Issues to consider when positioning subwoofers
Subwoofers are pretty much omni-directional (equal sound output in all directions). With high frequencies the sound waves are directed by the horn flare (and waveguide) of the speaker. We don't have this control with Subs and low frequencies. Low frequencies also have longer wavelengths, so generally travel further. That’s why you will often hear the low frequency thump outside a club but not the higher frequency content until the door opens.
Most people want speakers hidden out of the way so they don't interfere with the decor or theme of the room. So what do they do? They put them up high in corners and put subwoofers in the corner out of the way. But is that doing more harm than good? Giving a subwoofer a boundary to work with could change the Subwoofers behaviour greatly.
If we place a speaker floating in midair away from all surfaces this is called free space and adds 0dB.
If we place a speaker next to 1 boundary whether it be a wall or a floor this will add 3dB and is called half space. The reason you would use half space is that instead of putting the sound out spherically (360 degrees horizontally and vertically), you are now only throwing the sound out in a 1/2 sphere so you are using the same amount of power but not wasting any energy where you don’t want it. The sound waves are now being put out into effectively half the amount of space this is how you increase level by 3dB. You also have 1/4 space as an option, this is two boundaries and will add an additional 6dB to the subwoofer.
You can go one step further and add 9dB by putting a sub in a corner this would be using three boundaries (2 walls and the floor). Some people will think ‘’wow that’s great you would be silly not to do this” and that a 9dB corner solution should be the selected positioning due to it giving higher output levels and more LF.
This does not always come as an advantage! Sometimes you just need to put the sub or the sound where it needs to be, you may find by putting the sub in a corner you are actually putting the speaker twice as far away as it needs to be (doubling the distance from the audience area or the listener means you're losing around 6dB in loss over distance).
You may also find that by adding the 9dB and placing your subwoofer in the corner of a room you might be exciting the room too much with your low frequency content. All rooms have resonance, frequencies that get reinforced by the physical shape and materials of the space and generally lower frequencies are what cause acoustic issues,so it's crucial to get your speakers in the correct position. It's always good to experiment with subwoofer positions and simply using your ears to listen will generally give you the best results.
So in a nutshell use boundaries to help with performance but do be cautious as sometimes less is more. Flying subs can waste energy and by simply placing them on the ground you could also save on installation costs as you won't need any additional bracketry.