any experience with B&W type backwave absorbers?

[terryj]

Gee, dunno if I even have the terminology right!

What I mean is the type of chamber that is some sort of cone shaped with stuffing.

As far as I can tell, it is designed to absorb as much as possible the backwave of the driver (I am considering it for my mids).

Are there any rules?, design hints, links?

Much more importantly, is it worth the effort over any old chamber for the mid. At a guess would it be fair to say that a lot of the reasons why some prefer OB IS because there is no return of the backwave thru the diaphragm? If that has some truth to it, then it does tend to suggest that it could be an avenue to explore.

I am nearing the end of the construction of my boxes, so little details like this need to be finalised. If I have a few different options to try, then it should not be too hard to do some experiments and see if there is a worthwhile difference.

Hopefully you can decode my poor descriptions and work out what it is I am asking about!

Here is a link to one guys implementation of it, within it you can download a drawing of how he constructed it, basically many layers of mdf with holes in each, which when sandwiched together provide the sculpted out tapering chamber (it also allowed him to fold it back on itself, to save size presumably)

If there are benefits to this type of construction, would he not have gotten better results if he did not use hard reflecting material like mdf? If say he had substituted compressed f/glass with the same cutouts, would he not have gotten an automatic improvement in absorption?

[fjhuerta]

Link?

[terryj]

duhh, sorry about that.

http://home.wanadoo.nl/dezaire/ultiem/ultimate.htm

go down to enclosure, and find this.....1. Microsoft word document with drawings

that shows how he did it

been doing internet browsing, it seems data is thin on the ground. Luckily there is a cutaway picture of the nautilus on wikipedia, maybe can try 'monkey see monkey do' with it.

Interestingly enough zaph seems a fan, that has piqued my interest a bit more

http://www.htguide.com/forum/showpost.php4?p=260213&postcount=31

must have a hunt around his site, maybe he has written something on it.

[jkrutke]

I built this (see attached) specifically to gain the benefits of a B&W reflection-free style of enclosure. It's been sitting in my basement unfinished for a year and a half.

That old message of mine that you linked to is another one with the same concept applied to both the mid and the tweeter with a custom pole piece enclosure. Ironically, it's also down in the basement in a cabinet that had a failed veneer attempt. It's speaker carcass city down there.

I believe it works, but I have not been able to find substantial objective measured proof that there's enough of a benefit there to make it worthwhile over a well damped sealed rectangular enclosure. I can indeed see a smoother impedance curve, but only if I amplify the Y scale 100 times and at that point I suspect I can see things that are not audible. So really, there's no proof. B&W doesn't really offer any solid proof either, only concept. About all I can say for sure is that it looks cool and interesting. Probably not the answer you're looking for.

[terryj]

well it is, if it means don't waste my time and effort!!!

I do have a 'related' question. As in your pic, why does it seem that everyone outs the absorption material directly onto the walls of the chamber (be it mid or bass)??

I will give my reasons, it might help show where my thinking is astray (or not)

First off, is it true to say the main reason is to 'remove the backwave'? (still not sure if I have the terms right) The main mechanism for that would be to convert the energy to heat yeah?

For that to happen optimumly, we want to have the wave push thru the absorbing material, and the force that would provide that push has to be pressure differential. But if the stuffing is directly on the wall, that pushing mechanism has to be severely curtailed doesn't it?

A bit like hanging f/glass panels in your room, if you space them off the wall you get better absorption. Or the asc tube traps. The sealed internal void very quickly gets a pressure differential between it and the room, and so provides the pressure equalization gradient. (or so says the blurb)

I guess the limiting factor here is box size?? But as a theoretical discussion, would it not be way better to space any absorption material off the box walls?

I understand you are a measurements guy (and so am I, if that is of any importance), but did you ever simply compare the two different styles side by side by listening? Could you hear any difference?

(I don't know how you would measure the 'removal' of the backwave. would something like that show up on a FR sweep?? or do we need 'sneakier' types of measurements)

I think I will just quickly knock up some rough test boxes, and play the two side by side. If I am gonna try this, I may as well do something exotic like your pic (else why bother? )

so quick question, do I need the same internal vol behind the driver as I would have had in a rectangular box? I only need 6-10 l or so, not a big deal.

[johnathanwinter]

i cant really remember but i believe the same thing can be done with mounting the speaker to a piece of pvc pipe and then filling it with denser and denser insulation to the end. i believe it has the same effect.

(off topic but kind of related to insulation. by taking a smaller box and stuffing more insulation in it the properties of the box change and act like a larger box. am i correct about this? or am i misunderstanding something i read.)

