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La Scala resonance problem


Dave A

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I will have to get a picture of these to post. They are so small they might as well not be there. I can see the sound waves hitting that back side and ricocheting their way out to the sides and getting garbled on the way out. It is a weird problem and I wish someone with deep acoustic design knowledge would chime in here with the precise effects splitters create.  One thing I am pretty sure of though is there was a lot of time spent figuring out the right splitter size and the ones I have seen have all been pretty consistent until this one.

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Just a thought here:  Check the wire from the woofer...see if it is possibly against one of the panels inside the doghouse...where when the volume hits a point it is slightly rattling against the panel.  I remember this driving somebody nutso in a LaScala doghouse before a few years back.

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3 hours ago, jason str said:

Reflectors do nothing under 200 Hz, the sound waves are too long.

 

Like i said midrange is what it will effect if at all.

It is mid range for sure.

 

3 hours ago, HDBRbuilder said:

Just a thought here:  Check the wire from the woofer...see if it is possibly against one of the panels inside the doghouse...where when the volume hits a point it is slightly rattling against the panel.  I remember this driving somebody nutso in a LaScala doghouse before a few years back.

Nope it is serious resonance and not something like that and on both of them.

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Play something that gets the noise activated and use a stethoscope to determine where the noise is emanating from.

 

If a stethoscope is unavailable a long screwdriver, steel rod or similar tool will work holding one end up to your ear and the other end against the cabinet.

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It might could.  The only thing that could still suggest otherwise in my mind is if it's the splitters and they came that way originally, then the speakers must've been "singing along" all along.  That somehow seems unlikely to me.

 

I'd find a way to make them sing steadily while I poked around.

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10 hours ago, Dave A said:

It is mid range for sure.

 

Nope it is serious resonance and not something like that and on both of them.

I did not see where you isolated the bass cabinet only (disconnected the mid and tweet) Your statement about the mid range had me going down a different path. LSi at that time were using the K55M (I think). Those mids had a tendency to rattle as the back plastic plate would get loose. A simple tap with your knuckle on the back of them will let you know. I had one driving me crazy and it took me several tries to locate it. Good news is that if that is the issue, a small bead of silicone around the edge fixes the problem. Not nearly as much work as some of the other possible remedies. Just a shot in the dark but worth a try. Good luck.

77M.thumb.jpg.0c78f54c7dc3274556f4d291f6f3ea73.jpg

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39 minutes ago, MookieStl said:

Those mids had a tendency to rattle as the back plastic plate would get loose.

Makes more sense than anything else.  I've heard that before.  A dried driver to horn gasket will make noise too or loose driver to horn.

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50 minutes ago, CECAA850 said:

The internet has a bunch.  You can pick whatever sine wave frequency you want and there will be a test tone on you tube or the like.

 

Back in the good 'ol days of car and home audio, I owned one of the Rockford OSC2 tone generators.  Direct RCA connection to the amplifier and could simply sweep the frequency in a quick fashion.  Was great for finding odd resonance frequencies in cars and rooms and, of course, finding the tuning frequency of a port in an enclosure in the vehicle...especially back in those SPL days.  :D

 

image.png.32238cb9b4b71f5bab0699933a61d0ee.png

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9 hours ago, MookieStl said:

I did not see where you isolated the bass cabinet only (disconnected the mid and tweet) Your statement about the mid range had me going down a different path. LSi at that time were using the K55M (I think). Those mids had a tendency to rattle as the back plastic plate would get loose. A simple tap with your knuckle on the back of them will let you know. I had one driving me crazy and it took me several tries to locate it. Good news is that if that is the issue, a small bead of silicone around the edge fixes the problem. Not nearly as much work as some of the other possible remedies. Just a shot in the dark but worth a try. Good luck.

77M.thumb.jpg.0c78f54c7dc3274556f4d291f6f3ea73.jpg

Well I did the ear test and sticking my ear in the mid horn A-OK but the bass bin was another story. When I say mid range I mis-spoke since what I should have said was the top end of the woofer output in the bass bin.

 

9 hours ago, avguytx said:

Do you own, or have access to, a tone generator? 

