[BoomieMCT]

New OB plan

I have a new concept for open baffle bookshelf speakers using what I've learned with (and since) my 6.5" open baffle speakers. Here is a list of the things I've learned / opinions I've build on this topic.

1. You can't have small OBs without some sort of bass augmentation. The baffle step diffraction and interference from the front and rear waves hurts you too much.

2. Building a "bass module" just results in placement issues that I'm not willing to deal with (see #1).

3. I'm personally not as moved by OB bass as content below 600 Hz or so.

So my idea is to build a 2-way speaker that has a 3" full range driver in an OB and an 8" driver in some kind of enclosure for the bass section. I want the crossover below 1000 Hz because that is below where the ear is most sensitive and hopefully I can use the x-over point to help combat baffle step diffraction instead of putting in a LR circuit and padding.

I've included a picture of my "rapid prototyping" test stand using a budget 3" TB driver and borrowing a friend's Dayton 8" reference driver. Right now each driver is run off a different channel so I can change the balance by shifting the L and R. When I buy some more alligator clips I'll make a rough crossover. I know this is far from ideal but it will give me an idea if I'm on the right track before investing money in parts.

 

[JCD]

Some general comments/questions from a novice..

1) How high does the full range go? I didn't think most full ranges went as high as most would want.
2) have you looked at Linkwitz's Orions. I think his method of compensating for the bass output is to create an "open box" and to eq the bottom end. Even then, he recommends a subwoofer with his system too. There is something about the open box that boosts the bass output without loosing much (any?) of the benefits of an OB system.

JCD

 

[BoomieMCT]

Some general comments/questions from a novice..
1) How high does the full range go? I didn't think most full ranges went as high as most would want.

According to manufactuers specs, it goes up to 20kHz. I'm verifying that right now (I'll report when everything is broken in). This is a 3" "full range" which doesn't make much output under 100Hz. That's why I'm using another driver to reproduce the low octaves.

2) have you looked at Linkwitz's Orions. I think his method of compensating for the bass output is to create an "open box" and to eq the bottom end. Even then, he recommends a subwoofer with his system too. There is something about the open box that boosts the bass output without loosing much (any?) of the benefits of an OB system.

I'm familiar with the Orion. It is an speaker with an excellent reputation. However, open baffle speakers are not known for having lots of bass - quite the contrary, they tend to be bass lean. The Orion has some fancy active equalization built in that compensates for this fact. Although I hear it works well, this is a bit beyond my skill and price range. Instead, I'm attempting to use the 3" driver in an open baffle but crossing over to a sealed enclosure just above the point where the OB bass rolloff begins. In my past OB experiments I used an additional speaker to act as bass augmentation - here I'm trying to make it all built in.

 

[JCD]

According to manufactuers specs, it goes up to 20kHz. I'm verifying that right now (I'll report when everything is broken in). This is a 3" "full range" which doesn't make much output under 100Hz. That's why I'm using another driver to reproduce the low octaves.



I'm familiar with the Orion. It is an speaker with an excellent reputation. However, open baffle speakers are not known for having lots of bass - quite the contrary, they tend to be bass lean. The Orion has some fancy active equalization built in that compensates for this fact. Although I hear it works well, this is a bit beyond my skill and price range. Instead, I'm attempting to use the 3" driver in an open baffle but crossing over to a sealed enclosure just above the point where the OB bass rolloff begins. In my past OB experiments I used an additional speaker to act as bass augmentation - here I'm trying to make it all built in.

1) That's a pretty good range! I can't wait to see how it performs for you
2) Agreed on the bass issue. He does EQ the bottom end a "touch" to get it to go as low as it does. I did miss that you were going with a sealed design for the bass augmentation. That should help a lot.

With the baffle dimensions that you're using, where would you need to get some BSC? My quick calculations say you'd need a baffle >27" wide/tall before BSC starts to kick in at 1,000hz (f3 would be ~170Hz).

JCD

 

[BoomieMCT]

2) Agreed on the bass issue. He does EQ the bottom end a "touch" to get it to go as low as it does. I did miss that you were going with a sealed design for the bass augmentation. That should help a lot.

I was under the impression the Orion had 6 dB / octave boost below a certain point to make up for open baffle roll-off. That is enough that I wouldn't want to use a passive x-over as you would be throwing away an awful lot of power.