[Kal Rubinson]

i cant really remember but i believe the same thing can be done with mounting the speaker to a piece of pvc pipe and then filling it with denser and denser insulation to the end. i believe it has the same effect.

(off topic but kind of related to insulation. by taking a smaller box and stuffing more insulation in it the properties of the box change and act like a larger box. am i correct about this? or am i misunderstanding something i read.)

Yes. Actually, KEF built a midrange speaker like this in 1967. It had a 65mm dome with an 830mm long corrugated, flexible pipe stuffed with long fiber wool.

Kal

[terryj]

kal, have you heard the nautilus? if so, was there anything 'special' about the midrange, that might somehow back up the marketing claims (I realise it can only really be compered with the same driver, one in a normal box the other in this tapered tube...)

[Kal Rubinson]

kal, have you heard the nautilus? if so, was there anything 'special' about the midrange, that might somehow back up the marketing claims (I realise it can only really be compered with the same driver, one in a normal box the other in this tapered tube...)

I have not really heard them. The only time was brief and informal and more of a viewing experience.

OTOH, when I was into DIY, I customized an IMF Monitor clone I built by doubling the length of the midrange tube enclosure to great effect.

Kal

[MVKFluent]

Check out http://www.bowers-wilkins.com/displa...788&artid=1577 , its a bit sketchy but should get you pointed in the right direction.

[ch83575]

kal, have you heard the nautilus? if so, was there anything 'special' about the midrange, that might somehow back up the marketing claims (I realise it can only really be compered with the same driver, one in a normal box the other in this tapered tube...)

I probably don't have Kal's experience, but I have spent some time (1-2 hr in store demo) with the 800D which has a closed transmission line on the midrange, but not the woofer as in the nautilus. I have to tell ya, it isn't a speaker I would hold up and say "this is the reason we need closed transmission lines on all of our midranges." I find it far to forward in the upper midrange and lower treble. I don't know if it is a linear error or a non-linear distortion that I am reacting to, but if I had to guess I would say that it has something to do with the fairly high crossover to the tweeter (4k). After that it could be the directivity of the large midrange operating up to that point, any linear corrections they have tried to make to adjust for the increased directivity under the crossover point, or just plane old non-linear distortion setting in as you approach the kevlar woofers resonant peaks. In any case I find the 800D and most other B&W speakers I have heard very fatiguing.

After all that, I think I should weigh in and say that I think a backwave suppression design would be well worth the effort. I am under the opinion (though I don't have any specific measurements to back it up) that the backwave being reflected through the cone is a fairly large contributor of tonal character in a driver or system. I say driver because the reflections off of the chassis and motor systems are so physically close to the original source that their time delay when reaching our ears is so low that our brain does not distinguish them from the original soundwave. The reflections off of the back of the box in a standard speaker are also within this range (it extends to about 1' to 1.5' (less than 2ms) according to most studies) but closer to the outside of the range, perhaps making them even more destructive. I think that box designs that physically suppress the backwave as well as driver designs that do the same (new Illuminators are the best example) will have very audible effects that are very difficult to measure!. That is where you get on thin ice... especially around here :W

Sorry for the tirade,
-Chad

[terryj]

it was a bit 'silly' to ask that question. we can only tell the worth of the tl chamber (if that is what it is on the b&w's) by hearing the same driver in a variety of boxes, which is what I'm slowly resigning myself to do.

I am kinda curious now it's popped into me 'ead, could turn out to be a waste of time and effort.

but I will have satisfied my curiosity one way or the other.

you did raise a point I hadn't considered, maybe a cheap and easy thing to do is to treat the basket and magnet assembly. may not do anything either, but that at least is quick and easy. at the least it should not do any harm.

Ohh, it's that sort of site is it?

good.

[ch83575]

Don't worry. I consider myself the king of inconclusive measuring tests. I have two of them posted on my website: http://www.cmhaudio.com . I haven't finished writing my synopsis of the plinth experiment, suffice to say I could find no measurable quality to correlate with the listening tests. Likewise, I know the H1212 tweeter sounds different (maybe not better... but surely different) with the phase shield removed, but can I find any difference with my microphone that is within my range of hearing? Nope!

