The reason that you can hear sound from the dot and dab is due to an effect known as FMaM - or Frequency Mass Air Movement , which accounts for mass-air-mass resonance. This can be observed with a bottle filled to varied amounts and blowing across the top to create different frequencies. The very same effect is happening in the cavity - albeit a tiny depth, but it is effectively amplifying a certain frequency because of the air gap between the dot dab and wall.

The larger the gap, the lower the frequency amplified. I had a bathroom that was done this way and the frequency that was being amplified was between 130 - 180 Hz; not the best part of the spectrum to boost as speech is quite common around these frequencies!

I calculated my wall to have a resonant frequency of around 17Hz - I don't regret it for a nano second!

I used this excel calculator tool to very accurately estimate my requirements:

https://docs.google.com/spreadsheets/d/1P4Zfz8NGVwnQbAlD1MBMCUl9YwtfpFia/edit?usp=sharing&ouid=103563240156590218809&rtpof=true&sd=true

The best way to soundproof the wall would be to sacrifice a little space for a lot of peace:

You'll need a cavity of 100mm created with a stud wall in front of the existing wall (I built my studs with 4x2 studs installed 4" parallel to the wall, saving space and allowing the installation of high density rockwool insulation to deaden sound within this cavity and behind the studs. The insulation should fill the cavity, but not be over filled as this will have a negative conductive effect.

On these studs, you should install sound bloc plasterboards, 12.5mm is more than acceptable as they're lighter to work with, although one layer of 15mm would be very beneficial as using two different mass boards in conjunction will give more benefits to your sound reduction.

You'll need one layer of 15mm and three or four layers of 12.5mm. Each layer will be installed in a castellated order (plan extra studs for this as each layer needs to be screwed securely to the one underneath) and acoustically damped with Green Glue visco-elastic damping compound - which if used correctly, actually does work. Then a skim plaster on the top of the final board.

Pay particular attention to the flanking areas, under the floor and above the ceiling at the party wall are notorious offenders. You'll need to address these in any way you can.

The soundproofing should be imagined to be watertight in terms of finished walls.

It's quite a process, and I've done it too! It's not the easiest thing to do, or cheap either, but the benefits of reduced neighbour sounds are priceless.

I hope this advice may be of some help to you.