Concept renderA MEMS mic port with a cavity behind a narrow vent can form a Helmholtz resonator. TDK/InvenSense AN-1003 says the PCB sound hole should be at least 0.25 mm and warns that a wide gasket can cause a high-frequency peak. Estimate the peak with the vendor formula, then measure it.
Put a MEMS microphone behind a case opening and you have built an acoustic part. The hole, the tunnel and the air volume behind them can add a peak to the frequency response. This guide collects what one vendor application note says about port design, computes the resonance for a set of sample geometries, and lists the measurements to run on your own enclosure. It contains no Duobay price and no measured result.
This guide is written for mechanical and acoustic engineers on a wearable. If the product records speech, the port design is only one part of a consent and indicator-light design. Recording must be visible to the people around the device. The port layout should never be used to hide a microphone.
What does the vendor say about the port?
The only primary document we read in full is TDK/InvenSense application note AN-1003, "Recommendations for Mounting and Connecting InvenSense MEMS Microphones," revision 1.0, release date 12/31/13. It covers InvenSense bottom-ported MEMS microphones. We read a copy hosted on a distributor site. We did not read the design guides of other microphone vendors, so we quote none of them.
| Topic | What AN-1003 says | Note | Source |
|---|---|---|---|
| PCB sound hole diameter, lower limit | Response is not affected by hole size as long as the hole is not smaller than 0.25 mm (0.010 inch). | Hole size only matters below this limit, according to the vendor. | AN-1003, as of October 31, 2026 |
| PCB sound hole diameter, typical | 0.5 mm to 1 mm (0.020 inch to 0.040 inch). | Typical range from the note. | AN-1003, as of October 31, 2026 |
| Gasket between mic PCB and case | A wide gasket forms a Helmholtz resonator and may cause a high-frequency response peak. Keep the gasket as small as possible, or place the board directly against the device case. | The vendor says to avoid the peak unless the designer deliberately seeks it. | AN-1003, as of October 31, 2026 |
| Longer acoustic path | When industrial design forces a longer path, keep the effective path diameter close to or smaller than the case opening. Multiple small holes can replace a single vent. | Figure 3 of the note shows a recommended gasket design. | AN-1003, as of October 31, 2026 |
| Gasket seal and material | A good seal between case and gasket, and between PCB and gasket, is important. Gasket stiffness has a negligible effect on microphone performance. | The note lists rubber, silicone, neoprene and closed-cell foam as examples. | AN-1003, as of October 31, 2026 |
| Dust mesh or membrane, tunnel sweep on a real mic | Vendor did not publish numbers in the document we read. | Vendor did not publish this. Measure it on your board. | AN-1003, as of October 31, 2026 |
How do you estimate the Helmholtz resonance?
A Helmholtz resonator is a wide cavity with a narrow vent to the outside. AN-1003 gives this estimate for the resonance frequency:
fb = c × D / ( 4 × sqrt( π × V × ( L + sqrt(π) × D / 2 ) ) )
fb resonance frequency, Hz
c speed of sound, about 340 m/s = 340,000 mm/s
D vent diameter, mm
L vent length, mm
V cavity volume, mm³A caution on the formula. A plain-text extraction of the PDF drops the square-root signs, and the resulting expression is dimensionally wrong. We checked the formula against the page image of the PDF (page 3) and used the form printed there. The vendor says the result is an estimate of where the resonance is likely to be, and that real parts can differ because of nonrigid gasket walls, leaks and other imperfections.
Worked example, recomputed by us: D = 1 mm, L = 1 mm, V = 10 mm³. The term inside the root is π × 10 × (1 + 0.886) = 59.3, its root is 7.70, and fb = 340,000 × 1 / (4 × 7.70) = 11,042 Hz, or about 11.0 kHz. These inputs are an example for illustration.
Inputs, and which are yours to replace
| Variable | Meaning | Status | What to do |
|---|---|---|---|
| c | Speed of sound | Published in AN-1003 | About 340 m/s, entered as 340,000 mm/s so the units match. |
| D | Vent diameter, mm | Your assumption | Replace with the vent diameter in your design. |
| L | Vent length, mm | Your assumption | Replace with the length of the tunnel through the case and gasket. |
| V | Cavity volume, mm³ | Your assumption | Replace with the air volume between the mic port and the vent, from your CAD model. |
| fb | Estimated resonance, Hz | Computed | Estimate of where the peak is likely to sit. The vendor says calculated and measured values can differ because of nonrigid gasket walls, leakage and other imperfections. |
Where does the peak land for sample geometries?