I for one would be very interested in any investigation you do on the backwave dilemma, so be sure to post any findings. I truly believe you will hear differences that you will not be able to measure. After all, if our ear and brain can not tell the difference between the reflected sound and the original sound within the first 1-2ms how can we expect to separate the two in our measurements? After all our ear/brain mechanism has been developed over millions of years of evolution to distinguish minute spacial and timing cues in order for us to survive. I think it is naive to think that a Beringer mic and soundcard can come close to being able to resolve all of the information our ear/brain does. Specifically, gateing is ineffective at these times scales as the resolution would not extend low enough in frequency to show the reflections we are looking for. Perhaps we could use a 3d array of microphones and a spacial correlation algorithm to find what part of a signal originated on the cone surface and what reflected off of an internal structure, but that would still be difficult because any internal reflections must be reflected back through the cone. I know that some of this information should be available through a 2d FFT like a CSD. But I do not think that a standard CSD would resolve a single reflection of sound off of an internal structure, it seems to me that it would be swamped by the energy storage of the of any number of resonant systems in the driver or system. But does that mean it is not audible? I don't think it does. Our brain is extremely sophisticated and very good at detecting and ignoring some types of energy storage and discriminating between reflections and reverberations and the original soundwave, but only after aprox 2ms (I personally think it is probably a bit lower than that, maybe between 1-1.5).

One place I see evidence of this is in current driver designs. Look at the recent evolutions of drivers from SEAS and Scan. Each are working very hard to minimize motor and chassis reflections. But we as a DIY community are so obsessed with our microphones measure the new designs and say "they don't measure as well as the old ones" sometimes we ad "at least not enough to justify the price increase." Do we use them and then make our final decision based on finish system evaluation? No, not usually. I personally feel that this behavior is a very off hand way to totally discredit a lot of hard work done by a lot of smart people.

Now I am really sorry for my tirade, and I really don't want to start a flame war. My only point is that while the things that go on in "other forums" thankfully don't hold much weight here, perhaps we are a bit dogmatic as well. I, like everyone here, wish I could measure everything with my microphone in order to quantify it and design for it, I am just not convinced that I can.

-Chad

[Dennis H]

A bit like hanging f/glass panels in your room, if you space them off the wall you get better absorption. Or the asc tube traps. The sealed internal void very quickly gets a pressure differential between it and the room, and so provides the pressure equalization gradient. (or so says the blurb)

I guess the limiting factor here is box size?? But as a theoretical discussion, would it not be way better to space any absorption material off the box walls?

It's true that room absorbers work better if spaced off the wall but that's a bit deceptive. A 6" panel spaced 6" off the wall works better than a 6" panel right on the wall. But (big but) a 6" panel spaced 6" isn't as good as a 12" panel right on the wall. If your budget is limited to 6" panels, they work best spaced from the wall. If you have 12" of space to use, you'll get the best absorption by filling the whole space with fiberglass. As for the claims for ASC tube traps, independent testing has shown that their performance would be improved by stuffing them with fiberglass.

[TacoD]

Vivid Audio is also doing the same. One of best sounding speakers I've heard.

[Kal Rubinson]

I probably don't have Kal's experience, but I have spent some time (1-2 hr in store demo) with the 800D which has a closed transmission line on the midrange, but not the woofer as in the nautilus. I have to tell ya, it isn't a speaker I would hold up and say "this is the reason we need closed transmission lines on all of our midranges." I find it far to forward in the upper midrange and lower treble. I don't know if it is a linear error or a non-linear distortion that I am reacting to, but if I had to guess I would say that it has something to do with the fairly high crossover to the tweeter (4k). After that it could be the directivity of the large midrange operating up to that point, any linear corrections they have tried to make to adjust for the increased directivity under the crossover point, or just plane old non-linear distortion setting in as you approach the kevlar woofers resonant peaks. In any case I find the 800D and most other B&W speakers I have heard very fatiguing.

With a trio of 802Ds in my living room, it should be obvious that I have different opinion and, as set up in my room, do not hear the problems that you do. The behavior of the Kevlar mid is unconventional and controversial and I do not know if the usual "modes" of analysis apply. It is supposed to 'break up' but in a more controlled pattern
than with rigid cones. Nonetheless I am not sure any of this has anything to do with the enclosure except that a flexible cone might be a more critical test of back wave problems than would a conventional rigid cone.

Kal

[jkrutke]

I think that box designs that physically suppress the backwave as well as driver designs that do the same (new Illuminators are the best example) will have very audible effects that are very difficult to measure!. That is where you get on thin ice... especially around here :W

Sorry for the tirade,
-Chad

Holy cow Chad, you really have something to get off your chest. No problem, everyone is welcome to their opinion. :B

Now here's my opinion. Very audible effects to me are generally very easy to measure and very easy to see in the measurement. But also, nearly inaudible effects are easy to measure and still at least visible. I can measure and see the differences in well damped tapered enclosures compared to well damped rectangular enclosures, but I can't hear them. Do I like tapered enclosures? Yes, but mostly as a style statement. I don't bother building tapered enclosures that will be hid inside a box because I don't see the point. I do reserve the disclaimer that maybe there really is an audible difference, but for some reason I just haven't noticed it in any of the 6 tapered enclosures I've built so far.

