Carbon Fiber open baffle dipole panels

[thadman]

I spoke with one of the Materials Science professors at my University and apparently they have some surplus Carbon Fiber I might be able to use. The material is currently being used by his Grad students for fatigue testing for military aircraft. I am not sure of its exact parameters, however I believe it is very expensive. The splinters are not very fun to pull out of your hands.

Would carbon fiber be a good material for an open baffle? Intuitively, I would not assume it to be optimal since a resonance will probably occur within the bandwidth of the loudspeaker. However, I'm not sure what would be more optimal other than maybe Corian.

How many plys would you recommend? 8?

0, +45, -45, 0, 0, -45, +45, 0

He also has a large amount of honeycomb Zylon I might be able to use for a core. However, I'm concerned the panel may become excessively thick (1/4")...or maybe I'm overthinking things.

Would you use a Zylon core?

How would you mount the drivers to the Carbon Fiber? I'd like to use the SS 15Ms I have laying around.

I've got a pair of Peerless HDS tweeters I could use for the HF. However, they wouldn't be dipole. Alternatively, I could build a small DIY open back ribbon and bond the CF directly to the C1010 steel used in the magnetic circuit. He has some VERY strong epoxy (can withstand 20,000lbs of force).

If I used the SS 15Ms in a MTM format, how long would you make the ribbon? I'm thinking a 1500hz crossover would be a good place to start.

Should I let the baffle "swing" or use steel bracing for floor mounting? If braced, the resonance may be directly excited.

Thanks,
Thadman

[twest820]

The most interesting application I see for composites in a dipole is shaping H and, to a lesser extent, U baffles---if you think about it, as usually made they're basically poorly built horns and should benefit from reduced rectilinearity. Can you clarify your design, the design goals, and link the 15M datasheet? From the above it sounds like you're thinking a flat baffle two way but desired SPL levels and so on aren't specified. If you check this forum there are several of active or recently active dipole build threads that cover baffles extensively.

Most carbon fiber's neither particularly cheap or particularly expensive, though if the research in question is using milspec weaves it'll be expen$ive. But for an audio baffle I wouldn't necessarily expect any great advantage of fiberglass, kevlar, or other composite layups. I'm curious, however, of your motivation for keeping baffle thickness under 6mm.

[Few]

I'm working on open baffles made out of kerfed mdf as a core and several layers of fiberglass as a skin. Your carbon fiber should work fine, but its main advantages (high stiffness to weight ratio) may not be used to full advantage in your application.

The choice of core--material, thickness, etc.--will have as much impact on your baffle's behavior as will the fiber skin. I think Rockport is really going about this the right way so you might check out their site: highly damped core with thick fiberglass or carbon fiber skin. Of course if you're of the "stiff and light baffle" school then you might take a different approach. I just don't happen to be a member of that school.

I see twest820 posted while I was typing. I second his question regarding the thickness of your baffle. Why so thin? Stiffness scales very nonlinearly with thickness so a little extra thickness goes a long way.

Few

[thadman]

Originally Posted by Few

I see twest820 posted while I was typing. I second his question regarding the thickness of your baffle. Why so thin? Stiffness scales very nonlinearly with thickness so a little extra thickness goes a long way.

Few

Airflow.

At high frequencies, the magnetic circuit will begin to look like an H-frame. As a result, standing waves and diffraction will result if the baffle is too thick. For 20khz reproduction, the full depth should not exceed 4.5mm.

With midrange / midbass drivers, the baffle is often chamfered behind the frame. This is done to minimize turbulence and maximize airflow. If the baffle is made thin, chamfering will not be required.

[thadman]

Originally Posted by Few

I
The choice of core--material, thickness, etc.--will have as much impact on your baffle's behavior as will the fiber skin. I think Rockport is really going about this the right way so you might check out their site: highly damped core with thick fiberglass or carbon fiber skin. Of course if you're of the "stiff and light baffle" school then you might take a different approach. I just don't happen to be a member of that school.

I hold a similar opinion. Resonances are unavoidable above 500Hz. It's simply unrealistic to assume a baffle could achieve such things. We might minimize the amplitude of the resonance by damping as you suggest.

At low frequencies, however, light and stiff would be the way to go.

I understand these materials are not optimal per say, but you can't beat free.

[twest820]

Originally Posted by thadman

Airflow.

Not exactly clear what the previous post is about; taking a tweeter apart to better understand how they're built before proceeding further might be in order. If you're referring to the ribbon case I would suggest taking a closer look at the Neo3.

[thadman]

Originally Posted by twest820

Not exactly clear what the previous post is about; taking a tweeter apart to better understand how they're built before proceeding further might be in order. If you're referring to the ribbon case I would suggest taking a closer look at the Neo3.

I'm familiar with how tweeters work. The ribbon is perhaps the simplest mechanical transducer in terms of topology. We might think of the magnetic circuit as a "duct" in which the ribbon is located.

http://www.linkwitzlab.com/images/graphics/hframe1.gif

At high frequencies, the thickness of the baffle approaches and often exceeds the physical wavelength of sound waves. Since the maximum depth for the duct is already defined (<4.5mm for 20KHz), the depth of the magnetic circuit will decrease as the depth of the baffle increases. Obviously this is undesirable since we are losing linear Bl.

The idea is to place the h-frame resonance at least an octave outside the bandwidth of the transducer, since it cannot be equalized.

[twest820]

Can you post a figure describing this design and explain the requirements to baffle the ribbon and why the ribbon magnet structure determines the thickness of the baffle everywhere rather than just near the ribbon? Seems to me this approach makes things unnecessarily difficult.

[thadman]

Originally Posted by twest820

Can you post a figure describing this design and explain the requirements to baffle the ribbon and why the ribbon magnet structure determines the thickness of the baffle everywhere rather than just near the ribbon? Seems to me this approach makes things unnecessarily difficult.

My last post may have been a little unclear. I was talking about distribution of variables. If the baffle depth (as a design variable) increases, then the magnet depth (as a design variable) must decrease to satisfy our total depth requirement (4.5mm).

baffle --> magnet --> baffle

A little more clear?

Here's the Scanspeak pdf

http://www.madisound.com/catalog/PDF...w_8530k01e.pdf

I've also got a pair of Beyma TPL150 AMTs that I've been wanting to use. They've been up for sale for a while, but no buyers, so I might as well use them if they'll work. However, they're probably overkill. I only need 100dB peak as I live in a small apartment. I doubt the other renters would appreciate any more than that.

I see you've got a Neo3 in your avatar. Do you have a lot of experience with that particular driver? How do you think it would handle a 1500Hz crossover point dipole?

[thadman]

Has anyone tried out the SB Acoustics 4" midrange?

http://www.madisound.com/catalog/PDF/SB12NRXF25-4.pdf

It's impedance doesn't appear to have any significant aberrations, except the mild dip at 250Hz. The price is nice as well.

Any thoughts on a 3-way

SB17NRX35 --> (500Hz 2nd order) --> SB12NRXF25 --> (2500Hz 2nd order) --> Neo3