Why is soundproofing bad even with a cavity party wall

[jacoscar]

I have lived in my 1960s terraced house for three years now and based on what I can hear from nextdoor (steps on stairs, doors slamming, people’s voice when shouting) I have always assumed we had single brick party walls.
In the loft you can clearly see a single brick wall pattern with no bricks placed at 90 degrees to tie into another wall.

However, having recently cut a hole in an external wall (but very close to the party wall) to fit a rectangular duct, I realise there is indeed a cavity between the houses, I could put my hand in there.

What strikes me is that if there is a cavity, sound insulation should be much better than what it is as the houses are basically decoupled/detached

What could be the reason?

 

[Wayners]

Sound travels through solids like bricks and air gaps.
Also can travel 100s miles through water. It's a solid.
To stop sound you need a floating solid that will soak up the noise.
So metal in sponge will stop sound.
Sound block plasterboard (a solid) on resilient bars not touching sides (floating) will stop sound.

You could have many cavities but that will reduce sound each time it goes through but would need many to stop sound.
You can be 5 floors above a party and still hear it with all that concrete and walls below. Solids

 

[jacoscar]

I think what you want to say is that sound can propagate through matter but not in a vacuum

Bricks are solid, water is normally found in liquid state and air is usually found in gas state. Yes, sound can propagate through all this stuff.

Now, between the houses there is mostly air (the cavity).

If they were two detached house only 10cm apart, soundproofing would be amazing: you could hammer on one outside wall and not hear it in the other house (provided there are no windows)

So there must be something joining the two leafs of the party wall, what could it be? Wall ties? Joists resting on both leafs?

 

[Wayners]

Foundation joins.
Glass double glazing let's sound through worse than bricks

 

[^woody^]

Air is not a magical barrier, we can hear people whispering across a room for instance.

Attached buildings carry sound through the structure and also across any cavity. You can't get around that fact.

Another significant cause of sound transmission is whether the neighbour has flooring that promotes sound travel, such as wood or laminate, or attenuates it such as thick carpet and underlay. Likewise for wall coverings.

The most effective form of sound insulation is to stop it getting out, not coming in.

 

[DIYBloggerUK]

I have lived in my 1960s terraced house for three years now and based on what I can hear from nextdoor (steps on stairs, doors slamming, people’s voice when shouting) I have always assumed we had single brick party walls.
In the loft you can clearly see a single brick wall pattern with no bricks placed at 90 degrees to tie into another wall.

However, having recently cut a hole in an external wall (but very close to the party wall) to fit a rectangular duct, I realise there is indeed a cavity between the houses, I could put my hand in there.

What strikes me is that if there is a cavity, sound insulation should be much better than what it is as the houses are basically decoupled/detached

What could be the reason?

Yes, 2 layers of brick are much better than one.

But...

Sound does not see that cavity wall as much of a barrier. It simply vibrates through it without loosing much energy. Basically, the more dense a material is, the better it will be at blocking sound. Bricks are fairly dense, but there is just not enough material there to dampen the sound as much as you would like. Sound can also be dampened slightly when it has to transition from one material to another. Or when materials are not joined to each other.

Currently this is the path your sound takes:

air - brick - air - brick - air.

And like others have said, is all connected allowing sound to transfer between the walls easily.

If you really want to sort this out, then I think you have only one option. But it may not work as well as you want!

If you are wanting to reduce the sound going through a party wall, then maybe you need to create a stud wall your side filled with heavy 100mm thick acoustic grade rockwool. You could leave a 25mm - 50mm air gap between your brick wall and stud wall to help stop the transfer of sound (decoupling). Then use heavy acoustic grade 15mm thick plasterboard over the stud wall.

So it means the sound now takes this path.

air - brick - air - brick - air - 100mm thick acoustic grade rockwool - 15mm thick acoustic grade plasterboard - air

I am not allowed external links on here I think. But I have an example project on my DIY blog. It is called Warton Woodworks DIY Blog, then search for soundproofing.

I go into more details about what types of materials are better at blocking certain frequencies, such as low frequency (bass) sounds, or higher frequency sounds.

I hope this info helps.

Jamie

 

[jacoscar]

Yes, 2 layers of brick are much better than one.

But...

Sound does not see that cavity wall as much of a barrier. It simply vibrates through it without loosing much energy. Basically, the more dense a material is, the better it will be at blocking sound. Bricks are fairly dense, but there is just not enough material there to dampen the sound as much as you would like. Sound can also be dampened slightly when it has to transition from one material to another. Or when materials are not joined to each other.

Currently this is the path your sound takes:

air - brick - air - brick - air.

And like others have said, is all connected allowing sound to transfer between the walls easily.

If you really want to sort this out, then I think you have only one option. But it may not work as well as you want!

If you are wanting to reduce the sound going through a party wall, then maybe you need to create a stud wall your side filled with heavy 100mm thick acoustic grade rockwool. You could leave a 25mm - 50mm air gap between your brick wall and stud wall to help stop the transfer of sound (decoupling). Then use heavy acoustic grade 15mm thick plasterboard over the stud wall.

So it means the sound now takes this path.

air - brick - air - brick - air - 100mm thick acoustic grade rockwool - 15mm thick acoustic grade plasterboard - air

I am not allowed external links on here I think. But I have an example project on my DIY blog. It is called Warton Woodworks DIY Blog, then search for soundproofing.

I go into more details about what types of materials are better at blocking certain frequencies, such as low frequency (bass) sounds, or higher frequency sounds.

I hope this info helps.

Jamie

What you have done (and I also have done) by building a new stud wall is basically decoupling; the sound transmission is not reduced so much because of the insulation but by the fact that the structure is (mostly) separated from the party wall.

But having a party wall is basically a very good mean of decoupling structures (although properties would be joined at the external wall), hence why I was surprised that it did not work so well.
Had I known previously that the part wall was cavity, I wouldn’t have built the soundproofing studwall because its effect must be minuscule compared to what the cavity (and I mean the presence of two almost independent walls, not the fact that there is air in between) is already doing

 

[tony1851]

Whatever you do to the party wall,you'll still have flanking transmission via the front and rear external walls.