Doppler effect for moving vehicles

[kfarr]

A few good writeups:

• Doppler effect in threejs: https://github.com/mrdoob/three.js/issues/16289
• Web audio: https://gamedevelopment.tutsplus.com/tutorials/creating-dynamic-sound-with-the-web-audio-api--cms-24564
• JS example: https://github.com/cemrich/js-motor-sound-generation

First MVP:

• Write a basic A-Frame component that alters pitch and/or playback rate for an entity's looping sound