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Old 30th November 2007, 10:26 AM   #1
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Default cabinet tweaking... and a two second tweak.

ok y'all.

So I was playing with horns tonight. plugged in a "finished" version of the ugly duckling right after an "unfinished" version. "finished" meaning the oil on the wood. the shiny stuff.

the sound totally changed. things sounded all chilled out on the finished version. even though everything else between the two pairs of horns was identical.

I have always noticed that the resonant frequency of the wall of the cabinet changed (fewer upper harmonics) (judged by knocking) after the application of the oil, but I had no idea that it made this much of a difference. the difference was subtle, yes. but at the same time, not. the effect was especially felt in the mid/high range.

So my question for everyone is why one would consider finishing the inside of the cabinet? Other than to "seal" the enclosure against long term moisture and subsequent glue break down. course, I have always figured that glue nowadays is so nice, that old school intricate (furniture) joinery, and internal moisture protection considerations can be left out of the equation to some degree.

I see around the web all sorts of different ways of finishing the insides and outsides of cabinets. I am asking for people's opinions on this matter. It is no doubt an issue that involves taste to some degree, but what sorts of effects can we imagine for ourselves?

obviously, I am asking about acoustic effects here. not visual effects of the finish.

In the musical instrument world, some hold the idea that the inside of the sound chamber should be left unfinished. Letting the roughness of the wood and little glue spots etc soak up any high frequencies. That is the thought I have prescribed to since playing with acoustic musical instruments on my own but eh, who the hell knows?

I mean, obviously, the type of finish product is important in and of itself, but where do we draw the lines? what happens with these things?

___TWO SECOND TWEAK___

It has come to my attention lately that there is a matter also, important to acheiving a chiller sound from full range drivers. I don't mean to give away trade secrets, but eh. that which you give away comes back in a different form, I suppose.

Basically, the torque of the driver mounting screws is of vital importance. I feel that it takes a whole lot less torque than most people think. If you have bent the area of the driver frame surrounding the screw, then back off!!!

For cast frame drivers, a more precise torquing method can be employed, involving a torque driver.

for me, though, i use just enough torque to secure the washer to the point that I cannot slide it around with my finger nail, and then just a smidge beyond that. obviously, it should be enough to provide an airtight seal, and to prevent anything from rattling (like the washers).

I have found myself tracking down early customers and doing torque tweak for them, or instructing them how to do so. It is SO important.

the drivers sound instantaneously broken in compared to before. I have found the effect of torque on frame to baffle to be VERY audible (in a subtle way). compared to a tightly cinched down driver, a thoughtfully mounted driver sounds like it has 40-100 hours more of break in, right out of the box. things only get better too...

Basically, it is all a matter of destressing components.

In other aspects of the horn, proper square, glue amount, clamp pressure, all amounting to not unnecessarily forcing a panel to warp during clamping. all this plays HEAVILY into the sound of a BLH. All this infinite variance though is scientifically frustrating enough to make any (relative) perfectionist like me go insane. All these considerations make me want to apprentice for a violin or harpsichord maker. Not that they would by definition know any better, but these sorts of things are hard to model, and hard to know the ultimate effect of without the hundreds of years of trial and error tweaking that musical instruments have been subjected to.

Also, I am wondering people's opinions on various driver mounting methods (t-nuts, screws, driver on the front of the baffle, on mounted on the back of the baffle, etc.)

alot of these little tweaks most manufacturers keep rather secret, as frankly, they are the hardest learned lessons (that is, if they even think about these sorts of considerations in the first place...) so I hope that people will step out for a minute and share advice.

anyways, thanks in advance for the hopeful discussion this will start.

Later,

Clark

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Old 30th November 2007, 11:19 AM   #2
marce is online now marce  United Kingdom
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T-nuts, neopreme closed cell gasket, and attempt to decouple the driver as much as possible. Looking at rear mounting to de-couple further, ala rear mounted tweeter in my Changs.
Sorry for short reply in Library and time up.
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Old 30th November 2007, 03:23 PM   #3
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I have been thinking of mixing wood glue with sand and applying it to the inside of a cabinet. I am rebuilding my bass guitar cabinet, and I hope it will dry without flaking off inside. Has anyone tried this?
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Old 2nd December 2007, 07:19 AM   #4
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Default finish.

my finish is a polymerized linseed oil. hand rubbed, no solvents. It is called "tried and true" varnish oil.

i think it sounds good. I am sure that there are other options though, acoustically speaking. health wise speaking, I stick with the non toxics. one of our stronger theories for why my old boss Terry Cain RIP got ALS (lou gehrig's disease) was that working for 30+ years with heavy metal and solvent laden finish products damaged his nervous system. while there are no truely conclusive studies to this effect, ones like this seem to point in a general direction. I mean, the stuff usually talks about nervous system damage on the can.

http://query.nytimes.com/gst/fullpag...=&pagewanted=2

So I guess I am wondering about people's experiences with the acoustic viability of various finishes, and whether or not people finish the insides of their speakers, and why or why not.

