La Scala resonance problem

[Dave A]

I have a set of LSI's that are driving me nuts. Cabinet is tight no leaks or gaps. K-43 woofers are A-OK. I stuck a set of doghouse to side wall braces in there just to see if they would work as well as replacing the 3/4" sides with 25MM Baltic sides. The resonance problem was still severe and I was quite disappointed since I though it was the 3/4" plywoods fault. Fiddling around with it all and I noticed that the splitter is maybe 1/3 as big as normal. This thing is so small I cant believe it and I have never seen such a thing before and I have to wonder if this is the culprit and causing a standing wave issue with these. I can tame them somewhat with the room EQ on my Driverack Pro but these with braced sidewalls should not need this kind of help.

 

  These were 1984 builds and the AL has been replaced with a known good AA crossover sans Zeners which have been removed from the board. A set of my tweeters and K-55-M mid drivers which are also in good shape. Tweets and mids sound great and then purely awful when you stick you ear in front of the bass bin. Very worst resonating LSI's or LS's I have heard and those splitters are the only thing I can see.

[glens]

What frequency is the main culprit?

[Dave A]

Don't know and how would I check? Run a curve with my TrueRTA program? Rather than specific frequencies here I am thinking cabinet anyway since there is one clearly different part (splitter) used in these.

[jimjimbo]

Would you mind clarifying what you consider the "splitter"?

 

I have a pair of LSI splits and would like to look at this....I don't seem to have these same issues.

[Marvel]

All the splitters I've seen look about like  2x2 split on the diagonal. Not very big at all.

[jason str]

Finish the brace through the driver chamber as well so you have bracing running all the way through one side to the other.

[glens]

23 minutes ago, Dave A said:

Don't know and how would I check? Run a curve with my TrueRTA program? Rather than specific frequencies here I am thinking cabinet anyway since there is one clearly different part (splitter) used in these.

 

Well, the resonance(s) is(are) going to be tied to frequency(s).  Certainly you've got something on your laptop to generate frequencies to order.

 

I'm just spit-balling here...  Run a sweep, narrow down the start/stop points until you hit pay dirt.  You can use the resultant information a couple ways.  First, to "play" steady tones that make it sing while you poke, prod, and press around on things.  Second, to use the number(s) to mathematically determine dimensions (speed of sound in air, speed of sound in plywood) which may be significant and investigate that way in case "first" don't work for you.

[Schu]

As interesting as it would be to know the precise frequency, how does knowing what the frequency directly translate to a cabinet repair or modification?

 

I 

[Wirrunna]

Dave, You are not exactly a newbie here so I won't insult you by asking basic questions, but apart from agreeing with glens the only thing that may be a help is this link to plans -

which will enable you to compare the size of your splitter to a 1977 model.

[Dave A]

8 hours ago, jimjimbo said:

Would you mind clarifying what you consider the "splitter"?

 

I have a pair of LSI splits and would like to look at this....I don't seem to have these same issues.

In the back inside of the cabinet if you look through the slot the woofer mounts over you can see two triangular cut pieces of wood glued to the back side opposite of the slot. Perhaps splitter is incorrect but lacking the proper name that's what I call it.

[Dave A]

8 hours ago, Marvel said:

All the splitters I've seen look about like  2x2 split on the diagonal. Not very big at all.

Well that is what I have seen but these were ripped from 3/4 x 3/4" lumber and are tiny compared to all I have seen. What I am really asking is does the reduction in size make for standing wave resonance here and can this be serious enough to create this problem I am having with resonance.

[Dave A]

8 hours ago, jason str said:

Finish the brace through the driver chamber as well so you have bracing running all the way through one side to the other.

You mean inside the doghouse? Or front to back? I added braces where the slope is on the dog house to the side walls but not the rectangular slot past that going to the back. I did clamp wood to the outside where that slot is to dampen any resonance that might occur with no bracing there but it made no difference as far as I could tell. I have clamped wood in every direction I can where it makes sense and still the resonance problem.

[Dave A]

8 hours ago, glens said:

 

Well, the resonance(s) is(are) going to be tied to frequency(s).  Certainly you've got something on your laptop to generate frequencies to order.

 

I'm just spit-balling here...  Run a sweep, narrow down the start/stop points until you hit pay dirt.  You can use the resultant information a couple ways.  First, to "play" steady tones that make it sing while you poke, prod, and press around on things.  Second, to use the number(s) to mathematically determine dimensions (speed of sound in air, speed of sound in plywood) which may be significant and investigate that way in case "first" don't work for you.

