Try an electronic stud finder first.

IMHO the best approach is to remove the plaster so you can seal the back of the outlet boxes and the perimeter framing at the other side of the wall (if it is really a stud wall) then fill the cavity with insulation and then install resilient channels and two layers of 5/8" type X gypsum board to provide a 2-hour fire resistance rating between the properties.

If you seal the edges of the perimeter framing before installing the drywall you should nave a very sound resistant wall.

The red stuff might be hollow structural clay tile used for fire separations 100 years ago.

An alternative is to use molly bolts to secure strapping or resilient channels to the existing plaster surface. I prefer the IsoMax system. This approach also avoids modifying a fire rated wall which would require a building permit.

Materials like QuietRock and Green Glue are intended to achieve an improvement in sound isolation in the thinnest possible space. If you can give up an additional 2 inches of space the resilient mounted channel system will provide better results.

If there is any type of masonry in the wall is was very common to apply the plaster directly to the masonry.

Special blocks with a square keying surface were used.

I wouyld not be surprised at all for a 110 year old row house to have plaster directly on masonry.
It will not have good sound isolation when installed this way.

Plaster has excellent sound isolation when it is on wooden studs.
The mass of the plaster limits the amount of sound that can be coupled to the studs.

You may need to simply build a wall just inside the existing plaster wall, and then consider isolation mounting of the drywall, along with some sound absorbing fiberglass fill.

I have a little Black & Decker stud finder. It does OK on drywall, but not great. It does worse on the plaster, but that may be because there are no studs in there. I think it's quite likely, as brickeyee suggests, that the plaster is directly on the masonry and there is some kind of raised surface in places -- the square keying surface -- and this is what is stopping some of my screws.

The reddish stuff could be some kind of patch applied long ago -- who knows. Or some weird chemical reaction.

I don't want to give up 4+ inches on either side of the room, and the plaster seems very strong, so I'm now inclined to glue & screw furring strips to the plaster and then add two layers of drywall as I described. This is not going to eliminate all sound and won't allow me to blast a home theatre system or anything like that, but should be adequate for privacy. I'll be able to dig the outlet boxes out and insulate behind those.

As for removing the plaster, it's not clear how much space that would net me, since those square keys seem to be only about an inch deep in the plaster. Though perhaps I could work around those with framing so they wouldn't be a factor. Another consideration is that I don't want to cause too much disruption through this process to the neighbors, so that also dissuades me from breaking off the plaster.

Thanks to both of you for your replies.

"I don't want to give up 4+ inches on either side of the room..."

There is no requirement to build a full thickness wall.

Steel studs turned on the flat work well, and you would only give up ~1.5 inches (they need to be barely clear of the old wall surface) plus drywall layers.

I've lived in and owned a number of 19th Century homes built roughly the same way--plaster directly on masonry party walls. Trying to stop the sound transfer with drywall on resilient channel was never very successful. That's because there are so many flanking pathways for sound to travel through, particularly common attics, crawlspaces and sub-floor planking Plus, researchers have commented that just one misplaced screw can short-circuit the whole system. I've used Quiet Rock to good effect, but it is very costly in small quantities. Green Glue has similar tested results, but I have no experience with it.

(I've found growing old helps, as your hearing diminishes. And if you're "lucky" enough to get tinnitus along with it, you're always so awash in a sea of noise emanating from your own head that the neighbours hardly matter.)

The hollow structural clay tile I mentioned was a red brick-like/terra cotta material with grooves on its face for the direct application of plaster or stucco. Entire houses were constructed of it instead of wood studs. I suspect in your case it might have been used by itself for a fire resistant party wall or as an infill material between bearing studs. Natco was the major producer in the US.

Here is a link that might be useful:

latest thought is to rough up the plaster with sandpaper and use liquid nails to glue the furring strips to the wall, without screws. liquid nails doesn't dry hard so I would think it would not be great at conducting sound. That, in combination with the Green Glue between the drywall layers, should suffice, if I caulk everything well. Yes some sound will go up through their ceiling and down through mine, and a little bit from below the floor, perhaps, but not much.

I'll get short enough screws so they don't go through to the plaster.

I'm going to seat the bottom course of drywall in a strip of foam pipe insulation and then cut the excess. That should also take a little weight off the wall.

re: Natco, I think there's a good chance that's what it is, or a similar product.

Why not green glue the drywall to the existing plaster? Try a sample first.

I don't think Green Glue is intended to be used as an adhesive in place of screws, despite the name.

Green Glue application.

yes, the sheets must be screwed down because the compression spreads the Green Glue thin. "Drywall sheets must be screwed together while the material is still wet."

You may have old gypsum Pyrobar block, or could be black iron with a stretched metal lath between the upright iron. In either of those scenarios case there's no studs. Very common in NYC (not sure where you are).

At this point, you could laminate 5/8" drywall and Green Glue as suggested. Also seal the outlets.

Here is a link that might be useful: Wall Soundproofing Options

I wanted to report that the addition of furring strips plus two layers of drywall with green glue between the layers did not have any effect. The approach does not work, since all I achieved was addition of a relatively small amount of mass to an already massive wall. More likely what I needed was to create a larger air cavity by using studs rather than furring strips (along with insulation in the cavity). I erred in thinking that the green glue product would help make up for the lack of a larger air cavity, but it definitely did not. For much, much more detail about my (failed) project, see this thread: http://www.avsforum.com/avs-vb/showthread.php?t=1299436

You need to decouple the new wall form the old surface completely.

Steel studs turned flat and held off the wall, along with sound absorbing fiberglass and 5/8 drywall will go a long way.

Part of the problem with solving problems like this is that you generally need to overdo things since going back later when a fix is not adequate more than doubles the work.

The sound insulation will help with sneak paths through the existing wall, and decoupling so the new heavy surface is not attached to the wall (floor and ceiling only) also helps.

You are likely to have to pull electrical boxes forward into the new wall.
Since the old wall is going to be concealed damage is not a big deal.

Great question. I'm having the same problem. Plaster walls, and no studs or bottom board to nail my baseboard into. The wall has a concave spot with about a 3/8" gap, so I'm not sure caulk would look right. I thought of using a toggle bolt, but i'd have to use a washer because I've already mounted the baseboard, and it just wouldn't look right. I'm afraid liquid nails might pull the plaster right off the wall. It's a tricky problem. I might have to just use caulk.