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Issues on Active Noise Control Technology – Latency

  • 01 When the control signal reaches the error microphone later than the noise signal, it is considered non-causal. When a non-casual case occurs, it decreases ANC performance.
  • 02 Whether to be causal or non-causal is determined by latency.
  • 03 The issue of being causal or non-causal only applies to Feedforward or Hybrid systems that use reference microphones.
Causal or Non-causal

Factors Causing Latency

Typical analog LPF group delay
  • 01 Group delay of Anti-aliasing LPF & Anti-imaging LPF (in the case of Analog Mic)
  • 02Conversion time of A/D and D/A converters
    • Successive Approximation A/D converter: Fast conversion time
    • Sigma-Delta type A/D converter: Slow conversion time
  • 03 For digital MEMS Mic, a low-order decimation filter is required to reduce latency.
  • 04 Algorithm Delay
    • Delay caused by the time required for algorithm computation
    • Sample processing : 1 sample delay
    • Block processing : 1 block + 𝛼 (typically 𝛼 = 1 block)
    • Hybrid processing : 1 sample delay
  • 05 Speaker Group delay
    • Large-diameter woofers designed to reproduce very low frequencies can cause significant group delay.
  • 06 Secondary path propagation delay: Propagation delay from a control speaker to an error microphone
  • 07 Geometry-dependent
    Geometry-dependent
  • 08 Delayed error-based update
    • Use of Delay-compensated Algorithm
    Delayed error-based update

Issues on Active Noise Control Technology – Small Quiet Zone

  • 01 The diameter of the Quiet Zone created by ANC
    • 1/10 of the noise signal's wavelength
    • At 100 Hz: 34cm
    • At 1000 Hz: 3.4cm
  • 02 ANC Headphones or Earphones
    • The small Quiet Zone is not an issue due to the confined space inside the ear.
    • However, it becomes problematic when targeting high-frequency noise.
    • For example, with 4 kHz noise, the Quiet Zone radius is 4.25 mm, which is smaller than the distance to the eardrum.
    • In most cases, noise below 500 Hz is active-noise controlled, and noise above 500 Hz is passive-noise controlled (sound shielding by headphone earcaps)
  • 03 Technology to expand the Quiet Zone: Multi-channel ANC
  • 04Technology to move the Quiet Zone: Virtual sensing technology

Active Noise Control Technology – Perfect Cancellation

  • 01 It is fundamentally impossible to eliminate noise radiated in all directions from a noise source in every space.
  • 02 To eliminate noise radiating into the entire space, a signal that is present in the same location as the noise source, has the same amplitude as the noise source, and has the opposite phase
    • This is not practically feasible.
  • 03 Applications where perfect cancellation is theoretically possible: Duct environments
    • When a duct¡¯s cross-sectional area is narrow, the sound passing through the duct is relatively perceived as a plane wave.
    • In theory, canceling the sound on one plane can lead to the removal of the sound further along the duct.
    • The basic principles of Control-at-source method
      • Simultaneously apply passive and active noise control
      • Used in noise reduction applications that require heat dissipation or air intake/exhaust
      • The noise source is entirely enclosed with sound-absorbing materials to block noise emission, and then a duct is implemented in a certain area for intake/exhaust.
      • Noise leaking through the duct is removed with active noise control.

Active Noise Control Technology – Virtual Sensing

  • 01 In some ANC applications, it may be difficult to put an error microphone in a place where noise needs to be canceled.
  • 02In such a case, if the error microphone is placed away from the desired location, noise cancellation at the target location is not made appropriately due to ANC's small Quiet Zone characteristics.
  • 03 Virtual Sensing technology involves installing a virtual microphone in a place for noise cancellation, making it function as if a real error microphone is placed there.
    Single-channel or Multi-channel Active Noise Controller using a virtual microphone
  • 04 This technology ultimately involves estimating the signal at the virtual microphone location by using the signal received by the physical microphone.
  • 05 Two main technologies are typically used.
    • Remote Microphone (RM)
      • Less dependent on the tuning signal
      • Sensitive to changes in the acoustic path
    • Auxiliary Filter (AF)
      • Dependent on the tuning signal
      • Less sensitive to changes in the acoustic path
ANC with virtual microphone technology based on RM / Characteristics of transfer  between physical and virtual microphones