As you may already know, the energy sent backwards by a speaker hits the wall behind it and gets reflected. If the wavelength is long enough, this will add in-phase to the energy that the speaker sends forwards.
You can imagine it as if half of the balloon that expands outward from the speaker folds back on top of the other half.
These two halves add together, doubling the amount of energy. What you end up with can look like a low shelf boost at up to +6dB SPL. And, just like with an EQ in your DAW, you can bring it back down with a similar low shelf cut!
The same thing happens again when you place your speaker in a corner. Now there are two surfaces that reflect energy, and half the balloon is folded over again. All that energy is now concentrated in ¼ of the volume. Another potential +6dB SPL low shelf boost.
So what does that mean in practice when placing your speaker? In simple words:
As you move your speaker closer to a wall or corner, it’s going to sound like somebody is boosting your low end with a low shelf EQ.
If you don’t like it, simply bring it back down with the low shelf EQ on your speaker. Done.
But this doesn’t mean you HAVE to place your speaker at any particular distance from the wall. In fact, you might like a little bit of extra energy in the low end! And if you don’t, it’s easy enough to take care of.
Those Dreaded Reflections
But what about energy that gets reflected back from the front wall and ends up combining out of phase with the sound sent forward?
What you end up with here is a classic comb filter.
You know those mono-to-stereo mixing tricks that you’re not “supposed” to do because of mono compatibility, but you end up doing all the time anyway? You know, like the one where you double the track, pan the two versions out and delay one side? Yeah. Sounds great. Just don’t hit that “mono” button.
That’s what happens when the reflected energy off the front wall combines out of phase with the energy sent directly forward.
Except, the reflected energy is effectively low-passed because of the dispersion pattern of the speaker. It only sends bass backwards, remember?
As a result, that comb filtering only substantially affects the low frequencies.
And, like with any comb filter, the frequency of the first and lowest cancellation is determined by the time difference between the direct sound and the reflection.
The important thing to understand is that this first cancellation happens at the membrane of the speaker itself!
You see, two sound waves interact by the laws of superposition. That means they interfere at every point in space while continuously passing through each other. But for us listeners, the effect is only “cemented” into existence at the very moment and point in space where the energy is transferred to a membrane.
And depending on what that membrane is (the speaker membrane in this case), the interference effects can be quite different.
In this case, the cancellation literally affects the power output of the speaker. It changes the actual frequency response played into the room. Basically, it colors the speaker! And so that coloration stays the same no matter where you are in the room, as long as the speaker stays in the same place.
You may have heard of it. It’s sometimes called Speaker Boundary Interference Response or SBIR.
Note that this is not the same as your classic “mirror point” reflection, who’s effect does vary throughout the room as you change position.
Boundary Effects in Combination
So what does it actually look like as you move your speaker closer to the wall? You’d expect the cancellation to start low, because of the long time difference, and move up in frequency as you move the speaker closer.
Speaker Boundary Interference. The out phase part causes a comb filter that moves up in frequency as you move the speaker closer to a wall. The in phase part causes a 6dB low shelf boost. Image via GIPHY
So as long as your speaker is more that 4m (12 feet) away from the front wall, the fundamental cancellation notch lies beneath the audible spectrum. It’s probably outside of your speaker’s frequency range and doesn’t get created in the first place. So that’s good.
Unfortunately, the first harmonic above it still sits smack bang inside the audible bass region, wreaking havoc.
In between 3 meters (10 feet) and 1 meter (3 feet) in distance, the first cancelled overtone is now above 100Hz. And, the main notch moves right through the subs and bass. Not cool.
Once the speaker is less than 1 meter from the wall though, the main notch has moved above 100Hz as well. At those frequencies, simple insulation material absorbers on the wall are very effective.
Finally imagine what happens when you move the speaker all the way into the wall. Now the reflection doesn’t even happen in the first place, and you can simply ignore the entire issue. That’s the whole idea behind soffit mounted speakers.
So, to keep your front wall reflections from messing up your bass, place your speakers at least 4 meters away from your wall, or less than 1 meter away. Anywhere in between 1 meter and 4 meters and you are asking for extra trouble.
You can reduce the effect by placing an absorber right behind the speaker, but it will have to work at least down to 100Hz, or lower.
Optimizing For The Front Wall = A Game of Futility
This advice is all nice and dandy. The problem of course is that there are usually five more surfaces in your room apart from the front wall. And all of them cause the same effect.
In most typical home studios, like converted spare bedrooms, attics, garages or basements, you will neither have the space nor the ergonomic luxury to place your speakers to optimize for all 6 reflecting surfaces. (And often, not even for one.)
Even if you could do this, which surfaces and reflections would you prioritize? Which are the worst offenders?
As you optimize for the front wall, you may be missing the mark on the back wall reflection. And to actually get the oft-recommended “equilateral triangle” going, you end up completely ignore getting the right distance to the side walls. Finally, there’s the floor, and whatever you do, I’m sure you won’t be successful in setting up your speaker within 1m of the floor.
I think you can see what I’m getting at.
Ever wanted to have a great way to chase your tail for weeks in the studio? This is it. It’s a lesson in futility.
Sure, you might be able to squeeze a little bit of an improvement out of your setup with lots of patience and trial and error. But looking at the bigger picture, all of your efforts are going to be swamped by the effect of room modes anyway.