And for those of us allergic to glass wool / fiberglass ? 🙁
Also what is the likelihood of glass fibres or dust escaping through a bass reflex port into the room ?
I'm not seeing the point of the 2D QRD type internal diffusers shown earlier, if you use stuffing as well. All I have read on the subject suggests that when covered, diffusers don't behave like diffusers any more.
Otherwise I agree with speakerdave in his post a couple up about internal absorption. Tallies well with my experience too.
Otherwise I agree with speakerdave in his post a couple up about internal absorption. Tallies well with my experience too.
What about building in some internal baffles to effectively break up the resonant spaces into smaller spaces?
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Assuming you will put the woofer in the top of your cabinet as drawn, I think you are still in the same boat. The divided spaces you create will be resonant in the frequency range wher your damping is already very effective. The issue is effectively treating the lowest modes and you will still have a mode due to the longest path from top to bottom. It may be a little longer/lower with the zig zag path of your labyrinth, but that only makes the treatment situation slightly worse.
David
This is very interesting stuff David. As always, you have provided real sources of practical information that we can use in our projects.
Can you elaborate on airborne and structural born cabinet colorations? Are you talking about bracing? This is an interesting topic for me right now because I'm trying to kill a vibrating back panel on a vented box for a 15" woofer. The box is about 6 litres and the back panel is 26" away from the front, resulting in a 260 Hz wrinkle in the impedance plot.
Can you elaborate on airborne and structural born cabinet colorations? Are you talking about bracing? This is an interesting topic for me right now because I'm trying to kill a vibrating back panel on a vented box for a 15" woofer. The box is about 6 litres and the back panel is 26" away from the front, resulting in a 260 Hz wrinkle in the impedance plot.
Um, I actually don't have the woofer at the top - does it look upside down to you?
My understanding is that resonances require two unimpeded parallel surfaces. With the intermediate baffles, yes each will form a smaller resonant space, but the longest path would be without these requirements.
Reflections would still mostly be diminished with the top and bottom absorption material you would have already used, with only the middle space now needing it.
You're definitely right, it would have problems - I'm just wondering if they would be less or more problems than without.
It would be an interesting thing to try, IMO.
Thanks for the comment.
We talked about cabinet construction with regard to resonances, structural rigidity etc. quite a bit in this thread:
http://www.diyaudio.com/forums/multi-way/138111-what-characteristics-better-material-enclosure.html
You'll have to seperate the fact from fiction, though.
15" woofer in a 6 liter box sounds pretty snug (maybe 6 cubic feet?). Bracing or a front to back brace will certainly raise the primary panel resonances of the front and back panels. I'm guessing that if you have a resonance strong enough to see in the impedance curve it is a combinational resonance of front and back panel resonances and woofer mass. Bracing the back of the woofer magnet structure to the rear panel may help there.
My preference is always to go for damping rather than just stiffness. Damped decoupled masses such as ceramic tiles with a lossy glue coupling could be a good approach too.
Woofer decoupling might be a big help also. The first decoupled woofers we did at KEF where to improve a similar big resonance that the 12" woofer (B300) of the original Reference 105 had. The 105 mkII had the trochoidal woofer chassis and decoupling, and it dropped cabinet vibration quite a bit.
Good luck with that.
David S
Hey, whats more important, being able to breath or having good sound!
Most people are okay using plastic gloves for installation. If not there are some acoustical foams out there that are good. They tend to be more expensive so I've never pursued them.
Fiberglass out the port is a real factor but it diminishes over time as the loose stuff blows out. Cutting the material cleanly will reduce the problem. You can also line the cabinet and then cover that with cheesecloth (or just about any cloth). Some systems have used squares of fiberglass, each nicely wrapped.
David
Not at low frequencies. A "box" speaker will be almost omnidirectional at low frequencies. An omni speaker gives more radiation into the room than a dipole.... 4.8 dB according to Linkwitz.
Yeah, good point.
