wall acoustics

[colterphoto1]

I've read the web discussion re the low frequency sound absorbing panels and the problems with obtaining the OC materials. My question is more basic.

Assuming I am willing to build and install the appropriate sound absorbing panels, would I still benefit from creating a finished 2x4 stud/drywall wall along my concrete block basement walls? I'm a firm believer that a basement is just a basement, ie, don't put carpet and the best furniture down there. But would there be some degree of sound taming just from the flex in the drywall with perhaps some fibreglass batt between it and the concrete block?

Also, all the upstairs rooms in the house are double layer 3/8 plaster board with a skim coat of plaster- ie VERY stiff plaster walls and ceilings along with mostly hardwood floors. What type of acoustical treatment should I expect to install in primary/critical listening rooms (this would be my photo gallery, hearth/great room, and small bedroom theatre)

Thanks

[artto]

Concrete doesnt make for a very good acoustical material.

There are 2 schools of thought on the flex wall concept. And to some extent, I guess it depends on your speakers. The harder, thicker & denser the wall is, the more low frequency sound will be kept in. This can be a good or bad thing.

If you build the 2x4 stud walls I would definitely pack fiberglass or rock wool between the studs to damp resonances.

IMHO if you leave the concrete block walls bare, youre in for ear bleed type sound. Too many mids & highs, harsh, pingy sounding.

Acoustical control is not about only absorption. Or even just low frequency absorption. Its about treating all audible frequencies (ie: broadband) with both absorption and diffusion.

The hardwood floors upstairs (ceiling to the basement area) will have high impact sound transmission to the spaces below. The best place to stop impact sound transmission is at the source (ie: a sound barrier underlayment under the hardwood would be more effective than placing the same material between the ceiling drywall & the joists)

What type of acoustical treatment should I expect to install in primary/critical listening rooms (this would be my photo gallery, hearth/great room, and small bedroom theatre) No offense, but the room you are describing cannot possibly be for critical listening. You have too many conflicting uses assigned to the same space.

Welcome to the Forum

[phatbass]

[Klipschfoot]

I found a site that may be of help in your planning.

http://www.auralex.com/

[WMcD]

Art is the master.

I'm not sure he adressed your basic concern.

Concrete is certainly a good reflective surface. But so is sheet rock or plaster. Putting some batting behind sheetrock may have some little effect.

You should do a lot of reading about the different effects of rooms and where there can be structures which absorb or diffuse sound, and if so, at what frequency.

For example. Please stand in the middle of your basement or any other room lacking a lot of soft rugs and furnature, and clap your hands, forefully, once. Listen. You may well hear a slap echo. It is like a "boyy-ing" following the clap.

This is, roughly, the impulse reponse of the room. Your hand clap is sending in an impulse and you can hear the echos.

This is a decent and ear openning test of echos. Try this test on rooms around the house and at the office. It will give you some perseptive on room response and how echos are abosobed.

For the most part, we're stuck with walls which are parallel and hard. We can put up some absoptive and diffusive surfaces. However most of the grunt work is by furnature, drapes, and carpets.

Gil

[artto]

Gil, your comment about room furniture, drapes, etc. reminded me of a recently discovered a major mistake in my room while going through the process of it's 4th revision (which is not yet complete).

When I had completed the previous revision (around 1992) I was eager to get things back in the room & listen. I didn't even bother hooking up the center speaker or tube amps or whatever. Just a small Crown D60 on the flanking Khorns. I had dragged one section of a 6 piece section sofa back in the room to sit & listen. Virtually no furniture in the room other than the equipment racks & record cases along the rear side walls. I remember being quite amazed at the improvement in sound, both in terms of its openness, scale & low-end weight (this was the time I installed the polycylinders all around the top half of the front & side walls along with the large vertical floor to ceiling ones in front).

Now, as we all know, furniture & other substantial objects in the room can absorb & diffuse sound. This is usually assumed to be a good thing. The room's current revision, includes adding substantial bass trapping in the more 'ideal locations', rear vertical & horizontal corners loaded with Auralex LENRD's & MegaLENRD's. The low-end response was substantially improved. However, there still was something missing from that first 'run-thru' experience mentioned above.

All these years I left the large sectional sofa (4 seats & 2 ottoman) in the middle of the room because I thought it would be good at breaking up lower frequency standing waves due to it's size & composition.

As the 'theory' behind corner bass trapping goes, the bass wavelengths tend to 'pile-up' in the room corners causing excessive amplitude increase in these locations. The result of this in other parts of the room, especially in the middle of the room, is often just the opposite, a cancellation or reduction of amplitude at the same frequencies.

It recently occurred to me, could this substantial sized absorptive piece of furniture in the middle of the room be actually be contributing to acoustical problems instead of helping to improve it? Was it actually absorbing some of the lower frequencies in exactly the wrong location (where there were already cancellations occurring), as well as absorbing too much mid & highs?

I took out one of the seating sections. Not too much difference. I then took out the 2 ottomans. Now I definitely could hear a difference. It was more like the initial experience mentioned above. Unfortunately, there are often conflicts of interest between the rooms acoustical requirements, & our physical needs/likes/dislikes. The sound was much more open, larger sense of space & soundstage, with more weight & fullness when I preliminarily had only one sofa section in the room. Needless to say, Im going to be changing the furniture in the room to something lighter & less substantial.

[formica]

A oak church pew would probably be less absorptive...

[WMcD]

Very interesting read.

I thought that bass traps were put in corners because there is a reverse horn effect so that the pressure is greatest there. Just as PWK has mirror images of a horn in a corner, there are mirror images of absorptive surfaces. (I'm on a mirrors-as-an-analogy-for-acoustics these days.)

Acoustic design by experimentation doesn't seem so bad, or at least others have done it.

Perhaps you can comment on the following:

It used to be that concert halls would be built only after a one-third model was built and tested. But Leo Beranek thought he knew everything and designed Avery Fisher Hall without an actual model. It did not sound good.

Years later there was a vast remodeling to solve problems.

My conclusion is that the ancients realized the problems of predicting response and therefore a model was necessary. Of course it would be tough to build a one-third model of a living room and test the effect of, say three-foot ceilings.

Smile,

Gil