AVAudioEngine, using 3D Audio - swift

Question

I would like to use AVAudioEngine for a 3D audio effect, where the sound source circles the user head. The source appears to move from left to right but I've been unable to figure out how to make it circle the users head. The audio source must be mono or it cant work.
I dont understand AVAudioMake3DVectorOrientation and AVAudioMake3DAngularOrientation.
I thought my math was correct but I suspect that if it were, I would have gotten the results I was looking for.
This is bare bones, so there isn't much error checking.
Would someone provide guidance to get me on track?
Thank you,
W.
import AVFoundation
class ThreeDAudio {
var _angleIndx = 0.0
var _engine : AVAudioEngine!
var _player : AVAudioPlayerNode!
var _environment : AVAudioEnvironmentNode!
var _circleTimer : Timer!
func initTimer()
{
_circleTimer = Timer.scheduledTimer(timeInterval: 0.10, target: self,
selector: #selector(ThreeDAudio.updatePosition),userInfo: nil, repeats: true)
}
#objc func updatePosition()
{
let centerX = 0.0
let centerY = 0.0
let radius = 10.0
let degToRads = Double.pi / 180.0
let angle = _angleIndx * degToRads
let x = centerX + sin(angle) * radius
let y = centerY + cos(angle) * radius
let z = 0.0
let posInSpace = AVAudioMake3DPoint(Float(x), Float(y), Float(z))
_angleIndx += 1.0
_player.position = posInSpace
print("angle: \(_angleIndx) , \(posInSpace)")
if(_angleIndx == 360.0) { _circleTimer.invalidate() }
}
func getBufferFromFileInBundle(fileName: String, fileType: String) -> AVAudioPCMBuffer?
{
// audio MUST be a monoaural source or it cant work in 3D
var file:AVAudioFile
var audioBuffer : AVAudioPCMBuffer? = nil
let path = Bundle.main.path(forResource: fileName, ofType: fileType)!
do{
file = try AVAudioFile(forReading: URL(fileURLWithPath:path))
audioBuffer = AVAudioPCMBuffer(pcmFormat:(file.processingFormat), frameCapacity: AVAudioFrameCount(file.length))
try file.read(into: audioBuffer!)
} catch let error as NSError {
print("Error AVAudioFile:\(error)")
}
return audioBuffer
}
func outputFormat() -> AVAudioFormat
{
let outputFormat = _engine.outputNode.outputFormat(forBus: 0)
let nChannels = outputFormat.channelCount // testing, will always be 2 channels
let sampleRate = outputFormat.sampleRate
return AVAudioFormat(standardFormatWithSampleRate: sampleRate, channels: nChannels)
}
func setupEngine(_ audioBuffer: AVAudioPCMBuffer )
{
_engine = AVAudioEngine()
_player = AVAudioPlayerNode()
_environment = AVAudioEnvironmentNode()
_player.renderingAlgorithm = .HRTF
_engine.attach(_player)
_engine.attach(_environment)
_engine.connect(_player, to: _environment, format: audioBuffer.format)
_engine.connect(_environment, to: _engine.outputNode, format: outputFormat())
_environment.listenerPosition = AVAudioMake3DPoint(0.0, 0.0, 0.0);
_environment.listenerVectorOrientation = AVAudioMake3DVectorOrientation(AVAudioMake3DVector(0, 0, -1), AVAudioMake3DVector(0, 0, 0))
_environment.listenerAngularOrientation = AVAudioMake3DAngularOrientation(0.0,0.0, 0.0)
do{
try _engine.start()
} catch let error as NSError {
print("Error start:\(error)")
}
}
func startAudioTest()
{
var thisFile = (name: "", fileType: "")
thisFile = (name: "sound_voices", fileType: "wav")
thisFile = (name: "Bouncing-Ball-MONO", fileType: "aiff")
let audioBuffer = getBufferFromFileInBundle(fileName: thisFile.name, fileType: thisFile.fileType )
if ( audioBuffer != nil )
{
setupEngine( audioBuffer! )
initTimer()
_player.scheduleBuffer(audioBuffer!, at: nil, options: .loops, completionHandler: nil)
_player.play()
}
}
}