I believe the gap has to be wider than a wavelength - or at least 1/2 of it. If it is narrower, it will be as effective as a cattle grid wrt an elephant walking by.
In that logic a 2" gap would disappear/close for anything lower than 5 kHz.
Well, here is an opportunity for some testing with cardboard!
Even a narrow slot relative to the wavelength will have some effect of making the response more like a dipole, i.e., it'll be leaky. the effect of the size of the remaining baffle will also play a role. I would not dismiss it as having to be 1/2 lambda to work.
X, Can you simulate this in AkAbak?
Glad I had a look-in to see if there were any new goings on... I bought a 4' x 8' sheet of 1" thick MDF, which was cut into four 2' x 4' pieces in store. Two pieces I've already cut down to 20" x 38", which was the basic baffle size I wanted.
From what's been said, no real point in me trying any cut-outs in the baffle yet... It appears that having a narrow baffle to help directivity would necessitate a larger number of drivers, at least a FR & mid woofer. Then we'd end up with something closer to the likes of the Orion or LX521 etc. Yes?
From what's been said, no real point in me trying any cut-outs in the baffle yet... It appears that having a narrow baffle to help directivity would necessitate a larger number of drivers, at least a FR & mid woofer. Then we'd end up with something closer to the likes of the Orion or LX521 etc. Yes?
That is a great question. I can simulate most things but AkAbak can only simulate baffle edge effects for a simple baffle. I don't know how to simulate a gap then a baffle. That will require a 3d capable acoustic sim program. There may be a workaround like superimposing two baffles on top of one another. I will have to think about it.
Rudolf was suggesting that we let the woofer cross higher up. But then you lose the point source effect if the woofer creeps up above 300 Hz - it becomes more directional. That is, no longer a FAST.
Just downloaded ABEC3 demo from R&D team. Seems like it would do the job but... This is not easy to use!
Heavy learning curve.Cutting foam core and measuring would be quicker and, I'll bet for this problem, more accurate.
I could not get AkAbak to run even in compatibility mode on my 64bit Win 7 system. I did not research how to get the 32-bit emulator working. So I cant compare the two.
Anyway ABEC3 completely script driven, even the drawing- just that alone is a pain in the ***. But perhaps AkaBak is the same way and you have already mastered this? I was able to run some of their ABEC3 examples. But even that was not intuitive. I guess the nice thing is since the scripts are just text files the fact that you can't save the results in the demo version is not so important because your model is all text and you don't lose that.
But what does it say about us, that this is what we are doing on a Friday night?
That is why we run basic test cases with known results. Like sealed, bass reflex, TL, then BLH, etc.
AkAbak is all script based but that is where the power is. You can resuse much of the code by copy and paste - so new designs are all variants of old design of the same alignment type. There is a learning curve but start with a sample MLTL and BLH script and you can do most things from there. For the Karlson I had to develop some totally new code to model the aperture though.
An OB or slot OB is pretty basic though. There is the limitation of a basic rectangular baffle though.
AkAbak is all script based but that is where the power is. You can resuse much of the code by copy and paste - so new designs are all variants of old design of the same alignment type. There is a learning curve but start with a sample MLTL and BLH script and you can do most things from there. For the Karlson I had to develop some totally new code to model the aperture though.
An OB or slot OB is pretty basic though. There is the limitation of a basic rectangular baffle though.
Try to understand that the acoustic wave is a pressure vector that for the most part moves parallel to the baffle. It is not like a pressurized tank, where the pressure is acting in all directions with the same force. Sure there will be a small leak at the narrow slot, but most of the sound will run over it like the wind over a harvested field.
Wind and Elephants...
Rudolf,
I love your analogies! Or perhaps they are Deutsch idioms? Very witty.
And what you say is true, at some frequency the slot becomes opaque. However, there is also a vector on the backside with the reverse pressure and my conjecture is this makes the width of the slot more significant. I'm not up to doing the analytical math; perhaps X can model it. I hope to get around to running an experiment in a couple of weeks where I keep cutting the slot wider about a fixed size baffle and see what occurs.
I'm pretty sure more of the elephant slips through than you expect.
Rudolf,
I love your analogies! Or perhaps they are Deutsch idioms? Very witty.
And what you say is true, at some frequency the slot becomes opaque. However, there is also a vector on the backside with the reverse pressure and my conjecture is this makes the width of the slot more significant. I'm not up to doing the analytical math; perhaps X can model it. I hope to get around to running an experiment in a couple of weeks where I keep cutting the slot wider about a fixed size baffle and see what occurs.
I'm pretty sure more of the elephant slips through than you expect.
Hi everyone, I spent yesterday cutting and gluing MDF... luckily the gluing wasn't because I'd cut to much off
The build I've gone for is the basic MJK OB design, will be using an active 2-way system for XO and power... From MJK's pdf, the Eminence Alpha 15A XO is 2nd Order at 200Hz, the FR unit's XO is 2nd Order at 500Hz.
