Use of carpet padding or foam under MLV for airborne noise

[Debbie]

I want to soundproof my bedroom from airborne noise coming from the room below me and am planning to put MLV on my bedroom floor underneath the carpet. In another thread it was recommended that carpet padding be put under MLV to help reduce airborne noise coming from a room below. I was wondering if someone could explain the principle behind putting carpet padding or foam under MLV to reduce airborne noise from below? I understand how carpet padding under MLV would reduce impact noise - the soft padding would help to absorb impact noises such as footfalls and would "decouple" the floor from the MLV. But I don't actually understand how carpet padding underneath MLV would help reduce airborne noise coming from below, given that the carpet padding itself isn't going to be much of a sound blocker.

If someone could explain the soundproofing principle behind using carpet padding or foam underneath MLV to reduce airborne noise coming up through the floor, I'd really appreciate it.

Thanks very much,

Debbie

[Randy S]

Sure Debbie,
Its real simple, MLV has an STC rating of 26 however when MLV is placed on a hard surface that STC rating drops by half. The padding allows MLV to remain flexable and decoupled from the hard surface.
Sound is not only airborne noise it is also vibration that can pass through mass, without decoupling the vibration can easily pass through MLV.

[Debbie]

Hi Randy,

Thank you very much for your reply. I understand now why carpet padding or foam is needed under MLV.

I'm planning to put down a total of four pounds per square foot of MLV on my bedroom floor. What weight and thickness of carpet padding would you recommend for putting underneath this much MLV?

Thanks so much for all of your help.

Debbie

[Randy S]

Debbie,
For this amount of MLV sound barrier to put down, I would recommend using #2 MLV - 2 layers with 8lb. 1/2" carpet padding between each layer. So padding/MLV#2/padding/MLV#2/padding/carpet. You should check to make sure this is not to much build up this will be about 2 1/2" or more.
I would also use lead tape on the MLV seams and on the plywood seams before you put down your first layer of carpet padding.

[Debbie]

Hi Randy,

Thank you very much for your reply. There actually is a height issue in that the existing carpet and padding sit on a finished hardwood floor, so I don’t have a lot of room to work with.

Do you think I could get away with just putting down one layer of the 1/2 inch carpet padding underneath the four pounds per square foot of MLV? If I were to only put down the one layer of carpet padding under the MLV and not put down the second layer between the MLV, how much of an impact would you say this would have on the effectiveness of this project? And if I do only use one layer would it better to use 10 lb carpet padding or should I stick with the 8 pound?

Thank you again for all of your help.

Debbie