Originally Posted by Garman
Question about soundproofing... I am sure I will be calling a couple of people I know, but we got the go ahead to do the basement, and looking for some good ideas for soundproofing and making the "Media" room sounding decent. I heard Denim installation works great and using steal beams horizontally across and not driving the screws into the stud but the beam works wonders. Open to any suggestion as I want to get the best sound out my Dynaudio's!
What are your soundproofing objectives? Are you trying to keep noise out of the media room, or sound from the media room out of the rest of the house? If the former, what/where are the major sources of noise? In my case, my downstairs room had an opening to the stairwell and I had to seal that up with some framing and a DIY acoustic door to keep noise from the furnace & utility room out. I also have a sliding glass door to the outside that I considered treating or replacing but it's facing the woods and there's not enough noise coming in to worry about. Finally, the main water line into the house passes over my listening room and I haven't been able to effectively mitigate the cavitation noise from it when somebody is running water. The problem is that it's rigidly mounted to the joists above the room and I'd have to re-do the run of plumbing to vibration-isolate it. I tried absorbers on the ceiling but they didn't help.
If you're trying to keep sound from the media room out of the rest of the house, then I suggest concentrating on the ceiling. You could fill the space between the joists with rock wool insulation, and then double-drywall the ceiling with an acoustic drywall mounting system that isolates the two layers.
You also mentioned wanting to get the best sound, but that's more a function of room treatments and shape than soundproofing. Do you have control over room dimensions at this point in construction? If so, then you're lucky. You can use an online room mode calculator to help select room dimensions that give you a more even distribution of room modes. You can also estimate where the nulls are going to be based on where you think you'll put the speakers and listening position. Room treatment can't make up for bad geometry or bad placement.
Room treatments can either be purchased as finished pieces and attached to the walls and ceiling, or they can be built into the walls and ceiling, or they can be built over existing walls and ceiling. I went for a combination of the second and third. Here's a few guidelines on room treatment based on my experience with it:
- You can never have too much bass trapping. Corners are the best place for it, or anywhere where two or more surfaces meet. Tube traps are easy to stand in corners, but don't forget the wall/ceiling junction. Put in soffit traps if you can.
- For broadband bass trapping, use thick absorbers with free air space behind them for best efficiency and facing on the absorbing material to minimize high frequency absorption.
- For narrowband bass trapping, use membrane traps. But bass trapping for room modes below 50 Hz is generally futile. The lowest mode I was able to treat effectively was the first axial floor-ceiling mode (0,0,1) at 70 Hz - I used two 2x4 foot tuned traps on the ceiling.
- Use absorbers to treat reflections if the reflecting surface is close to the speakers or listening position. Use diffusors if the reflecting surface is further away.
- The most important first reflection in my room was the ceiling. Treating the ceiling yielded a much bigger improvement in imaging than treating the side wall reflections.
- If it's a rectangular room, it will have slap echo. You can use either diffusors or absorbers to take out slap echo, but be mindful of the next point.
- Make sure the total amount of absorption in the room is balanced wrt frequency. A common mistake, especially in custom home theaters, is to have too much high frequency absorption and not enough low frequency absorption. The result is a dead sounding room that tilts the perceived frequency balance. The best way to avoid this is to add absorption incrementally and measure RT60 vs. frequency while you are in the process of adding treatment. If you can't measure, you can try to do it by ear.
Here's a few DIY tips/tweaks;
- Spacing absorbers off the wall so there is an air gap behind them improves their efficiency at the lower end of their absorption band. A 2" thick absorber spaced 2" from the wall is nearly as effective as a 4" thick absorber placed on the wall.
- You can turn a high frequency absorber into a mid frequency absorber by covering the face of the absorbing material with a membrane. If you have a pre-finished absorber panel, you can remove the fabric cover, attach a plastic sheet with spray adhesive, and re-cover it. If you're making your own, just buy rigid fiberglass panels with foil facing.
- You can further reduce the high frequency absorption by covering the face with a sheet of cardboard. You'll lose absorption in the midrange too, but the low frequency absorption is largely unaffected.
- You can achieve a mix of absorption and diffusion by starting with a faced or covered panel and cutting holes or slots in it. Best to use a somewhat random pattern otherwise it's more of a diffraction grating than a diffusor.
- You can hide a lot of the ugliness of room treatment by framing around it and stretching acoustic fabric over the frame. Here's an old picture of my room showing that: