Testing of directional microphones under free-field conditions at low frequencies is problematic if an anechoic chamber is used beneath its cutoff frequency. In accordance to IEC 268-T4 guided waves in a correctly terminated duct can be used to produce a plane, travelling wave at low frequencies. An active absorber system is introduced which reduces the length of the tube and improves the reflection coefficient. First measurements in a test setup are presented.
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https://aes2.org/publications/elibrary-page/?id=6899
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