Concrete walls

[Maswov]

I am looking at some houses to buy and most here are built from concrete. Is the bare concrete ok for sound or should i be look at installing some material in room to improve sound?

[MAL01]

Originally Posted by Maswov

I am looking at some houses to buy and most here are built from concrete. Is the bare concrete ok for sound or should i be look at installing some material in room to improve sound?

You should definately put something on a bare concrete wall, probably even drywall, if you don`t the sound will reflect and bounce off the walls, not good.
I`m sure there will be people more knowledgeable than me that will chime in on this.

[richteer]

Originally Posted by Maswov

I am looking at some houses to buy and most here are built from concrete. Is the bare concrete ok for sound or should i be look at installing some material in room to improve sound?

MAL01 is absolutely correct: bare concrete walls will sound terrible. You'll definately need some room treatments in a concrete room, and they're highly advised in a drywalled room too.

To get an indea of the problems you'll face, go into an empty concrete room and clap your hands. That long decaying echo (called a flutter echo) will screw up your speaker sound.

[inlayterms]

As members have suggested, drywall the concrete walls. But be happy about the fact that the walls are concrete, as they will provide great sound insulation. I had four concrete walls in one room in the basement where I used to have my setup and nobody in the house could hear much even though I played music/movies fairly loud. Since I've moved to another room with just drywall, even the sound of my playing my electric piano with headphones on (so only the sounds of me hitting the keys is heard) annoys my parents upstairs.

[Deciazulado]

etc

[kareface]

The main reason why you'd want to put up a layer of drywall is to reduce the reflections off that wall. Foam would be much worse for this then drywall, but I'd consider adding a second layer of drywall with a layer of green glue. The LF reflections will really hurt you with cement, GG is one of the best products to reduce transmission in the bass region. Doing a single wall wouldn't cost much more then just a normal single layer. If you layered that wall with foam, you'll do nothing to stop the LF reflections and only hurt your mids and highs. The highs will sound like they are missing from that 1/2 of them room.

[Twitch9]

Originally Posted by kareface

The main reason why you'd want to put up a layer of drywall is to reduce the reflections off that wall. Foam would be much worse for this then drywall, but I'd consider adding a second layer of drywall with a layer of green glue. The LF reflections will really hurt you with cement, GG is one of the best products to reduce transmission in the bass region. Doing a single wall wouldn't cost much more then just a normal single layer. If you layered that wall with foam, you'll do nothing to stop the LF reflections and only hurt your mids and highs. The highs will sound like they are missing from that 1/2 of them room.

I don't understand your logic that drywall will reduce the reflections over concrete, the only way to stop reflections are to add defuser or foam, carpet will also help but drywall is essentially the same as a concrete wall and has no absorption properties.

[kareface]

Originally Posted by Twitch9

I don't understand your logic that drywall will reduce the reflections over concrete, the only way to stop reflections are to add defuser or foam, carpet will also help but drywall is essentially the same as a concrete wall and has no absorption properties.

I'm talking specifically about LF reflections. Diffusers will help very little, but foam will not at all. Once you get into the LF region, the nature of acoustics changes greatly. For example, a room built with 16 OC studs will sound louder the the bass region then one with 24 OC, but the 24 OC will have greater peaks and dips in FR. A room built with traditional 1/2 gypsum will sound louder in the bass region then 5/8th.

There are several reasons for this. As the surface density of the material increase so does it's absorption coefficient, until you get to a tipping point where it becomes harder to pass energy through the material and you end up with more reflectivity then absorption. The problem with LF is it takes so much energy to produce the waves because of the relatively low propagation efficiency and with wave lengths that are longer then most rooms, traditional insulation or foam is almost completely ineffective. So if it's not for it's absorption properties why does gypsum offer an advantage over concrete? The simple answer is, transmission, degrees of reflectivity and structural dampening.

