[hrosenba]

Hi,
I'm not a DIY'er.  Have seen some designs (eg Donald North-- DNA Sequence uses 4 cone midrange drivers) that use >1 midrange driver, with or without a dedicated smaller tweeter.  Do the possible advantages (efficiency, room interaction) outweight costs or complexity? I see that Bob Brines has discussed an OB using the Alpair 7.3 with an H-frame 18 inch Goldwood woofer; curious if any advantage in using more than one of the Alpair drivers.
Thanks,
Harvey

[-Richard-]

Hi Harvey ~

I have very little technical knowledge and rely on the generosity and creative intelligence of our incredibly helpful and knowledgeable OB DIY community on Audiocircle.

That said I can share a "philosophical" perspective with you. This OB thread was started with people, like myself, who where interested in making a leap from "commercial" hi-fi/audio, with its hype and high-prices... with its nasty habits of advertising audio as an elite sub-culture of financially "arrived" professionals who use audio as status jewelry... so that we could enjoy music on the highest possible level without spending outrageous amounts of money... with the emphasis being on loving music and the pleasure of music.

The OB paradigm is perfect for us precisely because it is simple and open to a relatively simple DIY approach... even someone like myself with my very limited technical background could feel confident enough... thanks to this wonderful forum... to jump in and experiment.

I mention this by way of sharing an insight with you. Simple is elegant... simple is "direct"... simple is easier to "fix"... easier to understand. So to answer your question from a non-technical point of reference... I would stay away from complex solutions that promise "more"... more of this and that... more straight-as-a-ruler frequency control... more frequency re-produced at the extremes and so on. And instead think about the most elegant simple solution that brings the music to life... "living" music... "alive" music that throbs with passion and emotion and intelligence.

Admittedly, that may seem entirely irrelevant to your question. But for me is the entry point... the starting point of my own excursion into DIY OB magic... and bringing music to life in the home... and beyond that... to learning the easiest and simplest way of doing it that bypasses the commercial trap... and learning more about audio than purchasing an endless series of commercial speakers will ever reveal to you.

With Warmest Regards ~ Richard

[InfernoSTi]

I'm going to be trying a "simple" MTM (mid tweet mid) configuration with my Hawthorne Audio Reference OBs.  It appears that two benefits happen, but I'm waiting until I make the conversion to say if it is true.  I'll be using two 15" SSI mids and one AMT ribbon tweeter per side.

First, the MTM configuration "couples" to create a greater output volume per watt (I believe MTM is calculated at 3 db...from memory so be careful with that).  This creates greater dynamics than just the single driver.  I think the same is true with four drivers coupling, as well.  Second, the greater sensitivity allows me to reduce the padding on the tweeter so it will be a little more direct.  This will add to its simplicity and clarity (or so I hope).

Best,
John

[JohnR]

Hm, the Alpair 7 is a fullrange driver, not a midrange. Rather than use multiples, it would be better to use the larger and more sensitive 12P.

[jimbones]

Hi Harvey ~

I have very little technical knowledge and rely on the generosity and creative intelligence of our incredibly helpful and knowledgeable OB DIY community on Audiocircle.

That said I can share a "philosophical" perspective with you. This OB thread was started with people, like myself, who where interested in making a leap from "commercial" hi-fi/audio, with its hype and high-prices... with its nasty habits of advertising audio as an elite sub-culture of financially "arrived" professionals who use audio as status jewelry... so that we could enjoy music on the highest possible level without spending outrageous amounts of money... with the emphasis being on loving music and the pleasure of music.

The OB paradigm is perfect for us precisely because it is simple and open to a relatively simple DIY approach... even someone like myself with my very limited technical background could feel confident enough... thanks to this wonderful forum... to jump in and experiment.

I mention this by way of sharing an insight with you. Simple is elegant... simple is "direct"... simple is easier to "fix"... easier to understand. So to answer your question from a non-technical point of reference... I would stay away from complex solutions that promise "more"... more of this and that... more straight-as-a-ruler frequency control... more frequency re-produced at the extremes and so on. And instead think about the most elegant simple solution that brings the music to life... "living" music... "alive" music that throbs with passion and emotion and intelligence.

Admittedly, that may seem entirely irrelevant to your question. But for me is the entry point... the starting point of my own excursion into DIY OB magic... and bringing music to life in the home... and beyond that... to learning the easiest and simplest way of doing it that bypasses the commercial trap... and learning more about audio than purchasing an endless series of commercial speakers will ever reveal to you.

With Warmest Regards ~ Richard

Perfectly said Richard!
I contacted a designer of loudspeakers and inquired about designing me an OB. He wanted $$ up front and provided scant details so much so that I could not tell what I was getting fo my money. I understand that he can't give details away cause then someone just goes and builds it. So I decided I would do the design build "myself" with the help of community. I can say that I have found everyone on this board and others to be very generous and wanting to help. I am hopefull that my project will please!! Us DIY'rs have to stick together!!

[-Richard-]

Hi Jimbones ~

Wonderful to hear that you are "jumping in"... congratulations on your readiness to take a journey of discovery.