With the baffle dimensions that you're using, where would you need to get some BSC? My quick calculations say you'd need a baffle >27" wide/tall before BSC starts to kick in at 1,000hz (f3 would be ~170Hz).

If I were to design it such that the crossover was right at where baffle step diffraction begins then you are 100% correct and that may be what I end up doing (see attachment). If I went forward with this I would try to use a combination of having a more efficient woofer and an assymetrical x-over to shape the response to counter baffle step diffraction (hopefully with a thinner baffle). Obviously, some testing and experimentation would be required.

 

[BoomieMCT]

I think I've come to the end of where these tests can take me. Here are my impressions.

1) Cardboard boxes leave much to be desired when making enclosures for the low freqencies. Frankly, it makes the bass sound downright muffled. The Dayton 8" wasn't as loud as I thought it would be - I chalk a lot of that up to the cardboard.

2) I'm not sure a 3" works well in OB (for me). My 6.5" OBs seemed to have more presence and seemed to create a larger soundstage. I'm not sure if a 3" can move enough air to reproduce this effect.

For grins and giggles I've attached FRs for the paper box setup. The crossover is a bargain bin passive subwoofer x-over - I think [email protected] ohms was the x-over frequency. The second graph is the same setup with a L-pad setup to bring the tweeter down a bit.

The nice thing about this driver is it had no problem getting up to 20kHz. Of course, neither did the 4" drivers in my TABAQs (and they had less freqency rise) - maybe I'll test those out too.

I'm not giving up but I have to find some more parts to play around with before I go forward with this.

 

[JCD]

I was under the impression the Orion had 6 dB / octave boost below a certain point to make up for open baffle roll-off. That is enough that I wouldn't want to use a passive x-over as you would be throwing away an awful lot of power.

No, you're correct -- I think I'm not being clear. :duh:
And without EQ, I couldn't imagine doing this with a passive x-over
And I agree, going with a sealed box will dramatically increase the bass ability of the system.

I think I've come to the end of where these tests can take me. Here are my impressions.

1) Cardboard boxes leave much to be desired when making enclosures for the low freqencies. Frankly, it makes the bass sound downright muffled. The Dayton 8" wasn't as loud as I thought it would be - I chalk a lot of that up to the cardboard.

LOL! Yeah, I guess there is something to be said for good ole MDF.. I'd imagine the actual physical back and forth of the driver itself would to an extent mute the response of the driver.

2) I'm not sure a 3" works well in OB (for me). My 6.5" OBs seemed to have more presence and seemed to create a larger soundstage. I'm not sure if a 3" can move enough air to reproduce this effect.

I think you're probably right -- might this be alleviated by using more drivers?

I'm not giving up but I have to find some more parts to play around with before I go forward with this.

Definitely looking forward to your experiments! :T

JCD

 

[Anthony]

Interesting test. So that's what you do with my drivers when I go out of town?

I loaned him the Dayton driver last weekend to break-in and test. It seems we need to build a test stand that will hold the drivers off the floor and then put the cardboard baffle around it. That should alleviate most of the motion-cancellation effects you were seeing.

My OB design will have two of the Dayton's, which should help with the bass response. Still not sure on the baffle design yet, but we'll play around until we come up with something.

 

[Bob in St. Louis]

So that's what you do with my drivers when I go out of town?

HA HA :rofl2:

It seems we need to build a test stand that will hold the drivers off the floor and then put the cardboard baffle around it. That should alleviate most of the motion-cancellation effects you were seeing.

YES! What some guys do is make aN MDF skeleton with legs/feet to hold the driver steady while they can A/B compare "quick change" cardboard baffles. The cardboard won't 'sway in the breeze' since no mechanical moving devise is attached to them.
One of the slickest methods I saw recently was a PCV pipe attached to a base plate. The pipe went up a few feet and turned 90 degrees. then the fellow made a slit in the pipe so it would slide over the magnet, then he used a steel hose clamp to secure the magnet inside the pipe. Very neat.
I can find the pics if anyone is interested.

