Hi Khalid,

There are a couple distinct things going on regarding the "boundary effect."

At low frequencies, the presence of nearby boundaries increases the amount of bass. This is true (and easily measured/heard) because the energy that would (in free space) be emanating in all directions gets reflected back by the boundary. Since the waves are very long, the reflection is essentially in-phase with the direct sound and reinforces it.

At somewhat higher frequencies, however, the reflection is no longer in-phase (since the waves are shorter), causing a comb filter of alternating reinforcement and cancellation as you move to higher frequencies. You can hear this for yourself by moving a Bluetooth speaker playing pink noise closer to the wall and then away from the wall. The "phasey" coloration that changes as you move the speaker is the changing comb filter.

In-wall speakers pretty much eliminate this second effect since there is no opportunity for the sound to wrap around the speaker and be reflected back into the room. Of course, they still get the bass boost benefit from being in "half space" (2π) and should be voiced accordingly by their designers. 

On-wall speakers behave as you might expect from the description above: they get the bass reinforcement since those waves are very long compared to the small distance from the face of the speaker to the wall. But they also tend to have a rather sharp notch in their response at a frequency that is determined by the distance they stand proud of the wall. For example, if the speaker were 6" deep and on the wall, there would be a sharp cancellation at the quarter wave frequency (about 565 Hz in this case). At 1/4 the wavelength, the bounce off the wall will be exactly out of phase with the energy coming from the front of the speaker. This happens again and again at multiples of that frequency, hence the comb filter.

Even though this wall bounce is really startling in measurements, it is surprisingly benign in most cases. The human ear/brain mechanism hears comb filters all the time in nature. We seem to process them simply as part of the environment unless they are changing (as was the case with the moving Bluetooth speaker).

Anyway, to answer your last question: It has been my experience that flush mounting (whether in-wall speaker or a well-made baffle wall) is the best way to go in terms of getting smooth response. However, you also should consider the advantages of being able to aim the speakers at the listening area. If you cannot (due to in-wall mounting), you need to make sure your speakers have really wide, uniform dispersion.

This response was probably more than you asked for… sorry about that.

The speaker we are installing almost 20 inch in depth after what I understand from your detailed answer I will go with baffle wall.

Thank you for your time