How does Apple’s “spatial audio” or other “3D audio” work when there are only two speakers?

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How does Apple’s “spatial audio” or other “3D audio” work when there are only two speakers?

In: Technology
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Humans locate sources of sound in space through frequency response cues (i.e., the bass/midrange/treble balance; for example, noises heard from far away will sound more muffled due to air absorbing the upper treble), the difference in time it takes for a sound to reach one ear before the other, the way the sound changes as you move your head around, and the difference in loudness between ears.

Headphones and earphones can only simulate the last one and (to a much lesser extent) the first one because instead of letting sources of sound interact with your outer ear and other body parts naturally, you’re piping sound directly into each ear. However, those interactions can be turned into a mathematical model by measuring how sound changes as you move a sound source around either a real or dummy human head (called a *head-related transfer function* or *HRTF*). You can apply this HRTF to audio digitally to simulate listening to something from speakers or live when listening to headphones.

Also, sound bounces around real rooms, whether it’s a home theater or an auditorium, which creates its own changes in when and how sound waves reach the ears. This can also be simulated through digital processing.