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Good point about the order of experiments.
I was thinking again about this 30dB limit - which is the goal I take away from the BBC paper. As I said somewhere above, I don't think this goal is good enough. It may be adequate when comparing one box speaker to another, but the BBC did not compare box against no-box. In order to get colourations down to open baffle (room treated) levels it may be necessary to do better than 30dB down.
I would think that this was a perceptual issue. It would be unrelated to cabinet or no cabinet, it would be purely tied to the audibility of resonances versus their level (and arrival time and direction). If the resonances are below the threshold of audibility then you are done (unless you are an audiophile for whom "good enough" is never good enough) independent of cabinet type.
You still assume that you can let the rear radiation out into the room and treat it to a level that is lower than what good absorption in the box would achieve. You need to rethink that.
David S.
You still assume that you can let the rear radiation out into the room and treat it to a level that is lower than what good absorption in the box would achieve. You need to rethink that.
David S.
Another perspective is to utilize the rear radiation and this is the only reason I can think of why an open baffle design might be a good idea (or not).
I'm not passing judgement on open baffles but I do think it is important that any rear radiation is directed to arrive well delayed and from the sides, otherwise there will be coloration issues.
What I do take exception to is any assumption that room treatments can catch all the rear energy and reduce it to levels lower than a well stuffed closed box. Unless your back wall is to full anechoic construction then that won't be the case.
Correct that, even with an anechoic back wall the rear energy will diffract around and we will have comb filtering of the front radiation.
David S.
What I do take exception to is any assumption that room treatments can catch all the rear energy and reduce it to levels lower than a well stuffed closed box. Unless your back wall is to full anechoic construction then that won't be the case.
Correct that, even with an anechoic back wall the rear energy will diffract around and we will have comb filtering of the front radiation.
David S.
That is what I have tried to do in my dipole setup, and doing that I found that even the best damping materials do reflect energy at higher frequencies, a lot more than expected.
Only if we have a dipole that was not designed for constant directivity, that is one which is operated above the dipole peak where all the comb filtering will start. IMO that is not how a dipole should be designed.
How did that damping look like? I would think you need to treat the whole front wall to a considerable depth. Or build a absorber housing around the speaker.
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You are talking about how the driver affects the environment - through the backwave. But Cal pointed out that the environment also affects the driver. By removing the box from behind the driver he found the sound much improved and this may be because the driver no longer 'feels' the box (through resonances, unequal front-back acoustic impedance).
Which is the more controllable, the back-wave escaping into the room to colour the sound, or the box colouring the output from the driver ?
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I have covered the entire front wall, 1/3 of the side walls and ceiling in the speaker end of the room with 15 cm Rockwool plus 3 cm Rockwool acoustic tiles on top of that. So it should be pretty dead down to approx 100 Hz, right? But in fact it is not. I found that even with this treatment, the front wall would give high frequency reflections (mainly above 3 kHz) just 20 dB down at 1 to 1,5 meter reflection path. At shorter path like in a speaker cabinet, the reflection would be just a few dB below the source. Not a problem for woofers, could be a problem for midranges, and definitely a problem for open-back tweeters.
I went a little further and treated the main reflection point with deep wedge-shaped acoustic foam (Auralex Venus bass traps actually). That helped a lot, and the reflection now got down into the -30 dB area, which for me is good enough.
Depends on the flow resistivity of the Rockwool panels. The tiles are probably too reflective. I would remove them. Is there a air gap behind the fluffier panels? If not then there should be one. This can considerably extend the absorptive properties at lower frequencies.
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ah, you mean like the BOFFLE
which further corroborates the idea that 30dB down is the right target.
Rockwool emits dust, and you dont want that dust in your lungs... so the Rockwool must be covered. I measured all available samples of acoustic tile I could find, and chose the least reflective of them. Generally, the more expensive they get, the worse is the acoustic performance. Cheapest is best in this case. I also tested various fabrics, none of them better than the cheapest acoustic tile.
I have an air gap behind the fluffy Rockwool yes.
In addition there are "corner killers" made the same way, acoustic tiles are placed at 45 degree angle in the corners, and the space behind is filled with Rockwool. Maximum depth is about 1 meter.
I have only four of those, and I got them for free from a studio that was closing down... 🙂
Absorbing the back wave in the box has a much greater chance of reducing coloration (as I and others have said to you numerous times). All of the back energy can be directed into FG or equivalent of a depth sufficient to absorb a very high percentage of the sound. You can never achieve that in your room. Can you cover all of the wall surfaces that the woofer sees with 6" of fiberglass? You can easily do that in most cabinets.
As to the box itself having resonances and causing issues, this needs to be treated, but don't forget that any baffle that an OB driver is mounted on will also have its series of mechanical resonances. They are no better in this regard.
Leaving the back off the cabinet does not improve the matter of cabinet coloration!
David S.
Industrial Noise Control | Noise Control Products & Systems | Noise Control Materials | Industrial Noise Control Inc.
This site has some interesting products for application.
This site has some interesting products for application.
... did you mean to say "sound coloration"?
Surely, you must agree that a cabinet missing any of it's wall elements will sound different than one that is completely enclosed. Further, that as each element is removed, the box coloration (resonance) generated for that particular panel is eliminated.
Is the key here finding the ideal frequency range for driver/resonance introduction? ... don't have LR resonance induced by a bass cab (well braced etc) .. while (LFr) perfectly acceptable for mid/tweeter enclosures (not braced as well .. loose so to speak)? Neither driver able to excite a large resonant mode in it own house.
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