Many Acoustical Panel Companies say to Put an Absorptive Panel at the Mirror Points; Should I? (No replies)
There are situations where the inclusion of absorption at a mirror point is beneficial. It depends on the quality of your speakers and which reflection you are talking about. It's important to point out that side reflections are not the evil they have been depicted over the years. In fact, the energy bouncing off of the side wall can be quite beneficial by expanding the sound stage width and enhancing spaciousness. Placing absorption at the mirror point may actual reduce this pleasing enhancement of the soundstage. The consequences of absorption on focus and envelopment are discussed in my article on the HAA Portal blog: http://homeacoustics.org/2017/02/acoustical-focus-and-envelopment/&newtab.
Scenario 1) Some speakers have very good on axis frequency response, but are not so good off-axis. This has been a fairly common problem with many (even high-end) speakers over the years. Better technology and speaker design have resulted in good speakers with very good off-axis response but it is not yet a common trait in even very expensive speakers. The sound quality of a speaker with poor off axis response may actually be improved by absorbing the reflection of it's poor quality off axis response signal directed at the wall.
Scenario 2) Speakers with very good off-axis response can actually sound quite good without any treatment. That is not to say that management of the reflections is not still beneficial. In these cases, a strategy of absorbing some reflections and yet allowing others to remain strong can be a great idea. The bottom line is that a speaker that has similar response off axis to on axis is much easier to put into a less than properly treated room, so more flexibility. This falls into my formula for getting the best performance; if something is easier to calibrate it will, in all likelihood get calibrated properly and result in the best high performance sound quality. Treatment is still beneficial but the design of treatment show include plenty of reflections; use less absorption and be picky about where it should go. That is likely, not the first reflection point.
Scenario 3) Some speakers are more directional than others. In the case of some dipole planar speakers, the addition of an acoustical panel at the mirror point may have negligible effect since so little energy is directed there. A more complex derivation of the proper positon of the speaker is to allow the rear wave of the speaker to reflect off of the back and then side walls which reveals a more consequential position of the mirror point. My advice is to let those rear reflections fly as they can be quite an enhancement of the soundstage and spaciousness of the speaker. This scenario can also be a potential issue with horn loaded speakers used in a small room as they tend to have a narrower directivity, although not as narrow as some planar speakers.
One of the problems that is commonly discussed with regard to reflections is their effect on frequency response; namely SBIR. Speaker Boundary Interference Response or SBIR is the distortion of the low frequency response due to interference with the direct sound at the listening position. Use of a low frequency absorber can reduce some SBIR, but they also reduce the beneficial effects of the reflection. Many acousticians have adopted the use of a hybrid panel which can absorb some low frequency information yet allow the higher frequencies to be reflected. Food for another discussion.