Soundproofing an existing party wall between 2 apartments:

Problem:

Reduce airborne noise coming through existing party wall in apartment building

Possible Solutions:

Simple Solution: Add a layer of Mass Loaded Vinyl to the existing wall. Then add a layer of acoustic plasterboard over the top & paint. (for high level noise use heavier MLV or multiple layers of MLV & Plasterboard)

Next level solution: Add a layer of MLV to the existing wall. Then batten out the existing wall preferably with metal battens. Add lightweight absorbent acoustic material (11kg/m3 min) between the battens. Then add a layer of MLV across the battens. Then add a layer of acoustic plasterboard.

Ultimate Level:

Line the existing party wall with a layer of mass Loaded Vinyl. Then build a metal framework 50mm apart from the existing wall and not touching. Install medium density (11kg/m3 min) acoustic glasswool into the 50mm gap. Then add a layer of Mass Loaded Vinyl to the back of the studs. Then add acoustic batts between the studs of the new wall, followed by another layer of MLV a layer of acoustic plasterboard.

Let’s break down the scenarios and evaluate which one would provide the highest level of soundproofing and why.

1. Line the existing party wall with Mass Loaded Vinyl (MLV) followed by a layer of acoustic plasterboard.

This setup adds some mass to the existing wall, which helps with soundproofing. However, there is no decoupling, meaning sound can still transmit through the solid wall structure. MLV and acoustic plasterboard will help reduce mid to high-frequency sounds, but low-frequency sounds (like bass) will still transmit to a significant degree.

• Expected Soundproofing Level (Rw/STC): Approximately 45–50 STC.
• Effectiveness: This is the least effective of the three, as there is no decoupling or air gap to further block sound.

2. Line the existing party wall with MLV, add a metal framework with medium-density acoustic glass wool between studs, then another layer of MLV and acoustic plasterboard.

This option is more effective than the first because:

• MLV adds mass.
• Metal framework with acoustic glass wool creates some level of sound absorption, but since the studs are directly attached to the wall, there is no full decoupling.
• Another layer of MLV and acoustic plasterboard further increases mass, which helps block sound transmission.

However, the direct connection between the studs and the existing wall allows for some sound transmission through the structure, especially low frequencies.

• Expected Soundproofing Level (Rw/STC): Approximately 55–60 STC.
• Effectiveness: This is more effective than scenario 1, thanks to added absorption and mass, but it lacks complete decoupling.

3. Line the existing party wall with MLV, build a 50mm metal framework separated from the existing wall, fill the 50mm gap with acoustic glass wool, add another MLV layer to the back of the new studs, install acoustic batts in the new wall, then cover with another layer of MLV and acoustic plasterboard.

This option combines decoupling, mass, and absorption:

• MLV adds mass to the original wall.
• 50mm gap with acoustic glass wool decouples the two walls, which is crucial for breaking the direct path of sound transmission through the structure.
• Another layer of MLV and acoustic plasterboard on the new framework increases the wall’s mass and reduces sound transmission further.
• Acoustic batts inside the new wall enhance absorption.

The decoupled framework and multiple layers of MLV, combined with absorption, make this the most effective method for reducing sound transmission, including low frequencies.

• Expected Soundproofing Level (Rw/STC): Approximately 65–70 STC.
• Effectiveness: This is the most effective scenario, as it maximizes decoupling, absorption, and mass. It will provide excellent soundproofing, especially for low-frequency sounds like bass, which are usually the most challenging to block.

Summary

• Scenario 1 offers basic soundproofing but is the least effective due to the absence of decoupling and air gaps. It would be suitable to reduce TV / Talking sounds.
• Scenario 2 provides good soundproofing level, but lacks the full decoupling of scenario 3. It would be suitable for louder airborne sounds like TV being listened to by a hard of hearing person.
• Scenario 3 would produce the highest level of soundproofing due to the decoupling between the two walls, the use of absorbent materials, and the addition of multiple layers of mass. This would be more suitable for louder music, heavy bass or impact sounds like doors closing, chairs scraping etc.

The Rw/STC ratings are just guesstimates and have not been modelled. A normal 90mm stud wall with 10mm plasterboard either side and no insulation is about Rw32

Flanking:

With all 3 solutions there is the possibility of flanking, especially if dealing with impact sounds or very heavy bass sound. 

Areas of concern:

Floor structure:- Impact Sound may be travelling through the floor structure and radiating into the opposite apartment.

Ceiling voids:- Even though the party wall has been soundproofed, the 2 apartments will undoubtedly share a party wall in the void above the ceiling. It is possible for the sound to still travel through the party wall above the ceiling line and then radiate into the opposite apartment.

Ductwork bulkheads:- do the apartments share any cavities that have duct or pipework running through them?  If so, sound can find its way through here.

Soundproofing Products Australia online store sells a wide range of thermal and acoustic insulation products. We also sell Mass Loaded Vinyl, Nuwrap Pipe Lagging Insulation, Polyester Insulation, Fibertex Rock-wool, Knauf Earthwool, Fletchers Pink Batts, and Bradford Acoustic Batts.

If you have any questions or require a particular delivery quotation, don’t hesitate to Contact Us! You can also get a free obligation quote. Check out other insulation supplies in Soundproofing Products Australia

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Read more about Soundproofing:

Soundproofing Noisy Pool Pumps, & Air Con compressors etc
https://www.soundproofingproducts.com.au/soundproofing-noisy-pool-pumps-air-con-compressors-etc/

Soundproofing Guide
https://www.soundproofingproducts.com.au/soundproofing-guide/