[MJK]

Don't keep us in the dark, lets hear about some of those issues ideas!

How about actually spending time designing the OB size/shape, the crossover if any, the interaction of the drivers with the baffle, and predicting the SPL response in a room before any wood cutting or measurements.  I think most people are in the cut and try mode of design followed by building iterations and speculation of complex theories to explain what causes the good and bad things they might hear from the results.  Read through the posts in the thread and look at all of the differing opinions and speculations some of which have no basis in acoustics.  My opinion, and this should be no surprise, is that if people spent more time designing and understanding the physics up front and less time speculating the final results would be much better.  That is all I am trying to say, it is just my method of approaching a speaker design problem.

[nodiak]

Thanks Martin.

[nodiak]

John, broken car = no surf trip to Baja = disappointed teenager (and dad), a real character test for us. Life could be far worse, and is unfortunately for some. Wasn't going to make it to CR this year, but have a surfer friend there to visit sometime, he made good use of his inheritance from his grandmother and bought property on the beach.
The basket mod idea is lost in cyberspace somewhere, I wouldn't know where to look, it's been awhile. But it's as basic as it sounds. The point was to make the inside of the baskets "legs" rounded. Sure, split some dowels and even  just tape on. Got to admit Martins points make me less concerned now.
My suspended floor is a real problem. It resonates easily with heavy footsteps. even the walls resonate with mild pounding, and the paneling isn't firm either. It's an older building. In other places have suspended speakers from the ceiling in the past but have since felt mechanical grounding works better for bass transients. Since it's a narrow room (11.5') may work on some bracing that mechanically grounds the bass drivers at least partially to the bottom plate of the side wall to see if it helps tighten up the bass. Or just place them on heavy stone slabs. Am open to other suggestions that have succeeded. Hanging from the ceiling is easy so I may give that a try after all. But I think to a degree I'm just stuck in a resonant room. The good side is a sw for ht will be reeaalll effective!

Don   

[Bob in St. Louis]

The good side is a sw for ht will be reeaalll effective!

Install tactile transducers ON THE FLOOR!
That'll get ya' rockin'

Bob

[mcgsxr]

JohninCR - very valid point about my use of the amp AS the filter, I had not really thought of it that way.

Nodiak - pool table still very much a longer term goal these days, my basement bunker remains a concrete structure, with ribbed roof (floor joists for the main floor)... so no ability to engage myself in my passion - good 9 ball.

I am interested by this no baffle thing, and watch in earnest as you guys play on.

Who knows, once the space is a finished room, and my family starts to venture down there more often, the largish panels may have to go too...!

[Bob in St. Louis]

With my cheap 600W PA amp, it's a different story. 

I was thinking about robbing half of my amps power. Currently bridged to 900 watts for the sub.
I've heard it run 450 to the sub, sounded acceptable, so I figure 450 to a pair of Augies and 450 to the sub would suffice.
(Not runnng at the same time. I'd be loosing the point of OB bass. Just wired that way for conveniece when I do want the 'main' sub)
Bob

[markC]

So you don't want a crossover with its associated caps, inductors, and resistors but you will except a series correction filter?  Seems to be a little inconsistent. 


 

The small values of the components and the low count- 1 inductor and one resistor- are going to have less of an impact than a 2nd order network to be sure.
It is a compromise, (but the best one to date), as are most things in this hobby when searching for the current best performance from my rig. 


[/quote]

[Bob in St. Louis]

Gee, I remember a place once where a guy could go to, ... what was that phrase? Something like:
A friendly place, where folks can share recipes, and discuss speakers built on an open baffle

I know several of you from other forums, I respect the guys on both sides of this fence.

Let's work together and learn.

Bob

[scorpion]

If you want decent mid bass from this driver, you need baffle width. This is more than apparent to me when I fold back the wings on my baffles.
I agree completely, when I fold the wings back on my OB system ( http://www.quarter-wave.com/Project07/Project07.html ) the bass response is compromised.  So if we extend that thought, for a given driver producing bass frequencies, the size of the baffle will determine the depth and volume of the bass.  An infinite baffle is one extreme and no baffle is the other.  You will get bass out of both extremes but the question is how much and at what frequency.  Running with no baffle is fine if it meets your performance goals.  I'll stick with a baffle. 

Martin,

If I understand you right you state that your bass is significantly improved when sound travel distance round the sides is increased from about 51.5 inches to 68.5 inches (some average) as I measure from your baffles ? I also find your simulations regarding the Alfa-15 interesting and revealing for the Qts importance. Even though excursion for the Alfas is limited you will have ample SPL at hand, I suppose ?

Did you experience any serious resonance from your as read it solid 12 inches sides ?

/Erling

[scorpion]

Bob,

Well spoken !