Clark
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Old 2nd December 2007, 07:12 PM   #5
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At this very moment I am listening to a pair of five year old FE167E MLTL's. They have suffered temporary storage in an un-air conditioned garage in central Arkansas, been indoors for the heating season, been dragged to Dallas, Tulsa and Atlanta in the back of a pick-up and been rained on once. They show no sign of structural or cosmetic damage.

The insides are not sealed. That would be a little hard to do anyway, as I use a sandwich construction of MFD/"Liquid Nails"/HardiBacker.

I've tried a lot of different finishes: tung oil, "Danish" oil, acrylic, polyurethane and others. I get the best results with Minwax "Tung Oil Finish". My technique: Sand to 220 grit. Apply any stain and then a heavy coat of finish that saturates but not quite runs. Buff with 0000 steel wool and apply another coat of finish. This coat needs only be heavy enough to wet the surface. Repeat. Three coats is usually enough for a nice eggshell finish, although the grain may not be completely filled, particularly on oak and mahogany. For a super nice finish, apply a fourth coat, wait a week, wet sand lightly with 600 grit, then buff with white polishing compound.

Bob
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Old 2nd December 2007, 08:00 PM   #6
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Default finsh

It depends on whut I'm building.
How inert do you want the cabinet to be?

I've tried polys, varnish, lacquer, shellac, linseed oil blends (tung oil, etc.) and inside, tar undercoating.
If youíre gonna be building cabs professionally, try building a few identically, then finishing them differently. It can make a difference.
On sealed subs I go for inert, may go for a constrained layer of concrete backer-board, and then use tar based automotive undercoating inside.
I havenít built any horns yet, thatís next, but I plan to start doing the same as BLHs.
Back loaded horns I like to shellac the inside.
And sometimes Itís a pain, like the small parts of a Swan, ya know?
Why? Because people say Iím crazy, it sounds better
I donít know why. Wood resonates different? Waves travel different?
Robert
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Old 3rd December 2007, 02:55 AM   #7
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Default Re: finsh

Quote:
Originally posted by serenechaos
[Back loaded horns I like to shellac the inside.
And sometimes Itís a pain, like the small parts of a Swan, ya know?
Why? Because people say Iím crazy, it sounds better
I donít know why. Wood resonates different? Waves travel different?
If you are working in 15mm BB or thinner, sure the wood resonates. Will a soaking in shellac tone it down? Probably, but not like the finish on a violin will. The violin is much thinner.

In my case, i seriously doubt that the finish has any effect at all. The box is dead to begin with.

It is interesting that you think that damping your cabinets improves the sound. I have always thought that and have taken it to the extreme.

Bob
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Old 3rd December 2007, 12:45 PM   #8
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I don't think damping improves the sound in all cabs.
I went go extremes in things like the sealed box mentioned above.
The walls of a BLH are another matter.
My experiments (and ears) may differ from yours, and thus our opinions differ.
That's why I suggest clark try for himself, and test with HIS ears.
Robert
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Old 3rd December 2007, 03:08 PM   #9
BobM is offline BobM  United States
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I agree that you have to be careful with damping to make sure you don't suck the life out of the music. This is just as much an art , with lots of trial and error as most anything else in audio.

With stuffing and interior treatments like Black Hole 5 and others you can add or remove pieces until you find a nice balance. With oils and tar and other things that soak in, once it's done you can't remove it - so experience plays a big role there. If you want to get crazy I bet you will find that what works best in a bass or midrange cabinet is not the same as what works best for a tweeter.

Good luck,
Bob
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Old 4th December 2007, 08:54 PM   #10
SCD is offline SCD  Canada
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Default Resonance

Hello folks:
I have built a few cabinets over the last few years my techniques are a bit different from some people. I will attempt to describe some of the concepts I try to incorporate in my builds.
First and foremost I believe everything has a frequency of resonance. Differing materials resonate at differing frquencies.
Secondly I do not think you can eliminate resonance. I do think you can manage it though or at least influence it.

I think you want build as solidly as possible to deal with the simple stuff. So use good wood working skills and use the best joinery posible. Your enclosure is not a musical instrument it is an instument to reproduce as accurately as possible the music you are feeding it. It is not part of the music. This is not an easy concept for all to accept. But you really are not building a piano here.
Next use complicated materials that have good tensile strength and inherently low resonance. Baltic Birch plywood, dense fine grain real wood, bonded materials. The list is long. MDF or chip board is not really on the preferred list of materials.
Next try to make as small a panel as possible to reduce the flapping drum effect from large surfaces. If you look at Planet10 designs you will see lots of bracing in his designs. He is attempting to reduce the flapping drum effect in the cabinet walls. The concept here is to accept you are going to get some resonance, try to minimize the panel size and push the frequency up above the frequencies of interest. There are damping materials you can use but I am not familiar with them. I think Borosilicate is used by one manufacterer and I suspect the silent running product dave is using on drivers will also help.

there you go
Nuff said for now
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