I am a practical guy and so my first effort is to find out what is different that would cause this in construction of the cabinet. If everything was the same top to bottom in the cabinet build  compared to others  I have worked on then I would have to try what you are suggesting I guess. But there is one big difference and it is the splitter so I am asking for specific information on this topic before I start off into other areas. 

[HDBRbuilder]

Keep in mind that the production previous La Scala construction uses butt-joint glued and nailed construction.  Because the nails go thru a panel into the edge of another panel, there is a POSSIBILITY that the nails may have caused a separation of the plies that is NOT visible, but which COULD cause a resonance issue...especially over time as speaker had been man-handled, moved around, transported, etc....which may have caused an INCREASE in the ply separation which has the plys resonating against each other at certain frequencies.

 

This is one of the reasons which I used rabbeted joinery, along with dadoed joinery when I built my oak prototypes a decade or more ago.  The joinery stiffens up the construction, and no nails were used in the construction....therefore no possibility of splitting the plys apart over time. 

[jason str]

2 hours ago, Dave A said:

You mean inside the doghouse? Or front to back? I added braces where the slope is on the dog house to the side walls but not the rectangular slot past that going to the back. I did clamp wood to the outside where that slot is to dampen any resonance that might occur with no bracing there but it made no difference as far as I could tell. I have clamped wood in every direction I can where it makes sense and still the resonance problem.

 

 

Here.

 

[glens]

6 hours ago, Dave A said:

I am a practical guy and so my first effort is to find out what is different that would cause this in construction of the cabinet. If everything was the same top to bottom in the cabinet build  compared to others  I have worked on then I would have to try what you are suggesting I guess. But there is one big difference and it is the splitter so I am asking for specific information on this topic before I start off into other areas. 

 

What I suggested should help you pinpoint, quickly, the problem area(s).  All the other stuff is interesting enough in and of itself, and I'm the sort that likes to know such things, too.  But the immediate concern isn't so much "what could be different?" as "what the hell is it that's vibrating, and what'll make it quit?"

 

A piece (or layer) of wood somewhere has come unglued or some such - my guess.  Or are you saying a pair of cabinets are both acting the same?

[jason str]

Recheck the woofer dustcaps & leads.

 

The crossover can create noise as well if not secured properly.

 

A rubber mallet can be a useful tool for finding loose panels or parts. If you are worried about marring the finish put a facecloth over it securing it with a rubber band.

[Dave A]

6 hours ago, HDBRbuilder said:

Keep in mind that the production previous La Scala construction uses butt-joint glued and nailed construction.  Because the nails go thru a panel into the edge of another panel, there is a POSSIBILITY that the nails may have caused a separation of the plies that is NOT visible, but which COULD cause a resonance issue...especially over time as speaker had been man-handled, moved around, transported, etc....which may have caused an INCREASE in the ply separation which has the plys resonating against each other at certain frequencies.

 

This is one of the reasons which I used rabbeted joinery, along with dadoed joinery when I built my oak prototypes a decade or more ago.  The joinery stiffens up the construction, and no nails were used in the construction....therefore no possibility of splitting the plys apart over time. 

Can I take a small hammer and tap along all the joints and detect this separation?

[Dave A]

2 hours ago, glens said:

 

What I suggested should help you pinpoint, quickly, the problem area(s).  All the other stuff is interesting enough in and of itself, and I'm the sort that likes to know such things, too.  But the immediate concern isn't so much "what could be different?" as "what the hell is it that's vibrating, and what'll make it quit?"

 

A piece (or layer) of wood somewhere has come unglued or some such - my guess.  Or are you saying a pair of cabinets are both acting the same?

Well yes that is exactly what I have been saying and they both have itsy bitsy tiny splitters. So I am back to reminding everyone that I am primarily asking about what effect these runt sized splitters can have. They are sequential numbers so someone decided to do a wonderful different thing on these. I suppose the idea of nails splitting plys is possible but two in a row?

 

 

11 minutes ago, jason str said:

Recheck the woofer dustcaps & leads.

 

The crossover can create noise as well if not secured properly.

I have eliminated everything you can come up with except the idea of internal doghouse bracing which I don't think will solve this problem. The more I read the responses and think about all this the more I believe the splitters are the culprit. When I have time I will solve this splitter problem and report back.

[jason str]

16 minutes ago, Dave A said:

Well yes that is exactly what I have been saying and they both have itsy bitsy tiny splitters. So I am back to reminding everyone that I am primarily asking about what effect these runt sized splitters can have. They are sequential numbers so someone decided to do a wonderful different thing on these. I suppose the idea of nails splitting plys is possible but two in a row?

 

The only detriment with a smaller splitter i can fathom would be a slight loss of midrange performance if anything.