English 
Chinese Introduction
Omni‑directional microphones are widely used in two‑ or multi‑microphone arrays to enable beam‑forming and spatial sound capture. By detecting differences in the time of arrival (ToA) of acoustic signals, microphone arrays can enhance sound from a preferred direction while suppressing unwanted noise.
However, when physical space constraints limit microphone spacing—as in earphones, hearing aids, and compact wearables—phase matching becomes the dominant factor determining low‑frequency directional performance. Phase‑sequenced MEMS microphones provide a practical and scalable solution to this challenge.
Why Phase Sequencing Matters
Historically, matched‑pair and matched‑triplet electret microphones have been used in hearing aids to enable switchable omni‑directional and uni‑directional (beam‑forming) operation in the voice band. With the industry shift toward MEMS microphones, similar matching concepts have been applied—initially through tray‑packaged matched pairs.
However, tray‑based matching introduces significant challenges when migrating to tape‑and‑reel (T&R) manufacturing:
- Pick‑and‑place automation cannot guarantee placement of specific matched pairs
- Dropped or mis‑placed microphones break the matched set
- Manual intervention increases cost and reduces yield
Phase sequencing eliminates these obstacles by arranging microphones in a known phase order on tape‑and‑reel, enabling efficient, automated array assembly without per‑unit calibration.
Phase, Spacing, and Low‑Frequency Directivity
Consider a two‑microphone array integrated into an in‑ear or behind‑the‑ear device:
- Speed of sound in air: ~343 m/s
- Typical microphone port spacing: 15 mm
- Acoustic travel time over 15 mm: ~44 µs
At 400 Hz (the low end of the telephone band), the acoustic wavelength is approximately 0.858 m. A 15 mm spacing corresponds to only 1.7% of the wavelength, or about 6.3 degrees of phase difference.
To reliably resolve time‑of‑arrival differences at low frequencies, phase mismatch between microphones must be a small fraction of this phase difference. Excessive phase error directly degrades beam‑forming accuracy and low‑frequency directivity.
Directionality and Beam‑Forming Fundamentals
Directional microphone arrays commonly generate a cardioid polar pattern by applying an electronic delay to one microphone signal. When this internal delay matches the external acoustic delay between ports, signals from the rear direction cancel when subtracted.
Different ratios between internal (electronic) and external (acoustic) delay produce different polar patterns, including:
- Omni‑directional
- Sub‑cardioid (pre‑cardioid)
- Cardioid
- Super‑cardioid
- Hyper‑cardioid
These patterns belong to the limaçon family of curves, described by:
f(θ) = (1 − k) + k · cos(θ)
where k is the ratio of external to total phase shift. Phase errors between microphones distort the intended polar response, reducing rear‑null depth or introducing unwanted back lobes.
Phase‑Sequenced MEMS Reels
Phase‑sequenced MEMS reels offer a powerful alternative to:
- Software‑based initialization and calibration
- Tray‑packaged matched microphone pairs or triplets
In a phase‑sequenced reel, every microphone is 100% measured (typically at 1 kHz sensitivity and 200 Hz phase) and placed on tape in monotonic phase order. The microphone with the largest phase is positioned at the start of the reel.
This approach enables:
- Flexible population of PCB microphone arrays
- Automated pick‑and‑place without manual pairing
- Consistent directional performance across devices
Example: SISTC WBC4030DB26UJ0
As a practical example, consider the Silicon Source Technology (SISTC) digital MEMS microphone WBC4030DB26UJ0.
For this model:
- Phase variation within a 100‑microphone window is limited to ±1.5 degrees at 200 Hz
- At 400 Hz, the effective phase error is even smaller
From the earlier example, the total phase difference due to spacing at 400 Hz is ~6.3 degrees. A 1.5‑degree tolerance at 200 Hz represents less than 24% of the spacing‑induced phase difference, well within acceptable limits for stable beam‑forming.
In terms of delay ratio (k), the difference between cardioid and super‑cardioid behavior is approximately 26%. Even in the worst‑case scenario, an array built with WBC4030DB26UJ0 would only exhibit minor deviations from the intended polar pattern—differences that are extremely difficult to detect in real‑world use.
In practice, typical phase distribution is significantly tighter than the maximum specification, further improving array consistency.
Internal link:
- SISTC Digital MEMS Microphones: https://sistc.com/product/digital-mems-microphone/
Manufacturing and System‑Level Benefits
Although phase‑sequenced reels carry a modest cost premium compared to non‑sequenced reels, they often reduce total system cost by:
- Eliminating per‑device calibration
- Simplifying manufacturing logistics
- Increasing automation and yield
- Improving product‑to‑product consistency
These advantages are particularly valuable for earphones, hearing aids, and consumer audio products manufactured at high volume.
Conclusion
Phase‑sequenced MEMS microphones provide a scalable, production‑ready solution for beam‑forming applications where space is limited and low‑frequency directivity is critical. By ensuring tight phase control at the component level, manufacturers can achieve repeatable directional performance without costly calibration steps.
As MEMS microphones continue to evolve toward higher integration and AI‑enabled audio front ends, phase sequencing will play an increasingly important role in enabling compact, high‑performance microphone arrays.
Silicon Source Technology (SISTC) remains committed to delivering high‑precision MEMS microphone solutions optimized for beam‑forming, wearables, and next‑generation intelligent audio devices.
External reference:
- General background on beam‑forming: https://en.wikipedia.org/wiki/Beamforming
- SISTC official website: https://www.sistc.com
