Ferrite Tiles vs Pyramid Absorbers: Which One, Where?
Which frequencies the ferrite tiles, pyramid foams and hybrid absorbers used in anechoic chambers work at, and how they affect chamber size.
3 min readOctober 18, 2026
Why Validate the Chamber?
A semi-anechoic chamber is built to imitate an ideal open-area test site indoors. But it can't be assumed that the chamber actually behaves close to that ideal; absorber performance, chamber size and installation quality all affect the result. So a chamber can't be used for emission tests until it is proven by measurement.
CISPR 16-1-4 defines how that proof is obtained. The best-known method is NSA, normalized site attenuation, covering 30 MHz to 1 GHz.
How Is NSA Measured?
Two antennas are used, one transmitting and one receiving. The transmitting antenna is placed at different points in the volume the product under test will occupy, while the receiving antenna is scanned in height to find the maximum signal. The attenuation between the antennas is compared with the value expected at an ideal site.
The measurement is repeated in horizontal and vertical polarization at many frequencies. A deviation of up to ±4 dB from the ideal value is accepted. If this limit is exceeded, the chamber isn't considered suitable for that test distance.
Above 1 GHz: SVSWR
A different validation is used for emission tests above 1 GHz: SVSWR, the site voltage standing wave ratio. It shows how small reflections are within the test volume and directly reflects the absorbers' high-frequency performance.
The right choice of ferrite and pyramid absorbers is decisive for passing both NSA and SVSWR measurements the first time.
Handover and Periodic Validation
When a SAC-3 or SAC-10 chamber is handed over, NSA and SVSWR measurements are reported. Accredited labs present this report in audits.
The measurement should be repeated after major changes such as replacing the turntable, repairing absorbers or adding new equipment. Routine revalidation every few years is also recommended.
Frequently Asked Questions
How long does an NSA measurement take?
Depending on the test distance and volume size, it usually takes one to two working days.
Is NSA needed for a pre-test chamber?
It isn't mandatory where certification validity isn't required, but it's recommended to see how reliable the results are.
What if NSA fails?
Absorber layout, floor bonding or reflective objects in the chamber are usually examined; the measurement is repeated once the problem is fixed.
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