Rabbitz,
Thanks for taking the time to get that info, much appreciated 😉
I quickly modelled the XLS in that box in WinISD pro beta, and got slightly different results. ~60 m/s in the vent with 300w input with a 65mm vent. Hmmmmm interesting. Also managed to get <34 m/s with 90mm vent.
Still, the point is, B&W got it right with their design, if they can get a 65mm vent to do what I would have used a 90mm vent for. I will definately be exploring this option!!!
The tuning at 29 Hz seems a bit high, I wonder if they are in fact using the downfiring vent to lower tuning, as 25 Hz looks a bit better.
Thanks for taking the time to get that info, much appreciated 😉
I quickly modelled the XLS in that box in WinISD pro beta, and got slightly different results. ~60 m/s in the vent with 300w input with a 65mm vent. Hmmmmm interesting. Also managed to get <34 m/s with 90mm vent.
Still, the point is, B&W got it right with their design, if they can get a 65mm vent to do what I would have used a 90mm vent for. I will definately be exploring this option!!!
The tuning at 29 Hz seems a bit high, I wonder if they are in fact using the downfiring vent to lower tuning, as 25 Hz looks a bit better.
OK get this Paul which makes it even more remarkable:
On your sketch-
58 is actually 62
23 is actually 29
253 is get this 162
Total length including flares 253.
I did my modelling in LSPCad lite so maybe a bit different and I haven't worked out how to model this port. I had a rough idea of the Fb in the box it came out of so to lower the Fb I made some assumptions and increased the box size.
So Vb goes up, Fb goes down with the same port...... Newton's 18th law maybe ;D
On your sketch-
58 is actually 62
23 is actually 29
253 is get this 162
Total length including flares 253.
I did my modelling in LSPCad lite so maybe a bit different and I haven't worked out how to model this port. I had a rough idea of the Fb in the box it came out of so to lower the Fb I made some assumptions and increased the box size.
So Vb goes up, Fb goes down with the same port...... Newton's 18th law maybe ;D
Thanks for clarifying. I'm trying to work out the idea behind the way it is flared. In a flare there are 2 vectors. Supposing x is parallel to the diameter of the vent and y is parallel to the length of the vent. They have made x greater than y. Due to the inertia of air moving parallel to y, I would have expected this dimension to be greater. Any ideas why they would do it this way?
I suspect them paying more attention to the flare underneath is not as this is more optimal, but that the inside flare is just not as important.
I suspect them paying more attention to the flare underneath is not as this is more optimal, but that the inside flare is just not as important.
The inside vent is just one out of their parts bin common with a lot of their speakers and I'm sure the configuration of the external port would be determined from that it is downfiring.
I have noticed that they have gone away from ported subs for most of their models and are using sealed.
I have noticed that they have gone away from ported subs for most of their models and are using sealed.
Yes I noticed their subs are going that way as well. I heard one of their 10" subs which had a fairly high excursion driver and I liked the sound.
Today I did a bit of work on one of my prototype flares, all of them turned up on the lathe, made from MDF. One has an 18mm radius, two of them have 36mm radius (one of which I started experimenting with adding dimples for AB comparison today), and the other is 72mm flare radius. This is for a 100mm diameter vent. Like the B&W flowport vent, I will deviate from a strict radius profile on the flare.
I found MDF isn't a good material to put dimples into. First I tried drilling holes so that only the pointy part of the drill bit goes in. Then I found it was quicker and got the same result with a punch and hammer. Even better will be a custom made punch which has a more suitable shape. For the real thing I will use solid timber as MDF doesn't remain smooth.
I will have to get my amp before I can really test, however ...
I'm hoping that I can get the 36mm flare to sound as quiet as the 72mm flare.
Today I did a bit of work on one of my prototype flares, all of them turned up on the lathe, made from MDF. One has an 18mm radius, two of them have 36mm radius (one of which I started experimenting with adding dimples for AB comparison today), and the other is 72mm flare radius. This is for a 100mm diameter vent. Like the B&W flowport vent, I will deviate from a strict radius profile on the flare.
I found MDF isn't a good material to put dimples into. First I tried drilling holes so that only the pointy part of the drill bit goes in. Then I found it was quicker and got the same result with a punch and hammer. Even better will be a custom made punch which has a more suitable shape. For the real thing I will use solid timber as MDF doesn't remain smooth.
I will have to get my amp before I can really test, however ...
I'm hoping that I can get the 36mm flare to sound as quiet as the 72mm flare.
Oh believe me, I'm having fun with it!
I picked up a golf ball and noticed that the dimples were different. They were shallow and fairly smooth, all almost touching each other. The B&W dimples are deeper and spaced further apart. I have made mine a combination of both.
I bought some elbows for the PVC yesterday, but unfortunately they are not as smooth as they could be inside - they don't fit flush (what were they thinking???)
I'm making another larger vent to dimple as well ... the real version (if I find this thing works) will be solid timber, which handles dimples better than MDF.
