The CSS-FR125S in 4.5 litre aperiodic (planet10 design)

[Speedsmile]

Well first post here, I've to say I'm reading along here for a while and have learned a lot in the process.

Some 3 years ago I built 2 floorstanders from a kit (Vifa Filligran), these are still playing in my room as I speak, and I'm very happy with them.
However my hands started itching again and with the last exams in the pocket I have the time to do a little project again.

This time I want to built a small portable but still good quality speaker for use with my PC, and eventually to take it with me to a 3.5 month internship I'll be doing in Norway starting in September. The thought of being without decent quality sound for over 3 months kind of made me nervous....

So after some research I came to the conclusion that the FR125S is a very promising driver, and given my space/portability constraints the 4.5 litre aperiodic from planet10 seems like a perfect fit.

The problem is, the proposed enclosure on the planet side doesn't really fit my size constraints regarding depth and width. Therefore I redesigned the volume to a taller box, in the process dismissing the trapezoidal form factor. So the questions:

- Is it correct that this form factor was mainly chosen to reflect the waves going to the backwall down, and thus preventing reflected radiation through the driver itself as much as possible?
- How much of a problem am I going to get with my design (shown below), and are there any extra steps I can take to prevent this?
- As you can see I placed the 'vent' on the back, but I'm wondering if this is going to cause problems if I place them very close against a wall?. maybe it is better to vent to the side?

So to flourish this long story up a bit , here a few cad drawings, and a 3dsmax render of the finish I had in mind (nevermind the Reference
).





I'd like to add my mind is set on these drivers, and already ordered them, mainly because if this project is not going to work out I always can build something with less compromises like the Fonkens out of them.

Furthermore, if anyone is interested, because I'm doing a master in Mechanical Engineering I've access to quite a few fancy numerical (FEM) programs, and I plan on doing a (structural) eigenmode analysis on this design in the weekend just for the fun of it...

[planet10]

Originally posted by Speedsmile
- Is it correct that this form factor was mainly chosen to reflect the waves going to the backwall down, and thus preventing reflected radiation through the driver itself as much as possible?
- How much of a problem am I going to get with my design (shown below), and are there any extra steps I can take to prevent this?
- As you can see I placed the 'vent' on the back, but I'm wondering if this is going to cause problems if I place them very close against a wall?. maybe it is better to vent to the side?

1/ yes... the shape can be mutated as required
2/ you might have to play with damping a bit more
3/ as long as there is almost any clearance behing the terminus you should be fine.

dave

[chrisb]

Well, I can see that Dave sniped in a quick reply to your questions, so I 'd add the following:

This is quite a surprising performer for its size, and makes for a great nearfield / computer monitor or as part of a HT system with stereo subs, but it will not begin to approach the same driver in the mini-Onken design. (FWIW, allow me to be anal for a minute, as the christener of the design's name; the F in Fonken family moniker designates the use of Fostex drivers )

If you do build the little guys and find the performance missing a bit, and if the complexity of the onken style remains a deterring factor (having built more than couple pairs of them, I can understand that), you could always consider the more conventional flat front paneled GR design. In or listening comparisons, it's quite close to the beveled front design.

Of course if you're very tight for space, either of the larger cabinets will require some creative juggling - they definitely like several feet of free space surrounding them, and a decent floor stand.

It will be interesting to see what your FEM analysis of the design predicts, and how that correlates (or not?) to your final listening impressions of the speakers in their final environment.

[OzMikeH]

If space is limited you don't have to stick with a small enclosure, make it a 2 piece cabinet, the top half (with the driver) smaller than the bottom half.
Picture the top half as a top hat shape, with a flange.
The bottom half is a larger box that the top sits on and is screwed down to.

for transport you unscrew the top hat, tip it upside down and stick it inside the bucket. As a bonus it protects the driver.

[Speedsmile]

Thank you for your comments, so as I understand it I can get away with this configuration without to much problems, nevertheless plan on making a test cabinet from some scrap wood I've lying around, just to get a feel for the overall speaker.

