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Using multiple subwoofers to improve bass: The Welti / Devantier and Geddes approaches

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.

‘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.

Geddes Approach

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!

4 thoughts on “Using multiple subwoofers to improve bass: The Welti / Devantier and Geddes approaches”

  1. I’ve found in some instances when only one sub-woofer was available, that purposeful overlapping of the crossover points of the mains and sub could help smooth the response in the crossover region.

    For instance if you had a main with meaningful output down to 35-40hz, in room, you could cross the main at 40hz, LR4 and then sub at say, 80hz, LR4. Effectively from 40-80hz you have 3 emission locations of bass and this can tame the response in that region. Of course you will need to apply additional EQ in this region to smooth out the increase response you would get from the overlap, although many times this falls within an appreciable house curve low end rise.

    What I’ve found to be the most critical item with this approach is ensuring the time arrival of the signals are matched perfectly, if not arriving 2ms to 5ms sooner for the mains.

    I’m curious as to your thoughts on this approach?

    1. Hi Chris

      Thanks for reading and leaving a comment. I took a look at your website, keep up the good work on writing your blogs! It can be hard in the beginning but after a while it pays off in terms of traffic and business!

      As for your particular comment, I have not experimented a lot with overlapped crossover points. I guess if the mains and the subs were phase aligned through that region then there would be no harm in doing it the way you suggested. However some things may prevent that from happening, including:

      • Phase shift from the EQ applied inside the subs (most subs are EQ’d by the manufacturer to make them flat in the low bass as the drivers are in enclosures that are too small for a “naturally” flat bass alignment)
      • Phase shift due to porting of the main speakers
      • Phase cancellations due to speaker boundary interference

      Also, I tend to advocate multiple subs, placed in such a way as to drive room mode destructively (as per this blog). If you add another sound source to the mix then you can unbalance the way in which you are driving the room modes, and so they end up not being cancelled.

  2. The Geddes approach actually instructs you to overlap the mains with the BB subwoofer, then overlapping the subs with one-another and running the mains full range It works great, so long as your mains can handle it. It’s also prefereable to use a miniDSP to bandpass each sub, with overlapping XO points. This takes a great deal of strain away from each subwoofer because each sub is only playing part of the bass frequencies. This is also why the subs need not be the same, nor incredibly expensive.

    There’s many ways to get great bass, and they all require multiple subs. Welti and Devantier ‘s approach is also great, but at a higher cost to the end user. Geddes’ approach is by far the most cost effective way to optimal bass, very cutting edge stuff..just like the waveguide speakers he used to manufacturer.

    I really like your website, and the information that you provide.

  3. I think using multiple subwoofers works really well, I have 4 in my truck and the bass is ridiculous. But I think it all depends on the size and space you have as well.

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