A speaker back-box can be used to protect an architectural
in-wall/ceiling speaker from construction insulation and/or comply with commercial fire codes. And if the box is also audio-engineered for the speaker, you’re good. However, a generic back-box can severely compromise audio performance.
I sourced the following information from reputable manufacturers*, plus my 5 decades of audio experience.
The best manufacturers engineer their architectural speakers for the intended enclosure.
Unless an engineered cabinet is included, better in-wall speakers
are designed for a sealed acoustic suspension enclosure of 2.79 cubic feet. That’s equal to 2″ x 4″ wood studs, 16″ on-center, a layer of sheetrock, by 8 feet high. The sealed enclosure also prevents out-of-phase front/back sound waves from canceling each other. A smaller cavity compromises low-frequency bass performance.
Better in-ceiling speakers are designed for an infinite baffle enclosure.
This is based on 2 criteria. A typical residential ceiling meets both.
1. An enclosure volume of more than or equal to 10 times the Vas of the woofer.
try and find the Vas of your architectural speaker. However, I was once informed* by 2 reputable manufacturers, that 5 or more cubic feet qualify.
2. As the acoustic suspension design, isolate the front sound wave from the back wave.
Enclosure construction is also important. Wood wall studs & ceiling joists are a good start. But sheet-rock severely compromises the enclosure. Distorting out-of-phase sheet-rock resonance creates significant loss from the lower bass frequencies through the vocal ranges.
3 Solution Options
1. Rebuild the wall with a material of more mass/weight.
Medium-density fiberboard (MDF) panels work well.
2. Build and insert a reinforcing MDF sub-baffle on the back of the sheet-rock.
3. Insert a sand-filled SandTrap sub-baffle.
If the 1st option is not feasible, then consider option #2. However, option #3 is easier and more effective. Sand offers more mass than MDF, and re-tunes system resonance to a lower more audibly suitable frequency. That’s my choice. But I’m biased. I am the captain and owner of SandTrap Audio.
*Note: I recently tried to reconfirm the manufacture’s recommended cabinet data. I was confronted by phone-tree-staff that could only recite less than helpful scripted generic information. It appears the techs are no longer available.