Does a lot of damping material have any negative effects in a closed box?

[fpitas]

Thank you for the article but it doesn't answer my questions. If you have a look at bass reflex cabinets it is clear that you will also reduce the effect of the bass reflex port with to much damping material. But with a closed box it is different you don't want/ need the backside sound pressure level. So a lot of damping might be clever

It does illustrate the box mode resonances, though. You're right that bass reflex takes some restraint.

 

[test1223]

It does illustrate the box mode resonances, though. You're right that bass reflex takes some restraint.

Yes it shows the positive effects of damping material if you transfer the outcome to a closed box. But it doesn't says anything if there are or can be negative effects of to much damping material in a closed box.

 

[fpitas]

Yes it shows the positive effects of damping material if you transfer the outcome to a closed box. But it doesn't says anything if there are or can be negative effects of to much damping material in a closed box.

At some point the material starts reducing the box volume, if you cram it in.

 

[olieb]

So I guess there has to be a negative effect.

I guess the biggest negative effect is money. Less damping stuff is cheaper.
Bass oscillation can be controlled electrically by the woofer and damping is there to tame cavity resonances in higher range.
Of course you might be able to put too much. Imagine a stack of woven cotton (like denim). This will start to behave like a piece of wood.

 

[RickS]

Yes, it does, but as with many things, the answer depends and too much of anything is usually bad or wasteful…

For a closed box speaker, if the empty ideal box case makes it critically damped, more damping is likely to make it sound worse. Critically damped is what some audiophiles prefer to get optimal transient response, but most often, it does not sound great imo. So, this all starts with a specific driver and putting it in an enclosure of a chosen volume. Since you chose these before adding damping, this is the foundation for how a closed box speaker will sound. With too much damping material, you can get frictional losses and so would sound more “dead”.

Since ideal boxes do not exist, damping materials can be used to help control resonances too. Adding excess damping material might help reduce resonances in a marginal enclosure, but taking that approach will more likely get you to one of the aforementioned bad conditions.

 

[JeffGB]

The internal damping of a box works in conjunction with the Q of the driver chosen. Ported speakers use woofers with a lower Q (.29 for instance) whereas sealed speakers use a woofer with a higher Q (.7 to over 1.0). The final speaker Q for a flat frequency response is generally agreed upon as being about .7 with a higher Q being elevated bass response and a lower Q being a "thinner sounding", rolled off frequency response. In addition to the Q of the driver, the fs is also very important because the "system resonance", especially in a small box, rises.
The idea of a fully stuffed speaker comes from "air suspension" systems, which use a high Q, low fs woofer in a small box, filled with damping material to bring the overall system Q to approximately .7 (or a flat frequency response).

There are many variations on this including using a low fs, low Q woofer in a small box with electronic equalization to give it a flat system response.

 

[test1223]

At some point the material starts reducing the box volume, if you cram it in.

From my experience this is impossible if you stuff the typical material. Even if you use very dense material like footfall sound insulation plates wich are dense you will increase the virtual volume of the box.

I guess the biggest negative effect is money. Less damping stuff is cheaper.
Bass oscillation can be controlled electrically by the woofer and damping is there to tame cavity resonances in higher range.
Of course you might be able to put too much. Imagine a stack of woven cotton (like denim). This will start to behave like a piece of wood.

Yes you are right if the material is in some way processed that it is more like a piece of wood it definitely wouldn't work as intended.

You also have to be aware that you have a frequency dependency of every damping material and every stuffing. In the tweeter frequencies you might run into problems with to much stuffing but for the midrange and woofer I don't think to can run into problems with stuffing of the typical material?

As with many things, the answer depends and too much of anything is usually bad or wasteful…

For a closed box speaker, if the empty ideal box case makes it critically damped, more damping is likely to make it sound worse. Critically damped is what some audiophiles prefer to get optimal transient response, but most often, it does not sound great imo. So, this all starts with a specific driver and putting it in an enclosure of a chosen volume. Since you chose these before adding damping, this is the foundation for how a closed box speaker will sound. With too much damping material, you can get frictional losses and so would sound more “dead”.

Since ideal boxes do not exist, damping materials can be used to help control resonances too. Adding excess damping material might help reduce resonances in a marginal enclosure, but taking that approach will more likely get you to one of the aforementioned bad conditions.

The internal damping of a box works in conjunction with the Q of the driver chosen. Ported speakers use woofers with a lower Q (.29 for instance) whereas sealed speakers use a woofer with a higher Q (.7 to over 1.0). The final speaker Q for a flat frequency response is generally agreed upon as being about .7 with a higher Q being elevated bass response and a lower Q being a "thinner sounding", rolled off frequency response. In addition to the Q of the driver, the fs is also very important because the "system resonance", especially in a small box, rises.
The idea of a fully stuffed speaker comes from "air suspension" systems, which use a high Q, low fs woofer in a small box, filled with damping material to bring the overall system Q to approximately .7 (or a flat frequency response).

There are many variations on this including using a low fs, low Q woofer in a small box with electronic equalization to give it a flat system response.

I don't quiet get it. Are you referring to the damping factor of the box? You can adjust it with air volume and damping material so what gets measurable worse if you do it with a lot of damping material and have the same damping factor in the end??

 

[LTig]

The internal damping of a box works in conjunction with the Q of the driver chosen. Ported speakers use woofers with a lower Q (.29 for instance) whereas sealed speakers use a woofer with a higher Q (.7 to over 1.0).