Truly... what I have learned building my own simple Open Baffle speakers is astonishing! Once built and lived with... every box speaker will sound contrived, limited, overly directional, and "boxy"... it is strange to realize that every designer of boxes already has a serious problem to overcome just because they are working with a box (killing the back wave, bracing, room interactions, overly directional frequency gamuts are just a few).

So may I share some simple suggestions with you... take your time... experiment... give yourself time to experience the "new" sound of the Open Baffle paradigm... it is different. You may have to, or want to change a few things as you become familiar with the sound... perhaps the amplifier, or "source", or cables, or position of the baffles in the room, or toe in, even the size of the baffle. Please think of it as a "learning" experience so that you approach it "creatively". Reproduced musical sound is so perceptually personal... but I think you will be amazed at what you hear... the music will live... will breath... will come alive!!! released from its box-coffin into the living space of your room.

That "new" Open Baffle presentation may take a little time to fully-appreciate... after all we are hearing the musical-sound freed of its former constraints.

Deborah can never go back to listening to boxed-speakers... she hears how caved-in and fragmented the presentation is... immediately... and with highly rated and regarded examples of the box-speaker commercial offerings. Deb is as far from an "audiophile" as imaginable... but she is an attentive listener and she loves music.

Good luck, Jimbones... and keep us informed. I am also planning a new OB for myself... and I am investigating highly sensitive drivers that will allow me to use low watt SET amplifiers again. This is fun!!!

With Warmest Regards ~ Richard

[hrosenba]

Dear Richard, Jimbones, John, and JohnR,
Thank you all very much for your comments.  I admire, or envy, those who DIY but even if I had the time to invest in acquiring the skills, I would not be interested in "reinventing the wheel."  I also think that what is simple and straightforward to a proponent of OB design would differ from a proponent of line arrays ("What, only 4 drivers?!") or active loudspeakers ("what could be more straightforward than a direct connection between the amplifier and the driver?"). 
Regarding the Alpair 7 being a fullrange driver, seems it might also shine in a 2-way system, with low frequencies handled by a woofer, and might outperform its larger, more sensitive sibling in such an application. 
BTW, I tend to keep my gear for a long time.  My current speakers were purchase >20 years ago, Genesis Tech IM-8300 3-way standmounted, after listening to many speakers/systems.  Their midrange/treble ability to naturally portray acoustics instrument/voice sold me on them.  At the time, I also liked some planars -- the ET LFT3 and the Maggy 3.3's, but no go with my better half. My interest in OB kindled by reading about the Danny Richie's Super V, Bastani speakers, and a brief listen to the Emerald Physics 3's at the recent THE Show in So Cal.
Best regards,
Harvey

[JohnR]

Regarding the Alpair 7 being a fullrange driver, seems it might also shine in a 2-way system, with low frequencies handled by a woofer, and might outperform its larger, more sensitive sibling in such an application. 

Hi, my comment about it being a full-range driver was with respect to the treble, with multiple fullrange drivers you will get interference patterns. On open baffle, a woofer for low frequencies will be a must

[hrosenba]

Hi JohnR,
Thanks.  Your DIY 4-way active OB + sub system looks way cool 
From your experience/knowledge, are there any geometric arrangements of, or distances between identical full-range drivers on an OB that avoid the interference?
Harvey

[JohnR]

From your experience/knowledge, are there any geometric arrangements of, or distances between identical full-range drivers on an OB that avoid the interference?

As far as I know, there isn't a way around it, unless you do something like this:

http://www.trueaudio.com/array/index.htm

Or, you could lowpass filter all drivers except one, but then you are using them as mids and not fullrangers.

Consider that an Alpair 12P has around 6dB more sensitivity more than the Alpair 7, and 2.5 times the cone area. If sensitivity is a concern, then that's the way I would go.

Having said that, the 12P seems to have vanished from the Madisound store 

BTW: I have not heard either of these drivers.

[richidoo]

are there any geometric arrangements of, or distances between identical full-range drivers on an OB that avoid the interference?

My informed but amateur opinion is that the center to center distance of the drivers is the problem. It creates areas in the projected soundfield where soundwaves combine destructively and create cancellation lobes. The higher the frequency for a given center to center distance, the greater number of lobes.
https://en.wikipedia.org/wiki/Acoustic_lobing

DAppolito array uses these lobing patterns to put the cancellation lobes pointing at the ceiling and floor to reduce audibility of reflections, but it is a precise formula to achieve this, not just any drivers in MTM. 15" mids and a tweeter cannot achieve a true DAppolito array. The center to center distance of the 15" cones separated by a large tweeter will make the lobes happen way down into the midrange frequencies.

The audibility of lobing is said to begin when the center to center distance exceeds 25% of the wavelength being played from the separated drivers. It is said to be tolerable up until 50% of the wavelength. So higher frequencies want smaller center to center distance to remain sounding good. 

Of course, the drivers can be very far apart and not hear any problem at high frequency if your ear is exactly the same distance from each driver. But the farther away the adjacent drivers move from each other, the smaller the zone your ears can be in while remaining within 1/2 wavelength of each driver.

