Low frequency sound field enhancement system for rectangular rooms, using multiple loudspeakers

The scope of this PhD dissertation is within the performance of loudspeakers in rooms at low frequencies. The research concentrates on the improvement of the sound level distribution in rooms produced by loudspeakers at low frequencies. The work focuses on seeing the problem acoustically and solving it in the time domain.

Loudspeakers are the last link in the sound reproduction chain, and they are typically placed in small or medium size rooms. When low frequency sound is radiated by a loudspeaker the sound level distribution along the room presents large deviations. This is due to the multiple reflection of sound at the rigid walls of the room. This may cause level differences of up to 20 dB in the room. Some of these deviations are associated with the standing waves, resonances or anti resonances of the room. The understanding of the problem is accomplished by analyzing the behavior of low frequency sound in rooms both in the time and the frequency domain. This is done by implementing the finite-difference time-domain method (FDTD) in order to simulate and predict the sound field produced by loudspeakers in rooms at low frequencies.

The outcome of this work is a novel and effective solution named Controlled Acoustically Bass System (CABS). The system utilizes loudspeakers at the front wall of the room and extra loudspeakers in anti phase at the opposite wall. Differently from the traditional room correction systems, CABS acquires a more even sound level distribution at low frequencies throughout the room. The effect of the room resonances in the reproduced sound has been decreased remarkably. The traditional one subwoofer setup in an IEC standard listening room presents typically differences in spectral magnitude up to more than 20 dB below 100 Hz. By using CABS the spectral standard deviations have been reduced to ± 2.1 dB. The CABS system works in the time domain therefore it performs as well for transient signals as for steady signals. The system can be integrated into stereo or multichannel systems. The study discusses the implementation and the performance of CABS in two standard listening rooms and the simulation of CABS in irregular rooms.