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In Reply to: RE: Subwoofer Placement posted by Mister Pig on February 26, 2020 at 10:57:54
I would not place any more than one sub in a corner. I'd rather have none in corners than two in corners.
Duke
Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.
Follow Ups:
A single sub in a corner is a bad thing - it will excite all modes.
A pair of subs placed in adjacent corners of a rectangular room will work to cancel all odd-order room modes associated with the room dimension they span. For example, if the room is 17' long, you're going to have axial room modes at approximately 33Hz (1-0-0), 66Hz (2-0-0), 99Hz (3-0-0), 132Hz (4-0-0), etc. If you place the subs in the right front and right rear corners of that room, they will mostly cancel the 33Hz and 99Hz modes. Considering the lowest order axial mode is usually the strongest in most rooms, cancelling the 33Hz mode would be really beneficial.
Adjacent corner placement will also cancel some of the tangential modes: 1-x-0, 3-x-0, 5-x-0, 1-0-x, 3-0-x, 5-0-x, etc.
But you can do even better than that. If you place two subs in opposing corners e.g. right front & left rear, they will try to cancel the odd-order modes associated with BOTH the length and width dimensions.
Opposing corner placement is one of the two theoretically best placement options for two subs. The other best option is placing the subs symmetrically at the midpoints of opposing sidewalls. A sub located at the midpoint of one room dimension is in a node for all odd-order axial modes associated with that dimension, so it simply doesn't excite those modes at all. And because the two subs are symmetrically placed on opposing walls, they cancel the odd-order axial modes for the other dimension.
These placement options are most effective in a rectangular room. With the OP's room opening up to another room on the left side, the axial modes associated with the width of the room could be unpredictable.
OK I can place two subs in adjacent corners. Now should they be the two HGS 12 or one of them the HGS 18. Third location cannot be on a midpoint of side wall. But can be inside left front speaker but only the front wall to load off. Should the HGS reside there, or be in that front right corner.
Your other email clarified the room layout for me. Since you don't really have symmetrical corners, I would try two setups:
1. Both HGS12's up front, one in the right front corner and the other against the front wall near the left speaker. HGS18 goes left rear.
2. Both HGS12's in the back, HGS18 up front, on the front wall, between the speakers. The HGS18 might suffer from too much room gain in the front right corner, but you could try it.
I'm not sure which will sound better, and it may take some experimenting with level controls to get the best LR balance and FR fade.
Use ratios of lengths of the most irrational number Phi, powers and reciprocals thereof. (1+sqrt(5))/2
Make ratios like like 0.618...
Yup!! Imo that's the theoretically ideal ratio to use.And 1 - .62 = .38, so if a sub is .62 times the wall length from one corner, then it's .38 times the wall length from the other.
So by way of example, we might do one in a corner; one .38 x the long dimension from another corner; one .62 x .38 x the long dimension from a third corner; and one .62 x .62 x .38 x the long dimension from the fourth corner.
I'd still want two of them to end up along the walls opposite that first corner (the one with a sub in it), as that was among Earl Geddes' recommendations.
Duke
Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.
Edits: 02/27/20
Your suggestions sound like they come from Todd Welti's investigations of symmetrical subwoofer configurations, which is fine.
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.
This is one of the reasons why Earl Geddes advocates deliberately asymmetrical placement... it's going to be acoustically asymmetrical anyway.
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.
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.
Also, the ear is especially sensitive to differences in SPL at low frequencies, such as those arising from the in-room peak-and-dip pattern. This sensitivity is revealed by equal-loudness curves, which bunch up south of 100 Hz. So a difference of 5 dB at 40 Hz is perceptually similar to a difference of 10 dB at 1 kHz! In other words, techniques which significantly smooth the in-room bass response (distributed multisub configurations, bass trapping, EQ) often pay larger perceptual dividends than the raw measurements suggest.
For the record, I have NOTHING against pursuing a Todd Welti symmetrical configuration. Give it a shot if it's practical for you! Todd did not investigate asymmetrical configurations because there are too many possible variations.
Interestingly, Todd was developing his symmetrical multisub concepts at exactly the same time that Earl Geddes was developing his asymmetrical multisub concepts. They were each unaware of the other's work.
Duke
Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.
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.
