Issues
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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.

Factors Causing Latency

- 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
- 08 Delayed error-based update
- Use of Delay-compensated Algorithm
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.
- 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
- Remote Microphone (RM)
