Thinking About H-Frames

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cogitech

cogitech

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#1
I am considering following in the footsteps of Mr. King and Mr. Griffin and doing a FROB + H-frame build.

The FR will be Alpair 10.2, and I think I can handle designing the OB for it.

Based on Mr. King's comments in http://www.quarter-wave.com/Project08/Jordan.pdf I think I will go for 15" Goldwoods in the H-frame.

Which brings me to my question; what are the ideal H-frame dimensions for the Goldwood GW-1558?

I have tried to work with Mr. King's free sample worksheets and they are beyond my abilities, so I would very much appreciate it if someone could help me with this.

Also, am I right in assuming this was one of the 15" Goldwood drivers that Mr. King was referring to in the following quote? (emphasis added):

The Goldwood line of drivers from Parts Express contained a pair of 15” woofers and an 18” woofer that all met my needs. After running a number of simulations, the Goldwood GW-1858 18” diameter woofer was selected. A comparison of the Goldwood GW-1858 and the Eminence Alpha 15A measured Thiele/Small parameters is provided in Table 2.
In my opinion, any of the drivers mentioned above would work very well in an H frame design and could be substituted into this type of system. In fact, one of the 15 inch woofers would lead to a further reduction in the external size of the H frame. You would be trading a few Hz of low frequency output for a 3 or 4 inch reduction in the width and height of the H frame.
Click to expand...
 
#2
I have read MJK's OB theory paper and recall the important thing was to select a woofer that is at least 6 to 10 dB more efficient than the full range driver in order to balance the low and high outputs. The Alpair 10's are 87 dB efficient so you will need 93dB efficient woofers minimum, and the other recommendation was to have a high Qts on the woofer around 1+. I think the Eminence Alpha 15a satisfies this with 97 dB and Qts of 1.26. Not sure if the Goldwoods can satisfy these requirements. The thing that I got from reading the paper was that it is a system design and if you go off reservation, things are really different. It would probably be best for someone familiar with MJK's OB sims to run one with your Alpair 10 and an Eminence Alpha 15a to get the baffle dims, cross over specs, etc. Otherwise it's anyone's guess how well it turns out. FWIW, personally, I think use of a nice Alpair 10 in an open baffle is kind of a waste of it's unique capability to provide it's own bass from a nicely designed cabinet. You can achieve the upper end of an OB with many much less exotic or expensive drivers. They just need to be about 90 dB efficient and flat from 100 Hz and up. I can think of many $30 to $40 drivers that would fit this bill.
Sounds like a fun project and glad your understanding wife is ok with an OB. 🙂
 
#4
Thanks for the response, XRK.

My rationale behind this is:

A) Mr. Griffin used the A10.2 as the FR in his project and he is happy with the results.

B) Mr. King used the Goldwood GW-1858 in his project (an 18") aand is happy with the results, but specifically stated that two separate models of 15" Goldwood drivers would be suitable for such a project. To me, it looks like the GW-1558 is one of the ones he considered.

C) I already have the pair of Alpair 10.2.

D) I like the idea of keeping the H-frame size down a bit. According to Mr. King, a 17" H-frame is a reasonable size for one of the 15" Goldwoods.

E) There are better bass drivers out there, but I can afford the GW-1558 and if a Goldwood is good enough for Mr. King, it is good enough for me. Furthermore, my SPL requirements are not extreme.

F) Nicely veneered and grilled, I think the OB + H can look quite nice. Mr. Griffin's are quite attractive. I hope my wife agrees.
 
#5
Jim Griffin said:
The Alpair 10.2 sounds very nice open baffle when crossed over from an H-frame woofer. You cross is in the 150-200 Hz range from the woofer which would likely be a 15" Eminence Alpha for a starter system. My project used a more expensive Eminence driver but it had a DSP active crossover to provide equalization.

My project is at:

http://www.diyaudio.com/forums/full...rk-audio-alpair-10-2-open-baffle-project.html
Click to expand...

Thank you, sir.

I am very, very familiar with your thread. However, as explained, I am intrigued by the prospect of the Goldwood 15" drivers as Mr. King discusses in his paper. The reason why is because they are readily available for a decent price from a Canadian supplier.
 
#9
I am not sure how much this matters, but I think it might be meaningful for me to confide that I spend a minimum of 6 hours a day listening to my system. I am home from work at 3pm and unless something out of the ordinary happens, I am listening to my system from then until I go to bed - which may be anywhere from 9:30 till 11:30. I have thousands of hours on my current speakers after less than 2 years. Over 10,000 songs and I am basically bored of most of them. I may be cooking, or cleaning, or helping the kids with homework, or soldering a new project, but the system is on and playing at a solid level.

A significant build will not go to waste, is what I am saying.

Note: Weekends, summer, and hunting season are different. It's all up for grabs then which means I have to settle for the junky system in my truck and/or trailer most of the time. I'm OK with that, though. 🙂
 
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#10
cogitech,

I'll answer your question on the two Alpair 10.2 projects and talk about my development directions as of late in my discussions below.

My 2011 Alpair 10.2 open baffle project was my first open baffle design and I was really impressed with its sound and capabilities. But open baffle/dipolar sound steps up the woofer size/enclosure to be much larger than an enclosed box speaker. Plus the open back speaker needs to be at least 4-5 feet away from the wall behind the speaker to achieve the best sound. So open back speakers do have some negative WAF attributes which have to be considered.

The Alpair 10/10.2 MLTL project dates back to 2009-2010 and was a natural follow-on to my various earlier MLTL projects. The Alpairs sound very nice in this enclosure and would be acceptable in most living rooms. The sound isn't quite as pleasing to my ears as the open baffle configuration but I could live it as a daily listener if my room and WAF considerations influenced my thinking.

All that being said, one of my current daily listen-to speakers is a step beyond the A10.2 open baffle. It is a MTM with open mids as shown in posting #8 at:

http://www.diyaudio.com/forums/multi-way/231056-looking-mid-woofer-match-raal-tweeter.html

I use the same H-frame woofers as before but used a Linkwitz Phoenix like MTM set-up.

My most recent project is a MTM MLTL using the excellent CSS drivers. This project likely has the best MLTL sound of the various projects that I've built.
The thread for that project is at:

http://www.diyaudio.com/forums/multi-way/226173-triton-mtm-grows-legs-mltl-design.html

Bottom line is that I prefer the sound of open baffle speakers but I would be
happy with a MLTL speaker if my situation limited my space and WAF.

Jim
 
#11
Thank you very much, Jim. I really appreciate your input.

The 4-5 feet distance from the wall requirement is basically a show-stopper. I thought I might get away with 1.5 feet, but it seems I will have to rethink things.

The driving force behind all of this is the desire to take a large step forward in impact and dynamics. I love my dual-EL70 microTowers. They do a very admirable job, especially now that I got my amplification figured out. I just feel that I am often pushing unnatural amounts of bass through them to get the depth and impact that I want.

What I don't want to do is buy a bunch of nice wood and spend a bunch of time building an MLTL with the 10.2 drivers and then end up with slightly different sounding speakers with the same limitations.
 
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