Also, as a minor side comment I've generally been impressed with B&W's higher end offerings that I've heard, and that's without the excellent build quality and looks swaying my opinion.

[terryj]

As for the claims for ASC tube traps, independent testing has shown that their performance would be improved by stuffing them with fiberglass.

thanks dennis for the amplification os spacing off the walls.

the last statement made me smile, so a lot of marketing in there you reckon?

any link?

[terryj]

I can measure and see the differences in well damped tapered enclosures compared to well damped rectangular enclosures, but I can't hear them.

John, can you comment on these series of measurements a guy has done??

http://www.diyaudio.com/forums/showthread.php?postid=1782412#post1782412

[ch83575]

Holy cow Chad, you really have something to get off your chest. No problem, everyone is welcome to their opinion.

Wow! I read through my comments and you are right. I am really not that much of a d-bag normally (at least I hope not), I think I am usually a pretty reasonable dude. Must have been a bad day or something, sorry about that. Just to be clear, here is my belief about measurements in a more reasonable tone: I think that there are some things that are resolved more clearly by a measurement system than the ear. Absolute amplitude at a particular frequency for instance, or electrical impedance, which is only an indirect way of illustrating things that may or may not be audible. Then there are things that the ear/brain mechanism is much more accurate at measuring than a computer, not that the computer can't try, its just that our brains have much more advanced way of gathering, processing and interpreting some types of data than a computer. In my eyes these things are mostly phase related; spacial cues, reflections and reverberations and others. After all, that is a big reason why we have two ears. But, I am glad to leave the rest of that discussion for another time and place as this is about enclosures.

John, can you comment on these series of measurements a guy has done??

http://www.diyaudio.com/forums/showt...12#post1782412

Great find terryj, that looks like a very compelling study to me. I guess I need to learn more about a wide variety of measuring techniques before saying something is difficult as in this case it looks like it wasn't very difficult after all ops: . I too want to hear the experts view on the matter, but I would also like to verify that I understand how these tests might be working. By moving the mic very close to the cone you can detect the standing waves by making their phase different enough from the cones radiation as to cause cancellation? At 1m these effects are still there but don't really look like much because they are way down in level and now the phase difference is so small that they basically sum with the signal? Now of course none of this is proving that the effect is audible :W , but I find the progression of changes in the results very compelling indeed and would love to hear a more experienced perspective.

-Chad

[ch83575]

Check this out!

http://home.wanadoo.nl/dezaire/ultiem/ultimate.htm

It is the project that the study linked to above from DIYaudio was done for (or so it seems. Looks like the same drivers he referenced in the post).

-Chad

[terryj]

actually chad, I just want it made clear that I too am no expert!! I mean I can measure the response of my speakers etc, but that's it!

Would love to hear Johns perspective on it, so I can get it clearer in my head.

That link was the starting point, but did you check out this build?

http://picasaweb.google.com/collier6/Concave?feat=directlink#

I guess when you have a cad router, you may as well use it right?? Love the crossover board.

Anyway, back to this topic, have a look at the midrange cavity.

It is a lot of work for thee and me (I am assuming you don't have a cad router either ) so again, the big question is if it is worth it (the effort)

Data seems to be thin on the ground really (which is why those graphs are particularly interesting?), so I think it is time to do some listening tests and experiments.

[joecarrow]

It is my understanding that a box speaker with inadequate internal absorption will exhibit increased linear distortion separate from the linear distortion of the unloaded cone. The frequencies at which this may occur should be fairly sharply defined by box geometry, and should have a time delay before the delayed sound is present. I believe that Linkwitz went over this somewhat in the development of his Pluto speaker.

The audible result I would expect, although I've never had a listening room adequate to really hear my speakers, is that any instrument or vocal to excite these specific frequencies (somewhere in the hundreds of hertz, where absorption material is least effective and box geometry allows resonances, and rear walls may be large fractions of a wavelength from the cone) will sound less defined. I'd expect it to sound kind of "boxy", not quite an echo, but a coloration to the sound.

Overall, I'd say that a lot of things would have to be done right before you need to start worrying about the midrange transmission line.