JohninCR,

Lets put the -3 dB point at 300 Hz in your experiment. It will be near the thruth. What is your impression of putting 6 db/octave compensation from there together with a sharp bass roll-off at 30 Hz ?

/Erling

[JohninCR]

Bob,

Well spoken !


JohninCR,

Lets put the -3 dB point at 300 Hz in your experiment. It will be near the thruth. What is your impression of putting 6 db/octave compensation from there together with a sharp bass roll-off at 30 Hz ?

/Erling

Erling,
I'm not so sure about that as a -3db point.  300hz is Fequal (OB spl = box).  With just a tone sweep and ear, I came
up with 280hz as the starting point of the SIcoax roll-off up in the air.  I'm unsure of the effect of the early reflection
forward of about 1/3 of the rear wave due to the construction, so it's no use measuring until I have a better structure.

I only did a quick sweep on the Augie's to verify no big peaks or dips, and there's no use trying to use sweeps by ear in
the bottom octave due to the change in ear sensitivity down low.  They're fun to play around with using subs and to track
down room rattles and room modes, but not much use in tuning.  To compensate the Augies, I used music including one
bass tester track for nice full balance bass on the bottom, and added higher frequency sweeps to aid in adjusting the
top end.

Impression- Great, clear, distinct bass as always using OB bass, with no signs of strain at significant volume, just vibration
with the lightweight structure.  Keep in mind that it was really just a proof of concept.  The key will be proper integration
with the top driver, since I have to use the bass driver much higher than with a baffle.  Graphically, for the Augie I created
a gradual slope in response the match the coax, and rolled it off steeper at 280hz and steeper still at 400hz.  The 2nd order
at 400hz improved the sound enough that I don't believe only a 1st order filter will work for this driver combo without a baffle. 
I believe the only real sonic problems with my test mules are that early reflection off of the structure and the edge diffraction
occuring at the same point due to the circular shape.  I believe I'll be able to cure the diffraction effects using thick rings,
and a better structure will eliminate the reflections.

I want to try it with my B200's too, but with wide as well as thick rings.  I do have 4 B200's so I can see just how far a
B200 can augment another B200, obviously not in a NoBaffle setup.  With a deep U at the bottom transitioning to a shallow
angled U at the top, it could be quite a compact unit.

Lots of measurements to come.  Once I learn how and practice a little, I'll post my planned methodology to solicit advice
or other ideas and make sure I don't have any gaping holes in approach.  It will be a lot of work and I want meaningful
real world results that are useful to everyone.

[MJK]

Hi Erling,

If I understand you right you state that your bass is significantly improved when sound travel distance round the sides is increased from about 51.5 inches to 68.5 inches (some average) as I measure from your baffles ?

When the wings are extended the low end of the bass is improved.  I am not sure how to determine the equivalent baffle width with the 12' sides projecting back from the front baffle.  But there is a noticeable improvement with the wings rotated out to form a bigger front baffle area.

I also find your simulations regarding the Alfa-15 interesting and revealing for the Qts importance. Even though excursion for the Alfas is limited you will have ample SPL at hand, I suppose ?

Even at loud volumes, a subjective term since my loud might not be your loud, you have to touch the 15" drivers to convince yourself they are really moving.  The deflection of the Alpha A15 woofers is very very small, probably because I have two of them per side, for the acoustic jazz I enjoy.  Even with drum solos where you can feel and hear the drums, the motion is very difficult to detect with the eye, again a subjective measure since my vision is not so good.

Did you experience any serious resonance from your as read it solid 12 inches sides ?

I have not noticed any panel resonances when music is being played.  Like any large piece of wood panel, there are probably resonant frequencies and mode shapes but I have not noticed that they are excited and having a degrading impact on the system's sound.  Maybe having some of the panels sitting on carpet improves this potential problem.

Hope that helps,

[nodiak]

Gentlemen, take it to the Fight Club please.

[scorpion]

Martin,

Thank you !
Membrane surface helps. With regard to resonance I didn't express myself very clear, what I meant was the possibility of an air cavity resonance between the 12" sides. Well, I just calculated the hypotenuses + half main baffle width but I don't know either if that is a meningful calculation. 

/Erling

[Brad]

There is some useful information in this thread.
Give me a few minutes to split off the 'unproductive' posts


Okay - done - took me a coupla minutes to RTFM

[scorpion]

JohninCR,

Just some simulations I have done poiting to my raised proposal. Between 300 and 280 Hz there would only be a .3 dB difference again according to my simulation. You wouldn't be able to judge by hearing only. 

However the simulation presents a much smoother frequency response than I first envisaged.