Imagine you're in a house built with nothing but paper walls. You would hear almost no reflections from the sub because all of the energy would pass right through the walls. From the outside of the house you could hear every bass note. Now imagine you're in a hose built of nothing but wood, you hear some of the reflections, but from the outside of the house you can hear some of the bass from the sound transmitting through the wood. The sound that was passed though the material removes energy from the wave each time it strikes the wall. Now imagine you are in bomb shelter, you can see where this analogy ends. Concrete is just too solid. Even if you build a wall in front of it, you'll still get some reflections, but it'll be reduced dramatically by comparison as the energy will transmit through the studs into the ceiling and floor. The more reflective the walls are the more times the wave bounces before dissipation. Bass in normal rooms is overly reflective, bass with cement is unmanageable.

If you try and correct a room with cement walls with nothing but bass traps and diffraction you'll end up with a terrible sounding room. This is because of the energy needed to produce LF waves is so much greater. The amount of dampening you'd need to effectively reduce the excess bass energy will be many times greater then what you would need for the mids, and many times greater then that for the highs. As a result you end up with a slope, starting high around 20hz, and declines to almost nothing on the upper range. Diffraction can help, but it is impossible with nothing but diffraction and foam to correct excess bass in 95% of cases, and it's only made worse by cement or concrete.

The second reason why a traditional wall is better then a cement wall it the mass-air-mass system of traditional double leaf walls. The pocket of air acts like a spring, the system becomes what is known as a harmonic oscillator. If you are familiar with Hooke's law, you'll understand that some of the energy that enters the system will be dissipated by the work. The down side to double leaf walls is they create a resonance system, which can negatively impact the room and the sound transmission at the fundamental resonance, and corrosponging multiples. You have to keep this in mind, things like increasing the air cavity or stiffening the system result in a lower fundamental resonance, which is preferred. Other products like green glue can help both in reducing transmission in the bass region (one of the hardest regions to deal with) but also lowers the fundamental resonance and helps with bass absorption. It's one of the reason I suggested it for his situation.

Once you get down to LF, the most important things for acoustics become room dimensions, speaker positioning and structural dampening. You can use bass traps that don't rely on mass absorption, but rather sympathy or displacement equalization as they don't adversely affect the mids and highs. A good example of this are ASC tubetraps or membrane traps.

[kareface]

If you are interested in things of this nature, I heavily recommend the peer reviewed publication: Journal of Sound and Vibration. They have great articles like, "Sound transmission through lightweight double-leaf partitions: theoretical modeling" that detail exactly what I mentioned above, but in a more articulate and accurate fashion. It is a science journal tho, so it's not laymen friendly. Cheers.

[Twitch9]

I understand the theory of LF waves, i was responding to the first sentance in you post where you did not mention LF reflections, The main reason why you'd want to put up a layer of drywall is to reduce the reflections off that wall if you would have stated LF reflections in this statement i would have agreed and never posted, i was responding to reflections in general with concrete vs drywall and how just hanging drywall would make no difference in how each would reflect sound off the surface area.

It's all good and your post will help people understand how materials can capture sound. It nice to meet other geeks who appreciate sound theory and put it to use.

[MAL01]

Kareface,
When you built your room did you double drywall the walls or just the ceiling? Also, what thickness drywall did you use?

[kareface]

Originally Posted by Twitch9

I understand the theory of LF waves, i was responding to the first sentance in you post where you did not mention LF reflections, The main reason why you'd want to put up a layer of drywall is to reduce the reflections off that wall if you would have stated LF reflections in this statement i would have agreed and never posted, i was responding to reflections in general with concrete vs drywall and how just hanging drywall would make no difference in how each would reflect sound off the surface area.

If you read the whole post I said:

The main reason why you'd want to put up a layer of drywall is to reduce the reflections off that wall. Foam would be much worse for this then drywall, but I'd consider adding a second layer of drywall with a layer of green glue. The LF reflections will really hurt you with cement, GG is one of the best products to reduce transmission in the bass region. Doing a single wall wouldn't cost much more then just a normal single layer. If you layered that wall with foam, you'll do nothing to stop the LF reflections and only hurt your mids and highs. The highs will sound like they are missing from that 1/2 of them room.