[BigguyZ]

That link was the starting point, but did you check out this build?

http://picasaweb.google.com/collier6/Concave?feat=directlink#

I guess when you have a cad router, you may as well use it right?? Love the crossover board.

I don't want to drag this OT, but I have to say that although I think the laminate he used is a pitty, when there are so many nive veneers available. But I wish some day I can do something that cool/ extreme.

That's just awesome.

[orbifold]

+1 on the laminate
+1 on the awesome--er maybe shock and awe level

did you notice the real Nautilus (not an 800 series?) :E:E

end of OT :roll:

[terryj]

I too thought the laminate, after all that work, was a shame. He doesn't do it halfway, imagine the effort to get those things up on that stand to measure them.

I'm too lazy to even haul my speakers to the midpoint of the room!

That nautilus look alike he built? Eeyuk I'm afraid, the same reaction to the original.

Overall, I'd say that a lot of things would have to be done right before you need to start worrying about the midrange transmission line.

That is my concern! A lot of effort for maybe little return over simply good damping? Still, I do like to test 'theories', and will be able to at least report back my findings, dunno if it will be of interest.

One point on that tho, whilst acknowledging your point, I have in fact done a lot of work and this is the final decision to be made. (almost wish it hadn't popped into my head...but would have been worse to be left wondering after I'd finished them I guess)

[terryj]

this has been given as a link to the b&w white papers on the topic

http://bwmedia.keycast.com/download/Libraries/3/800s_l2_w0_h0_2.pdf?dm=633108447502000000

I tried downloading it, and got gobbledegook. Others who know computers better may get readable results.

In any case, here is the 'formula' that the guy used for the midrange cabs in the build above

In brief:
The internal volume is based on an IB Q=0,5 for the specific driver (notice that this is different from the B&W whitre paper).
Then the pipe is calculated as closed transmission line, with a mouth area which is identical to the cone area of the driver (devided by 2 since there are 2 lines).
The shape of the internal is designed that way that most of the waves are guided into the dampened lines.
The goal here is to lower the standing waves and decrease the backwave to the cone.

[AJINFLA]

Hi Terry,

Welcome aboard. An article from someone in your neck of the woods
Volume filling a reflex box.
Don't forget to look at what happens on the other side of the cone, when mounted in an enclosure...then placed in a reverberant room.

Btw, Chas, consider this. Everything your ear/brain "hears" listening to recorded music, was captured by a microphone (with the exception of any signal chain distortion).

cheers,

AJ

[Dennis H]

this has been given as a link to the b&w white papers on the topic

http://bwmedia.keycast.com/download/Libraries/3/800s_l2_w0_h0_2.pdf?dm=633108447502000000

I tried downloading it, and got gobbledegook. Others who know computers better may get readable results.

Works for me. It's a .pdf file titled "Development of the B&W 800D". Pretty interesting reading. You need Adobe Reader installed.

[terryj]

I used adobe, but kept getting rubbish. However someone sent me a link to a different reader program and that did the trick, so can read it now.

Thanks AJ, Rod elliot has a site well worth browsing.

It seems that data is a bit hard to find, so I will 'bite the bullet' and build a few different chambers and mainly just have a listen and see what turns up.

I may try a few nearfield measurements as well, but will mainly be concentrating on the listening aspect. In audio I'm really a bang for the buck guy (tho in this case we're not talking bucks but time to build), I simply cannot be bothered with straining to hear a slight difference (usually not better or worse, just different) so unless there are obvious and immediate improvements I won't spend much time auditioning each different box. Any obvious improvement of a particular box construction then stays, and the next box is put on the other one, till at the end we have the 'last man standing'...bit like a pool player who keeps the table until he loses, then that player keeps the table.

Worst case scenario is that it simply will not be worth the effort for these fancy schmancy chambers (esp as in my case the shape will be hidden inside the box), but still as a learning experience that is valuable.

will let you know.

[Kal Rubinson]

Works for me. It's a .pdf file titled "Development of the B&W 800D". Pretty interesting reading. You need Adobe Reader installed.

I have the book somewhere. :W

Kal

[A9X]

Hi TJ,

I'm looking forward to hearing the results of your experiments. If I can finally get my SC to work with the MoBo and V64, I'll bring up Soundeasy and see if we can find out anything more via CSD and Z plots.

I have a bit of spare time now eh? Thanks to Mr40%.

[terryj]

realised that if it's gonna happen, it will be soon. Am on to the front baffle now, and depending on how big the back chamber is it could affect how close I can get the tweet to the mid, another factor that might have more bearing on the sound than any benefits from the chamber itself.

all that means is that for the build to continue I need these answers, then on to the baffle.