I purchased the Behringer Europower amp with 650w for each sub ... hopefully pick it up today so I will have some serious power to test these vents with 😀
I picked up a golf ball and noticed that the dimples were different. They were shallow and fairly smooth, all almost touching each other. The B&W dimples are deeper and spaced further apart. I have made mine a combination of both.
I bought some elbows for the PVC yesterday, but unfortunately they are not as smooth as they could be inside - they don't fit flush (what were they thinking???)
I'm making another larger vent to dimple as well ... the real version (if I find this thing works) will be solid timber, which handles dimples better than MDF.
I purchased the Behringer Europower amp with 650w for each sub ... hopefully pick it up today so I will have some serious power to test these vents with 😀
This is exactly what I'm doing. 2 flares with 36mm radius - one dimpled, one regular and the same with a 72mm flare radius (doing the 2nd flare today) ... in earlier comparisons I found the bigger flare was better, although the biggest improvement was from an 18mm - 36mm radius.
paulspencer said:
If there is turbulence in a vent, there will be distortion because we introduce an nonlinear lossy term in the system. In fact, the bassreflex system is highly dependent on a flow through the vent that is non-turbulent and that is why we flare the mouths. Now, introducing dimples is a way of ameliorating a situation when a flow actually separates. A golfball spinning through the air has a fully developed turbulent field, separation and wake - quiet the opposite of the situation we want from our vent. My take on the B&W dimples is that hopefully their function is pure cosmetic. Otherwise, the vent would not be optimally designed since it would allow the flow to separate and hence the formation of turbulence. But it looks nice.
/Kranis
Get ya dimples here!
If you live in aus. you can buy the 1.25litre bottles of "Sprite" lemonade. They have dimples. If you cut carefully, you get 14 wedge shaped strips, each with 7 dimples graduated in size. These could be glued inside a port for experimentation. Should look OK too!
If you live in aus. you can buy the 1.25litre bottles of "Sprite" lemonade. They have dimples. If you cut carefully, you get 14 wedge shaped strips, each with 7 dimples graduated in size. These could be glued inside a port for experimentation. Should look OK too!
Kranis,
Thanks for the comments!
You are not the first to comment on distortion of vented designs, but the only distortion measurements in a comaparison of vented and sealed subs showed all the vented subs with much lower distortion, especially in the bottom octave where the vent reduces cone excursion. Distortion is in fact in favour of vented designs as the non linearities of the driver at higher excursions is far greater than anything from the vent if reasonably designed. Even at high SPL.
B&W have a write up on the flowports on the paper on the development of the Nautilus, including a full page description of the development of the technology. It is very much a performance thing. As I understand, the small eddies created actually cause the air to follow the shape of the flare, rather than separate. Hence they create very small eddies to in turn prevent a very large one which is what causes large scale turbulent flow.
Sounds quite convincing. But based on Rabbitz comments on his vent and my own understanding of the theory, dimples do work in vents. I will find out, not so much if they work, but if MY attempt of dimples works this week.
In their smaller speakers you are probably right - they don't need dimples. They do look cosmetic only on such speakers.
Collo,
Now there's an idea "out of the box!" They could also be used as a press. I could put clay over my MDF flares then press those dimples in. I could then use it to make a mould for a fibreglass flare!
Hmmm sounds like a bit of extra difficulty .... still I might just have a look at those sprite bottles.....
This would make a great diy audio add ...
"can I help you sir?"
"oh, just looking for a soft drink bottle for my subwoofers"
"for your subwoofer???!!!"
"yes, it prefers dimpled bottles over the ordinary ones ..."
(strange look from storeman)
Then the blurb comes up ....
diy audio
...
projects by fanatics, for fanatics!
Thanks for the comments!
You are not the first to comment on distortion of vented designs, but the only distortion measurements in a comaparison of vented and sealed subs showed all the vented subs with much lower distortion, especially in the bottom octave where the vent reduces cone excursion. Distortion is in fact in favour of vented designs as the non linearities of the driver at higher excursions is far greater than anything from the vent if reasonably designed. Even at high SPL.
B&W have a write up on the flowports on the paper on the development of the Nautilus, including a full page description of the development of the technology. It is very much a performance thing. As I understand, the small eddies created actually cause the air to follow the shape of the flare, rather than separate. Hence they create very small eddies to in turn prevent a very large one which is what causes large scale turbulent flow.
Sounds quite convincing. But based on Rabbitz comments on his vent and my own understanding of the theory, dimples do work in vents. I will find out, not so much if they work, but if MY attempt of dimples works this week.
In their smaller speakers you are probably right - they don't need dimples. They do look cosmetic only on such speakers.
Collo,
Now there's an idea "out of the box!" They could also be used as a press. I could put clay over my MDF flares then press those dimples in. I could then use it to make a mould for a fibreglass flare!
Hmmm sounds like a bit of extra difficulty .... still I might just have a look at those sprite bottles.....
This would make a great diy audio add ...
"can I help you sir?"
"oh, just looking for a soft drink bottle for my subwoofers"
"for your subwoofer???!!!"
"yes, it prefers dimpled bottles over the ordinary ones ..."
(strange look from storeman)
Then the blurb comes up ....
diy audio
...
projects by fanatics, for fanatics!