I just finished doing a quick eigenmode analysis in Unigraphics NX4, the results can be found under the link.
Nothing unexpected though, the modes are mainly equivalent to the usual plate modes. I also want to make a analysis with the driver itself in place, but I don't have enough info on the basket material and bulk weight of the driver to do that now, does someone have more info on this subject?

Eigenmode analysis

So I would think that if this is any indication on the sound of the speaker, it would sound a little bit brighter in the upper mid range 600-1000 [Hz], which is not necessarily a bad thing.

Furthermore for the folks who are less familiar with structural vibration modes I made some movies of the first 4 modes to illustrate them...

Mode 1
Mode 2
Mode 3 Mode 4

[consort_ee_um]

Nice pictures and nice FEM modelling Not sure I believe the bending mode because of the stiffness of the corners will tend to stop that but the panel "breathing" modes seem right.
Can you try fitting bracing to the model and see what happens?
The FR125 has a phase plug and air can leak out between the coil and plug i.e there is no dustcap. The cabinet loss due to leakage is therefore high which affects any bass alignment you care to try.

[Speedsmile]

Originally posted by consort_ee_um
Nice pictures and nice FEM modelling Not sure I believe the bending mode because of the stiffness of the corners will tend to stop that but the panel "breathing" modes seem right.
Can you try fitting bracing to the model and see what happens?
The FR125 has a phase plug and air can leak out between the coil and plug i.e there is no dustcap. The cabinet loss due to leakage is therefore high which affects any bass alignment you care to try.

You're right regarding the 2nd and 4rd mode, these will probably not occur very fast, and only do show up this low because there is such a big hole in the upper part of the cabinet, with the drivers modelled along I expect them to completely disappear.

[binarywhisper]

I would think that bracing would make a substantial difference in the modeled results.

You think much air passes thought the voice coil gap? I'd be interested to see that modeled.

I have a pair of 12" woofers with no dust caps running in sealed boxes. I really can't imagine that much air passes thought that space. Many of the vintage woofers had very flimsy dust caps and I've never seen one appear to act, or react, in any way. Not very scientific but none the less, being a huge fan of sealed inclosure's I would have thought I'd have seen a dented cap on one of many old 12"ers I've owned at least try and pop out when I was thrashing out the metal tunes on em.

It's an interesting thought. Sounds like a great project for an engineer to figure out

[Speedsmile]

Originally posted by binarywhisper
I would think that bracing would make a substantial difference in the modeled results.

You think much air passes thought the voice coil gap? I'd be interested to see that modeled.
.........

It's an interesting thought. Sounds like a great project for an engineer to figure out

1. Bracing will probably make a significant difference, the problem is, I don't have much room for it. However as you see in the drawing I'm planning to use 12 mm MDF on the front and 8 mm for the remaining of the cabinet, an interesting question could be what would be more (weight/volume) efficient? Partially braced 8 mm, or plain 12 mm.
But I'm not planning to model that before I know more of the driver itself because the metal will seriously effect the stiffness in that part of the cabinet. So again, anyone of the CSS owners..how much does that driver weigh? CSS doesn't list it in the spec, searching on internet gives 1.3 kg as an approximate...

2. I think the airgap around the voicecoil will not have a very big impact if you consider how small it is, furthermore the spider limits airflow even further. If I had some exact dimensions I could probably model the static flow through it, and from the timescale involved say something about this effect in dynamic behavior, but I doubt it will be small enough to effect say 60 Hz bass. A thought experiment with a 'squeezing' baloon shows exactly what I mean....

3. Indeed a great project for someone like me, finally all things I learned come to good use....

[binarywhisper]

Sorry, I don't have any non-installed css125's to weigh although I have 5 of them ordered.

They do not have a metal basket, its plastic so I'm not sure how much reinforcement they will add to the hole. Some obviously but its not a particularly ridged plastic although it does appear quite inert.

well on a box that small I don't think there is a need for a regular brace. Two, maybe three hardwood dowels would probably be plenty. Use one as a cross brace, another from the magnet to the back wall of the box and maybe a third bracing the front panel to the rear below the driver. Just a thought. I'd personally brace the driver against the back panel at the least.

If my drivers show up before yours I'll weigh them and post.