[JMH]

Hi,

I'm new to this forum and kind of new to speaker building. I tried to get some help on diyaudio with my project, but there was no interest. Hopefully you guys can help me a little 

I'm planning to build some open baffle speakers with one Eminence Beta 15A per side. Eminence Beta 12cx coax for mids and PRV D290Py-S compression driver for highs, crossed at 800-900Hz. I love the look of the Spatial Audio M3 and M4 speakers, so the goal is to build something similar. I'm going active because I would like to have some flexibility in design and driver selection, and because I have very little experience with passive crossovers.

I would like to have the bass extend to 30Hz or very close to that, and from my understanding this can be difficult. Another goal is to keep baffle width 45cm or under. I really like the simple flat baffle, but could also consider a U-frame. Is this possible with one Beta 15 per side? I will have the option to eq, but will it require more excursion than Beta 15's 4mm xmax? Or will I run out of power before that with my 90 watts per channel in 8 ohms for the woofers?

And another thing. I may sound really stupid now, but I also really like symmetry... The coax I want to use is 12" while the woofer is 15". What would happen if I rear mounted both drivers in 11" holes? Kind of like emerald physics 15" woofers with 6" holes in front. This is just a crazy idea, I have no idea how to model the response of this and I'm thinking it probably won't work, but it would be nice.

[richidoo]

Welcome to AudioCircle, JHM!

Sounds like a cool project. We'll need lots of pictures, of course...   

Beta 15 has relatively high Qts of .58, which allows it to play lower and louder than typical pro audio speakers with Qts around .4, making it better suited to passive xo OB, but it will still be rolling off naturally above 30Hz.  I had two 12" per side on 15" baffle with Qts = .7 and it needed 2dB boost to be flat FR down to 30Hz. You'rs will probably be flat to ~45Hz without boost on a 20" baffle. That's pretty good! But since you are using active XO, you can apply bass boost to EQ the bass to whatever response you want, driver and amp willing. The driver is high sensitivity, so you have a good amount of headroom for boost. Single 15" cone with boost should be enough for moderately loud listening in a normal size room. If you want to blow the walls down you'll need more bass drivers. OB is very inefficient in the low frequencies.

Use this to see if your driver excursion is adequate for the SPL and extension you desire. You have to derate it for open baffle, the adjustment varies with frequency. The lower you go, the more adjustment you need due to OB. This calculator is for sealed box. But it will be accurate above the frequencies where OB inefficiencies begin (>300Hz on a 20" baffle.)
http://www.baudline.com/erik/bass/xmaxer.html

Use this to see if your amp has enough power. But set the loudspeaker sensitivity to the bass driver nominal sensitivity minus the boost you intend to apply to account for the losses from baffle step diffraction and dipole cancellation. In theory (iirc) that would be -6dB for the baffle step and -12dB/oct for the dipole cancellation. But because of the high Q driver, I think you'll need <6dB boost with single 15 cone at Qts .6, but that's just a rough estimate.
https://www.crownaudio.com/en/tools/calculators#amp_power_required
You also may not need it to be flat to 30Hz. -6dB at 30Hz is flat to 40hz which is adequate for most music listening. With active you can easily fine tune this to your taste and headroom limitations.

Use a measuring system like OmniMic, or a SPL to monitor FR while you fine tune the EQ. But in the end the ear is what matters, as long as driver/amp can take it.

Since you are using pro audio high efficiency drivers I would think 90 watts is sufficient for home use, as long as the amp has low distortion at full power. If you put a lot of boost on the bass it will eat up the amp's headroom and will audibly clip the mids even on moderate SPL peaks. But certainly worth a try. Depends on how much bass boost. But it can add up quickly. Power doubles with every 3dB you add to the bass boost. 

I'm not a big fan of U frames. The reflection between them is audible to me. Flat baffle is fine, especially if you are using active EQ to adjust the FR. 

The nominal size of a driver is the outside diameter of the mounting flange. So your 12" might fit an 11" hole, but the 15" bass driver would sound better with full sized 14" hole. I am not familiar with the undersized baffle holes you mention in the emerald physics speakers. If you wanted to simulate it you could try hornresp   but no promises on accuracy since it's not a horn.

Did you see the Fane full range 12 (12-250TC) and 15" (FC-152F01TC) drivers? Opinions seem positive on the SQ. Beaming not too bad with the double wizzers and treble extension pretty good too. I have not heard either, so don't take my mention as endorsement. But these might sound better, easier to implement and lower cost than the coax/CD, ymmv.
Good luck with the pro-ject!

[gregfisk]

Welcome to AC, while I own OB speakers and share your passion someone else will need to help you with your questions.

I know a lot of other people around here love OB as well.

Have a great time building.