In my opinion it was clear what reflections I was referring to.

Originally Posted by MAL01

Kareface,
When you built your room did you double drywall the walls or just the ceiling? Also, what thickness drywall did you use?

I used 2 layers of 5/8ths on all the walls, green glue and 16 OC. I'd say for a room my size in the future I'd go with 24 OC studs, I have no problem with rigidity hurting the LF response.

[kareface]

I'd like to add the line where you said "drywall is essentially the same as a concrete wall and has no absorption properties" is true, but very misleading. It would be like saying a bike is faster then a car because it's harder to peddle a car. The basis of comparison is silly, you should be looking at other properties to make a realistic comparison. I'm being blunt right now, don't confuse this with upset, lol. I don't have any animosity, I'm just trying to point out the incongruities.

[MAL01]

Originally Posted by kareface

If you read the whole post I said:

In my opinion it was clear what reflections I was referring to.

I used 2 layers of 5/8ths on all the walls, green glue and 16 OC. I'd say for a room my size in the future I'd go with 24 OC studs, I have no problem with rigidity hurting the LF response.

Just for kicks, what would happen if you went with 3, 4, 5 etc. layers? Would it get better or worse?

[kareface]

Originally Posted by MAL01

Just for kicks, what would happen if you went with 3, 4, 5 etc. layers? Would it get better or worse?

It would provide better mass dampening to reduce transmission. I can't imagine it would hurt the acoustics. As long as you have the space, it's cheaper just to use a larger air cavity to reduce resonance. Trying to install outlets in a wall with 3 layers of drywall would be a nightmare. Recessed lighting would be funny to see. There are things that for the money could provide either better isolation or acoustics depending on which route you're going with the construction.

[Twitch9]

If i had the money to do my basement the way i wanted to i would have went with quite rock drywall, but $100 a sheet was not in my budget.

[kareface]

Originally Posted by Twitch9

If i had the money to do my basement the way i wanted to i would have went with quite rock drywall, but $100 a sheet was not in my budget.

I've used that stuff a lot, I can tell you from personal experience, don't bother. It works on the same principle that green glue does, but it's a pain in the ass to work with. The sheets weigh several times more then a normal sheet of drywall. Most installers will charge you a lot more to work with the material. You don't get the flexibility of selecting the rigidity of the product to compensate for dampening. Its cost is much higher then green glue per foot^2, but it's effectively on par (if that). The only advantage is it offers a smaller footprint.

Transmission loss tests for a quiet rock style product:
http://www.greengluecompany.com/imag...820_Report.pdf

Transmission loss tests for green glue in the same scenario:
http://www.greengluecompany.com/imag...035_Report.pdf

Both tests conducted by a 3rd party lab.

Quiet rock has a whole host of products, and I have yet to work with their THX certified panels. However from my personal dealing I'll tell you, never again. Unless someone insists on them, I'll always go different route to achieve better results for the cost.

[Twitch9]

I worked on a recording studio using it and i agree it is heavy, they were installing 1 3/8" THX sheet rock, i believe it was around $400 a sheet. I have used the green glue and every other type of silicone clamming results but the green was the best by far. What type of work do you do, to understand and know as much as you do.

[Big Daddy]

I believe you should all read this article.

http://www.greengluecompany.com/soundAbsorption.php

[kareface]

Originally Posted by Big Daddy

I believe you should all read this article.

http://www.greengluecompany.com/soundAbsorption.php

I've combed through all the content on their site, it contains a lot of great info. I'm going to have to dig up a good article published in the journal I listed above that discredits a few of the comments they made on that page tho. The comment they made about mass-air-mass resonance is only partially true, as it can exhibit both properties depending on the structure of the wall. I also would of preferred a graph with higher resolution (1/6th smoothing or greater) and focus more on <400hz, but that's just me being picky.