- Acoustic Foam Panels - 
Second Story Floors and Stairs
Under Hardwood Floor
- 1-3mm thick - otherwise may be too thick. 
- Carpet underlayment is a no-go for hardwood flooring. 
- You can use carpet as underlayment for hardwood -  - but not carpet padding.
- Frothed urethane foam - 
- Cork - good for soundproofing as a natural material - . Sourcing - . Finished cork floor? Yes, from dense cork, but not cork underlayment.
- Rubber - 60 cents/sf, equivalent to 6 mm cork 
- Styrofoam - cheap - . But use seam tape, as its own tape doesn't work? 
- Felt - by Roberts - 33c/sf - 
- Acoustic caulk for gaps - . Sourcing - 
- Insulation or fibeglass - around stairs
- Underlayment between stringers and steps
- Use non-slip carpet stair treads 
- Use carpet - 5 hours for diy with chisel as in this video, including taking old carpet off. Underlayment is used.
- Sound Transmission Class - 60+ is great -