[hubert]

Hello all,

I'm currently thinking about building a 3-way using the new Seas CA12RCY loaded in a separated box, with a "partly-open back". The depth will be 11.26" (1/4 the 300hz wave lenght, the lowest fr of the mid-Xover).

I do not want to use damping materials because they decrease the Qms of the driver and thus the dynamic.

Also, I do not want a real dipole, only hole(s) allowing to let the wave-pressure "escape" out of the inside.

At the moment I think that the most efficient shape could be a kind of double tapered line going from the middle of the box to the rear, making two fine and wide ports, in a similar way than the B&W nautilus 800 sphere/tapered tube in their midrange box.

Any other ideas/links?

Thanks,

  Elsass beers are good, white wines are better.

[timbley]

I'm glad you're asking this question, because I too am trying to figure out how to optimize the enclosure for a midrange driver cutting off at 300Hz. The terminated line seems like a good idea, but there's so much to consider as far as the taper and shape of the enclosure. Some say that tapering doesn't really do anything for the sound, just as long as there's adequate depth of absorptive material to attentuate the lowest frequency backwave.

[Hogg]

These types of questions are best posted on the Madisound forum.

http://www.madisound.com/cgi-bin/discuss.cgi

                                                            Jim

[JohnR]

Also, I do not want a real dipole, only hole(s) allowing to let the wave-pressure "escape" out of the inside.

Well, if the wave escapes from the box, then it will interact with the front wave in some way. Damping material reduces the amount of the back wave... I guess I don't understand you reasons for not using it - ?

[_scotty_]

You might try an aperiodic enclosure. Here is a link to an explanation of the design.    http://www.elitecaraudio.com/article.php?sid=18
Hope this helps. Scotty

[hubert]

Thanks gents for your answers.

The real matter of this post could lie in these few considerations:
To get the higuest accuracy from the midrange, we have obligatory to cancell the rear waves; there is so much pressure inside a box !!! The shape of the box has little/no influence on the amount of the energy, even spherical
http://www.bwspeakers.com/index.cfm/fuseaction/products.techfeatures/ObjectID/3A7556B4%2D4779%2D11D4%2DA67F00D0B7473B37
 or irregular
http://www.eventusaudio.it/tecnologia/sacc_ingl.htm#
The problem is this:
- if you use a huge amount of damping material, you get a huge damped rear wave, but you decrease the Qms of the driver and you get ... less dynamic. (so, Scotty, no aperiodic enclosure; sure, for a car subwoofer, yes; for my midrange, not.) And, JohnR, did you listen to Hales speakers? Hales guys allways claimed: "no reflex, only sealed"; they were right but used to much damping materials, so their speakers sounded very dead.
- if you use an open baffle in adequate sizes, you get your goal, but also you get a dipole. I believe that the point source is the best system to get right soundstages and image; so, no open baffle for me.

I think really that B&W guys are right with their sphere and terminated tapered tube.
 

[JoshK]

I believe that the point source is the best system to get right soundstages and image; so, no open baffle for me.

What does point source have to do with open baffle or non open baffle?  Two totally different topics. You can have point source and open baffle and non-point source and non-open baffle.

You can also dampen the rear wave of an open baffle if you are a little creative.

[hubert]

What does point source have to do with open baffle or non open baffle?

Well, nothing in an ideal world (free field, no room wall bounces)
In my relatively small listening room, the open baffle would add some undesirable delayed waves.

You can also dampen the rear wave of an open baffle if you are a little creative

Tony Gee, about its "Progress":

I added some absorptions to the rear wave coming from the open baffle midrange unit...result...a little gain in clarity, at the cost of dynamics and spaciality, so I decided to leave them out.

BUT, I agree that it doesn't exist one or more ideal conception about any acoustical matter because of the interactions in physical laws (action/reaction). What we all try to make is to go near what we believe the better and to discuss together about.
So, Josh, if you think that you can help me, make it please and give me  your knowledges.
Regards,
 

[_scotty_]

An aperiodic enclosure is an unpressurized enclosure and could be damped internally with Spectra Dynamics Deflex panels.   http://www.oregondv.com/Spectra_Dynamics_Deflex_Panel.htm                 I would damp the enclosure
with Deflex internally and start drilling holes until the impedance rise at resonance stabilized and quit there. This should result in a reasonably sized enclosure with excellent transient response.  You can always experiment
with a number of different designs until you find something you like as well.
Depending on the Theile-Small parameters the driver has, a sealed enclosure
is not a problem.  The size of the enclosure as well as the low frequency cut off of the driver determine the amount of pressure that is in the box.  The lower the frequency the greater the excursion and the higher the pressure.
With 300Hz crossover and 12 dB/oct slope your excusions shouldn't be excessive.  Assuming  5inX5inX 11.5in internal measurements which put your box volume equal to the VAS on this driver you still may be over damped. I think your total system Qts will be under .5 with this box volume.   I would agree with Hogg you should probably pose your questions at Madisound forum for a more detailed answer..  http://www.madisound.com/cgi-bin/discuss.cgi
Scotty

[hubert]

Hello Scotty, thanks for your answer.
What do you mean with "unpressurized enclosure" about the aperiodical loading?
As far as I understand, the membrane behind the woofer lowers the air flow, so the impedance peak is decreased at the resonance frequency of the system driver/box.
To lower the air flow obligatory decreases the Qms of the driver. When Qms goes lower, the mechanical work of the driver is perturbed, it does not work well, it is overdamped, so dynamics are weaker.
IMO, this system is good for a sub-woofer, to make it work in small spaces, not for bass and upper range.
What makes you think at good transients on the range up to sub-bass?

And OK, I will perhaps pose a question at Madison forum.

Regards,