This article continues the theme of the last two blog postings in whic we outlined some ways in which subwoofers can be used to improve sound quality. Last week's installment 'Using Subwoofers to Improve Sound Quality: Part 2 Room Modes
' outlined a technique whereby two subwoofers are positioned within a room to destructively drive a particular room mode, nullifying its frequency response peak and time domain ringing. This week we introduce the Welti / Devantier and Geddes multi-sub methodologies. Both involve using multiple subwoofers as a way to smooth bass response and reduce the impact of modal resonances.
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'Welti / Devantier' Approach
Todd Welti and Allan Devantier worked at Harman International at the same time as Floyd Toole (a respected acoustics expert and author of the highly recommended book Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms). The authors' paper, 'Low Frequency Optimization Using Multiple Subwoofers', explored ways to reduce the sound pressure level variation, or Mean Spatial Variation (MSV), between seats in a home theater. In a home theater situation seats are spread across the length, width and height of a room. This means each seat interacts differently with modal resonances, resulting in some seats having good bass response and others poor bass response. Equalization cannot be used effectively in this situation since any change applied will affect the frequency response at each seat equally. The authors built a mathematical simulation that examined the impact of a different subwoofer numbers (up to one thousand!) and placement on the MSV. The results of these simulations showed that a configuration of two or four subwoofers in a rectangular room could significantly reduce the MSV such that global equalization could be effectively employed to produce a smooth frequency response. This result is instinctively expected as an extension of the simple model we introduced last week where two subwoofers were used to destructively drive a particular room mode. For more details follow this
link to a PDF of the paper.
Earl Geddes, of GedLee LLC
, has come up with a slightly different take on the multiple subwoofer approach. The basic approach is to integrate three additional subwoofers into a system such that there are multiple low frequency sources in the room, each of which interacts with room modes in a different manner. The first sub is placed in a corner. The location second and third sub is determined empirically through trial and error, viewing the impact of the adjustments on a high resolution frequency response chart. Further details on the methodology can be found on this
page. For a single listener there is less of a requirement to use multiple subs to get smooth bass response since we can heavily equalize the response at the listening position as explained in our earlier article 'Room Correction: A Primer'
. I wonder if our readers know about any other interesting subwoofer placement methodologies? If you do please share!