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Robbie010

La Scala Bracing - Off Centre??

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Hi All,

 

I've been testing my clone la scala while waiting for the mid-horn to turn up and think they definitely need to the cabinet bracing.

 

I thought I read somewhere that the bracing has to be off-set veritcally in order to avoid some kind of unwanted interference with the low frequency signal, is this correct?? and if so, by how much??

 

For a better bracing effect, I was thinking of doubling up on the braces i.e two pieces of sandwiched 18mm ply cut in to the appropriate wedge, then fixed in place, probably glued and screwed. is this overkill?

 

Thanks.

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18 minutes ago, Robbie010 said:

Hi All,

 

I've been testing my clone la scala while waiting for the mid-horn to turn up and think they definitely need to the cabinet bracing.

 

I thought I read somewhere that the bracing has to be off-set veritcally in order to avoid some kind of unwanted interference with the low frequency signal, is this correct?? and if so, by how much??

 

For a better bracing effect, I was thinking of doubling up on the braces i.e two pieces of sandwiched 18mm ply cut in to the appropriate wedge, then fixed in place, probably glued and screwed. is this overkill?

 

Thanks.

Actually most prefer 1 horizontal brace in the center of the cab -between the sidewall and the dog -house panels -

 

PWK10001.jpg

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placing the brace off centre will result in two different frequency side panel resonances rather than two panel resonances at the same frequency as would result from a centrally mounted brace. It is a bit of a moot point however as a centre mounted brace will push the first cabinet resonance above the bandpass of the cabinet so the resonance will never get excited by the woofer to begin with. If you want to do something useful double the thickness of the top and bottom of the cabinet with a layer or 3/4" plywood as those panels ave very flexible. Have fun and enjoy your project.

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4 hours ago, Robbie010 said:

Hi All,

 

I've been testing my clone la scala while waiting for the mid-horn to turn up and think they definitely need to the cabinet bracing.

 

I thought I read somewhere that the bracing has to be off-set veritcally in order to avoid some kind of unwanted interference with the low frequency signal, is this correct?? and if so, by how much??

 

For a better bracing effect, I was thinking of doubling up on the braces i.e two pieces of sandwiched 18mm ply cut in to the appropriate wedge, then fixed in place, probably glued and screwed. is this overkill?

 

Thanks.

Well, with my reasoning, anyway (not worth much), exactly centered vertically would double the resonant frequency of the sidewall.  That's probably good enough, but if they are off center by a bit, the resulting resonant frequencies would be slightly less than double and slightly more than double.

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1 hour ago, codewritinfool said:

Well, with my reasoning, anyway (not worth much), exactly centered vertically would double the resonant frequency of the sidewall.  That's probably good enough, but if they are off center by a bit, the resulting resonant frequencies would be slightly less than double and slightly more than double.

a single centrally mounted side wall brace has historically resulted in both stiffening of the dog house walls as well as the cabinet side walls so as to completely remove any call for complaint. This modification has been used by a very good number of owners. Extending the brace out toward the mouth of the cabinet will also significantly stiffen side wall flexing at the mouth of the horn which is of importance in horns of this size and larger. Your efforts to reduce panel motion will be greatly rewarded with improved clarity and dynamics.

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7 hours ago, Robbie010 said:

Hi All,

 

I've been testing my clone la scala while waiting for the mid-horn to turn up and think they definitely need to the cabinet bracing.

 

I thought I read somewhere that the bracing has to be off-set veritcally in order to avoid some kind of unwanted interference with the low frequency signal, is this correct?? and if so, by how much??

 

For a better bracing effect, I was thinking of doubling up on the braces i.e two pieces of sandwiched 18mm ply cut in to the appropriate wedge, then fixed in place, probably glued and screwed. is this overkill?

 

Thanks.

I personally think braces look horrible exposed on a  la scala's but making the braces the least intrusive and hiding them behind a grille ala  klipsch Belle  -- can make the cab both more better sounding and great looking -

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13 hours ago, Randyh said:

I personally think braces look horrible exposed on a  la scala's but making the braces the least intrusive and hiding them behind a grille ala  klipsch Belle  -- can make the cab both more better sounding and great looking -

it is all a matter of perspective. I see good brace work and I am delighted that the design has been well thought out and executed. I believe if you are going to do something then do it as well as you possibly can and make your effort worth while. Folks can choose to do as much or as little as they are comfortable doing, as the owner you get to decide you are in control of the modification process. Personally I think the LaScala looks better with two braces per side of the doghouse but that's just me and I like the thick cabinet walls of the LaScala ll over the 3/4" walls of the LaScala. It's a great speaker however you build it.