[FullRangeMan]

I will have the option to eq, but will it require more excursion than Beta 15's 4mm xmax? Or will I run out of power before that with my 90 watts per channel in 8 ohms for the woofers?
It will depend how loud you will hear or how large are your room.
If the VC beat in the botton of magnet motor you will hear.
The coax I want to use is 12" while the woofer is 15". What would happen if I rear mounted both drivers in 11" holes?
Dont do it!! made all drivers front mounted.
Put the Beta15 very near the floor; I suggest this panel:
Plans on Visation site or here:
https://www.blackdahlia.com/html/tip_55.html

[JMH]

Welcome to AudioCircle, JHM!

Sounds like a cool project. We'll need lots of pictures, of course...   

Beta 15 has relatively high Qts of .58, which allows it to play lower and louder than typical pro audio speakers with Qts around .4, making it better suited to passive xo OB, but it will still be rolling off naturally above 30Hz.  I had two 12" per side on 15" baffle with Qts = .7 and it needed 2dB boost to be flat FR down to 30Hz. You'rs will probably be flat to ~45Hz without boost on a 20" baffle. That's pretty good! But since you are using active XO, you can apply bass boost to EQ the bass to whatever response you want, driver and amp willing. The driver is high sensitivity, so you have a good amount of headroom for boost. Single 15" cone with boost should be enough for moderately loud listening in a normal size room. If you want to blow the walls down you'll need more bass drivers. OB is very inefficient in the low frequencies.

Use this to see if your driver excursion is adequate for the SPL and extension you desire. You have to derate it for open baffle, the adjustment varies with frequency. The lower you go, the more adjustment you need due to OB. This calculator is for sealed box. But it will be accurate above the frequencies where OB inefficiencies begin (>300Hz on a 20" baffle.)
http://www.baudline.com/erik/bass/xmaxer.html

Use this to see if your amp has enough power. But set the loudspeaker sensitivity to the bass driver nominal sensitivity minus the boost you intend to apply to account for the losses from baffle step diffraction and dipole cancellation. In theory (iirc) that would be -6dB for the baffle step and -12dB/oct for the dipole cancellation. But because of the high Q driver, I think you'll need <6dB boost with single 15 cone at Qts .6, but that's just a rough estimate.
https://www.crownaudio.com/en/tools/calculators#amp_power_required
You also may not need it to be flat to 30Hz. -6dB at 30Hz is flat to 40hz which is adequate for most music listening. With active you can easily fine tune this to your taste and headroom limitations.

Use a measuring system like OmniMic, or a SPL to monitor FR while you fine tune the EQ. But in the end the ear is what matters, as long as driver/amp can take it.

Since you are using pro audio high efficiency drivers I would think 90 watts is sufficient for home use, as long as the amp has low distortion at full power. If you put a lot of boost on the bass it will eat up the amp's headroom and will audibly clip the mids even on moderate SPL peaks. But certainly worth a try. Depends on how much bass boost. But it can add up quickly. Power doubles with every 3dB you add to the bass boost. 

I'm not a big fan of U frames. The reflection between them is audible to me. Flat baffle is fine, especially if you are using active EQ to adjust the FR. 

The nominal size of a driver is the outside diameter of the mounting flange. So your 12" might fit an 11" hole, but the 15" bass driver would sound better with full sized 14" hole. I am not familiar with the undersized baffle holes you mention in the emerald physics speakers. If you wanted to simulate it you could try hornresp   but no promises on accuracy since it's not a horn.

Did you see the Fane full range 12 (12-250TC) and 15" (FC-152F01TC) drivers? Opinions seem positive on the SQ. Beaming not too bad with the double wizzers and treble extension pretty good too. I have not heard either, so don't take my mention as endorsement. But these might sound better, easier to implement and lower cost than the coax/CD, ymmv.
Good luck with the pro-ject!

Thank you som much for your thorough answer, richidoo. This clarifies a lot, and now I know more of what to expect. I think Beta 15 will be fine, and I'll probably just try them on a flat baffle. And maybe I should just have flat response to 35-40hz as a goal instead. I think I'll stick to the appropriate sized holes. I have been thinking about the Fane full range speaker, but I really want to try the coax/CD combo. Btw, which 12" woofers did you use on your open baffle?

Welcome to AC, while I own OB speakers and share your passion someone else will need to help you with your questions.

I know a lot of other people around here love OB as well.

Have a great time building.

Thank you, gregfisk! Yes, I think I've come to the right place.

I will have the option to eq, but will it require more excursion than Beta 15's 4mm xmax? Or will I run out of power before that with my 90 watts per channel in 8 ohms for the woofers?
It will depend how loud you will hear or how large are your room.
If the VC beat in the botton of magnet motor you will hear.
The coax I want to use is 12" while the woofer is 15". What would happen if I rear mounted both drivers in 11" holes?
Dont do it!! made all drivers front mounted.
Put the Beta15 very near the floor; I suggest this panel:
Plans on Visation site or here:

Thanks, FullRangeMan. If the holes for the drivers are appropriately sized, is there any reason not to rear mount the drivers? I want to router the edges 45 degrees, so they look like this:

[richidoo]

Btw, which 12" woofers did you use on your open baffle?

