Freefield Measuring Rooms: A new Standard for Vehicle Acoustics

For more than 50 years acoustic engineers and scientists seem to have accepted that anechoic (freefield) chambers are equipped with porous or fibrous wedges or pyramids the depth of which being determined by the lower limiting measuring frequency.

This conventional acoustic lining technology requires a large portion of occasionally more than 30 % of the room's raw volume with no other practical benefit whatsoever.

Apart from this, delicate fibrous structures have to be carefully protected and sealed against abrasion and damages. With an increasing need for technical sound sources to be tested at very low frequencies (even below 50 Hz) and the manufacturers' ambitions to maintain attractive and durable test-bed environments for their products and personnel called for a fundamental innovation in the acoustic performance and design of anechoic linings.

This prompted a group of researchers at IBP to develop over the past 20 years a new concept for the general layout of freefield or semi-freefield rooms. It enables a marked reduction in lining depth by employing Compound Panel Absorbers CPA for primarily damping the low-frequency room modes below, say, 250 or 500 Hz.

These are individually tuned and spatially adapted to the geometry of the respective test environment. In combination with an additional Asymmetrically Structured Absorber ASA cladding with more than 99% absorption efficiency for normal as well as oblique sound incidence this guarantees precision measurements according to ISO 3745 down to 50 Hz for narrow bands and down to 25 Hz for third-octave bands. 

More than 50 anechoic rooms have so far been equipped with the new absorber modules, most of them in the European and American automotive industry.

Together with their license and cooperation partner, Faist Anlagenbau, the Fraunhofer IBP researchers have completed aero-acoustic wind tunnels and other anechoic test facilities at BMW in Munich (completed 1987), FKFS in Stuttgart (1993), AUDI in Ingolstadt (1998) and DaimlerChrysler in Detroit (2002). Design and planning have started for another wind tunnel for Renault and Peugeot near Paris. Two Complete Vehicle and Power Assembly noise test stands at FAW-VW in Changchun will round up projects with BMW (engine test stand), DaimlerChrysler Research and Development Center (6 rooms) and VW Acoustics Center (7 rooms). 

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