Bob

 

[BoomieMCT]

Small update - I got some Hi-Vi B3N's to play with. The Qts is higher then the Tangbands I used before making them better for sealed and (hopefully) OB enclosures. Right now they are breaking in (they come super tight) but here are two graphs - one with no baffle and the other with a two foot cardboard baffle. Can you guess which is which?

I cannot emphasize how different the big baffle sounds or how much better these sound in an OB alignment then the other 3"ers.

 

[BoomieMCT]

Here are two of my tests - a big OB and using a passive sub and cheap sub x-over with just a driver. I kind of like the latter.

 

[Bob in St. Louis]

Graph #2 is cardboard....right

 

[BoomieMCT]

So here is my current plan for my further OB experiments. Behold Boomie's $100 OB's!

The "tweeter" is the Hi-Vi B3N in an open baffle. Many people like the B3S in a sealed enclosure. I tested the B3S in both sealed and open baffle and like the sound. At $9 each this is a bargain.

The "woofer" is this 12" heavy duty woofer. It models well in a sealed enclosure, has good reviews and is only $20 each.

The crossover is based on a DVC crossover I got on clearance for $5. The x-over point @ 8 ohms is 400 Hz which is good for my purposes. I'm going to add a notch filter on the B3N as recommended on Zaphaudio.com. I can tweak these as I play with them.

The baffles are 18" x 48". The front and side pieces are made from one sheet of pine faced ply. I can build this and test it all open baffle. If I'm not satisifed with the bass response I can easily enclose the woofer segment so it is sealed (my original plan).

 

[JCD]

I'd consider moving the drivers closer together. I have nothing to back that up but an inkling that the drivers might not couple so far apart.

JCD

 

[BoomieMCT]

Well, here is a first stab without sides, notch filter and the woofer unsealed. Oh, not broken in either. I did put a bit of an L-pad on the tweeter. On some notes I'm getting a ringing in the woofer's frame from the tweeter. I'm hoping this will go away when I add the sides and the whole thing gets stiffened up a bit.

 

[Anthony]

What are the different colors? I see a nasty region of no mid-bass. Any idea what's causing that?

 

[BoomieMCT]

What are the different colors? I see a nasty region of no mid-bass. Any idea what's causing that?

Green = woofer
Red = tweeter
Purple = both together

Part of that hole around 100Hz is because of my low untreated ceiling and the fact the single speaker is in the middle of the room. As you can see from this graph, it gets smaller when measured from a slightly different location. Hopefully more will go away when I stiffen up this board with the side pieces (and also increase the size of the baffle).

I have noticed that room modes seem exaggerated with open baffle speakers when compared to regular monopole speakers in the same location. Although I like open baffle speakers a lot I've never really been impressed witih dipole bass - part of this experiment was to see if I could change my mind on that. I will probably end up sealing the lower section if the wings don't change things.

 

[BoomieMCT]

I've never really been impressed witih dipole bass - part of this experiment was to see if I could change my mind on that.

Okay, this doesn't really seem like a fair statement - let me clarify before I get flamed. I've never heard dipole bass that is good enough to justify me spending a lot of extra money to get it done. I'm sure I could get two sub amps and two Hawthorne Auggies (for about $500) and make something I'd be happy with. As it is, I'm pretty confident I'll be happy with this setup sealed and that costs a lot less ($40 of drivers). Perhaps when Anthony gets his DDR knock-offs or Phoenix knock-off done I may change my mind. :bigsmile:

As to the dip @ 100 Hz - I'm pretty sure it is mostly room modes. It does seem much wider then normal - there may also be some issue with the flexing of the board. Maybe I'm hitting its natural frequency. When I add wings that should stiffen this up quite a bit. They should also add about two feet to the width of the baffle around the woofer. Maybe that will help too?

 

[BoomieMCT]

Let me take back anything negative I said about this project before. I added the wings which made the baffle bigger and helped stiffen it up a lot. I also added the same notch filter perscribed for the B3N on Zaphaudio.com. The stiffer structure took out a lot of the 100Hz suckout (only leaving my room mode) and let the woofer play much lower. Note I have not sealed the woofer as previously planned.

 

[BoomieMCT]

So I got one of these finished and put together. I did a quick mono test by ear. Adding the cross-brace and feet really cleaned up the bass - I'll get an FR after I get the other one put together. Here are some pics.