UMIK-1 measurement mic

[manoj.p]

What are you planning to use the rockwool for? How thick you plan to make it?

 

[Santy]

Mainly corner bass traps. At the front, I do not have much space on the side of PJ screen, so it would be around 1.2 x 1.2 x 7.5 ft feet and the rear can be 1.5 x 1.5 x 4 ft.

 

[manoj.p]

okay. If space is premium, have you given a thought to other alternatives? like adding one more sub or using membrane type bass traps? The membrane types will only absorb low frequency and reflect the high frequencies, helping in keeping the room alive.

Either ways, nice to see you getting the bass right. Keep us posted how you do it and how the treatments interact with the response.

 

[Santy]

One sub is already overkill for my room- I couldn't go beyond 20% on volume.
The traps are for the corners so not much of reflections would be lost I guess. I have DIY panels placed at the first reflection points are able to make the sound cleaner but at the cost of narrowing the soundstage. I think with the mic we should be able to understand more and accordingly set it up. If the corner traps with triangle cross section is not chunky enough to tame the resonances, may be a square pillar can be formed like (increases volume by 2x) this but it really kills the aesthetics.

Lets see the graphs to see how bad the ringing is and then take a call.

 

[sdurani]

The thicker the absorber, the less dense it needs to be to remain effective at absorbing bass.

For example, at four inches of thickness: 6 pcf (94 kg/m3) rockwool doesn't absorb as much bass as less dense 4 pcf (64 kg/m3) rockwool. For eight or ten inches of thickness, the material needs to be even less dense.

Do you know the density of the rockwool you have?

 

[Santy]

Ofcourse I am aware denser material is needed to absorb more bass especially in the lower end. At the same time, I think the surface area matters as well. The rockwool I have is of 96 kg/m. Any calculation possible as to what volume of rockwool can minimize the decay to what extent at what frequency band? I think it is pretty complicated. Or may be I have to do lot of reading.

 

[keith_correa]

You guys are obsessed. You know that, don't you?

 

[sdurani]

The thicker the absorber, the less dense it needs to be to remain effective at absorbing bass.

Ofcourse I am aware denser material is needed to absorb more bass especially in the lower end.

Something must have gotten lost in translation.

 

[Santy]

You guys are obsessed. You know that, don't you?

We are all obsessed with one thing or other in life, aren't we?

The thicker the absorber, the less dense it needs to be to remain effective at absorbing bass.

Do you mean to say a 10 cubic ft absorber of say 96 kg/m would be less effective less than a 5 cubic ft absorber of same density?

 

[sdurani]

Do you mean to say a 10 cubic ft absorber of say 96 kg/m would be less effective less than a 5 cubic ft absorber of same density?

Yes, depending on panel thickness. I know it is counterintuitive. I'll use absorption coefficients from Roxul brand of rockwool to give you an example:

If you were making 2-inch thick panels, 96 kg/m rockwool would be about 20% more effective at absorbing bass than 64 kg/m rockwool.

If you were making 3-inch thick panels, 96 kg/m rockwool would only be 10% more effective at absorbing bass than 64 kg/m rockwool.

If you were making 4-inch thick panels, 96 kg/m rockwool would be about 5% less effective at absorbing bass than 64 kg/m rockwool.

See the pattern? The denser rockwool became less effective at absorbing bass as the panels got thicker. By the time we reach 4 inch thickness, the less dense rockwool was absorbing slightly more bass.

Forum member manoj.p uses 48 kg/m density for his 4 inch thick panels. The riser for his back row doubles as a bass trap. Since it is 8 inches tall, he had to use 10 kg/m to maintain bass absorption.

The only reason I'm bringing this up is so you don't waste your 96 kg/m making bass traps columns that are 1.2-1.5 feet thick. They'll absorb everything but bass.

See the following thread:
Insulation density confusion - Gearslutz.com

 

[Santy]

Interesting Sanjay, thanks for the info.

I suppose we are talking about acoustic panels on the wall face; placed typically at reflection points. What about corner traps. Almost all corner bass traps I have seen are chunky, like this

 

[captrajesh]

The thicker the absorber, the less dense it needs to be to remain effective at absorbing bass.

I'll use absorption coefficients from Roxul brand of rockwool to give you an example:

If you were making 2-inch thick panels, 96 kg/m rockwool would be about 20% more effective at absorbing bass than 64 kg/m rockwool.

If you were making 3-inch thick panels, 96 kg/m rockwool would only be 10% more effective at absorbing bass than 64 kg/m rockwool.

