Ribbon buzzing from change in humidity.. Can we fix it?

[Ar9Jim 6,520]

Seems to be common with Carver ribbons to start buzzing and are effected by humidity changes.. Sea (Ed) has data on this and can predict issues based on conditions reliably.. The question is this, why when the frame swells slightly is the ribbon tension reduced and the buzz occurs? If it swells should it not get an increase in tension?

 

I have seen a similar situation when making some aircraft parts from thick wall aluminum tubing (temp related but still similar). I think I can explain and maybe we can cure the issue..

 

While the frame is expanding as it absorbs humidity (HDF,MDF or whatever) and becoming larger overall, it is really expanding in cross section.. While the outside will grow, it is possible for the inside to shrink at the same time because of the material change in cross section...

 

I have seen thick wall tubing grow from warming on the OD while the ID lost diameter at the same time.. The humidity is doing this to our porous material.

 

Sea, Can you dry these in a room with an electric heater and seal the frames with lacquer or maybe some kind of green product?. Should help stabilize these for a more consistent performance.. If they could be in a room with low humidity and then sealed it could help alot.. Maybe you could offer it as an option on your rebuilds? 

[weitrhino 1,445]

In my mind I think the term "swelling" is misapplied. Yes, strictly speaking when humidity rises materials such as wood and MDF will absorb moisture and shed it when relative humidity drops. But we must remember that wood is imperfectly constructed so there's no reason for 'swelling' to be uniform. In all likelihood what we have is simple warpage which should be enough to unevenly alter the tension on the ribbon substrate especially considering the ribbon function is so closely related to how it its tensioned within the frame.

[UncleMeat 1,174]

Two possibilities come to mind. The MDF is being clamped together, so the expansion direction can only happen parallel to the ribbon membrane. If the dowel groove that holds tension is further toward the center than the outer edge of the frame it may move inward, lowering ribbon tension. The clamping bolts are outboard of the dowels, encouraging expansion movement toward the center also.

 

Another consideration is cold air in addition to moisture. As the metal case gets cold it could be shrinking in addition to changes in the MDF.

[sea 683]

I don't know why. I agree it would seem that moisture would cause the MDF to swell but it also might make the MDF softer lowering the effective tension. I don't know. I will leave you engineer types to debate. I do know for certain that one of my 60 inch ribbons will buzz if the humidity is above about 60 percent. In the summer time, I have to run a dehumidifier in my basement area pretty much full time. My basement is not a totally underground basement. It is what is sometimes referred to as a daylight basement. The back of the basement opens to the outside.

[sea 683]

I am not sure about sealing the MDF in a low moisture environment. I think the ribbon tensioning might depend to some extent on the aluminum rod being a being able to "press" down into the opposing MDF channel.

[loner_t 2,571]

Slightly outside the box question. BG and Genesis also use ribbons, in some cases (e.g. Genesis) OEMed from Carver.  Do these speakers show similar issues? Are there design issues which did not envisage such longevity for the Carver speakers?   

[UncleMeat 1,174]

All materials, particularly plastics like kapton are prone to a permanent stretch caused by molecular bonds slipping. If it happens quickly it's called plastic defomation. If it happens over a long period if time at a low level of strain it's called creep.

[Ar9Jim 6,520]

 

 

In my mind I think the term "swelling" is misapplied. Yes, strictly speaking when humidity rises materials such as wood and MDF will absorb moisture and shed it when relative humidity drops. But we must remember that wood is imperfectly constructed so there's no reason for 'swelling' to be uniform. In all likelihood what we have is simple warpage which should be enough to unevenly alter the tension on the ribbon substrate especially considering the ribbon function is so closely related to how it its tensioned within the frame.

Agree.. Humidity causing a change in the frame seems to be the issue in either case.. Sealing wood products against moisture offers some control and protection from those forces of nature.. If they vary enough to fail at 60 % humidity as measured, than they are likely not sealed well and would likely get worse with time and age.. A coat of something thin like Thompsons water seal might control the variable. If I ever have one apart I'll try it..

[UncleMeat 1,174]

Think of a hardwood floor that absorbs too much moisture from being wet or humid, and the floor runs out of room to expand at the walls. The floor is going to get larger regardless of the fixed boundary so it will buckle in the center. The MDF frame is bounded by the clamping screws and metal case, so the only direction for expansion to take place is to move toward the center. Movement inwards will reduce tension on the ribbon.

 

I don't think expansion is the only cause of the problem. If the ribbon has proper (OEM) tension the expansion of the frame should not reduce tension enough to cause buzzing. Either the ribbon has slipped through the clamping rods and lost some tension or the material has stretched over time resulting in lower tension.

 

MDF is not a hardwood material, it's very unstable dimensionally. The clamping forces on the frame can cause it to change shape. I've got a broken 48" ribbon that has an indentation in the MDF where the daughter board attaches, because it was clamped so tight to the frame. The pressure and shape change of the MDF caused the ribbon and traces to tear.