Type of foam
Start with fiberglass insulation. This is probably the cheapest option, especially when mounting speakers in an existing wall that already has insulation. Battery insulation will reduce some vibrations through the wall, but it will also not work for low frequency frequencies. If you are installing speakers with large woofers that will reproduce very low sounds, you should choose more than just battery insulation. Try to create your own acoustic foam with different materials. Standard egg scrub foam works well as an acoustic absorber. Use a serrated knife to cut the foam to size. You can cut several pieces and fold them together so that the tops in one piece fit into the valleys of another piece to increase the thickness. Fold the foam with fabric, such as cotton or wool, to make it a little thicker if needed. Think of other materials such as foam board insulation, high quality Styrofoam or even packing of peanuts wrapped in fabric.
If you need more specialized foam, buy ready-made acoustic foam. Many companies, such as Auralex, make special acoustic foam. This foam costs more, but it is designed to reduce certain frequencies. You can buy custom foam for which frequencies you have the most problems with. Cut the foam to size and fit it around your built-in speaker cover.
Mounting considerations
Separate the built-in speaker from the structural points of the wall. The foam is intended to be an acoustic insulator, but if the speaker housing touches a wall body, the vibrations will be transmitted regardless of the foam. Make sure your built-in speaker is separated from the walls as much as possible using acoustic foam.
Verify the sound quality before ending your speaker mounts. Run some sound through your built-in speakers to check if your acoustic foam has the desired effect — before replacing any screws. Listen to frequency balance and noticeable rattle. If the sound quality does not suit you, fix it now before installing or installing the wall speaker.
Adjust your foam as needed after checking your built-in speaker with an audio source. You may need a higher density foam, or you may need to pack more of it into the wall. Since the foam will be hidden behind the speaker, it does not have to be aesthetically pleasing visually. Cut it up and wrap it. Do what it takes to get the right sound. Remember that your ears are the ultimate arbitrators. In-wall speakers provide a space-saving and efficient alternative for listening environments such as recording studios and home theater systems. When choosing built-in speakers, acoustic insulation is paramount. Proper insulation reduces unwanted bleeding and resonances from the speakers. Use of proper acoustic foam treatment ensures the most likely sound reproduction, while improper use of foam can result in uneven response. Do-it-yourself acoustic foam ranges from simple insulation to expensive custom-made materials.
Source:danspela.com