[Lonely Raven]

Ok, so I'm confused as to what the open cell foam is supposed to do when glued to the inside walls of these LLT cabinets.

It can't be for standing waves...it's a joke to think 2" of foam is going to do anything for bass wavelengths.

I can't imagine it's for dampening the panels...not enough mass to make any change to the density of the panels....

So what is it for? :dunno:

 

[Mike P.]

It damps the back wave of the sub and helps to stop the sub from sounding "boomy" for lack of a better word. I tested this on 2 Shiva sonotubes, one with egg crate foam, one without. The difference was night and day.

 

[Lonely Raven]

It damps the back wave of the sub and helps to stop the sub from sounding "boomy" for lack of a better word. I tested this on 2 Shiva sonotubes, one with egg crate foam, one without. The difference was night and day.

I wonder then, if it's affecting harmonics. Because 2" of foam isn't going to do anything to 40Hz tones.

 

[avaserfi]

Back wave resonances in loudspeakers are created by waves that are able to actually fit in the cabinet. To fit in the cabinet there must be a dimension that is equal to or greater than 1/4 the wavelength within the passband. This can be seen here.

Egg crate foam is not an efficient absorber of such reflections. Rather using a material like OC705 or 8lb mineral board would be far more efficient.

A subwoofer cabinet would need to be 7 feet tall for 40Hz resonance of this type to be an issue. Of practical concern in LLT design is these resonances is the 65-80Hz range which could be alleviated by use of about 4" to 6" of a high grade acoustic material on both ends of the cabinet, if possible.

 

[Lonely Raven]

Back wave resonances in loudspeakers are created by waves that are able to actually fit in the cabinet. To fit in the cabinet there must be a dimension that is equal to or greater than 1/4 the wavelength within the passband. This can be seen here.

Egg crate foam is not an efficient absorber of such reflections. Rather using a material like OC705 or 8lb mineral board would be far more efficient.

A subwoofer cabinet would need to be 7 feet tall for 40Hz resonance of this type to be an issue. Of practical concern in LLT design is these resonances is the 65-80Hz range which could be alleviated by use of about 4" to 6" of a high grade acoustic material on both ends of the cabinet, if possible.

Now we're talkin!

So you're in agreement with me the 2" egg crate is just taking up airspace then, right?

I'm guessing my JM814 (equivelent to OC703) isn't dense enough to make a difference.

My sub is probably going to be something like 4' X 4' X 15" or something along those lines. (think coffee table).

Now, I'm not knocking poly-fill. Because from what I understand, that makes the *airspace* seem larger to the driver...which can be a good thing to fake an extra couple cubes of space....that is, if I'm understanding that as well.

 

[avaserfi]

So you're in agreement with me the 2" egg crate is just taking up airspace then, right?

Pretty much.

I'm guessing my JM814 (equivelent to OC703) isn't dense enough to make a difference.

If enough is used it would be sufficient, but such applications are about efficiency so a more dense material would be better.

My sub is probably going to be something like 4' X 4' X 15" or something along those lines. (think coffee table).

With this size you will likely have back wave resonances around 70Hz. If you place 4" of wrapped OC705 or 8lb mineral wool against the walls on the axes that are 4', these resonances will be attenuated sufficiently.

Now, I'm not knocking poly-fill. Because from what I understand, that makes the *airspace* seem larger to the driver...which can be a good thing to fake an extra couple cubes of space....that is, if I'm understanding that as well.

Use of polyfill effects the apparent volume only as it interacts with Qtc. Approximately 0.1 cubic feet increase for every pound of polyfill used, a maximum increase of about 10% can be achieved. It really isn't too useful.

-Andrew

 

[Lonely Raven]

With this size you will likely have back wave resonances around 70Hz. If you place 4" of wrapped OC705 or 8lb mineral wool against the walls on the axes that are 4', these resonances will be attenuated sufficiently.

-Andrew

Andrew, thanks!

It sounds like I'm understanding the concepts for the most part.

Now, my next question...doesn't the 4" of OC705 take up airspace and effect my tune? And in another thread you said something about this:

"Do note that excessive use of such a material will reduce the ability of the box to act as a spring air-mass system. Meaning it will no longer be able to resonate properly."