If you were making 4-inch thick panels, 96 kg/m rockwool would be about 5% less effective at absorbing bass than 64 kg/m rockwool.

See the pattern? The denser rockwool became less effective at absorbing bass as the panels got thicker. By the time we reach 4 inch thickness, the less dense rockwool was absorbing slightly more bass.

Going by the above, the lighter the material, the thinner it can be at absorbing a given amount of Bass (in dB I guess?) Is this assumption correct?

 

[Santy]

Going by the above, the lighter the material, the thinner it can be at absorbing a given amount of Bass (in dB I guess?) Is this assumption correct?

Well not exactly. What is meant by sdurani is
Till a point, denser materials absorb more. Beyond that point in panel thickness, their coefficient of absorption starts dropping.

http://www.bobgolds.com/AbsorptionCoefficients.htm
In this database, some of the brands follow the above pattern while some don't.

 

[captrajesh]

Well not exactly. What is meant by sdurani is
Till a point, denser materials absorb more. Beyond that point in panel thickness, their coefficient of absorption starts dropping.

http://www.bobgolds.com/AbsorptionCoefficients.htm
In this database, some of the brands follow the above pattern while some don't.

That's precisely what I was trying to get: "To a point" To what point? That's not clear. You've articulated better than me.

 

[sdurani]

I suppose we are talking about acoustic panels on the wall face; placed typically at reflection points. What about corner traps. Almost all corner bass traps I have seen are chunky, like this

I wasn't talking about on-wall absorper panels vs corner bass traps, just making a general point about density vs bass absorption.

Your 96 kg/m rockwool will start to absorb less bass once it is more than 4 inches thick. This is not a case of diminishing returns, where going more than 4 inches thick doesn't buy you much improvement. Instead, going more than 4 inches thick makes the performance slightly worse.

Look at the pic of the corner trap you posted. From the front surface of the absorber to the corner, is that more than 4 inches deep?

 

[sdurani]

Going by the above, the lighter the material, the thinner it can be at absorbing a given amount of Bass (in dB I guess?) Is this assumption correct?

It's the opposite. The lighter (less dense) the material, the thicker it can be. The denser the material, the thinner it needs to be.

I'm using density as a short hand; what we're really looking for is gas flow resistivity. The "gas" in this case is air, which is the medium that sound travels through.

The denser the material, the more difficult it is for air to travel through it. So if you are going to use dense material, don't make the panels too thick. Otherwise, air won't be able to travel through it and the panel won't be an effective absorber.

If you are going to make the panel thick/deep, then use less dense material so that air can travel all the way through it.

Like I said, forum member manoj.p had an 8 inch air gap in his riser that he wanted to exploit as a bass trap. For that much thickness, he had to use 10 kg/m material. If he had used denser material (for example 48 kg/m), it wouldn't have been as effective at absorbing bass.

 

[Thad E Ginathom]

I like your explanation, but I'm unsure about this...

The "gas" in this case is air, which is the medium that sound travels through.

Sound travels through other media too. ISTR once reading that water is a better conductor of sound than air.

 

[sdurani]

ISTR once reading that water is a better conductor of sound than air.

What does water have to do with a discussion of density and low frequency absorption? Is your home theatre underwater?

 

[Santy]

@sanjay

The theory is fairly logical, in the sense when a denser material is thicker, it would start to bounce back the energies instead of absorbing them. So one need to be careful not to overdo the job. Thanks for the tip. :thumbsup:

Regarding the pic shown, with due respect, the sides touching the walls will be atleast 1 ft wide which means the deepest point from the hypotenuse will be 8.4" which slopes down to 0. And this is the smallest corner chunk trap I could find. Assuming the sides are 1.5 ft wide, the exposed area will be 2 ft wide and the max. depth will be 13.4" as per my calc! May be they use lighter material.

Anyways, I am going to restrict the sides to 1 ft and then cut the corners to leave some gap behind, so that advice given herein is taken care. Also will be making tricorner traps of 100mm slab.

 

[manoj.p]

Santy,

What I would do is a 2 or 4" thick verical wall panel, as wide as the diagonal that you want and straddle the wall with with it. Its a simple, rectangular, tall panel to build and does the job very easily.

Also, corner traps is not the only bass trap. You can build membrane type bass traps. These do a very good job at absorbing bass, leaving high frequencies alone. Keeps the room lively.

I prefer this order for any acoustics issues. Placement, then room treatment and equalization as last.