Modal Coupling of Directional Subwoofers in Rectangular Rooms

Open Access
Feurtado, Philip Andrew
Graduate Program:
Master of Science
Document Type:
Master Thesis
Date of Defense:
June 17, 2013
Committee Members:
  • David Carl Swanson, Thesis Advisor
  • sound
  • acoustics
  • audio
  • bass
  • subwoofer
  • room modes
When listening to music, either as an end user of high fidelity consumer audio or as a professional recording engineer, accurate reproduction of low frequencies below 100 Hz is essential. To this end the most common practice is to devote these frequencies to a single loudspeaker, or subwoofer, which is only responsible for the reproduction of low frequencies. However, interactions between the subwoofer and the natural resonances of the listening room can lead to changes in the frequency spectrum at the listener location such that certain frequencies may be strongly reinforced by the room or strongly attenuated. This can be problematic for consumer audio as it may lead to misconceptions about the quality of the source material or the sound system itself. In a professional audio environment this can make it very difficult for an engineer to obtain an accurate perception of the source material, which is essential to their profession. One possible solution to this problem involves the use of directional subwoofers which only radiate sound in certain directions, thus exciting only certain room resonances. Modal coupling was investigated by modeling subwoofers as acoustic point sources and calculating the acoustic pressure at a listener location by using the green's function for a rectangular room with modal damping. It was determined that the directionality of some sources did indeed dictate the excitation of modes, thus affecting the frequency spectrum at the listener location. Furthermore, it was found that a particular configuration of the listener and a single dipole source can create a nearly flat response at the listener location below the second axial mode of the shortest room dimension. For many applications this can cover the entire operating range of the subwoofer. It was also found that two sources that are closely spaced but time delayed on the order of tens of milliseconds exhibit periodic amplitude changes that can be exploited to radiate sound effectively in frequency bands with very low modal densities.