Yes but not that high. Q must be far below 0.7 because the housing increases it, so starting with 0.7 you cannot reach 0.7 in housing.

 

[Trdat]

But, doesn't all these measures of "Q" go out the door once you DSP or does the original "Q" of the cabinet have some influence regardless of the new roll off with the DSP or in this case the stuffing that might bring the peak down?

 

[olieb]

In the tweeter frequencies you might run into problems with to much stuffing but for the midrange and woofer I don't think to can run into problems with stuffing of the typical material?

I would agree. It seems difficult to put too much of the "usual" stuff. Maybe if you use a tube cabinet (like LXmini) and place one or two sheets of dense fiber material as below behind the driver you can produce significant - and frequency dependent - damping of the driver impacting the FR.

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But aside from such exotic design I do not see a problem.
In a sealed box on the other hand the sound of the cavity resonances has to travel through the cabinet walls or the diaphragm of the driver. Therefore a moderate damping might be sufficient.
But you might want to be on the safe side and I do not see a problem of „dead“ sound either. The sound energy inside will eventually be dissipated by friction (fibrous/porous or electrical in the driver motor) anyways. It is only a question of how quickly this happens. Stored energy normally is not an advantage.

 

[fpitas]

Stored energy normally is not an advantage

No, it's not. And sound can indeed make it's way through the cone, especially with a material like paper.

 

[egellings]

Hello,

from my experience a lot of midrange and bass enclosures sound much better with a lot of damping material in it. I often use very dense material for the walls and stuff a lot of usual material in the box so that it is very compressed with the exception directly behind the driver.
The sound for me is "smoother" you can see some changes in the impedance plot which is also a bit smoother.

Hardly any company puts a lot of damping material in such boxes. Consistently doing a stuffed damping isn't very easy so this might be the reason? But I wonder if there are any objective negative effects like none linear behavior which is dependent on the sound pressure level or on the cabinet temperature? I have often heard the anecdotal "evidence" that to much damping destroys the liveliness of the box. The claimed lack of "liveliness" may be the absence of small resonances or are the any other subjective negative effects?

Best
Thomas

My guess is that if you put too much stuffing in the box, that will decrease the box internal volume enough to throw off the design of the speaker. Then there's the excessive damping effect too much stuffing would provide, too.

 

[RickS]

The Quality factor can be measured and is depicted here…

https://audiojudgement.com/images/2016/01/Frequency-response-QTC-sealed-enclosure.jpg

For details, see the damping section of the article here: https://audiojudgement.com/sealed-enclosure-closed-box/

 

[olieb]

No, it's not. And sound can indeed make it's way through the cone, especially with a material like paper.

Yes, but it is much worse if your box has a hole in it for a bass reflex port.
Even in a design with strong back wave like the LXmini Linkwitz arrived at a decent attenuation (40 dB if I recall correctly) for the first reflection through the cone with moderate amounts of stuffing. But that is an aluminium cone.

My guess is that if you put too much stuffing in the box, that will decrease the box internal volume enough to throw off the design of the speaker. Then there's the excessive damping effect too much stuffing would provide, too.

More fibrous/porous material will increase the effective volume as the behavior of the air goes from adiabatic towards isothermal.
But to throw off the design you will need a lot of stuffing and even then this can be EQed easily.
And I was under the impression that the OP was talking about designing speakers not about modifying existing commercial ones.

 

[fpitas]

On topic:

Wayback Machine

 

[fpitas]

Ans I was under the impression that the OP was talking about designing speakers not about modifying existing commercial ones.

I was, too. If you start cramming an existing design full of stuffing, better be prepared to play with the crossover etc.

 

[egellings]

Yes, but it is much worse if your box has a hole in it for a bass reflex port.
Even in a design with strong back wave like the LXmini Linkwitz arrived at a decent attenuation (40 dB if I recall correctly) for the first reflection through the cone with moderate amounts of stuffing. But that is an aluminium cone.

More fibrous/porous material will increase the effective volume as the behavior of the air goes from adiabatic towards isothermal.
But to throw of the design you will need a lot of stuffing and even then this can be EQed easily.
And I was under the impression that the OP was talking about designing speakers not about modifying existing commercial ones.

True, so long as the stuffing is not so tightly packed as to not have much air space in between the fibers to allow that type of action.

 

[voodooless]

But, doesn't all these measures of "Q" go out the door once you DSP or does the original "Q" of the cabinet have some influence regardless of the new roll off with the DSP or in this case the stuffing that might bring the peak down?

It’s worse The Q goes out of the door the moment you put the speaker into a room obviously the original Q has some bearing on how it sounds, but in the end the room is dominant.

Also, you can alter Q of a closed box with DSP or analog electronics, for instance with a Linkwitz Transform: https://sound-au.com/linkwitz-transform.htm

As for stuffing: my guess is that it’s mostly the altered response that you hear. With lots of stuffing the efficiency also goes down a little bit as well. All this is audible and if not compensated for, may sound dissatisfactory. The rest, the imagination will take care of

 

[LTig]

My guess is that if you put too much stuffing in the box, that will decrease the box internal volume enough to throw off the design of the speaker.

AFAIK adding damping materials somehow (don't ask me) increases the volume seen by the driver, hence the lowering of the resonance frequency.

 

[egellings]

AFAIK adding damping materials somehow (don't ask me) increases the volume seen by the driver, hence the lowering of the resonance frequency.

Agree, so long as the stuffing is not overdone.