Could you clarify, maybe give an example of what you mean by "no longer able to resonate properly"?

 

[avaserfi]

Andrew, thanks!

It sounds like I'm understanding the concepts for the most part.

Now, my next question...doesn't the 4" of OC705 take up airspace and effect my tune? And in another thread you said something about this:

"Do note that excessive use of such a material will reduce the ability of the box to act as a spring air-mass system. Meaning it will no longer be able to resonate properly."

Could you clarify, maybe give an example of what you mean by "no longer able to resonate properly"?

In this case, I did not recommend excessive damping that would result in critically damping. If a driver is over damped in a sealed system it will actually act as if it is in an infinite baffle situation. If this is done in a ported system the same will happen, until the port becomes effective and the unit begins to resonate as a ported alignment.

The materials recommend are actually regular insulation just compressed and glued together. Due to this the volume does not need to be accounted for when adding it. Do keep these materials about 6" from the port though as they will impede airflow.

 

[Lonely Raven]

In this case, I did not recommend excessive damping that would result in critically damping. If a driver is over damped in a sealed system it will actually act as if it is in an infinite baffle situation. If this is done in a ported system the same will happen, until the port becomes effective and the unit begins to resonate as a ported alignment.

The materials recommend are actually regular insulation just compressed and glued together. Due to this the volume does not need to be accounted for when adding it. Do keep these materials about 6" from the port though as they will impede airflow.

Already (mentally) noted about giving space around the port.

Mike P and I are chatting about my next project. While he's helping me plot drivers, I'm designing details in my head. I want to make sure I understand the concepts before I dive in.

So if my Wicked One folded horn sells this weekend, I'll soon be starting a build thread about a coffee table with an 18" air piston. :devil:

Thanks again for explaining things. You're a great teacher by the way.

 

[Mike P.]

I have to dig up some reference material for you guys. I believe it was a Weems publication on how to test if a ported sub is damped enough by placing absorbant material on the walls of a sub. I shall search for it.

 

[avaserfi]

I have to dig up some reference material for you guys. I believe it was a Weems publication on how to test if a ported sub is damped enough by placing absorbant material on the walls of a sub. I shall search for it.

Resonances created by internal reflections are actually fairly easy to test.

In this thread if you look at the impedance trace you will note a ripple at about 170Hz in the unpadded measurement. This ripple is also visible in the frequency response data. The ripple corresponds with a quarter wavelength internal reflection.

Such an anomaly can be measured by an impedance trace, or by near field measurement (under 3"), ideally both. Simply place the microphone near the driver and run a sweep. The resulting measurement should show resonances created by internal reflection.

This is the exact methodology used by the OP in the linked thread.

-Andrew

 

[Lonely Raven]

I just got this E-mail from the guy who's supposedly buying my Wicked One sub. Totally a scam, and I'm going to give him an earfull (as much as I can over the internet):


------------------------------------------------------------------------------------------------

Hi Raven, thanks for the mail..I have just transferred the funds via paypal so just get back tome immediately the item has been shipped today via USPS Global Express Mail Service. You will be shipping out the item to my colleague who is currently transferred to our branch in west africa (British American Tobacco Company)so kindly get the item shipped to him. I have added $250 to the payment for the shipment:
Here is the shipping address:

Full Name : Yemi Alatise,
Address : No 1 Akinyemi Way. Ring Road,
City : Ibadan,
State : Oyo State,
Country : Nigeria,
Zip Code : 23402.

Thanks and i will be expecting back your mail asap.. Thanks..

------------------------------------------------------------------------------------------------


So much for my starting my new sub build anytime soon.

 

[Bill Fitzmaurice]

Ok, so I'm confused as to what the open cell foam is supposed to do when glued to the inside walls of these LLT cabinets.

It can't be for standing waves...it's a joke to think 2" of foam is going to do anything for bass wavelengths....

So what is it for? :dunno:

It's for absorbing midbass and midrange reflections above 300 Hz or so. Even if that's above the cab's passband the motion of the cone will create harmonics that can reflect back to the cone, or out the port, in both cases adding to the cab's THD.
As far as wavelengths longer than a couple of inches of foam can damp out, they're best controlled by the internal geometry of the cab, with bracing serving double duty to reduce internal pathway lengths so that 1/4 and 1/2 wavelength paths don't exist in those frequencies too long to be damped by the foam.