Dave_K, thank you for your in-depth reply. I'll just comment on a few things.You asked: "Where does [Floyd Toole] mention that anecdote?"
Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms, First Edition, pages 208-212.
DaveK: "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."
Quoting Toole, page 211, on the topic of this particular room:
"The minimum for the first-order mode is not where it should be: at the halfway point down the length of the room. It is shifted by slightly more than 2 ft (0.6 m) toward the wall having a [heavy solid-core] door. This behavior suggests that the room is "acoustically" longer than the physical dimension and that the extension is at the end where the door is located. A boundary that moves is a membrane absorber, absorbing a portion of the energy falling on it, and reflecting the remainder with a phase shift. In this case, at this particular frequency, the phase shift has the same effect as moving the wall by some distance beyond the physical location. If this is so, there should be a corresponding lowering of the frequency at which the resonance is observed. This is confirmed in Figure 13.8b, which shows that the first modal peak in frequency-response measurements is substantially lower than the predicted frequency."
DaveK: "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."
In my experience real-world measurements of individual subwoofers at different locations in the same room do not necessarily have the peaks all lining up at exactly the same frequencies. Please click on the link below and scroll down about 3/5 of the way,. You will see a graph with curves for "front left sub" (blue); "front subs" (red); and "all four subs" (green). Click on the graph so you can see it clearly.
Notice that the curve for "front left sub" has a peak at 45 Hz while "front subs" has a peak at 40 Hz. I don't have an explanation for this, but in my experience this sort of thing is typical.
Next, notice that "all four subs" has 4 peaks between 20 Hz and 80 Hz, whereas the other two curves only have 3 peaks in that region. Thus for "all four subs" some of the peaks ARE closer together. Notice also the generally reduced peak-to-valley variation for "all four subs"; in other words, the peaks ARE indeed smaller.
Before moving on, scroll down to the second graph and click on it. Here you will see the response of the same sub(s) as before, but measured in a DIFFERENT listening position. As you will see, in the 20-to-80 Hz region, ONLY the 23-Hz peak from the first graph is still clearly identifiable. Where are the other "modal" peaks? Basically with "left front sub" or "front subs" you now only have two big peaks, at either end of the 20-to-80 Hz region. With "all four subs" you have a mild intermediate double-bump peak, so arguably 3 peaks instead of the original 2. And once again the peak-to-valley variation is considerably smaller with "all four subs".
Taken together, these two graphs illustrate that a good distributed multi-sub setup can improve the bass response in multiple locations simultaneously. Incidentally, one way to mitigate the generally rising bottom-octave response in both graphs, which peaks at 23 Hz, would be to reverse the polarity of one of the subs.
Now quoting Earl Geddes from the paper entitled "Why Multiple Subs?", posted on his website (gedlee.com):
"The use of multiple source locations in the modal region will globally yield a response curve that is closer to the natural power response of the sources and the room. Said another way, if we use multiple source locations that the frequency response at any given location in the room will become closer to the true power response (read smoother) the more sources that are used. Basically if I have one source which has a variance, V, of the frequency response (the variation of the response from the average or smooth response) of say 6 dBs, that by adding a second source we will reduce this variance by half to 3 dB. Adding a third source reduces this to 2 dB, etc. Basically the variance goes as V/N where N is the number of "independent" sources. A key requirement here is "independent". If the added sources are close to the first source then they are not independent. And two sources in opposite corners or symmetrical locations are not as independent as two sources placed in non‐symmetrical locations. It is impossible to have two sources that are completely independent at LFs in a small room, so the effect is never as good as the formula suggests."
I have customers who have tried Welti-style symmetrical as well as Geddes-style asymmetrical placement strategies. The majority settled on asymmetrical, but I include Welti's two smoothest four-sub configurations in my setup guidelines, along with several other options.
Duke
Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.
Edits: 03/02/20 03/02/20 03/02/20 03/02/20 03/02/20 03/02/20 03/02/20 03/02/20 03/02/20 03/03/20
"These placement options are most effective in a rectangular room. With the OP's room opening up to another room on the left side, the axial modes associated with the width of the room could be unpredictable."
You have presented a useful synopsis of rectangular room mode math. Better than that, you noted that it all goes out the window as soon as we introduce openings to the room.
Hopefully, readers will learn something from your post.
:)
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