Untersuchungen zum Einsatz von Flachlautsprechern an schallreflektierenden Grenzflächen

Abstract

During the last two decades multichannel audio reproduction systems and the trend for flatter design of media devices have lead to an increased demand in flat panel speaker technologies.With a depth of a few centimeters these devices shall allow a space saving and unremarkable integration into rooms while providing the acoustic quality of a conventional conus loudspeaker. Current flat panel speakers can be designed with a depth less than 10 cm.However if they are mounted directly in front of a wall or maintained within a flat case an unacceptable quality loss occurs.In this thesis the reasons for the deterioration of sound quality have been investigated.Therefore existing flat panel speaker technologies have been analysed and possible solutions have been proposed. If flat panel speakers without enclosure are maintained directly in front of a wall, reflections and interferences between sound parts from the front and the back of the membrane lead to an increased acoustic short circuit and to a comb filter effect. On the one hand this effect can be prevented applying a loudspeaker enclosure. On the other hand the air volume inside the enclosure acts as an additional spring and influences the vibration behavior of the membrane. The smaller the case is the greater is the spring's counterforce. This leads to an decreased sound pressure level in the low frequency range. Because of this a new flat panel speaker technology based on array like arranged miniature transducers built in a case is proposed. By using many single transducers enough driving power can be realized counteracting against the air volume's stiffness. Additionally the distribution of the driving force and the subdivision of the sound radiating area in partial areas allows a piston-shaped vibration behavior and a optimized radiation characteristic. The developed flat panel speakers dimensions are 75 cm * 55 cm * 2,4 cm. The frequency range is 100Hz to 20 kHz (-6 dB limiting frequency). Mounted on a wall the speakers' mean sound pressure level is 88 dB/1W/1 m. Combined with a subwoofer the new developed flat panel speaker can replace a conventional loudspeaker. To achieve the required sound pressure level within cinemas or concert halls multiple flat panel speakers have to be combined.The stereo setup of the final flat panel speaker developed within this thesis was presented at the "Internationale Funkausstellung" 2009 in Berlin