Most commercial speaker designs shoot for 1/2 wavelength separation. A speaker like Tannoy concentric has perfect ceenter to center distance because the tweeter is in the center. Coherence and natural tone  of a quality coax or full range driver is excellent. A speaker like Danley Synergy horn uses cleaver design to make a three way, full range speaker with with 7 drivers, all within 1/4 wavelength distance.

If you only care how it sounds when sitting down in the sweet spot, then the distance can be larger than 1/2 wavelength.  A pair of Audax PR170M0 mounted vertically is said to sound excellent. That would be ~8 inch separation, so 1/2 wavelength is ~900Hz. Frequencies above 900Hz will have potentially audible and annoying lobing. If you were to compare this arrangement to a single driver you would easily hear the difference. But there are good benefits to paralleling drivers for efficiency and tolerance for lobing varies.

Lobing sounds bad because it causes dips and peaks in the frequency response. This is most audible as the listening height changes like standing ves. sitting, and how the lobes affect room reflections.  single point sound source has no lobing so it has the smoothest FR in theory. There is no perfect sound source.

[Crumbs]

First, despite what most of this post is about I agree with most of what is posted. Simple is better, single driver reduces interference patterns, etc. I suggest you look for Edge, an open baffle simulation program. You can test for interference and lobing via simulation.

   http://www.tolvan.com/edge/help.htm

Data Explained
Second I attached some noisy in-room data. The below 1000 kHz data is crap due to room reflections. Rough distance to object probably less than 1 meter so you can calculate a more exact low f measurement cutoff. Microphone directly in front of loudspeaker at height of center of 2 rs100-8s, roughly 1m. Values shown are step and power response using MLS sequence. Three sets of data co-plotted,

   1) parallel peerless 831735. parallel rs100-8
   2) parallel rs100-8
   3) only lower rs100-8 (note: changes T-amp from 4 to 8 ohm drive load)

The data is noisy and imperfect measured in a real room. Don't take this data to be 'perfect.' All use the same amplifier and mixer settings, I disconnected power to the amp while I changed the speaker connections to avoid changing the volume. Pictures to follow.

Design History
Why do I have this data? When I ordered parts to test out open-baffle I went big with the expectation that worst-case I purchase tweeters and drop back to a 3-way MTM with a dual-driver low-f. I made it a science project. Long-term goal is 4 theater speakers, and a similarly designed center channel using the same drivers to maintain voicing. I'm in no hurry on the build and have tested a lot of concepts. I will eventually write this up in more detail but I started with a open-baffle dual-driver concept whereby I would drive one driver forward and one dipole driver backward. This was based upon the Linkwitz and other's tests with reverse mounted tweeters. I tested different phasing, etc, but first order impression was I liked the sound better when the drivers were in-phase. I have notes for the full write up. Then I built the monopole and have been playing with measured response for different crossover manipulations to the input signal.

Currently it is a dual-driver dipole with a dual-driver sealed monopole for low frequency content. I've been debating dropping the second dipole driver for all the reasons outlined by previous posts. I was actually sketching alternative solutions today in a meeting and come up with a sweet concept I'm going to model in Edge. Overall, I just haven't had a lot of time to investigate in full and move the project forward. At some point I'll get around to ordering crossovers parts and moving the prototype forward to a full build.

Image of the speaker prototype



Impulse Response Comparison (x axis = seconds)


Frequency Response (Normal X Axis)


Frequency Response (Semilog X Axis)



Hope this info puke is useful.

Bill

[JohnR]

Thanks Bill. The graphs would be easier to read if some smoothing were applied, but with regard to the blue curves, are you using a crossover between the Peerless drivers and the RS100s?

[Crumbs]

Sorry for the delay. I must have forgotten to check the notify me of replies button.

I know they would look better with smoothing but I haven't had time to rewrite the code. To answer your question, there is no crossover, this is still a prototype side project. The peerless are driven by one channel and the RS100-8 are driven by the other channel of a little T-AMP. To test the effect of different crossovers I've been manipulating the signal on my laptop and dumping that while running the soundcard in duplex mode to measure the corresponding response. All data shown was with filtering turned off.

Hope that helps.

Bill

[bladesmith]

I'm going to be trying a "simple" MTM (mid tweet mid) configuration with my Hawthorne Audio Reference OBs.  It appears that two benefits happen, but I'm waiting until I make the conversion to say if it is true.  I'll be using two 15" SSI mids and one AMT ribbon tweeter per side.

First, the MTM configuration "couples" to create a greater output volume per watt (I believe MTM is calculated at 3 db...from memory so be careful with that).  This creates greater dynamics than just the single driver.  I think the same is true with four drivers coupling, as well.  Second, the greater sensitivity allows me to reduce the padding on the tweeter so it will be a little more direct.  This will add to its simplicity and clarity (or so I hope).

Best,
John

John,

This is the same ideology that I have been researching.

Did you ever come to a result or conclusion in your research ?

Thanks..