Just need to go buy a handful of woodscrews this morning to mount the Faital Pro 4FE35's onto the baffle, then the fun can begin... listening to music & maybe some measurements also
Paul.
The build I've gone for is the basic MJK OB design, will be using an active 2-way system for XO and power... From MJK's pdf, the Eminence Alpha 15A XO is 2nd Order at 200Hz, the FR unit's XO is 2nd Order at 500Hz.
Just need to go buy a handful of woodscrews this morning to mount the Faital Pro 4FE35's onto the baffle, then the fun can begin... listening to music & maybe some measurements also
Paul.
Attachments
Paul,
Great looking build there! Can't wait to hear of the listening impressions, and of course, measurements from REW! I see you have a gap between baffle and floor - is that in the MJK design? You may want to close that up because you will lose your floor reflection symmetry and lose some bass gain. I like how you rear rebated the mounts for the Faital Pros - they are kind of ugly when front mounted. You will be one of the first in this forum to report on the 4FE35's - it might be the new FR driver to use compared to the Vifa TC9.
X
Great looking build there! Can't wait to hear of the listening impressions, and of course, measurements from REW! I see you have a gap between baffle and floor - is that in the MJK design? You may want to close that up because you will lose your floor reflection symmetry and lose some bass gain. I like how you rear rebated the mounts for the Faital Pros - they are kind of ugly when front mounted. You will be one of the first in this forum to report on the 4FE35's - it might be the new FR driver to use compared to the Vifa TC9.
X
The .pdf doesn't show any pictures of the build. However, in the projects page of MJK's site, he has a few photos of this build which is raised by a similar amount ~1.5" ... Link
I can always add a section to fill the gap if needs be... or I can cut the plinth off and flush mount it to the base of the baffle
The 4FE35 are uglier than the TC9FD... the FE's woven cone doesn't look very rigid but to the touch is, the proof as always is how they sound. Just about to wire them up and might get a first listen before going to my parents for the rest of the day... Probably let them run in for a week before I make any measurements & listening impressions.
Think I may have some spare MDF in my parents garage, might give the baffle with the U shape a go. Like Z, suggested, start with a smaller cut-out and gradually increase the sizing. Be interesting to see how it effects things. Can do this with the Vifa TC9 to keep it relevant to Rudolf & Z's bits.
I can always add a section to fill the gap if needs be... or I can cut the plinth off and flush mount it to the base of the baffle
The 4FE35 are uglier than the TC9FD... the FE's woven cone doesn't look very rigid but to the touch is, the proof as always is how they sound. Just about to wire them up and might get a first listen before going to my parents for the rest of the day... Probably let them run in for a week before I make any measurements & listening impressions.
Think I may have some spare MDF in my parents garage, might give the baffle with the U shape a go. Like Z, suggested, start with a smaller cut-out and gradually increase the sizing. Be interesting to see how it effects things. Can do this with the Vifa TC9 to keep it relevant to Rudolf & Z's bits.
Is this of real relevance? I just measured (from the listening position) the difference between my dipole standing on the floor and lifted 2" exactly:
The difference is 1 dB max. Perhaps I should not have raised the whole speaker but only closed/opened the gap below.
Rudolf,
Thanks for making that test with speaker raised. If your speaker has a lower frame that was raised also it will probably be better than a gap. It's probably no more than a 1 dB difference as you say so I guess not a big deal and Buildmesomething has nothing to worry about.
What are you using for your woofer and baffle? That is pretty good bass at 107 dB and 35 Hz.
Thanks for making that test with speaker raised. If your speaker has a lower frame that was raised also it will probably be better than a gap. It's probably no more than a 1 dB difference as you say so I guess not a big deal and Buildmesomething has nothing to worry about.
What are you using for your woofer and baffle? That is pretty good bass at 107 dB and 35 Hz.
You make a point there. I raised a H-frame, which might be less affected by the gap than a plain baffle.
Forget about those 107 dB. My measurements are not calibrated.
Never mind. I see now that your xover is active./
I too am interested in that faital fullranger. I wonder if it's high qts
would enable you to cross lower. Please try and report back. Of course it will work at the org. xover points, but I think that if you were to x it lower,
one would not hear such a big jump between bass and midbass notes (such as a bari sax jumping from octave to octave). I was bothered by hearing one note from the bass driver and the next note from the fullranger. The SQ and
the FR were great.
please report back. Maybe try 100 L P and 200 HP if that does not result in too much excursion on the fullranger. thanks
sundbyd2@gmail.com
I too am interested in that faital fullranger. I wonder if it's high qts
would enable you to cross lower. Please try and report back. Of course it will work at the org. xover points, but I think that if you were to x it lower,
one would not hear such a big jump between bass and midbass notes (such as a bari sax jumping from octave to octave). I was bothered by hearing one note from the bass driver and the next note from the fullranger. The SQ and
the FR were great.
please report back. Maybe try 100 L P and 200 HP if that does not result in too much excursion on the fullranger. thanks
sundbyd2@gmail.com