We swept the formula over vent diameters of 0.5, 1 and 2 mm, cavity volumes of 5, 10 and 50 mm³, and vent lengths of 0.5, 1, 2 and 5 mm. The geometries are our sample values, chosen to show the trend. Values are the computed estimate in kHz, using the AN-1003 formula, as of October 31, 2026. Compare each value with the top of the passband your product needs.
| Vent D, cavity V | L = 0.5 mm | L = 1 mm | L = 2 mm | L = 5 mm |
|---|---|---|---|---|
| D = 0.5 mm, V = 5 mm³ | 11.0 kHz | 8.9 kHz | 6.9 kHz | 4.6 kHz |
| D = 0.5 mm, V = 10 mm³ | 7.8 kHz | 6.3 kHz | 4.9 kHz | 3.3 kHz |
| D = 0.5 mm, V = 50 mm³ | 3.5 kHz | 2.8 kHz | 2.2 kHz | 1.5 kHz |
| D = 1 mm, V = 5 mm³ | 18.2 kHz | 15.6 kHz | 12.6 kHz | 8.8 kHz |
| D = 1 mm, V = 10 mm³ | 12.9 kHz | 11.0 kHz | 8.9 kHz | 6.3 kHz |
| D = 1 mm, V = 50 mm³ | 5.8 kHz | 4.9 kHz | 4.0 kHz | 2.8 kHz |
| D = 2 mm, V = 5 mm³ | 28.5 kHz | 25.8 kHz | 22.1 kHz | 16.5 kHz |
| D = 2 mm, V = 10 mm³ | 20.1 kHz | 18.2 kHz | 15.6 kHz | 11.7 kHz |
| D = 2 mm, V = 50 mm³ | 9.0 kHz | 8.1 kHz | 7.0 kHz | 5.2 kHz |
Computed from the AN-1003 formula (source, accessed 2026-10-31). Rounded to 0.1 kHz.
Three trends follow from the arithmetic. A larger cavity volume V lowers the estimate. A longer tunnel L lowers it. A wider vent D raises it. These are properties of the formula, so they show the direction of each change. Measure the size in your enclosure. A big cavity behind a small, long vent pulls the peak toward lower frequencies, which is the shape the vendor warns about when a wide gasket sits behind a small case opening.
What are the common port-design traps?
These follow from the vendor note and the formula above. These are engineering inferences from the note and the formula.
- Letting a thick gasket make a cavity. The vendor says a wide gasket between the mic PCB and the case forms a Helmholtz resonator and may cause a high-frequency peak. In a wearable, a gasket cut for easy assembly can create exactly this cavity.
- Choosing gasket material to tune the peak. The vendor says gasket stiffness has a negligible effect. The formula takes D, L and V as inputs.
- A leaky seal. The vendor says a good seal on both faces of the gasket is important. If the seal leaks, the real acoustic path differs from the one you modeled. Engineering inference: your calculated peak and your measured peak will then disagree.
- An outer opening narrower than the inner path. The vendor advises an effective path diameter close to or smaller than the case opening. A wide inner path behind a small case hole is the shape that makes a cavity with a vent.
- Treating the formula as exact. The vendor says to use it for an estimate of where the resonance is likely to be located. A tolerance stack on D, L and V moves the answer, and the table below shows how fast it moves.
- Shrinking the PCB sound hole. AN-1003 gives 0.25 mm as the lower limit for the PCB sound hole. A limit for the case opening is not published in AN-1003; measure it.
- Copying a rule from another mic family. AN-1003 covers InvenSense bottom-ported MEMS microphones. We did not read the design guides of other vendors, so their limits may differ and you should read the one for your part.
What should you measure on your own board?
