Conventional vs Directivity controlled Loudspeakers
Speakers tend to radiate uniformly into all directions at low frequencies where the radiating cones and baffle are small compared to the wavelength. They will beam sound at higher frequencies because the radiating cones are typically larger than 1/8th of a wavelength. Thus a speaker, which is flat on-axis, radiates a different spectrum at angles off-axis, where the response is no longer frequency independent.
Conventional speakers tend to strongly interact with their listening environment. Each wall exerts influence on speaker behavior especially in the low and sub frequency domain. Best known is that speakers may sound "boomy" and excessively resonant when placed too close to the wall. Another phenomenon is the cancellation effect for certain frequencies depending on how close or far the speaker is placed in front of the wall, as illustrated by the pictures on the right.
Optimizing conventional speaker performance is a difficult task and demands empirically moving speakers back and forth to their optimum position in the room where music appears to sound "best". Clearly, speakers placed 1 meter or more away from adjacent walls might not be acceptable in most homes and definitely not all family members will give you a round of applause.
Speaker Boundary Interference
Interference is at play when two sound waves interact by the laws of superposition. That means they interfere at every point in space while continuously passing through each other.
Exhibit: Speaker Boundary Interference Response (SBIR), whereas the out-off phase part causes a comb filter effect that shifts up in frequency as the speaker is moved closer to the wall. The in-phase part causes a 6dB low shelf boost. (Simulation via GIPHY).
In case of SBIR, 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 that coloration stays the same no matter where you are in the room, as long as the speaker stays in the same place.
Now what happens as you move your speaker closer to the wall? Basic physics tells you the cancellation starts low, because of the long time difference, and shifts up in frequency as you move the speaker closer to the wall. Imagine what happens when you move the speaker all the way into the wall. Now the reflection doesn’t happen, and you can simply ignore the entire issue. The best place to mount speakers and avoid front wall SBIR is.. in the wall.
Exhibit: How loudspeakers radiate sound as function of size and frequency. From the animations below (courtesy by University of Southampton), it can be seen that the radiation pattern of a loudspeaker is different at low, medium and high frequencies.
At low frequencies, the sound pattern radiated by a loudspeaker spreads out evenly in all directions (omnidirectional/ monopole). For this animation, the acoustic wavelength λ is about four times larger than the piston radius R. At higher frequencies, when λ = 1.5R, the sound pressure produced by a loudspeaker is mostly contained within a cone around the piston axis. So, at these frequencies, you will hear louder sound levels in front of the speaker. If you move to either side it will get quieter. At even higher frequencies, when λ < 0.5R, the sound field radiated by a loudspeaker is constricted within a narrower cone around the piston axis. Now, the sound pressure levels fall off very rapidly as a listener steps away from in front of the loudspeaker.
The Solution: Directivity Control
So, what to do if you don't want your woofers mounted in the wall?
There are several applications where low frequency monopole behavior of woofers/subs is unwanted. Such a situation occurs, for instance, in public address, where monopole radiation can lead to howl back because the loudspeakers radiate not only towards the audience, but also at close range in the direction of the stage.
Many sound engineers have acknowledged this problem and are using cardioid-like sub woofer arrangements to deliver less sound energy to the stage and more to the audience. Although cardioid-like radiating pro-stage speakers have been around for a while, their principle to reduce the emitted sound intensity towards the rear and optimize (bundle) sound pressure towards the front, has become an interesting feature to copy-cat into high-end home speaker designs. Until recently, it has not been possible to apply cardioid technology in home speakers cost effectively.
In order to obtain a cardioid-like radiation pattern from your speaker it is essential to create similar but time delayed acoustical energy that will add and subtract under the laws of superposition. When front and back firing speakers are in a "half λ" phase (physical distance D + electronic delay δ) and inverse relationship, the energy sent towards the front wall may turn to zero, whereas the acoustic energy directed towards the listening position will be maximized. So, without the existence of sound cancellation waves reflecting off the front wall, it is possible to position your cardioid-like radiating speaker with much more freedom as before, the speaker has become in effect "position agnostic".