The baffle is wide enough so this isnt the problem. the opening on the front can be small the it acts as a acoustic lens.
You want to make a small baffle and small woofers result no sub-bass. That is obvious the result.
I have the idea you not know what your doing and by asking here if you made the right choice for drivers it fix the lack theoretical back ground.
By buying expensive stuff, raal, bms it gives you the feeling building a super system but you are fooling your self.
What you need isn't expensive drivers but basic knowledge about simulation and XO building when you want to build a high quality system. Other wise it will be a disappointment.
Helmuth
Sorry I didn't realise you where creating those response graphs, thanks, what software are you using to do that?
You are right in saying, "I dont know what I am doing". But I am trying to learn, it doesnt come easily though.
The Raal was recommended, the Saba I already have & love, the bass is the issue..
I assumed that by having an 20cm opening and a 30-38cm driver behind the opening I would get distorted sound because it is pushing thru the opening.
I am happy to use cheaper drivers as long as they do the job.
The BMS was the only driver I could find that went close to the requirements that I thought I needed.
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You can do what you like I only want to warn for decisions that easily could be prefented.
when the sound pressure has to come through a slit smaller than its own wave length it will bent and have a better of axes. It isn't a advantage for low frequency they already have good of axes, but I do not aspect problems with it.
Hi, just a small question here. I may be misinformed, but I believe I read somewhere that open-baffle / dipole bass does not benefit from room gain at all? The way I understand it, room gain is a function of wave lengths greater than the room dimensions creating positive pressure. However a dipole creates the same negative pressure at the back as it does positive pressure at the front, thus equaling zero net room pressure. I think. Am I barking up the wrong tree here?
I did measure a set of magnepan magnetostatic dipole's there was clear room gain.
you send of sound it will reflect and come back in phase or not depends how far away the back wall is. There is one major advantage of a dipole, at the side of the dipole the radiated power is near null so there are no reflection side effects.
But with room gain I thought at that moment that the woofers are placed near the floor.
I am over asked here.
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Well, that's one more measurement of dipole bass than I've ever made, so you probably have more experience with the topic than I do. I'm just going off things that I read, and my basic scientific understanding/guesswork of how things work.
I think that's boundary gain, not room gain / cabin gain. Has something to do with the fact that the sound waves are only expanding along a smaller angle of steradians, therefore covering less area. and less area --> more pressure. (p = F/A). Cabin gain on the other hand has something to do with wavelengths exceeding room dimensions, and.... well, that's kind of where my understanding ends.
Perhaps somebody else more knowledgeable on the subject could clear it up for me.
If you had a dipole with VERY small dipole length, this would probably be true. But this dipole would have VERY big difficulties to emit any deep frequency in the first place.
If you have a dipole with a rather large dipole length (say a 15") and your ear is very near to the cone, you can hear very deep frequencies even in a very small room.
So yes it is true that a dipole can't pressurize the whole room like a closed box speaker can below the lowest room mode. But the dipole does not immediately stop to contribute at that frequency either.
John Kreskovsky has looked into the details: roomgain
Rudolf
Why do you say that? remember I'm a beginner wanting to learn.
What do you think of the idea of a 38cm driver mounted in a U frame, pushing out thru a 20cm opening infront of the U frame? As I would need to do if I maintained the design I like the most.
Am I correct in saying? that with a passive design, I am better off sticking to a single larger drive (38cm) in a Uor H frame to get the lower frequencies, than using two smaller drivers in a U or H frame?
I have been following this thread. I was trying to find a diplomatic way to break it to you, but that failed. So I will just say it.
You have conflicting requirements and will never find a solution.
Hoffman's Iron Law basically states that you can't have high efficiency, low F3, and a small box. OB makes that worse, not better.
My recommendation is to find one of the OB calculators and play with a number of likely driver suspects, and input different values for D.
I think you will be appalled how quickly a single 12" reaches Xmec at 40 Hz.
Personally, I think you can get to 60 Hz with your design and go IB with 4 by 15", or go for a sealed sub.
Good luck with your project.
Doug
You have conflicting requirements and will never find a solution.
