I build SCIFs. Fiberglass insulation and extra layers of drywall remain about the cheapest method of building walls with a higher STC rating.
Most of the products cost more than adding another layer of drywall for a total of 3 layers.
Are you hearing low frequency noise (footsteps) or higher frequency noise (voices)?

Thanks for your answer. I guess I am more concerned with the high frequency noise. I can hear for example the baby cry and the loud voice of the 3 year old when he is playing. A bit of the television also. I can hear low frequency noise from time to time (but not footsteps; only when they seem to be dropping something heavy; the staircases are away on each side of the semi-detached, so from this standpoint, it is good); the low frequency one is not as frequent and does not bother me as much.

Would a few layers of drywall over an old wall which is made of concrete covered by a thick layer of plaster do the trick? I am afraid not to have any air to stop the sound... I am ready to put a few more dollars if I can be guaranteed some quietness... Thanks.

mtl -

I did research this last yr and did some soundproofing in a stand alone residence -

in addition to the help I received here - I have a bro in law who is a master carpenter and has rebuilt many of the SONY studios in NYC

Here's what I've found
as stated above different frequencies/noises/origins
- Perhaps the cheapest and most effective would be batt insulation - although I would not use fiberglass - I would use Mineral wool - I used brand name Roxul (made in Canada i believe) it has been & still used today for firestopping btwn floors - most people say ICCCK when you mention it as it was not until the recent past that it became available in Batt form - I found it for same pricing as fiberglass - I think it is rated R-15 if I recall. That I used btwn joists in LR ceiling
We lost largest ceiling in our home 2ndary to flooding - it was plaster which has excellent sound proofing qualities
After the Roxul btwn joists - I put up 5/8" resilient channel running on the perpendicular to joist - applied a green sound proof tape to that (similiar to weather proofing window tape) - then hung 5/8" sheetrock applied to channel - idea is to remove the reverb through the joists - Wouldn't you know IT WORKED!!!!

LR ceiling

In recent basement remodel - I wanted to soundproof the heck out of it!! 2 reasons - 1 kiddies (we have lots of those) there play/HW/hangout area
2. Future use of a real home theatre

We did Roxul btwn joist to ceiling and wall btwn work area of basement & play area (other walls are exterior fieldstone )
then applied Mass Loaded vinyl (1/8"thick) to joists - NOT an easy task by any stretch of imagination - this stuff is heavy, heavy, heavy & you are working overhead - I thought the contractor would walk off the job!

here comes the woops part - A grid system was spec'd to go to ceiling - well without visualizing the joists it makes for great difficulty getting the grid up over the MLV but it got done - then a homosote acoustic rated tile to that

No not totally sound proofed - because we have a stairwell which is a tunnel - the only way around that would be to put a door in at the bottom of the stairway - perhaps some day - it would be squishy though

Roxul btwn joists

applying MLV

MLV to joists

MLV to a floor would have been MUCH easier but not an option -

What bro in law used a lot at Sony was
Multiple layers of drywall - to be effective it needs to be done in varying layers of thickness 1/2" over 5/8" w/staggered seams

I posted a co. website that I purchased MLV & acoustic tape from and got the Roxul locally.

Quiet rock $$$$ not sure of it's effectiveness - green glue sounded really messy, messy, to work with

Windows and doors are also great sources for sound travel

Good luck
you really need to determine where sounds are coming from and type of sound in order to appropriately soundproof

Here is a link that might be useful: soundproofing

Lead foil, believe it or not, is also very effective. Expensive no doubt, but more compact than mineral wool, which I know is also very good.

Advantage of the foil is I guess you put the foil up, then drywall straight over.

Wall hangings like rugs might also make quite a difference.

Mtl,

As I've gutted and redone each room in my semi-detached, I've added Roxul 'Safe n' Sound' to the common wall and it makes a definite difference. Not spectacular, but still noticeable and worthwhile - it's not a cure-all and I still hear footsteps, voices and music through the wall but they are muffled and at a reduced volume. An added bonus is that it adds some fire protection as well.

In most cases I've also used resilient channel either horizontally or diagonally. I'm not convinced it makes a huge difference and am not sure I would use it if I were doing it again. I didn't use it on a bedroom wall since I built two small walk-in closets in front of that wall, and though a subjective assessment I don't notice any difference in sound level coming through the wall.

Here is a link that might be useful: Roxul Safe n' Sound

I know this will sound a bit strange but it is an inexpensive and stylish way to soundproof. Cork tiles. There are a lot of styles to choose from, just make sure you get the tile form (glue and stick) as opposed to the floating click type. I have done a lot of home theatres and this is a common way to soundproof. You can also lay rolls of cork down as a subfloor under any type of hard surface that is being installed to help reduce and deaden sound and echoing. A lot of higher end condominiums actually require cork underlayment before laying tile or wood.

To reduce sound transfer requires mass, resiliency, and/or absorption.

Insulations provide absorption but it takes a lot of it to make much of a difference. It does help absorb sound that enters at one point and exits at another point some distance apart since that sound might have to travel laterally through many feet of insulation. Cork might reduce reverberation inside the room but will absorb too little sound to be effective in reducing sound transfer between spaces.

To design added mass or resiliency I would need to know exactly what is there now and how well it is sealed especially if it has structural or plumbing penetrations. I would also need to know how much space is available on your side of the wall and how many obsticles there might be to adding materials. I would also need to know how much money you are willing to spend and if a professional installer will be involved.

It won't be easy because you only have access to one side of the wall and you probably can't add continuous materials so you should not expect textbook or manufacturers' solutions to work as promised.

I went through an acoustics class last semester. Here's what I learned about soundproofing:

1) Attack the weakest link first. The composite transmission loss equation of a mixed sound barrier is dominated by the weakest link.

2) Sound is transmitted in 3 ways
a. Direct (through the wall)
b. Flanking paths -- things that you wouldn't necessarily think about, such as poorly-installed outlet plate covers, ductwork, and floor spaces.
c. Leakage (air gaps under a door, etc.)

I doubt that you have sound leakage (air gaps) in the common wall, but you may well have flanking paths. In particular I would make sure that electrical outlets and switches on opposite walls of the shared wall don't also share the same intra-stud space. This will probably be your weakest link. Once that is remedied, I'd attack the wall itself.

A standard gypsum wall (one layer of gypsum on each side of a 2x4 stud) give you an STC of approx 35.

Add insulation (always leaving approx 1/4 airspace--if you pack the insulation it becomes a sound transmitter as well) and you get to ~STC 40

I wouldn't recommend resiliant channels--I think to do it properly you'd have to tear down the neighbor's drywall as well--probably not an option.

If you do staggered stud and no insulation, you go from STC 35 to ~STC 42.

Staggered stud AND insulation = ~STC 49.

Keep in mind that decibel levels are a log 10 scale, not a linear scale, so going from STC 35 to STC 49 is very significant. Roughly speaking, a 6 dB change upward is perceived as a doubling of sound level.

Resiliant channel works very well if you add enough mass to the new layers. On a concrete wall it wil be very hard to get enough mass on the wall to provide much benefit.
Look for the leakage (AKA 'sneak paths').
If you can block these you wil have a very large drop easily.
Even expanding foam will help with these paths. You need to close off the gaps.
Using a cardboard tube to provide some directional hearing can be a good way to work around and find exactly where the sound is comming from. Put some cotton in one ear and use the tube on the other. Move the open end of the tube around and you can quickly find 'leaks' in a wall.