What is a reasonable expectation of decibel reduction when sound proofing?

Question

Background

I'm currently renting an apartment in a very old building and have a lot of trouble sleeping because of the noise. My apartment is underneath the main entry way; and people come in and out of the building at all hours of the night. I also have neighbours directly above me and hear a lot of noise from them as well.

I've spent some time collecting data with a cheap analogue sound meter.

• General noise from above - 60dB
• Loudest bangs from above - 70dB
• Foot traffic from hallway - 75dB
• Loudest door slam from hallway - 80dB

I don't have any problems with subwoofers or loud music being played. I do know that low frequencies are harder to block out, but I believe most of the banging noises I hear would be low frequency.

I've been doing research on sound-proofing options available to me. Aside from the noise, the place is perfect and I'll be in this general area for at least another 4-5 years (so I'm willing to invest some time and effort). My landlord does not care what I do, so long as I can 'undo' it when I leave.

I'm considering constructing a large 9-foot x 11 foot 'room-within-a-room' to become my bedroom; but I'm questioning what level of sound-reduction I can expect?

My budget is around ~1000USD / ~800EUR (for the construction; I'll worry about disposal later).

Apartment Details

• Ground floor (carpet over concrete)
• Three neighbours - above, left and right (I never hear anyone except the neighbours above)

Proposed Construction

I'm open to suggestion here - but this is what I had in mind. I'll frame the room with 2x4s - using staggered stud construction (including the ceiling). I was debating skipping the floor; but I'm not sure about that. Then 5/8ths" drywall for the outside walls/inside walls and filled with insulation. I'll include a door (a heavy external/outside door).

If that's not quiet enough - I figured I could apply an adhesive like Green Glue and another layer of drywall on the inside of the room.

I also have a lot of acoustic foam. I don't think it will be very effective with this frequency range - but I intended to throw it on the roof of my 'box'.

I was hoping to avoid taping/sanding the drywall; so my plan was to fill the joints/corners with acoustic caulk. I'm also worried about ventilation (since I intend to sleep in here).

This is a scale model of my apartment (where the green room is the to be the sound-proof room)

My Concern

I've been reading about soundproofing and things like STC level (http://en.wikipedia.org/wiki/Sound_transmission_class). But I've also read the STC level does not indicate low-frequency performance well. My room might achieve STC level 60; but I might might only get a 20dB reduction in low frequency banging. I don't want to do all this work and spend all this money if I'm still going to have loud noises I can hear. This chart illustrates what I'm talking about

I'm not sure if I understand how dbs work; so maybe my math here is wrong...but...I feel like I need a to get the loud 80db door slam down to 30-40db level for this to be worth doing for me. Depending on the frequency of the sound; I think the chart would be able to tell me what I can expect - but I don't know what it would be.

TL;DR

With a $1000 budget - how effectively can I 'block' 80dBs of relatively low-frequency sounds? (IE would I perceive it to be 70db or 50db or 20db?)

And, given my situation what is the best way to do it?

Answer 1

First off, a reality check; not trying to discourage you, just trying to set some expectations here:

• First off, the actual pressures you're dealing with are very small. Absolute pressure is measured in Pascals; 1Pa = .0001psi. Your 80dB sound is inducing pressure changes of approximately .2Pa = .00002psi (that's two hundred-thousandths of a psi).
• Second, the percentage changes you need to make to these small pressures are very large. The Bel scale is logarithmic, based on a "reference" pressure. One Bel - 10dB - represents a tenfold increase in pressure. So, a 90% reduction in the pressure of your 80dB sound only reduces it to 70dB; to get the sound level below 10dB (you or I would call that "silence"; in laboratory conditions the threshold for human hearing is 0dB) would require reducing the pressure differential by 99.99999%.
• This level of isolation is possible but extremely cost-prohibitive in most applications; million-dollar recording/mixing studio spaces are built with lower levels of soundproofing than this. You normally see this level of isolation in medical and sound engineering research labs, where they literally build a room within a room, suspended on vibration-dampening struts, with a near-vacuum maintained between outer and inner walls of the chamber.

The upshot of all of this is that, no, a reduction in sound of 80dB is just not possible on a $1000 budget.

Now, all that said, don't lose heart. First, you seem to have much more ability to make changes to your rented space than the average apartment dweller would have (though I'd double-check your rental agreement; if you're required to return the space to its original condition - or pay damages - when you leave, think carefully about studding in a new wall).

Second, much smaller reductions in SPL can make a very big difference. Soundproofing that reduces an outside noise by 20dB makes the noise sound only a fourth as loud. This is easily doable in your budget (add some sound-insulating material such as mass-loaded vinyl between the existing wall and an extra sheet of drywall or wall covering), add a sound-insulating curtain or three, etc and you'll start hearing results pretty quickly. Two layers of mass-loaded vinyl will reduce most sounds above 125Hz by 18dB just by itself.