In small rooms, disturbing low-frequency sound waves cannot be attenuated by means of conventional absorbers. PTB has now developed an optimized procedure where special resonators are finely tuned, correspondingly dimensioned and positioned in the room. This considerably helps reduce not only the sound level, but also the decay process of sounds.
In small rooms, acoustic resonances (room modes) cause various problems which, to date, have not been solved in a satisfying manner. Firstly, an impulsive excitation (for example due to the loud banging of a door) can lead to resonance vibrations which, depending on the quality of the resonance, can linger on for several seconds. Secondly, the sound transmission of music and speech in the frequency range of the room modes can, due to constructive and destructive interference, generate level variations depending on the position; the quality of the sound thus varies depending on the place from where a person listens to it. Finally, disturbing sounds caused by electric devices (such as engines or transformers) with spectral components close to the room resonances can be additionally amplified in neighbouring rooms or flats if the rooms have the same geometric dimensions.
Solutions to all these problems have been found at PTB. Hereby, special resonators are used which can be finely tuned as to their resonance frequency and attenuation. A patent application has been filed for the combined tuning mechanism for both the quality and the frequency of the resonators. In a first step, the sound pressure distribution inside the room must be measured. For this purpose, modal analysis procedures known from vibration technology (for example for the investigation of musical instruments) have been transferred onto the measurement of room modes. Then, a resonator which is tuned to the corresponding resonance frequency is set up in a local sound pressure maximum of the room mode in question. It modifies the frequency response of the sound pressure transmission at the level of the resonance frequency.
By varying the resonance quality, an optimization adapted to each of these three problems can be attained. This procedure has been tested in several rooms with different acoustic problems. Further applications can be envisaged for acoustics in the passenger compartment of vehicles, for recording studios and small music rooms or in the field of industrial noise protection.
Ingolf Bork
Department 1.6 Sound
Phone: +49 (0)531 592-1531
ingolf.bork(at)ptb.de
J. Klaus, I. Bork, M. Graf, G.-P. Ostermeyer: On the adjustment of Helmholtz resonators. Appl. Acoust. 77, 37–41 (2014)