AE Speakers Dipole 12. Best of the non-servo OB-specific bass drivers, imo. But the Qts is  .7, ideal for passive OB drivers. I don't prefer the sound of high Q drivers. Leaves too much detail on the table for my taste. I prefer to use lower Q driver with stiffer suspension, then apply electronic boost to force it to play lower and louder to compensate for the OB losses. Most people don't mind and adore the high Q bass OB drivers. Technically it is a great invention, makes passive OB speakers possible.

I want to router the edges 45 degrees, so they look like this:

I think that will work fine. The problem with the edges comes when the driver is playing high frequencies, when the wavelength is close to the size of the dimensions of the edge/bump/obstruction it will cause diffraction, or the spraying ("prisming") of the sound which changes the amplitude of some frequencies heard at the listening spot and thus changing the tone of instruments. It's not different than putting a tweeter into a rectangle box with narrow baffle and sharp box edges, it happens all the time on the highest pedigree speakers, because sharp edges make good WAF and that affects sales. Frankly the compression driver has it's own diffraction issues due to the shape of the exit hole and transition of the exit hole to the cone horn. People just accept diffraction because fixing it ain't easy/pretty/cheap. The 15A will be playing up to 300Hz or so, which is 4 feet wavelength, so 2" obstruction will be invisible to diffraction. The tweeter band is where smoothing the edges makes an audible difference. Since a compression driver is a miniature horn itself it will direct the sound mostly forward, especially in the high frequencies it will beam forward because you're not using a controlled directivity horn. There will be a small amount of diffraction of the lower frequencies of the tweeter band from the wood lip on the hole edge, but if deeply chamfered should not be a big problem. You could experiment putting a layer of absorptive material on the perpendicular inside edge of the lip to attenuate the diffraction and see if it makes any change. Try to make the chamfer as large as possible to minimize the perpendicular edge of the hole. Using driver mounting cleats instead of flange mounting screws will allow you to move the mounting screws away from the chamfer so it can be as large as possible.

[FullRangeMan]

If the holes for the drivers are appropriately sized, is there any reason not to rear mount the drivers? I want to router the edges 45 degrees, so they look like this:
Rear mount is good, dont forget the top hat or similar cap:

[JMH]

AE Speakers Dipole 12. Best of the non-servo OB-specific bass drivers, imo. But the Qts is  .7, ideal for passive OB drivers. I don't prefer the sound of high Q drivers. Leaves too much detail on the table for my taste. I prefer to use lower Q driver with stiffer suspension, then apply electronic boost to force it to play lower and louder to compensate for the OB losses. Most people don't mind and adore the high Q bass OB drivers. Technically it is a great invention, makes passive OB speakers possible.

I actually looked at those the other day. They are unfortunately too expensive for my budget. And I would also like to try a woofer with Q around .7 anyways.

There will be a small amount of diffraction of the lower frequencies of the tweeter band from the wood lip on the hole edge, but if deeply chamfered should not be a big problem. You could experiment putting a layer of absorptive material on the perpendicular inside edge of the lip to attenuate the diffraction and see if it makes any change. Try to make the chamfer as large as possible to minimize the perpendicular edge of the hole. Using driver mounting cleats instead of flange mounting screws will allow you to move the mounting screws away from the chamfer so it can be as large as possible.

Thanks. I will chamfer as deep as possible and if I get a lip I will have some absorptive material there. And I will sand down the edges a little so there is a more smooth transition. I had not thought about flange mounting screws. But it says its for speaker grills. Do you think they can hold a woofer securely?

Rear mount is good, dont forget the top hat or similar cap:

Are you referring to the piece of plywood on top there? What is its function?

[richidoo]

I actually looked at those the other day. They are unfortunately too expensive for my budget. And I would also like to try a woofer with Q around .7 anyways.

Beta 15A is Qts .58. If you are using active EQ it will be fine. The higher Q will reduce the amount of boost needed from the bass amp. If you want really high Q, look at the Alpha 15, a favorite for OB experimenters. IMO, lower Q sounds better, as long as FR target is met.

Thanks. I will chamfer as deep as possible and if I get a lip I will have some absorptive material there. And I will sand down the edges a little so there is a more smooth transition. I had not thought about flange mounting screws. But it says its for speaker grills. Do you think they can hold a woofer securely?

To reduce diffraction the radius of the smoothed edge is comparable to the wavelength you want to reduce diffraction. So sanding 1/8" radius will only affect very high frequencies with wavelengths above 1/2" or so. If you want to reduce diffraction in midrange the edges should be like 6" radius. This is not possible in OB, so just don't worry about it. Bring the chamfer down as far as reasonably possible will help, but that's all you can do.

Those might be for grills. But you can apply the same idea to the frame mounting. Make your own larger cleats from hardwood or metal. This is what many pro speakers do. The only reason for this is because the full chamfer of the edge will reduce the thickness of the baffle where the flange screws would be, so they would penetrate the front of the baffle.

[FullRangeMan]

Are you referring to the piece of plywood on top there? What is its function?

Sound deflector.