Acoustic Cavity Design for MEMS Microphone Arrays

Sound Port Geometry, Sealing, Vibration Isolation & Anti-Resonance Best Practices

Author: Wuxi Silicon Source Technology Co., Ltd. (SISTC)
Version: 1.0 (Public White Paper)
Category: Explore / Acoustic Engineering / MEMS Microphone Array Design

Executive Summary

In microphone array products—smart speakers, conference systems, dashcams, smart home controllers, and industrial voice terminals—acoustic cavity design is one of the most underestimated drivers of real-world performance.

Even when the MEMS microphone itself meets excellent datasheet specifications (SNR, sensitivity, AOP, phase matching), the final product can still suffer from:

random ringing/buzzing noise due to structural vibration coupling
unexpected high-frequency peaks caused by Helmholtz resonance
degraded wake-word performance due to leakage and inconsistent sealing
beamforming instability due to non-uniform acoustic paths

This white paper provides practical structural design rules for acoustic cavities, sound ports, sealing, damping, and array layout, helping engineering teams shorten development cycles and avoid costly re-spins.

SISTC has been focused on MEMS microphone design and manufacturing for 15+ years, with strong capabilities in audio pickup technology, noise reduction algorithms, and hardware-software integration for acoustic systems.

1. Why Acoustic Cavity Design Matters

An acoustic cavity is not just a “hole” in the enclosure. It forms a controlled acoustic path between the external sound field and the microphone inlet.

In array systems, the acoustic cavity must achieve two objectives simultaneously:

Maximum direct pickup with minimal distortion or frequency coloration
Consistency across microphones, enabling stable beamforming and accurate phase alignment

In practice, poor cavity design often causes bigger performance loss than the microphone itself.

2. MEMS Microphone Selection Requirements (for Array Applications)

Before structural design begins, the microphone model must match array requirements. SISTC recommends the following baseline specifications for array products:

Sensitivity: > -40 dBV @ 94 dB SPL, 1 kHz
SNR: ≥ 65 dB
AOP: ≥ 120 dB SPL
THD: ≤ 1% @ 1 kHz
Phase coherence: < 3°
Omnidirectional pickup pattern
Frequency response variation: < 2 dB (100 Hz–8 kHz)

3. Sound Port & Acoustic Cavity Geometry (Core Rules)

3.1 The L/D Rule: The Most Practical Design Metric

A sound port is essentially a short acoustic tube. Its geometry is defined by:

L = depth of the sound port (or cavity length)
D = diameter of the sound port opening

SISTC recommendation:

D > 1.0 mm
L < 5.0 mm
L/D < 3 (critical guideline)

This means:

keep the port as wide as possible
keep the cavity as shallow as possible

Why it works:
A smaller L and larger D reduces acoustic impedance and minimizes unwanted resonant behavior, leading to smoother frequency response and better sensitivity stability.

3.2 PCB Sound Hole Size (Bottom-Port MEMS)

For bottom-port microphone structures, the PCB hole must match microphone sealing ring constraints.

Current common sealing ring inner diameters:

0.6 mm
1.0 mm

Considering manufacturing tolerance, recommended PCB apertures:

for 0.6 mm ring → PCB hole 0.4 mm
for 1.0 mm ring → PCB hole 0.8 mm

When PCB thickness exceeds 1.0 mm, prioritize microphones with 1.0 mm sealing ring for better manufacturability and sealing reliability.

4. Avoiding Helmholtz Resonance (Most Common Root Cause)

4.1 What is Helmholtz Resonance?

A Helmholtz resonator forms when a cavity volume connects to the outside through a narrow neck—similar to blowing across a bottle opening.

In microphone products, this typically happens when:

the port is too narrow/too long
the cavity behind the microphone inlet is too large
sealing is inconsistent and creates unintended volumes

4.2 Why it is Dangerous

Helmholtz resonance often creates:

high-frequency spikes
frequency response ripple
unstable acoustic performance across samples

SISTC explicitly recommends:
Avoid creating Helmholtz resonator structures, both for top-port and bottom-port designs.

Engineering checklist to avoid it:

follow L/D < 3
avoid narrow channels
eliminate unnecessary internal voids near the inlet
ensure consistent sealing so the cavity volume does not vary by assembly

5. Sealing, Gaskets, and Leakage Control

5.1 Uniformity is Mandatory in Arrays

For array products, each microphone must experience the same acoustic environment.

SISTC recommendation:

every microphone housing must be designed and installed identically
use silicone, gaskets, or foam seals between:
- panel and PCB
- silicone microphone and structure

5.2 Practical Sealing Principles

ensure direct vocal pickup at the sound hole and equal distribution to each microphone
prevent the sound holes from being blocked
use sealing materials that remain stable over temperature and aging
design for manufacturing tolerances (avoid borderline dimensions)

6. Vibration Isolation & Mechanical Noise Suppression

In real products, random ringing/buzzing often comes from mechanical coupling, not from the MEMS microphone.

6.1 Isolation Recommendations

microphone should be isolated from firmware/structure surfaces using silicone sleeve
avoid hard contact between microphone and enclosure
silicone thickness around the microphone should be > 3 mm

6.2 Positioning Guidelines

keep microphones away from vibration or interference sources
isolate from solid surfaces to reduce vibration transmission and improve sealing

7. Dust Filter / Dust Cover Design

For products where the microphone pickup hole faces upward:

add a dust cover to prevent long-term dust accumulation
select a filter with:
- flat frequency response
- minimal transmission loss

Tip: in mass production, dust cover selection should include acoustic verification across temperature and humidity conditions.

8. Microphone Array Layout Recommendations

Acoustic cavity design must work together with array geometry.

8.1 Line Array

all microphones face the same direction
all inlet ports aligned on a straight line

Recommended spacing:

2–4 mic line array: 30–60 mm
5–6 mic line array: 21.3–50 mm
best practice: 30–40 mm

8.2 Circular Array

Recommended radius:

30–40 mm

9. Speaker Interaction (If Product Includes Speaker)

When speaker + microphone coexist, acoustic cavity design must prevent speaker energy from coupling into microphones.

SISTC recommendations:

implement internal sound insulation between MIC and SPK
distance between MIC pickup port and SPK outlet: ≥ 100 mm
ensure MIC and SPK ports are ideally 90° (not aligned)
keep speaker-to-microphone SPL below 90 dB at MIC position
voice-to-speaker SNR at MIC should not be less than -25 dB

10. How SISTC Supports Customer Acoustic Cavity Design

During the structural design phase, SISTC recommends customers share:

microphone datasheet
amplifier / codec / speaker datasheet
structural drawings

Our engineering team can assist with:

acoustic cavity and sound port review
sealing and tolerance evaluation
array geometry validation
risk analysis for resonance and vibration noise
prototype tuning and acoustic testing guidance

Conclusion

A successful microphone array product depends on system-level acoustic engineering, not only microphone selection.

By following key design rules such as:

L/D < 3
avoid Helmholtz resonators
strict sealing uniformity
silicone isolation > 3 mm
proper dust filtering and array spacing

engineering teams can significantly improve:

wake-word robustness
beamforming stability
frequency response consistency
mass production yield

Call to Action

If you are designing a microphone array product and want SISTC to review your acoustic cavity and sound port structure, contact our team:

Website: https://www.sistc.com
Product page (internal link): Smart MEMS Microphone