[paulspencer]

As I see it there are TWO problems you should consider at the same time - one is keeping the sound in, the other is creating bass absorption! If it were possible to build a room that can keep all the bass in the room, it would sound awful. Bass absorption makes the task of getting good bass sane. Many are forced to use bass traps, but don't realise that the ideal is that the entire room acts as a bass trap. This actually works fairly well with the goal of sound isolation. Multiple layers of drywall with a flexible adhesive in between (like liquid nails) works well.

Your room looks very solid, and I suspect a real challenge. I'm expecting some serious peaks and dips in the response. If you can get some more bass damping then you can reduce the challenge down to something you can work with.

You should pay careful attention to any weak points that will "leak" - doors, windows and any penetrations (power points, downlight holes, services/ducts etc).

Bass is always going to be a challenge to keep in the room.

This may be a challenge, but ideally you want to build a false ceiling that does not touch the existing one at all - neither the structure nor the drywall. Of course, there are different systems available that are low profile. You can attach drywall onto low profile metal frame work with special mounting systems. I've looked into those in the past and the cost can really hike. Another factor is whether this is DIY or not.

Topics like these are covered in a book by Dr Earl Geddes
http://www.gedlee.com/Home_theatre.htm

I don't know of anyone else who actually has a PHD related to small room acoustics. Much of what is published about architectural acoustics is suitable for large scale applications, not domestic home theatres.

 

[paulspencer]

Is this a DIY project or are you having it done? This can have a big impact on the cost. If not diy, then labour cost will have a greater impact on budget.

When I looked into isolating mounting systems in the past, the cost was very high. It's a low profile option and the cost can look different if labour is factored in. However, if you can actually use staggered ceiling joists for example, you can completely isolate the false ceiling. Done cleverly, this will also be a low profile option where the staggered joists protrude just below the existing ceiling members. That would be my first choice in a DIY situation.

Bass traps are intrusive into a room, and it makes sense to try to turn the entire room into a bass trap. In essence, this is what we are doing in using drywall and isolating it from the existing cave-like envelope. So while I tend to disagree with ignoring the bass damping issue, thinking you can fix it later with bass traps, this is moot because isolated drywall walls and ceiling will in fact increase bass damping a great deal.

In terms of measuring, it actually makes sense. This allows a before and after - which you can't do later. You could measure sound transmission loss as well as internal acoustic response. Both will have value, and will give you some feedback on what you have achieved. You can't do this later if you get curious!

 

[paulspencer]

What a great reply. I agree with all these points. Maybe not so much the measuring part, I'll admit. But for sure very good point about the decoupled system absorbing low frequencies.

Thanks :bigsmile:
I'm a little surprised about the measuring part. If you're talking about measuring the ambient noise level, I don't see the point. If we are simply talking about sound proofing, then all it will do is show what you have achieved. I'd say that's worthwhile, mainly for the sake of curiosity. Especially when one wants to see what happens at 50 Hz! Probably a bit disturbing ...

The most benefit in measuring will come when the room is actually done, and it's time to work on the bass.

This is a simulation of a room that I used in the past:

The dotted line shows worst case scenario if it were built like a bomb shelter/garage. The magenta line shows a more typical room. This would be easy to eq mostly, except two deep nulls. The red line shows what we want to see - a room where the modes are sufficiently damped that we have a good chance of getting it right with eq.

Of course, you can't rely on simulations, but they are illustrative still.

These settings were used to show the eq required to get the bass flat:

Note the actual room response is the inverse. As you can see, it's pretty tame. This is a lossy room with timber floor and light timber framed drywall as well as double glass doors and reasonably large windows. No bricks or concrete.

The two charts above are for the same room. Keep in mind the eq/measured chart is more complex also as it has mains and subs overlapping, the mains able to get down to 23 Hz.

So if you get the room right first, this should be the first thing for the bass. Then I'd experiment with eq and placement. There's a good chance this will get a good result. Ideally if funds permit you'd do this in conjunction with a multi sub arrangement. Here is a measured example:

http://mehlau.net/audio/multisub_geddes/

Geddes suggests 3 subs are ideal. You can start with one sub which mostly meets your performance demands on its own. It's placement is less critical since it will most likely operate below problemmatic zones. The other subs will give some boost, but their main function is to improve the room response by "spatial averaging." They can in fact be compact, and this is a good place to diy since you can cleverly hide them if desired, and integrate them into the room creatively. You could have a stair riser sub, coffee table sub etc.

If after doing all this you are still having a case of audiophilia perfectionitis, then it's time to think about adding bass traps.

Do you like to use theater risers as bass traps? I'm not an in-room acoustics person but this topic comes up frequently.

The idea makes sense, but I haven't done it due to practical restraints. I also don't actually need add on bass traps. Chances are I will experiment with them to see if I can get an improvement. In that case I'd certainly measure before and after.

 

[paulspencer]

From a thermal comfort point of view, a concrete floor on the ground is a good thing. It acts as thermal mass, and when the room is heated it stores the heat - a flywheel effect. Some houses are designed with this in mind to avoid or eliminate conventional heating. It also works in cooling. Radiant cooling is the most natural and pleasant kind - ever been down deep in caves on a stinking hot day? Feels better than an air conditioned room. Of course, in a cold room it will feel cold, but rugs where you put your feet will help.

Concrete is very reflective acoustically, and should have something to tame it since the ceiling will normally not have anything. There are two schools of thought on acoustic treatment in general:

1. The first is the only one that many seem to know. Use whatever speakers you like best without considering the room as a factor then make the room fairly dead. Use a lot of acoustic treatement, mostly to absorb, but also some diffusion, with specific focus on the reflections that will cause the most problems.

2. Choose speakers with a room interaction which is optimal in a normal room. Speakers like dipoles, omnis or with controlled directivity and a very well behaved polar response. In this arrangement, reflected sounds are desired and radiated in a controlled and planned way, rather than trying to just kill them. You might still use treatement with this approach, but it becomes less critical.

 

[paulspencer]

Quick thought about the shaking things upstairs - what happens if you lay the sub across a couch? This will mechanically de couple it - what kind of difference does it make?