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On 3/22/2020 at 5:50 PM, Robbie010 said:

Hi All,

 

I've been testing my clone la scala while waiting for the mid-horn to turn up and think they definitely need to the cabinet bracing.

 

I thought I read somewhere that the bracing has to be off-set veritcally in order to avoid some kind of unwanted interference with the low frequency signal, is this correct?? and if so, by how much??

 

For a better bracing effect, I was thinking of doubling up on the braces i.e two pieces of sandwiched 18mm ply cut in to the appropriate wedge, then fixed in place, probably glued and screwed. is this overkill?

 

Thanks.

 

Ever played (with) guitar?  An unfretted string rings at it's tuned frequency.  One fretted in the middle rings at 2x the open frequency (+1 octave) and will chime at the 2x frequency.  Even 1 fret off the midpoint will no longer chime.  So, MUST a brace be off center, no.  But it's better. 

 

The brace doesn't stop an interference, it stops a resonance that adds extra output around 140 Hz. 


Glued and screwed is good.  Using 2 pieces of plywood for the brace is unnecessary.  What you're doing is tying down the front portion of the side panel so it can't resonate, or chime like a guitar string.  It takes little force to stop the resonance; a single thick brace is plenty. 

 

Any system that resonates is a spring with a mass attached.  Doubling the outer panel thickness increases its stiffness by the 3rd power (8x) while only doubling the mass, raising the resonant frequency many times above that of a single 18mm panel.  That would also work, but wouldn't look good to my eye. 

 

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IMO, the location of bracing is not very important.  That opinion is based on the experience described below.

 

When I first refurbished the La Scalas, shown below, for the local high school band practice room, they did not have braces.  After a week, the director called me to report a problem.  I went to investigate his concerns.  At first, I blamed garbage in garbage out.  

 

I could play play certain songs quite loudly, such as Devil May Care from Diana Krall’s Live In Paris, with no discernible issues, but a cheap Paul Simon CD sounded dreadful at any volume.  At first, he bought my diagnosis and explanation.  Eventually he called again.

 

Gradually, I became a convert to the existence and significance of the resonating sidewall issue.  The first proof to me resulted from wedging a small piece of 1/2” dowel between the “eaves” of the doghouses and the sidewalls.  The dowels were cut to be just long enough to be pressure fit into the narrow spaces between the doghouses and sidewalls just before the point where the “roofs” of the doghouses slope toward the peaks as the horns expand.

 

The quick and dirty 1/2” dowel trick solved 90% of the issues.  The crappy Paul Simon CD still sounded like crap, but the issues of sidewall resonance seemed to be tamed, if not eliminated.  The band director was satisfied, but I was not.

 

During the next summer break, I collected the La Scalas to perform a summer tuneup.  In addition to confirming tweeter operation, I fabricated the plywood braces visible in the gag photo below.  After marking their positions, I sanded the paint down to bare wood to glue and screw the braces into place.  With the bottom access panels removed, screws were inserted from the inside of the doghouse into the braces.  Screws were also put through the sidewalls into the braces.  Titebond II glue is holding the braces, the screws could probably be removed with no reduction in structural integrity, but they remain in place.  The speakers received a coat of Duratex and were returned to service with no further complaints from the director.

 

The dowels were not precisely at the vertical midpoint of the horns.  Neither are the plywood braces precisely at the midpoint.  Intuitively, it makes sense to not place the braces at precisely the midpoints for reasons articulated in earlier posts.  Practically, it doesn’t matter, IMO.  Any bracing, including the small dowels, is a vast improvement over no bracing.  Fretting over halving resonances is, IMO, akin to calculating angels dancing on a pin.  YMMV

 

FYI,  I used poster-board from the Dollar Store to fabricate patterns for the braces. The poster-board was cut into small pieces.  The pieces were placed in the horn mouth overlapping with straight edges against the sidewalls and the doghouse roof.  The overlapping pieces were then taped together.  The composite pattern was transferred to a single piece of poster-board to be used a pattern/template to cut the Baltic birch plywood  braces.  

 

The patterns, therefore the braces, were not symmetrical; they were not interchangeable.  Although the differences were minor, they were enough to require four custom braces, one for each horn mouth opening.  The cost to incorporate such braces at the production level would have been huge.  Nevertheless, it did “make a dime’s worth of difference.”

 

post-6832-0-93440000-1423926401_thumb.jpg

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Center bracing for me as it seems to do what I want and looks balanced. The other alternative requires much more work and that is to take a rubber mallet and beat the sides off and replace them with 25mm Baltic Birch. Both methods accomplish the same thing but replacing the sides looks much better.

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