The triple leaf effect is for drywall on studs so it would not apply to a brick wall.

Your minimum goal should be an STC rating above 50 and if the voids and penetrations in the brick wall are filled and sealed you will already have exceeded that rating.

One layer of 5/8" drywall on wood stud walls on each side of the brick wall with fiberglass thermal insulation will probably get the STC rating above 60, possibly 65.

If you want it higher try more drywall and/or resilient channels, IsoMax clips, or Green Glue but seal the perimeter and penetrations well.

One layer of 5/8" drywall on wood stud walls on each side of the brick wall with fiberglass thermal insulation will probably get the STC rating above 60, possibly 65.

The STC of a standard double brick wall is 42+/-. I would be curious to see any lab -tested results that bring the suggested extra 2x4 wall with insulation up to STC65.

But even if it does provide that result in a lab test, in practice you will notice very little difference, as bulking up the walls does nothing for flanking transmissions in the floor and ceiling.

I lived in and owned many such 19th Century/early 20th Century homes with exactly that construction. Old rowhouses are notorious for noise transmission--let alone the common attics that turn a fire in one unit into an inferno consuming the whole row at once.

Here is a link that might be useful: Technical Literature on Sound Transmission

Just something I found on the internet.

Here is a link that might be useful: masonry STC info

So, according to the linked item, steel studs on fiberglass and 8" modern block yields STC 59. No mention about flanking transmission.

I've added resilient channel and fg on several century homes converted to multiple-family dwellings. But without making improvements to ceilings and floors--let alone crawls and attics--there was little noticeable difference.

If I were doing the job, I'd suggest Renovator8's approach as a start and with the caveat not to expect a great improvement. Paying special attention to every crack, crevice, pentration and passageway between the units will help.

I lived in a townhouse separated from it's neighbors by old double wythe brick walls with no finish. The sound separation was very good but if you were standing near a crack or some bad mortar you could put your ear to it and hear normal conversations next door.

Bewitching to steel studs (even on the flat to reduce thickness build) increase the STC rating over wood studs.

Two layers of 5/8 drywall on steel studs with sound insulation is probably a very reasonable approach.

Just a few points here.

A triple leaf is defined by three layers of any mass, not just drywall.

The brick wall you have is simply a poorly sealed single leaf. Despite the bit of air in the current wall.

You will have some vibration flank under, over and around that wall.

Having said that, you can certainly improve your situation.

Build a new wall in front of the old masonry wall. Leave a 1" gap. You can use wood or steel studs, doesn't matter, since the final assembly is not a single stud wall. Also, could be 16" or 24" at this point. Again, doesn't matter.

Use simple R13 fiberglass, no more. I'd avoid spending more than you have to on the insulation. Cheap fiberglass will work as well as anything.

Make the new wall as heavy as practical, so I'd recommend double 5/8" drywall. Damp the drywall with a quality damping compound.

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

[Switching] to steel studs (even on the flat to reduce thickness build) increase the STC rating over wood studs.

Yep. Moves STC from 36 to 47 (See table in earlier link.)

With space at a premium, a layer of sound damped drywall such asQuiet Rock or regular drywall adhered with Green Glue could be effective.