Technical Column
Lower Limit Frequency for Measurements in an Anechoic Chamber
2025年5月27日
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- Lower Limit Frequency for Measurements in an Anechoic Chamber
Acoustic Power Measurement
Lower Limit Frequency in Anechoic Chambers
The “lower limit frequency” of an anechoic chamber refers to the lowest frequency at which accurate acoustic measurements can be performed within the chamber.
For example, if the measurement frequency range is 200–4000 Hz, then 200 Hz represents the lower limit frequency.
This parameter is essential because it directly influences key aspects of chamber design, including:
- The measurement range
- Absorber dimensions and configuration
- Measurement method
- Microphone positioning
Difference Between Lower Limit Frequency and Cutoff Frequency
Although often confused, the lower limit frequency and the cutoff frequency are not the same.
The cutoff frequency is defined by the acoustic performance of the absorber alone:
specifically, a normal-incidence absorption coefficient ≥ 0.99 and an absorber thickness of at least λ/4 (one-quarter wavelength) at the target frequency.
In contrast, the lower limit frequency is determined by the overall acoustic absorption performance of the chamber, not just the absorber unit.
Historical Context: ISO 3745:2003 vs. ISO 3745:2012
In ISO 3745:2003, Annex K3 (recommended), anechoic chamber design guidance included:
| ISO3745:2003 | ISO3745:2012 | |
|---|---|---|
| Absorbers installed on walls and ceilings must have a normal-incidence absorption coefficient ≥ 0.99 at the target frequency. | → | This requirement has been removed. |
| The absorber length (including air gap) must be λ/4 at the lowest frequency. | → | This requirement has been removed. |
As a result, the term cutoff frequency has gradually been replaced by lower limit frequency in modern chamber design and acoustic testing terminology.
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Introduction of the Manufacturer
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Hottinger Bruel & Kjaer
HHBK is a merger of two companies: Brüel & Kjær of Denmark and HBM of Germany.
Brüel & Kjær is one of the world’s leading manufacturers of acoustic and vibration measurement instruments, known as a total measurement chain supplier.Learn more about HBK
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Sonora Technology Co., Ltd.
Sonora Technology is a leading Japanese manufacturer of industrial anechoic chambers and anechoic boxes.
From design and manufacturing to installation and acoustic performance assurance, Sonora provides fully integrated solutions to build complete acoustic measurement environments from the ground up.Learn more about Sonora
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