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Old 19th September 2019
  #96
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Quote:
Originally Posted by TheCarlough View Post
Apologies for the hijack - but what room size would you consider "small" in terms of placing your monitors closer than the generally shared "at least 12 inches from the wall".
The underlying issue here is "SBIR", which stands for "Speaker-Boundary Interference Response". What that basically means is that at low frequencies the sound from your speaker "wraps around" behind the speaker, hits the front wall of the room, then bounces back and "interferes" with itself, causing phase cancellation for a certain set of frequencies that exactly match the distance to the wall and back. Thus, for any given distance from the speaker to the wall, you will get a deep null in your frequency response, because the wave cancels itself out at that frequency.

With a gap of 12" between the speaker and the wall, and assuming a typical speaker depth of another 12" or so, that places the front face of your speaker 2 feet from the wall. For a distance of 2 feet, the SBIR nul will occur at 141 Hz (because the wavelength at 141 Hz is 8 feet, and it is the half-wave that we need to look at, so 4 feet, and the distance is 2 feet to the wall plus 2 feet back again, = 4 feet). So at that frequency you will see a deep dip in the frequency response. If your speaker is further away, for example at 3 feet from the wall, then the dip will be even lower, at about 94 Hz. At 5 feet, it would be worse still, at around 56 Hz. To get the dip low enough that it isn't really a problem any more, you would need to get the speaker at least 10 feet away from the front wall, and the SBIR dip would then be at about 27 Hz, which is OK because there aren't many instruments that get down that low (only the grand piano and the pipe organ really). So, in order to have your speaker ten feet from the front wall and still have the mix position in a good spot (acoustically), the room would need to be about 30 feet long. That's well out of range for most home studios (not many people have rooms 30 feet long in their houses!). So any room shorter than 30 feet is a "small" room in this context.

Thus, for practically all home studio rooms, the only real option is to put the speakers as close as you can get them to the front wall of the room, hopefully with a gap of just 4" between the speaker and the wall, which implies a distance of maybe 16" between speaker face and wall for a typical speaker), and an SBIR dip at about 212 Hz. At that frequency, the dip is less of an issue for several reasons. Firstly, it is less noticeable in the mid range than it is down in the low bass range, secondly it is less intense, and thirdly it can be treated. Treating a wave with a wavelength of 64" is a hell of a lot easier than treating one with a wavelength of twenty feet!

So that's the reason for the recommendation to have your speakers as close to the front wall as possible unless you have a room that is large enough that you can get them more than 10 feet away from the wall.

Of course, the best of all is to flush-mount the speakers IN the wall itself (or rather, an angled extension of the wall): in that case you do not get any SBIR problems at all from the front wall, since the speaker is technically no longer in the room!

Here's a graphic put out by Neumann, that explains the issue briefly, then shows a table of frequencies with distances to avoid (red), distances that are sort-of maybe OK (blue), and distances that are good (green). In other words, avoid any distance between 8" and ten feet! Thus, the advice you saw of having your speakers 'at least 12" from the wall' is not correct. In fact, it should be 'No more than 12" maximum, unless it is also greater than ten feet".

Or you could just add a couple of subs to your room, and kill the SBIR issue like that...



- Stuart -
Attached Thumbnails
Just moved in!  New Studio Build-neumann_loudspeaker_boundary_location_v02-sbir-table-wall-bounce-distance.jpg