Sound leakage investigation of ANC headphones
Active noise control (ANC) headphones have received wide attention globally in recent years. Apart from developing more advanced noise cancellation algorithms, another challenge the ANC headphone industry faces is the selection of microphone positioning, which would significantly influence the performance of active noise cancellation.
The materials and structures of the headphone cups lead to in-homogeneous distribution of sound leakage. When the microphone is placed on positions with different leakage, it sends different feedback to the ANC system and results in different performance. Therefore, the positioning of the microphone plays a critical role in the noise cancellation effect. The particle velocity sensor can significant restrict background noise due to its high directivity, and thus suitable to apply for in-situ sound localization.
This study aims at obtaining a full understanding of the sound field around the ANC headphones to detect potential resonance and localize the main leaking spots.
In cooperation with HT Acoustics
Extracted from: Meng, F., Yu, A., Fernandez Comesana, D. (2020). Sound leakage investigation of ANC headphones using particle velocity sensors. In Proceedings of International Congress and Exposition on Noise Control Engineering (The 49th Inter-noise 2020).
- Fast leakage detection on ANC headphones
- Localization of resonances in a non-anechoic environment
- Accurate in-situ sound localization in 200 – 1000 Hz
- Understanding of the sound field around the headphone
- Optimization of the selection of the reference microphone position of ANC headphones
Reference microphone positioning
The reference microphone was positioned on Point A, B and O. With performing feedforward ANC algorithm, it can be seen that positioning the microphone at O outperforms A or B. The noise reduction difference can reach 10 dB at some frequencies. The reference microphone at various positions on the ear cup captured different sounds and feed in the forwarder ANC system, which leads to the variations in the noise reduction performance.
In-situ leakage detection
Voyager was used to conduct fast in-situ leakage detection. White noise was played by the loudspeaker in the ear cup on one side of the device, and the PU Voyager probe was placed 5 mm outside of the ear cup to measure the sound field. The probe was placed at the reference microphone position of 5 different headphones of different brands. Large variations exist between headphones. PVL (Particle velocity level) shows a larger dynamic range than SPL (Sound pressure level), due to the directivity of the particle velocity sensor, along with the intrinsic dependency on surface displacement perceived in the near field.
Focusing on one headphone, peaks in 600 – 1000 Hz and 1000 – 1200 Hz were observed indicating leakages. P9 (same as point O) shows anti-resonance at around 800 Hz in the PVL spectrum. On the contrast, the SPL spectra show no significant difference between the selected 5 points.
3D sound visualization
The sound visualization system used to capture the information hereby presented is Scan & Paint 3D.
In the 3D visualization of the particle velocity field around the headphone earcup, the bottom of the cushion is the main leakage spot. In 600 - 1000 Hz (left column; right column: 1000 – 1500 Hz ), the middle and bottom areas of the earcup side show anti-resonance, which is in line with the results measured with Voyager. Besides, it shows that the top left and right parts of the ring resonate in this frequency range.
The pressure field visualization shows the bottom of the cushion can be seen as the main leakage spot in 600 - 1000 Hz. Whereas in 1000 - 1500 Hz, the wide distribution of high pressure around the ear cup hardened the detection of the leakage.
Sound Intensity Field
The sound intensity fields are shown in 2200 - 2600 Hz (left column) and 4150 - 5130 Hz (right column). The bottom of the cushion remains the main leakage spot. Besides, the resonance on the side of the ear cup can be visualized, as well as two nodes on the ring of the ear cup (left bottom). In 4150 - 5130 Hz , a leakage spot, top left of the cushion, and two anti-nodes, top left and bottom right of the ear cup can be visualized. The middle of the ear cup shows anti-resonance pattern.
- Leakage and resonance/anti-resonance were localized in-situ by both Voyager and Scan & Paint 3D
- Particle velocity outperforms pressure in detecting leakage and modal behavior of the headphone ear cups
- Voyager provides a fast in-situ way of acoustic measurements of headphones
- Scan & Paint 3D is a complete solution to fully understand sound leakage and vibration of headphones with 3D sound visualization