Yes I have one and there is also a free one inside of Audacity.

 

2 hours ago, avguytx said:

 

Back in the good 'ol days of car and home audio, I owned one of the Rockford OSC2 tone generators.  Direct RCA connection to the amplifier and could simply sweep the frequency in a quick fashion.  Was great for finding odd resonance frequencies in cars and rooms and, of course, finding the tuning frequency of a port in an enclosure in the vehicle...especially back in those SPL days.  :D

 

image.png.32238cb9b4b71f5bab0699933a61d0ee.png

I have done that before I just didn't remember to do it here.

 

4 hours ago, JohnA said:

 

True.  A resonance is always from something loose or unrestrained.  Have you tried a different woofer?   

Yes a new Eminence Kappa 15C and an old known good k-43 and both did it. Funny thing is with room EQ using a Driverack Pro you can almost make it go away.

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I guess I am not understanding what the exact problem is. It sounds like the OP 1) added some bracing, 2) thickened the walls to one inch stock, 3) replaced the crossover with a compatible AA style (minus the diodes) and 4) noticed the the splitter appeared to be third the size that it should be. In the end 5) he hears some sort of "resonance"

 

Let's look at 1, 2, & 3. Folks have done this before (usually without before-and-after measures) and claim an improvement. These are reasonable steps and I have no reason to make sound worse. 

 

Looking at #4, when it is said to be smaller. Does this mean it is smaller than what would be shipped form the factory (implying someone at some point in time performed a modification). Or does mean that upon visual inspection it "just seemed unexpectedly small"? 

 

Looking at number #5, what are you hearing when you hear this "resonance" (I use this term in perhaps a more restricted sense)?  Is it a whistling sound, a screeching sound, a howling sound  or what? Can I assume this sound was not present before you made the modifications listed in #1, 2, & 3? Do you hear this sound from other La Scala cabinets? 

 

If the splitter is "smaller than a factory version" can you glue/screw a "shell" over it to make it "factory size"? It doesn't need to be fancy woodwork and I don't know how much room or access you have in there.

 

Some of the suggestions being made don't make much sense until we know the answers to #4 & 5.

 

Good luck,

-Tom

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1 hour ago, Dave A said:

Well I did the ear test and sticking my ear in the mid horn A-OK but the bass bin was another story. When I say mid range I mis-spoke since what I should have said was the top end of the woofer output in the bass bin.

 

Your ear to horn wont detect this vibration. It is the back of the speaker that vibrates which makes it hard to track down especially when it does sporadically. If you have K55Ms you need to give it a good hard look.

Again, it is just a shot in the dark but your symptoms sound like what I went through. It is very common with that driver.

Good luck.

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1 hour ago, PrestonTom said:

Looking at #4, when it is said to be smaller. Does this mean it is smaller than what would be shipped form the factory (implying someone at some point in time performed a modification). Or does mean that upon visual inspection it "just seemed unexpectedly small"? 

What it means is that from the factory it is maybe 1/3 as big as normal. I have been in the guts of many of these and I have never seen anything like this before. No it has not been replaced there are no signs of glue or stuck remnants of larger pieces of wood removed and the little bitty put in place.

 

  Guys look I don't understand why there is even a question as to whether it is way small or not. As many of these as I have been into I am telling you it is and that ought to be sufficient. When I have time to fiddle with this again I will get a picture and until then you just have to decide I know what I am describing or I just cant see what is in front of me.

2 hours ago, PrestonTom said:

If the splitter is "smaller than a factory version" can you glue/screw a "shell" over it to make it "factory size"? It doesn't need to be fancy woodwork and I don't know how much room or access you have in there.

I have pondered doing this very thing and it would be the easiest  quick way forward. It is very tight though and might be impossible to do without removing one side. I am leaning towards forgetting this problem for now and dealing with it when I have time to do a rebuild of the cabinet with 1" sidewalls.

 

2 hours ago, PrestonTom said:

howling sound

closest things and it reminds me of taking an orbital sander and going to the front of the sidewall and that lovely resonance you get then.

 

  K-55-M is not the problem as K-55-V's do it too an easy quick test done tonight.

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