Hoffman's Iron Law basically states that you can't have high efficiency, low F3, and a small box. OB makes that worse, not better.
My recommendation is to find one of the OB calculators and play with a number of likely driver suspects, and input different values for D.
I think you will be appalled how quickly a single 12" reaches Xmec at 40 Hz.
Personally, I think you can get to 60 Hz with your design and go IB with 4 by 15", or go for a sealed sub.
Good luck with your project.
Doug
I dont know, that was pretty diplomatic, sounds like Im trying to defy gravity. which is the sort of information I really need to hear. Thanks for the advice,
David
Can't say I've measured a lot of drivers, but I've yet to measure a low end dipole response that would be predictable from the datasheet with good accuracy. So I wouldn't put too much store in simulations, though Linkwitz' power limited SPL spreadsheet is excellent for roughing out driver selection for a given baffle. You'll need to find the balance between SPL and excursion which satisfies your distortion requirements, but the usuable excursion is probably someplace between xmax / 4 and xmax / 10 depending on how much distortion you're willing to accept. Normalization between geometric xmax and 10% THD xmax is desirable when comparing drivers.
Selenium 12PW5 or 15PW5 would be my choice here given the stated constraints, desire for pro efficiency levels, and lack of need to operate below those drivers' Fs. But as a rule of thumb going up a driver size results in about the same SPL boost as doubling a smaller driver or using an H baffle. If you're thinking three 15s you'll probably find using an 18 results in a smaller speaker. Dipole sub selection is usually xmax dominated and the price/performance ratio on the 18SWS1110 is tough to beat. Not many 18s with lower Rms, either.
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I was hoping to use 2 x 12" per speaker in a H or U frame. I dont need to achieve real LOUD music, I play mostly classical & probably only accasionally play them at preformance levels, also my room isn't huge about 200 square feet or 20m2.
At the monent I have 1 x 15" Altec in a "U" frame and it seems to do almost evervything I need. I haven't got my brain around the measurement side of life yet, so I really dont know what the Altec is achieving, but it sounds mighty fine with the cello playing low notes.
1 x 15" v 2 x 12", hmmmm ......
I don't see 2 x 12" is any less bulky. 1 x 15" can be used on very narrow baffle, too.
Using OB, you'd need every bit of cone area. To get similar or better performance, you need at least the same or as big as possible. And I think one big cone is tidier looking than multiple smaller ones. (Of course that's only my personal opinion.)
And, do I get it right? You want to dump Altec and take Selenium?
No I dont want to dump Altec, there great, I just that I wanted to go to 12" as per my first post. The other issue I have with Altec, is the pair of 15" I paid a lot of money for & a LOT of postage for get here from the US didn't live up to the description that the seller had on ebay, and he had a 100% reputation. I figure I shouldn't have that issue with new speakers, hopefully
I was hoping two 12" would give me the same response, & maybe be a bit more punchy, I dont always play classical. The other issue is that my design (for the moment) has them firing thru a slot. I was hoping that if Linkwitz could get great sound with 2 x 10". Maybe I could get close with 2 x 12".
Frankly said: In your place I would try to understand a little bit more about dipoles before starting such a project. Apart from the obvious sources like Linkwitz Lab or Music and Design you may download a small PDF from my place: the third download button below the red text.
Rudolf
I have been trying, reading is one thing, understanding is another, I have read the Linkwitz site front to back & many others, I will keep reading & hopefully it will sink in eventually. A lot is written for people who already have a general understanding, very little is written with the complete beginner in mind.
I would look at it under two aspects:
1. Is the cone area (including the surround), which is hidden outside the slot, less than 20 % of the whole cone area? In that case I would not bother much.
2. At a crossover frequency < 200 Hz I don`t see any danger of interference.
Apart from that, in my POV, here we are talking about wet hair while sinking in the mid of an ocean.
Rudolf
1. Is the cone area (including the surround), which is hidden outside the slot, less than 20 % of the whole cone area? In that case I would not bother much.
2. At a crossover frequency < 200 Hz I don`t see any danger of interference.
Apart from that, in my POV, here we are talking about wet hair while sinking in the mid of an ocean.
Rudolf
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