ventilation board

Shielding Effectiveness Numbers – What They Actually Mean and How to Tell If They're Real

You look at a datasheet for a shielding vent board. Big number. 60 dB. 80 dB. Looks impressive.

But what does that actually mean? And more important – how did they test it?

We test these things in our shop. Here's what shielding effectiveness really is – and how to tell if the number is real or just marketing.

What Is Shielding Effectiveness? In Plain English

It's simple. It tells you how much RF signal the vent stops. Measured in dB.

20 dB? The signal is 100 times smaller. 40 dB? 10,000 times smaller. 60 dB? A million times smaller.

So a vent that says 60 dB at 1 GHz means the signal coming through is one millionth of what it would be with no vent.

That's the idea. Simple.

But here's the catch – the number depends on how you test it.

What Affects the Number

Three things.

Cell size. Smaller cells block higher frequencies. 1/8‑inch cells cutoff around 1.5 GHz. 1/16‑inch cells cutoff around 3 GHz.

Depth. Deeper cells block more. A 1/2‑inch deep vent at 5 GHz might give 35 dB. Same vent at 1‑inch depth? Maybe 55 dB.

The edge seal. You can have perfect honeycomb, but a bad gasket ruins everything. The frame and gasket matter as much as the core.

So when you see a number, ask: what cell size? what depth? was the gasket included in the test?

How They Test It – The Real Way vs. The Cheap Way

There are standards. A real test uses a far‑field setup. Transmitting antenna on one side of the vent, receiving on the other. The vent is mounted in a wall between two shielded chambers. They sweep frequencies, measure the signal with and without the vent. That's the shielding effectiveness.

Some suppliers use a near‑field probe. Hold it an inch away from the vent. That's faster and cheaper. But it doesn't tell you how the vent performs against a real plane wave from a tower a mile away.

Near‑field numbers are almost always higher than far‑field. Sometimes by 10‑20 dB.

So always ask: far‑field or near‑field?

The Main Standards

If someone says they tested to a standard, here are the ones you'll see.

IEEE 299. Commercial standard. Used for enclosures and ventilation panels.

MIL‑STD‑285. Military standard. Common in defense and aerospace.

GB/T 34938‑2017. China's national standard specifically for waveguide cutoff ventilation panels.

GB/T 12190 and GB/T 30142‑2013. Chinese standards for shielding rooms and materials.

ASTM D4935. For planar materials like mesh and honeycomb.

MIL‑G‑83528. For gaskets.

If they can't name a standard, they probably didn't test properly.

What the Numbers Mean in the Real World

Here's a rough idea.

20‑30 dB. Basic. Good for some commercial stuff. Not for sensitive gear.

40‑50 dB. Solid. Most good honeycomb vents sit here.

60‑80 dB. High. Military, medical, critical.

100+ dB. Very high. Shielded rooms. Special construction.

Don't chase the biggest number. If you only need 40 dB, a 60 dB vent might choke your airflow for no reason.

What to Ask Your Supplier

When you see a shielding number, ask:

How did you test it? IEEE 299? MIL‑STD‑285? Or a probe in a garage?

At what frequency? 60 dB at 1 GHz might be 30 dB at 6 GHz. Get data at your frequency.

With or without the gasket? If they tested the honeycomb alone, the real number is lower.

Far‑field or near‑field? Far‑field is real. Near‑field is optimistic.

Can I see the report? A real supplier has batch‑specific test data.

Real Example – The 70 dB Lie

A customer bought a vent with "70 dB" on the datasheet. We tested it in our far‑field setup. At 2 GHz, it was 35 dB.

The supplier had tested near‑field. And they rounded up.

The customer didn't know until they installed it and got interference. They replaced it with our vent – same advertised 70 dB, but tested properly. Real 65 dB at 2 GHz. No more interference.

Why the Standard Matters

A vent that passes MIL‑STD‑285 is held to a different level than one tested with a probe in a garage.

The standard tells you how it was set up, what frequencies, what the margin of error is.

No standard? You're trusting a number with no context.

Shielding effectiveness is measured in dB. Tells you how much RF the vent blocks.

The test standard tells you how it was measured – IEEE 299, MIL‑STD‑285, GB/T 34938, or something else.

A 60 dB number from a real far‑field test is worth more than an 80 dB number from a near‑field probe.

Don't buy on numbers alone. Ask how they tested. Ask for the report. Test it yourself if you can.

We test to IEEE 299 and MIL‑STD‑285. We keep batch records. We know what we're shipping.

If you're not sure, send us your frequency and your requirement. We'll tell you what vent fits. That's what we do.