On the qualification of anechoic chambers; Issues related to signals and bandwidth

The qualification of anechoic and hemianechoic chambers requires selection of signal type and acquisition bandwidth to be used. This work demonstrates that, while a broadband random source may be used, the signal at points removed from the source are not composed of random combinations of direct and reflected waves, except in the limit of infinite bandwidth. It is shown that chamber qualification may be represented as finding the ratio of two transfer functions, where one includes the reflected waves, and the other is the free-field transfer function between the source and receiver. Consideration of this approach leads to a generic representation for the deviation from free-field performance where bandwidth is demonstrated to suppress the dominant modulation contributions of reflections. It is demonstrated that pure-tone qualifications will always exhibit a higher deviation from free-field performance than a broadband qualification. Finally, it is shown that the use of an incoherent source model in method-of-images simulations for the broadband performance of anechoic chambers is fundamentally flawed.