In this episode of Ask The Expert we're talking about the positioning and distance of loudspeakers from the rear wall, and the challenges that this can present.
These are exactly the kind of questions that fule the Ask The Expert fire, so be sure to keep us busy by submitting your questions via the 'Send us your questions' form at the bottom of this page, and we'll endeavour to address them in future episodes.
How much space should I leave between my speakers and the back wall?
The answer to this question depends on the room, just as much as on the speaker itself, meaning that you will have to experiment a little bit in order to get it right. Let's first discuss what impact a back wall can have on the performance of your speakers.
When you put a speaker close to the wall, the bass frequencies become more exaggerated. This is because the bass frequencies are dispersed from the loudspeaker in all directions – including backwards – much like a sphere. The higher frequencies, however, only travel forward.
Therefore, if you put the speaker closer to the wall, you're going to increase the reflections of the low frequencies from the back of the speaker. So, in terms of proximity to the wall, a balance has to be struck between having too much or too little emphasis on the low frequencies. This is what we meant by the need to experiment.
What is generally recommended is to start by putting the speakers very close to the rear wall, which is very unlikely to give the best results. Then, you can gradually start moving the speakers out from the wall, all the while listening to music that has some clearly decipherable and tuneful bass, and then stop at the point where the bass sounds best.
As you move gradually outwards, you should be able to hear that the low end becomes more and more defined and that it's increasingly easy to follow the bass notes. The trick is to keep increasing the distance of the speakers from the wall until you get to the point where the bass starts to sound worse, if not completely missing from the music altogether. At this point, you'll want to start going back towards the wall again, little by little, until you reach the optimal spot.
There exists some theoretical approaches to the distance between the speaker and the wall, including something called SBIR, which is Speaker Boundary Interference Response. What that means is that the sound coming from the back of the speaker hits the rear wall and is then reflected back into the room. That sound is, of course, slightly delayed compared to the direct sound coming from the speaker and, on its way back into the room, it gets mixed with the sound coming directly from the speaker. Depending on the wavelength and the frequency of that reflected sound, it could actually end up cancelling out some of the direct sound from the speaker, i.e some of the sound as you perceive it from your listening position will actually be missing.
The distance between the speakers and the rear wall can directly determine which of these frequencies are affected and how much they're affected, and that's the main reasons that it's so important to have the correct distance to the rear wall. As we mentioned earler, this can be calculated but, in reality, it's more efficient in the real world to do it by ear in the way that we have described here. If nothing else this is because theory and reality don't always correspond, and others factors such as the distance to the sidewalls, floors, and ceilings also have their part to play.
Another thing that can have an effect on all of this is the placement of the port on the speaker, but we will cover this in another episode of Ask The Expert.
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