Wednesday, April 15, 2009

Geddes and Harman Kardon on subwoofers, bass response

I am thinking hard about my system revision, and particularly the cornerhorn situation. I am reading a book by Floyd Toole, which I will be coming back to later. I have to finish reading it. :) The book is very interesting and is giving me quite a few concept changes about audio reproduction in the home. To paraphrase him shortly, you just have to do what you have to do to make it sound good and then it does sound good. There's a load of data and discussion to back it up, but that's not what I want to talk about. It just triggered me into realising that smooth bass response is a big issue for me (and music in general) and I need to adress it well.

With a single subwoofer or bass source, reflections and resonances in the room lead to peaks and dips in the frequency response, that also differ at varying listening positions. You can try to even them out with an EQ, but whatever is fixed in listening position A makes something else worse in listening position B. And you can take out a peak, but it's impossible to fully fill in the dips as they are cancellations. More power just means more cancellation.

Harman Kardon has a whitepaper about the use of multiple subwoofers and pre-determined, regularly spaced, placement in rooms, which leads to a similar frequency response in the bass range, for all listening seats. This response may be irregular, but it can be equalized flat and the result will be similar and linear bass response throughout the room. The strategy relies on a rectangular room and processing to get the final result right.

Earl Geddes suggests that listening rooms rarely are as predictable in their lay-out and characteristics as the testroom used by Harman Kardon and that this randomness will reduce the positive effects. He also points out that subwoofers placed in regular locations means they will have to be in sight and be very conspicuous. He suggests that randomizing subwoofer placement will work better in the random environment of the listening room, producing a better frequency response in all locations. Randomization basically means put the subwoofers where you think they are convenient. Also, this apparently gives a smoother frequency response even before final EQ and it seems people are getting by without final EQ.

Geddes has a simple approach to get started with placement and to work from there. There is a thread on diyaudio that describes the methodology and Earl Geddes participates in the discussion. There's also some measurements of results. I refer to thread that for the full background. One thing that I do want to mention here, is that Geddes advises to have one main subwoofer, that plays loudest and becomes the source, or the reference subwoofer. Other subwoofers are only there to break up roommodes etcetera, so are all adjusted to disappear into the main response. It is pointed out in the thread that dissimilar subs add to the randomization.

I want to employ this. A big hornsub in the corner and some frequency response smoothing via smaller subs here and there.

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