[-Richard-]

Hi Erling ~

Thanks for your further insights. I must try at least a temporary (experimental) extension of the bass "wings" to see how much more bass it can generate. I realize there is an ideal extension length before resonances begin to work against the benefits. Jim McCarthy has already suggested we should build a more "finished" baffle in the near future... so whatever I learn can be implimented in the newer design.

And Yes... I also wish everyone here on AC a most joyous new year... filled with happiness and great peace of mind... it would certainly be helpful if all the wars around the world could be ended... forever... why not? that way the predictions of an entirely new paradigm of consciousness which is supposed to unfold around 2012 could actually come about without the horrors of another world conflagration preceding it... or a pole shift resulting from the dangerous and alarming warming trends all over the world. One way or another there has to be a change of consciousness... either it will be the result of 2/3 of the world population being wiped out... through more wars or earth changes... or scientists will ultimately manipulate the genome to produce a race of clones designed to cooperate together... all are nightmares. We really do need to take full responsibility for how we respond to life... for ending the violence we carry inside us... which ultimately becomes the violence that is hoisted onto the world. What better time to mull over these important issues then on the threshold of a new year.

Warmest Regards ~ Richard

[panomaniac]

Hi Guys - Happy New Year from one of the last places on Earth that will celebrate it.  By the time we pop the cork, most of the world we be drunk or asleep. 

On the topic of OBees.  Yes, I think side wings are good, too.  Erling's deep, straight wings scare me a little.  I prefer to go asymmetric with triangular wings of different depths on each side.  Or even a shallow straight wing on one side with triangle on the other side.

Where you will notice the gain is mostly in the upper bass - at least that's where I hear it.

The dirty little secret of OB is that you need lots of surface area and lots of power compared to a box speaker.  That 6dB per octave loss caused by the open back really kills you.  On typical size open baffles that roll off happens pretty high - much higher than a box speaker, like 125-225Hz.  A real bass killer.

There are tricks to get around the high bass roll off point.
1) Wide baffle.  Not practical for may rooms and has its own set of sonic problems
2) Side wings.  A nice, simple solution.  Helps with bass, though  not with baffle step.
3) High Qts drivers.  These drivers are under damped, so tend to peak in the low end. That can be usefull for OB.
4) Multiple bass drivers with staggered crossovers.  Good idea, but large heavy and expensive.

So for typical OB bass you will need some sort of EQ to bring up the bottom end, or push down the top end for a flatter response.  That means you need extra gain, thus extra power.  Starting out with an efficient driver helps a lot here.  If you need 10-12dB of bass boost, starting with a 88dB/watt driver will get you into trouble fast.  Better to start with 94dB/watt or better.

Happy new year and stay baffled!

[scorpion]

Hi Pano,

I have had a discussion with Martin J King about this baffletype, the same as he uses for his Alpha 15 - Lowther OB.
Here is a link: http://www.audiocircle.com/index.php?topic=40827.0 .



The 'Volks-OB' this is a nearfield measurement of the right speaker's bassunit with crossover. No resonances here !

/Erling

[panomaniac]

That's a very nice looking FR plot!

But as you say, it's nearfield.  My FR plots look very nice in nearfield, too.  But move back a few meters and they fall apart.

My worry is that in nearfield at the front of the baffle you are not seeing what's coming off the back of the baffle.  Here is the challenge:
Turn your baffle around 180 and measure the nearfield response of the back side.  Are there any resonances?  If there are, you know that they are contributing to in-room response.

I'd love to see what you find, good or bad.  And it's easy enough for me to try the big, deep wings, so I may some day soon.

[Rudolf]

Turn your baffle around 180 and measure the nearfield response of the back side.  Are there any resonances?  If there are, you know that they are contributing to in-room response.

I´m afraid its not that easy
I measured my H frame nearfield at the front and back side. As can be seen the resonances at 850 Hz and 1,7 kHz show up in the front but are disguised by interference with the basket structure in the back:

[-Richard-]

I hope to be able to test out Erling's baffle suggestions very soon... probably use some form of rigid laminate-foam-sandwiched-between-paper material that cuts easily... like foam-core-board.

Listening to 60's tunes all day today while waiting for my stomach to calm down... the Beatles, Leonard Cohen, Janis Joplin, Dylan, Judy Collins and many more... with my B200/Alpha-15A's and Jeff Korneff's 45 SET amp the sound was utterly delicious... deep, rich, visceral, dynamic with incredible tonal color... it cannot get better than this.

Jim McCarthy needs to use sound absorption material for his in-house recording studio... as soon as he has constructed some panels he wants to come over to see what effect if any they have on the sound I am getting. I will report if anything interesting comes of it.

Interesting discussion about resonances and OB design strategy... thanks for sharing Erling, panomaniac and Rudolf... I hope you continue to go even more deeply into this.