I have actually measured the reflected energy off different types of acoustic tiles, done in a large near-anechoic room. At frequencies above 1 kHz some of the so-called acoustic tiles dont work at all. Pure fiberglass is excellent though, reflection is below -30 dB at 1 meter distance. That will however mean, that at 10 cm distance the reflection will still be significant.
Some more here (in the 2nd half). Note that Bradbury's work here has mostly been shown to be wrong.
Bradbury's Fiber Equations
dave
Gary Pimm research has him advocating UltraTouch recycled cotton insulation even better. Pricier thou. And available at Home Despot in the US.
We use it instead of fiberglass, and are starting to expeiment with it as a sub for pluff in TLs.
dave
I had considered sewing acoustically transparent grille cloth into covers for rock wool or fiberglass to cut back on loose glass fluff. For inside speaker enclosures, or frameless absorption panels in a room (hang from ceiling/wall with the eyelets like tarps have). You can buy the eyelets and an eyelet tool from Home Depot 🙂
Its true that fiberglass has high absorption but not infinite absorption. I went with an acoustician to a court room with a large perfect dome ceiling that you could stand in the center of. It had been lined with 1" thick high NRC fiberglass. Still had a pretty audible bounce back (the acoustician said "you should have heard it before!").
A 30dB reduction would be 0.1% return, in energy terms, or 99.9% absorption. Thats pretty good. What we are really looking for is a reduction in the standing waves internal to the box. Diaphragm mass also contributes attenuation through the woofer, so if some energy reflects from the cabinet walls it will still be further knocked down by going through the woofer.
David S.
Blonded Logic Ultratouch is better at lower frequencies than same thickness of fiberglass, and it's a pleasure to work with:
http://www.bobgolds.com/AbsorptionCoefficients.htm
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In my experience, builders will use whatever is the cheapest. Cheapest means labour + materials, and this is not the same as what a DIY speaker builder is concerned about.
When we talk of resonances..The three differing enclosures, sealed, ported, OB, each have there own battle with resonances. IMHO the ported being the most difficult. Consider the empty wine bottle...try this easy experiment, whistle into the neck...the Fb is apparent, but you can feel thru the bottle the vibration at that F ....The Fb has set the bottle in motion, & this motion will make its "own" sound. This probably why the tone is not a pure tone, like the tuning fork. All the non-linearities & harmonics will be created. My take on this is, if you can feel vibrations with your fingers on the enclosure...it ain't right, accurate....FAIL.
Long ago I did the simple experiment with the so-called fibreglas damping stuff, putting a wad of the stuff to my ear.....no attenuation..next, try it with some cotton batting...now we have attenuation. At the microscopic level, all we have is a random forest of solid glass "trees"....I think 'they' are trying to do the inverse square thing, hoping the sound will bounce around enough to attenuate.....next we have a "forest" of "trees" resembling giant ropes....which do you think will work best?
I for one would be hard pressed to build an enclosure at six cubic feet, ported that wouldn't vibrate at Fb...I'd start at two inches thick MDF, glued & pressed.....& keep making it thicker till no vibration is detectable. Using un-common materials is an option, but I've found MDF to be very "workable".
__________________________________________________________Rick...
Long ago I did the simple experiment with the so-called fibreglas damping stuff, putting a wad of the stuff to my ear.....no attenuation..next, try it with some cotton batting...now we have attenuation. At the microscopic level, all we have is a random forest of solid glass "trees"....I think 'they' are trying to do the inverse square thing, hoping the sound will bounce around enough to attenuate.....next we have a "forest" of "trees" resembling giant ropes....which do you think will work best?
I for one would be hard pressed to build an enclosure at six cubic feet, ported that wouldn't vibrate at Fb...I'd start at two inches thick MDF, glued & pressed.....& keep making it thicker till no vibration is detectable. Using un-common materials is an option, but I've found MDF to be very "workable".
__________________________________________________________Rick...
Yes, those numbers look pretty good. There are also some combination materals with a mass loaded membrane at front and absorption material behind, that can do very well at LF (less well above).
David S