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Speaker Asylum General speaker questions for audio and home theater. |
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Speaker Asylum General speaker questions for audio and home theater. |
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In Reply to: RE: Subwoofer Placement posted by Duke on February 27, 2020 at 12:34:00:
Your suggestions sound like they come from Todd Welti's investigations of symmetrical subwoofer configurations, which is fine.
I think these rules of thumb have been around a lot longer than Todd's paper. I learned them when I was a grad student in the early 1990s, and the first time I heard them from a dealer was around 2002. Regardless, they seem to be known better in high end HT circles than high end audio circles and I've never understood why.
As Inmate51 noted, rooms are seldom acoustically symmetrical at low frequencies. In fact Floyd Toole describes a dedicated listening room which Harman built using rather extraordinary construction techniques, including a very solid, studio-quality isolation door. At low frequencies the room's modal behavior indicated an effective length several feet longer than the actual physical length of the room. This was traced to the ultra-rigid door still being, in effect, a "soft spot" in the room boundaries. If this kind of door is a soft spot, then our normal home rooms are riddled with soft spots and outright holes in their boundaries.
Where does he mention that anecdote? The frequencies of standing waves are determined by the wavelength of sound and the room dimensions alone. The rigidity of the room boundaries affects the Q of the modes but not what frequencies they occur at. The only thing that can shift the frequency is absorbing material; because the sound propagation velocity is lower through the absorber, the wavelength is shorter. But you need a lot of absorber to make a small shift.
I've only measured a half-dozen listening rooms, but so far I've not seen anything too unpredictable. All of the rooms I've measured had conventional sheet rock over stud frame construction. They varied a lot in size and shape, but none were strictly rectangular. So far, I've found it relatively easy to predict where the lowest order modes are going to be based on room dimensions. I'm talking about the 3-5 modes (depending room size) below 80 Hz or so. Predicting their strength is harder, especially if the room boundaries are broken up, but predicting their frequencies is relatively straightforward.
Take my current living room as an example. It's in an open plan home, with openings to the kitchen, dining room, an entry way to the front door and bathroom, another hallway, stairs leading up to a landing, and there is a vaulted ceiling in the front half of the room but a conventional 8' ceiling in the rear half. Yet I measured 3 modes under 80 Hz, exactly where they should be given room dimensions. The lowest and strongest was the 1-0-0 mode across the wider of the two width dimensions, inclusive of the stairwell. I also found the weaker 2-0-0 mode, and when subs were placed in the rear half of the room I also found the floor to ceiling mode at 70 Hz (0-0-1). The 0-1-0 mode was MIA because the rear "wall" has more square footage of openings to other rooms than wall.
This is one of the reasons why Earl Geddes advocates deliberately asymmetrical placement... it's going to be acoustically asymmetrical anyway.
Symmetrical placement is for when you need to resort to cancellation.
But the first thing you can do is simply not place a sub in a location where it strongly excites a room mode. That applies for any number of subs. Even if you have a single sub, there are going to be locations that are predictably good and predictably bad.
If you're trying to cancel a mode, symmetry is ideal, but even if you can only get things sort of symmetrical it still helps.Speaking of which, Earl suggests raising one of the subs up so that it's closer to the ceiling than to the floor. This introduces a distribution of the bass sources in the vertical plane as well.
If your room has a uniform ceiling height and you're willing to raise a subwoofer off the ground, I would raise it approximately midway between the floor and ceiling because then it won't excite the floor to ceiling mode.
Something which probably is not obvious, which I should mention: The problem is not that there are typically too many room-interaction peaks and dips; the problem is that there are typically too few! So those peaks and dips end up being large and far apart, and the peaks in particular stick out like sore thumbs.
In contrast, at higher frequencies (shorter wavelengths) we have so many peaks and dips from room reflections that they effectively form a continuum, such that the ear doesn't hear them separately. They are close enough together that the ear averages them out.
A good distributed multisub system results is many more peaks and dips, which are much smaller and closer together... coming closer to the "continuum" that we have at higher frequencies. This peak-and-dip behavior mimics the sort of low-frequency response we might find in a large room. So a good distributed multisub system can make a small room behave like a larger room, in the bass region.
If these smaller, more numerous, and closer together peaks and dips are within about 1/3 octave of one another, the ear tends to average them out. So the perceptual improvements from a distributed multisub system are often greater than what we would expect from eyeballing before-and-after curves.
The peaks come from room modes. The number and location of the room modes is independent of the number of subwoofers you have. Adding more subwoofers does not make modal peaks smaller or closer together.
Dips are different. If you place a sub where boundary cancellation creates a null in its frequency band, then adding a second sub in a different location can fill in the null. Adding more subwoofers can ensure there's no uncovered nulls, I get that.
But the modes are where they are. I can't see any reason NOT to use smart placement to minimize their effects, regardless of how many subs you have.
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Follow Ups
- RE: Subwoofer Placement - Dave_K 14:05:51 03/02/20 (1)
- RE: Subwoofer Placement - Duke 15:24:45 03/02/20 (0)
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