound that enables even more information transmission. For example, it is implied that a listener in the 3D environment hears the sound with his or her ears, yet everyone hears differently due to their physical characteristics, and a “one size fits all” solution just won’t work in precision military applications.

Since most military applications that need sound are interactive, the 3D audio must also be interactive and thus must support a user (listener) who can interact in an environment by moving their head without the aural world around them shifting with their movements. Instead, objects should remain virtually stationary regardless of position, direction, tilt or movement of the head. This requires positional information about the listener’s head to compute what the listener should hear. This ability to interact is critical in military applications where a soldier cannot be strapped down into one position and told not to move his head.

Another quality that distinguishes different types of 3D audio is the level of overall precision. Precision in 3D audio can best be described by the accuracy with which a listener is able to localize sound. Since humans can localize to within two degrees of the actual location of a sound, delivering sound with this degree of precision is necessary. If you do not utilize 3D audio with this level of precision, the results can be drastic. The sound can be completely confusing to a listener and cause them to react in an inappropriate way.

The final characteristics that are important for 3D audio are on the system level. First, it should be noted that most 3D audio technologies are not versatile or scalable enough to be integrated into many applications. Simply put, if you cannot use the technology, you cannot gain the benefits. Scalable and versatile technology will easily integrate into every application from small wearable communication and navigation computers to large, complex and stationary networked tank battalion simulators.

Most 3D audio technologies use computationally cheap methods of generating 3D sounds that simply do not come close to accurately modeling sound. They often modify the audio with approximate delays, frequency shifts and gain losses in an attempt to deliver the illusion of sound emanating from around a listener. In general, however, there are several problems with this approach. The modifications are approximations and result in a lack of 3D sound positioning accuracy. Surrounding environments are not well accounted for and many ways in which audio is uniquely shaped by the path are ignored (reflections, refraction, diffusion, diffraction).

For example, sounds coming from the back hemisphere are often simulated by cutting off the high frequencies; no sound propagation simulation is computed at all. While sounds emanating from the back hemisphere often do have slight attenuation in the higher frequencies, there is still vital information that must be simulated and presented in the higher frequencies, particularly for generating cues for the perception of sounds at varying heights. In general, the value of 3D audio is greatly diminished by inaccurate technologies and the resulting sound is often unnatural, difficult to localize, and lacking in spatial correlation and organization, leading to confusion.

In many cases, a lack of precision causes many 3D audio technologies to deliver sound that is less helpful to a listener than simple stereo or mono sound because it confuses them. A lack of spatial organization and seemingly unnatural sound can be perceived entirely incorrectly. Multiple sounds may be perceived as emanating from inside the head and come across as completely jumbled. Often, visuals help reinforce the perceived location of an object generating sound, but when not precisely spatially correlated, the misleading aural cues are worse than none at all.

In mission-critical situations, the sound positioning must be exact. For example, when a fighter pilot is being approached by an aircraft from behind and slightly above, he needs to know this exact information. If his technology is limited to positioning audio without the correct elevation and places the sound directly behind him, he could make the mistake of maneuvering his plane directly into the path of an oncoming aircraft. Precise sound positioning is required to prevent confusion.

Another important shortcoming of many 3D audio technologies stems from the fact that there are varying levels of interactivity. Elementary systems assume fixed positions of both the listener and sounds. Others assume a fixed head position but simulate the motion of sounds. Fully interactive technologies account for both the movements of the head and sounds, alleviating a problem referred to as the “frozen perspective.”

For example, in a search and rescue mission where a wounded soldier (relaying his position via GPS) is being sought out in dense jungle, a sound beacon can serve to guide a searcher to the wounded soldier. Without complete interactivity, as soon as the searcher moves his position the beacon would no longer represent the location of the wounded soldier. An added bonus with this technology: the searcher does not have to stop walking to study a map and compare GPS readings. Finally, many existing systems capable of overcoming these shortcomings still remain either too costly, too voluminous or cannot scale and integrate into varying applications.

AuSIM's 3D Technology
3D audio has been around for a while, but its usefulness has increased with recent advancements. AuSIM3D is an audio simulation technology that generates extremely accurate, completely interactive and cost-effective 3D audio. Perceptual location cues are calculated by physically modeling the three areas of sound propagation—source propagation, environment propagation and listener filtering. The models are updated in real time and work well with position tracking devices for head and object tracking. The physical delivery forms of advanced 3D audio technologies have changed dramatically in the last ten years along with the quality, size and cost. The latest 3D audio technology from AuSIM is available in two COTS forms. Hardware-based client-server systems that offload all audio processing from a host typically serve as a scalable (one size fits all) form for use as development or runtime systems. The second COTS form is a cost-effective software-based 3D audio engine that can be installed on existing computing hardware and run either directly on a host or in a client-server fashion. The versatility of this second software form proves key for integration into a plethora of applications. It enables the technology to exist in peripheral boxes, integrated subsystems, standalone systems, end-user applications, application plug-ins, SDK libraries and software resources (DLLs) with an application-programming interface (API).
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