In real life, spatial sound is vital. When someone calls your name, you know which way to look. If you drop a coin on the sidewalk, you can tell which way it has rolled. Why? Because you can hear “spatially”–in other words, you can tell where in your environment the sound is occurring.
In a mixed reality app, you may have to look up, down, from side to side, or behind you to find something that’s happening. At times, it can be unclear where you should look. Spatial sound connects holograms more deeply to the mixed-reality world and supplies information about the environment and object state. You can provide cues from the direction you want to draw attention to; this helps users to maintain awareness of their real-world surroundings and can guide them to their next steps. This increases user confidence in gesture and voice interactions.
Use of sounds in mixed reality requires a different approach than in touch and keyboard-and-mouse applications. Key sound design decisions include which sounds to spatialize and which interactions to sonify. For detailed guidance, see our Spatial sound best practices article.
Spatialization
Spatialization is the directional component of spatial sound. For a 7.1 home theater setup, spatialization is as simple as panning between loudspeakers. But for headphones in mixed reality, it’s essential to use an HRTF-based technology for accuracy and comfort. Windows offers HRTF-based spatialization, and this support is hardware-accelerated on HoloLens 2.
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## Case studies
HoloTour virtually takes users to tourist and historical sites around the world. See the [Sound design for HoloTour](/mixed-reality/mixed-reality-docs/mr-dev-docs/design/case-study-spatial-sound-design-for-holotour.html) case study. A special microphone and rendering setup were used to capture the subject spaces.
RoboRaid is a high-energy shooter for HoloLens. The [Sound design for RoboRaid](/mixed-reality/mixed-reality-docs/mr-dev-docs/design/case-study-using-spatial-sound-in-roboraid.html) case study describes the design choices that were made to ensure spatial sound was used to the fullest dramatic effect.
## Spatializer personalization
The low-latency head tracking of mixed reality headsets, including HoloLens, supports high-quality HRTF-based spatialization.
HRTFs manipulate the level and phase differences between ears across the frequency spectrum. They're based on physical models and measurements of human head, torso, and ear shapes (pinnae). Our brains respond to these differences to provide perceived direction in sound.
Every individual has a unique ear shape, head size, and ear position. So the best HRTFs conform to you. To increase spatialization accuracy, HoloLens uses your inter-pupilary distance (IPD) from the headset displays to adjust the HRTFs for your head size.
## Spatializer platform support
Windows offers spatialization, including HRTFs, via the [ISpatialAudioClient API](/windows/win32/coreaudio/spatial-sound). This API exposes the HoloLens 2 HRTF hardware acceleration to applications.
## Spatializer middleware support
Support for Windows' HRTFs is available for the following third-party audio engines.
* A [Unity audio engine plugin](/mixed-reality/mixed-reality-docs/mr-dev-docs/develop/unity/spatial-sound-in-unity.html)
* A [Wwise audio engine plugin](https://www.audiokinetic.com/products/plug-ins/msspatial/)
## Acoustics
Spatial sound is about more than direction. Other dimensions include occlusion, obstruction, reverb, portaling, and source modeling. Collectively these dimensions are referred to as *acoustics*. Without acoustics, spatialized sounds lack perceived distance.
Acoustics treatments range from simple to complex. You can use a reverb that's supported by any audio engine to push spatialized sounds into the environment of the listener. Acoustics systems such as [Project Acoustics](/gaming/acoustics/what-is-acoustics) provide richer and more compelling acoustics treatment. Project Acoustics can model the effect of walls, doors, and other scene geometry on a sound. It's an effective option for cases where the relevant scene geometry is known at development time.
## Next steps
- [Spatial sound in Unity](/mixed-reality/mixed-reality-docs/mr-dev-docs/develop/unity/spatial-sound-in-unity.html)
- [Spatial sound best practices](/mixed-reality/mixed-reality-docs/mr-dev-docs/design/spatial-sound-design.html)