/Erling

[Wind Chaser]

There is some useful information in this thread.
Give me a few minutes to split off the 'unproductive' posts


Okay - done - took me a coupla minutes to RTFM

Good job, Brad.  Long live heavy handed censorship.  God forbid disparaging points of view be tolerated!  Like so many other people with an interest open baffles, I won't bother sharing or posting here anymore.

Farewell

[Brad]

Windchaser -

You and JohninCR apparently had a falling out.....
All I moved were the posts that were personal attacks.

There is some good information in this thread that I think a lot of people can benefit from.

I have no problem with disparaging points of view - I do have a problem with disrespectful, attacking posts.

[MJK]

Hi Erling,

With regard to resonance I didn't express myself very clear, what I meant was the possibility of an air cavity resonance between the 12" sides.

Now I understand your question, I have been asked this same thing before.  Everytime I revisit the question I think some more about the answer.  So today this is my opinion on this question.

BEGINNING OF OPINION/SPECULATION

The front baffle (24" wide and 48" tall) and the two sides (12" deep) form an open ended cavity with the floor at the bottom.  This space is open on two sides which are both very large surface areas.  If I think about the potential for resonances or standing waves in this space, I come up with the following possibilities.

• In the deep direction - the depth is 12" which would support a quarter wavelength standing wave at approximately 1775 Hz.  A standing wave is only generated if the end boundary condition can reflect the wave back into the space, like at the end of a transmission line.  However the open end of this space, assuming the top is closed for a minute, has an area of 24" x 48" which is sufficiently large that the acoustic impedance is highly resistive so no sound energy is reflected back.  Therefore a standing wave cannot be generated in this direction.
• In the tall direction -  the height is 48" which would support a quarter wavelength standing wave at approximately 445 Hz.  But the area at the top, again assuming the back is closed, is big enough so that the acoustic impedance is still highly resistive so no sound energy is reflected back.  Therefore a standing wave cannot be generated in this direction.
• Between the sides direction - the width is 24" which would support a half wavelength (volume closed at both sides) standing wave at approximately 1775 Hz.  But since the excitation, the drivers, are located at the middle of the span the first half wavelength mode will not be excited as a resonance. The second harmonic will be the first potential standing wave resonance, this is at 3550 Hz.  But with two open sides parallel to this direction of motion I do not believe a standing wave in this direction will be excited either.
• The woofers are rolling off at 12 dB/octave at about 200 Hz.  So by the time the frequency approaches the resonant frequencies shown above, the biggest excitation source will be significantly attenuated.  So I do not believe the woofers are a significant source of exciting for any standing waves.
• The Lowther driver does provide excitation above 200 Hz.  But it is so high on the baffle that it is really close to the top acoustic impedance boundary condition and only 12" from the rear acoustic impedance boundary condition.  So I do not believe that the Lowther supplies any significant energy to excite standing waves.

My conclusion is that for a cavity of this size, that is open on two sides, standing wave resonances do not occur.  I have not performed any measurements yet to prove this theory but I do not hear any one note response anomalies that would indicate an acoustic resonance occurring in the cavity.

One other interesting feature I have been considering about this geometry.  The sound waves coming off the front of the driver travel directly to the front side edges of the baffle.  The sound waves coming off the back of the driver must travel the additional distance to get around the 12" deep sides so they will arrive at the back side edges of the baffle later in time and with reduced magnitude.  I am wondering if this geometry produces a classic figure eight sound field for a dipole or a slightly unsymmetric version of the figure eight pattern.

END OF OPINION/SPECULATION

Everything I have stated above is strictly my opinion without any proof offered and definitely up for debate and questioning.

[JohninCR]

Hey Martin,

Thanks for that analysis.  From SL and JohnK's work, yes when the front and
back of the baffle aren't identical, you get away front the classic figure 8
pattern of dispersion for bass.  The area of greatest null exists where the
front and rear waves have travelled the same distance and remain directly
out of phase.  For flat baffles this is directly at the sides, resulting in the fig 8.
As you sweep your wings back, you move the lines of greatest null rearward.
Ultimately, in the case of a perfect U baffle (driver on the end of a short pipe),
excluding any 1/4 resonances you've stimulated, the area of greatest null will
be directly behind the speaker (not immediately nearfield), because the front and
rear have travelled the same distance resulting in a cardiod radiation pattern.

What you have is a hybrid, somewhere in between, and you should easily be able
to verify that your area of greatest null is somewhat behind the plane of the
baffle by walking around your speaker while it is playing.

I've built a number of these hybrid baffles, and GM once told me that having the
wings splayed open at least 1" for each 6" of depth should prevent resonances,
which leaves us with potential resonance only if this cavity shape is placed near
the wall.  Do you agree that this is a good rule of thumb?

Oh and btw, yes the above is my opinion, some of which is based on the work
generously shared online by others.