Warmest Regards ~ Richard

[panomaniac]

I´m afraid its not that easy

Ah, so...  Thanks Rudolf.  I suppose I was looking for resonances lower in frequency.  But given the dimensions of the wings, maybe I shouldn't have been.   

So maybe the big wings can help extend the bass without too much trouble in bass resonances, but we have to look out for higher frequency trouble?  That would certainly be important with low order crossovers.

BTW, is your H-Baffle 40 cm wide? 

[scorpion]

No reason to back down, Pano. This is still mostly uncovered territory which we are not so certain about in our statements.

Here is however the demanded measurement exactly as you suggested. It is bass only with the crossover in place. Microphone was placed
just outside the wings about 12" above floor. The graph looks good and hopefully will contribute to knowledge. I see there is a misprinted date on the graph,
measurement was taken as stated to the right 2008-01-03.



/Erling

[Rudolf]

Ah, so...  Thanks Rudolf.  I suppose I was looking for resonances lower in frequency.  But given the dimensions of the wings, maybe I shouldn't have been.   
So maybe the big wings can help extend the bass without too much trouble in bass resonances, but we have to look out for higher frequency trouble?  That would certainly be important with low order crossovers.
BTW, is your H-Baffle 40 cm wide? 

Yes, 40x40x40 inside. There has been a lenghty discussion http://www.audiocircle.com/index.php?topic=39094.0 about those resonances already.
What I took as some rules of thumb from that discussion and JohnKs Tech Studies:

• Don´t make the effective OB width larger than 2-3 times the driver diameter (two times preferred)
• Don´t make a H- or U- frame (or any other wings) much deeper than the baffle width
• Don´t extend the passband of a dipole driver beyond the dipole peak of it´s OB or x-frame – low pass at least with 12 dB/oct, preferably steeper

I know this will rule out all those single fullrange drivers on BIG baffles, but the masters - like SL and JohnK - somehow seem to think along the same lines, concluding from their own dipole designs.

[Rudolf]

No reason to back down, Pano.

Indeed not. Those resonance spikes that I measured (and that have been simulated by JohnK too) are less than 1/3 octave wide and will not be recognised by the ear when hearing voices or music.

Here is however the demanded measurement exactly as you suggested. It is bass only with the crossover in place. ... The graph looks good and hopefully will contribute to knowledge.
/Erling

[MJK]

Rudolf,

I don't believe those peaks are resonances generated by standing waves in the U baffle. A standing wave in the U baffle would produce a dip in the driver response not a peak. The peak would be seen in the response from the "mouth" of the U baffle. The frequencies are also high enough where I would expect the acoustic impedance of the "mouth" of the U baffle to damp them into small rounded humps. To my eyes these do not appear to be standing wave resonances due to baffle geometry but some other artifact of the measurement set-up.

Martin

[panomaniac]

Thanks for the graph Erling!

I took the liberty of rescaling and combining your 2 graphs.  I think I got the relative scales right. Please have a look.
On the graph below green is the front response, black the rear.  So they may be slightly different, if measurements were similar.  One would expect that, given the large rear wings.


BTW, you are using a 4th order low pass at 1K, right?  Any EQ or bass boost on the bottom end?

+++++++++++++++++++++++++++++
Thanks for the info Rudolph.  I figured the dimensions must be tied to 40 cm in view of your 850 and 1.7K resonances.  I'll look at the other thread as soon as possible.

[grantnsw]

I just got back from vacation, and there's so much happening here!

It looks like a great read, wow... so much to ketchup on, if I can
understand any of it! haha , I'm hopelessly out of my 'depth' but what
a hoot. Thanks guys.

I read in another forum post that someone 'bagged' the Eminence
Alpha 15... is this cause for concern?  just wondering...

[scorpion]

Thanks Pano,
Measurements are not quite identical since the speaker was moved a bit further into the room and turned around.
The 'Volks-OB' is a fully passive design, no EQ, simulated with MJK's Math Cad models, bass part by itself and fullranger part by itself.
The combination has turned out sensationally good.

Low Pass for Bass is calculated to 200 Hz and implemented as Linkwitz-Riley 2nd order and Bass is taken down some dBs
with a 3.3 ohm resistor in front of the LP-filter. High Pass likewise is targeted at 300 Hz L-R 2nd order. 30 uF Obbligato
capacitors contribute to the good sound for the Fullranger. The 1 m (in room right speaker) frequency plot (mic on fullranger axis) is like this:



/Erling

[panomaniac]

That's a very, very impressive low end response with NO EQ.  Bravo!

I ran the numbers on your A&D 15" and it looks like your 3.3R in series is raising the effective QTS to about 1.05 (Qes figured as 0.84). That's right were Martin likes to see it.  And it drops your sensitivity to about 91.2dB @ 2.83V.  Right in line with your 5" FR driver - nice.

To get the same from my Selenium 15WP3 it would take about 5 ohms in series.  Will have to give it a try when I get a chance.  Thanks for the info and the FR charts.