 

[Mike P.]

Resonances created by internal reflections are actually fairly easy to test.

In this thread if you look at the impedance trace you will note a ripple at about 170Hz in the unpadded measurement. This ripple is also visible in the frequency response data. The ripple corresponds with a quarter wavelength internal reflection.

Such an anomaly can be measured by an impedance trace, or by near field measurement (under 3"), ideally both. Simply place the microphone near the driver and run a sweep. The resulting measurement should show resonances created by internal reflection.

This is the exact methodology used by the OP in the linked thread.

-Andrew

The test I'm thinking of involved the use of a battery. Hooking the battery to the terminals of a sub would produce a "click - boom" in an unlined cabinet. Lining the cabinet would produce a "click -thud" sound. This information is from a David Weems book called "Building Loudspeakers", if memory serves me correctly. Recent information says this shouldn't happen, there is no difference. Perhaps Weems was wrong, and I didn't hear a difference in my tests. I have a hard time believing that, since the differnece between the two subs was so pronouced. Both cabinets were identical. Maybe a difference between the subs parameters? Something is at work here, that much I know.

 

[avaserfi]

The test I'm thinking of involved the use of a battery. Hooking the battery to the terminals of a sub would produce a "click - boom" in an unlined cabinet. Lining the cabinet would produce a "click -thud" sound. This information is from a David Weems book called "Building Loudspeakers", if memory serves me correctly. Recent information says this shouldn't happen, there is no difference. Perhaps Weems was wrong, and I didn't hear a difference in my tests. I have a hard time believing that, since the differnece between the two subs was so pronouced. Both cabinets were identical. Maybe a difference between the subs parameters? Something is at work here, that much I know.

This methodology will not be accurate compared to the one outlined and performed by user Ricci. As far as what you experienced, it could have very well been biase. If you read such a description before hand and completed the experiment in a sited manner it would have been impossible for anyone to avoid such bias.

Here is a test report on eggcrate foam: http://www.acousticsfirst.com/docs/egg.PDF

It is essentially useless below 400Hz and is fairly inefficient in the midrange octaves.

-Andrew

 

[Lonely Raven]

Cool, thanks for that info that backs up my theories.

That lab (if it still exists) is only 30 minutes from me.

I wonder if they are hiring. :innocent:

 

[Mike P.]

Andrew, that PDF does not appear to be egg crate foam, but rather a panel of egg crates, as in packaging material.

http://www.herpinvertsupplies.com/images/eggCrate.jpg

 

[avaserfi]

Andrew, that PDF does not appear to be egg crate foam, but rather a panel of egg crates, as in packaging material.

http://www.herpinvertsupplies.com/images/eggCrate.jpg

After looking at it more you are right. It was slightly vague and I didn't think anyone would actually waste their time measuring cardboard :scratch:. Guess I was wrong.

-Andrew

 

[Bill Fitzmaurice]

Here is a test report on eggcrate foam: http://www.acousticsfirst.com/docs/egg.PDF

It is essentially useless below 400Hz and is fairly inefficient in the midrange octaves.

-Andrew

I see the tested product described as 'eggcrates', not eggcrate foam. Judging by the date of the test and the results I wouldn't be at all surprised if the tested product was indeed wood pulp eggcrates. Nothing wrong with that, the best music ever made was recorded in Berry Gordy's eggcrate lined studio.
More recent testing of damping materials by Gerry Koonce and George Augsperger, amongst others, showed little difference in the effectiveness of foam versus polyester fiber, Acoustastuff,wool fiber and fiberglass. All that differed was the thickness of each required to reach the same level of damping, with foam falling in the middle of the pack.
Response anomalies caused by insufficient damping will show up as impedance peaks and dips. The impedance testing must be done outdoors, away from boundaries, as reflections will cause back EMF that also shows up as impedance peaks and dips.

 

[thxgoon]

More recent testing of damping materials by Gerry Koonce and George Augsperger, amongst others, showed little difference in the effectiveness of foam versus polyester fiber, Acoustastuff,wool fiber and fiberglass.

Since all the experts seem to be in one spot on this one, what is the effect of adding too much in a vented or LLT design?