I will keep you guys posted on what I actually find. Hoping to pick up my Behringer amp today, along with Ultracurve 😀
I wouldn't knock B&W's Flowport designs as they are one of the manufacturer's who aren't afraid of trying something different and have great designers. Even though their speakers look conventional on the most part, the technology isn't. There is an additional cost in adding these ports and from a commercial point I can't see their bean counters letting them if it didn't work.
I have never heard a bad sounding B&W speaker and have a lot of respect for B&W. I'm amazed what they can do with some quite ordinary drivers and boxes.
I can't comment on all the theories behind the Flowport but can say that I've got a ridiculously small port that should chuff like crazy but doesn't and yes I have tried a normal port which was a bit larger and guess what, it chuffed and huffed like the big bad wolf.
I have never heard a bad sounding B&W speaker and have a lot of respect for B&W. I'm amazed what they can do with some quite ordinary drivers and boxes.
I can't comment on all the theories behind the Flowport but can say that I've got a ridiculously small port that should chuff like crazy but doesn't and yes I have tried a normal port which was a bit larger and guess what, it chuffed and huffed like the big bad wolf.
I'm inclined to agree with Rabbitz. If I were to buy comercial speakers, B&W would be high up on my list of speakers to listen to. I'm a bit biased by style issues, I love the industrial design, which is something I have some interest in. The only thing I don't like about their speakers is I would prefer softer treble. If I had one of their speakers I'd probably put an Lpad on the tweeter. I have metal tweeters and found I liked the sound with Lpads - they aren't too bright anymore.
Getting back to my vent options, of the 3 I posted in the image, I like the one on the left. Can anyone see any problem with the location of the inside vent opening being so close to the driver?
Getting back to my vent options, of the 3 I posted in the image, I like the one on the left. Can anyone see any problem with the location of the inside vent opening being so close to the driver?
A vented system do lower distortion and increases the dynamic range, no question about that. It is such an elegant thing that I think one could use the principle on the basis of technical beauty alone. The question whether the dimples on the B&W speakers are only cosmetic or not will probably remain empirically unsolved since it would entail a rebuild of the flares on an actual speaker. Businesswise, the dimples do provide a unique selling point, which at the same time implies that other manufacturers' designs are flawed in this respect. While I hold the B&W products in high esteem, they do live on a competitive market. So, from a DIY point of view, there may not always be technical reasons behind a commercial design. However, this is DIY audio, so never mind. I for one look forward to here about your empirical results.
/Kranis
/Kranis
I think Rabbitz comments indicate pretty well that it works. It should be obvious when you have a 65mm vent that is tuned quite high (higher tuning means higher vent velocity), it would chuff with a 10" high excursion driver (XLS)! If you simulate it, you will find the vent velocity gets well above the acceptable range. I'm trying dimples not to see if they work, but to see if MY dimples work when using available techniques.
Agreed
I'll post soon.
I picked up my amp today. I have 2 x 36mm flare radius (only one dimpled) and 2 x 72mm flare radius (one needs to be completed and dimpled).
This description should be correct if the flow velocities exiting the vent will be at a high enough Reynolds number to become turbulent. Note that flaring the vent will also help prevent turbulence in the majority of the flow. The dimples will also prevent the central flow away from the walls of the vent from becoming turbulent -- as the dimples will trip the boundary layer (i.e. the flow very close to the wall) and cause it to become a turbulent boundary layer. The advantage of the turbulent boundary layer is that its thickness is quite small relatively speaking and it hugs the wall more and thus allows the rest of the flow (the central flow) to flow freely without obstruction.
Going back to the golf ball example, the dimples will also trip the turbulent boundary layer and while there is still flow separation, there is less of it as the flow hugs the ball more and thus there is less aerodyanmic drag. The less aerodynamic drag, the further it can go.
FWIW, my day job is as a mechanical engineer and I have had a lot of exposure to fluid mechanics. I hope this explanation was understandable. The hardest thing for an engineer is to break down something for an non-engineer to understand. 🙂
For you experimentalists, what is often done is to use smoke, e.g. cigarette smoke or some sort of fog, and hold the smoke source close to the vent. Then you can see how much difference there is in the flow for different vents. The fewer eddies in the main flow the better.
One other thing is that there are lots of other ways to trip the boundary layer and basically get a similar result. I think one reason the dimples were chosen by B&W would be it would be relatively inexpensive to make them once you get the right mold. The fine tuning would be in the depth and diameter and spacing of the dimples.
One other thing, if you have a big enough vent you won't have to really worry so much about the turbulence.
A little off topic but I remember a few years back Bose sued a bunch of other speaker manufacturers (I believe Harman international -- JBL, Infinity, Revel, etc. were some of those sued) for using an elliptical vent in a patent infringement case. I wonder what happened with that lawsuit.
A little off topic but I remember a few years back Bose sued a bunch of other speaker manufacturers (I believe Harman international -- JBL, Infinity, Revel, etc. were some of those sued) for using an elliptical vent in a patent infringement case. I wonder what happened with that lawsuit.
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