The vendor itself says calculated and measured values can differ, and the sources we read give no measured data for tunnel length, mesh or membrane. We have not measured these, and no result appears in this article. The pass line is set by you per product requirement, and you record the basis for it before the first run.
| What to measure | How to set up and capture | Raw data to record |
|---|---|---|
| Frequency response of the mic inside the real enclosure | Play a swept tone or broadband noise from a calibrated speaker at a fixed distance. Capture the mic output with the enclosure closed. Repeat with the same mic on an open board as a reference. | The raw sweep for both conditions, the speaker, the distance, the room, the firmware gain and the mic part number. |
| Measured peak versus the computed estimate | Take D, L and V from your CAD model and from a measurement of the built part. Compute fb. Compare it with the peak in your sweep. | D, L and V as modeled and as built, the computed fb, the measured peak frequency and the gap. |
| Tunnel length effect | Build enclosure variants that change only L. Run the same sweep on each. Keep the mic, gasket and board the same. | Which dimension changed, the L value, the raw sweep and the peak frequency for each variant. |
| Gasket seal quality | Build samples with a deliberately bad seal and a good seal. Run the same sweep. Check the seal by inspection after the test. | Seal condition, compression, gasket part number, the sweep for each sample. |
| Dust mesh or membrane | Run the sweep with and without the mesh or membrane on the same enclosure. The vendor did not publish numbers for this in the document we read. | Mesh or membrane part number, its position in the path, both sweeps, any change in sensitivity and peak. |
| Unit-to-unit spread of the path | Measure D, L and V on several built units and run the sweep on each. | Dimensions per unit, the sweep per unit, the assembly lot and the date. |
What should you decide before you cut the enclosure?
- Is the PCB sound hole at least 0.25 mm?
- What is the case opening diameter? AN-1003 publishes no lower limit for it, so plan to measure.
- Does the gasket leave a cavity wider than the case opening?
- Can the board sit directly against the case, or does industrial design force a longer path?
- If the path must be longer, is the effective path diameter close to or smaller than the case opening?
- Would several small holes in the case work better than one vent for the look and the mesh we want?
- Where does the computed resonance fall relative to the top of the passband your product needs?
- How good is the seal at both faces of the gasket, and how do we check it on every unit?
- Which mic vendor guide covers our exact part, and have we read it?
- Will the mic be used in a product that records people, and does the design include a visible indicator and a consent flow?
- Who runs the enclosure sweep, on which fixture, and who writes down the pass line before the first run?
Questions about mic port design
How big should the sound hole in the PCB be?
TDK/InvenSense AN-1003 says the response is not affected by the PCB hole size as long as the hole is not smaller than 0.25 mm (0.010 inch), and that 0.5 mm to 1 mm (0.020 inch to 0.040 inch) is typical. This applies to InvenSense bottom-ported MEMS microphones. Check the guide for your own part. AN-1003, accessed October 31, 2026.
Does a longer tunnel always lower the resonance?
In the vendor formula, a larger L makes the denominator larger, so the estimated resonance goes down. The table above shows this for the sample geometries. The vendor also says the estimate can differ from measurement, and it gives no data for real tunnels, so measure your own enclosure.
Does the gasket material change the resonance?
The vendor says gasket stiffness has a negligible effect on overall microphone performance. A good seal matters. Examples of gasket material in the note are rubber, silicone, neoprene and closed-cell foam. AN-1003.
Can I add a dust mesh and keep the same response?
We do not have a published number for this. The vendor note we read does not quantify mesh or membrane effects, and we have not measured it. Test with and without the mesh on your own enclosure and keep both sweeps.
Is the AN-1003 formula current guidance?
It is dated 2013 (revision 1.0, 12/31/13) and written for InvenSense microphones. We read a copy hosted on a distributor site, so also look for the vendor-hosted copy. Use it as an estimate and verify on hardware.
If you want help with the acoustic path of a voice or audio wearable, tell us about your device. You can also read about our audio and voice hardware development.
Sources
All pages accessed October 31, 2026. The note is dated 2013, so re-check the vendor domain for newer guidance.
- TDK/InvenSense, AN-1003, Recommendations for Mounting and Connecting InvenSense MEMS Microphones, revision 1.0, 12/31/13 (distributor-hosted copy): https://www.cdiweb.com/datasheets/invensense/Mounting-Recommendations.pdf (accessed 2026-10-31)
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