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Advanced Audio Recording

Boxed Absorbers

In modern studio design, absorbers are used to suppress unwanted frequencies. With the concepts below, certain frequency regions can be focused.  The fundamental design operates with the resonance of the air inside an airtight box.


helmholtz absorber 

A classical Helmholtz-resonator used as an acoustic absorber. It has no damping material inside at all and only makes use of the flow resistance of the moving air in the small hole. HHR's are very effective, but not easy to build, because they have a very steep absorption curve and will have to be tuned very carefully. Also should be noticed, that the HHR mostly acts with counter noise, which means there is a noise emission, especially after the release of the sound. This might create unwanted side effects.



Modified plate Absorber

plate absorber

A new concept based on a modified Helmholtz-Resonator is presented here. It has some damping material inside and works with the resistance of the air in both the hole of the enclosure and the damping material. The absorption curve is flattened in comparison to the HHRs leading to a higher bandwidth. Also the negative sound during the release phase is significantly lowered. This type of diffusor is easier to build but less effective. More diffusor elements might be necessary.



Membrane Absorber

membran  absorber

Another concept uses a wide open window and a light membrane covering the opening of the case which encloses a defined amount of air. It works with the flow resistance of the damping material only. The absorption curve has a medium steepness and might be flattened with more damping material. Heavy membranes lead to very low frequencies, so this concept seems appropriate for bass absorption.


membran absorber helmholtz resonator


Conclusion and Summary

 Optimized absorbers can be used to obtain an appropriate sound absorption profile in studios.


 You may also want to have a look at the article QRD-diffusers



© 2003 J.S.