Aperiodic alignment ? for speaker design gurus

[konut]

I am looking into designing an enclosure utilizing 2, CSS WR125STs. The goal is to have an enclosure as small as possible with flat extention to 80-90hz with a steep rolloff below that, as I'll be utilizing a sub. The specs of this driver can be found here http://www.creativesound.ca/details.php?model=WR125ST I would like to keep the baffle of the enclosure as small as possible and use the depth of the enclosure as the variable to attain the optimum volume. One of the reasons for this is to be able to have the option to use either a vertical or horizontal orientation for the top mounted naked tweeter that will be mounted on the same plane as the woofer's acoustic center. In my researching this subject I have seen references to aperiodic loading to achieve a smaller box volume for a given target extention. So far the only modeling program I have found that computes values for this type of alignment is BoxModel 1.0 for Windows. Does anyone know of another? As I am a complete neophyte at this kind of thing, are there any pitfalls to this type of approach? I would REALLY appreciate it if someone could model this for me as I have no experience with this sort of thing and lack any design software for this purpose. TIA!

[Ron E]

An aperiodic box can be considered asa lossy ported box or a leaky sealed box - it exists in teh grey area between these two types of enclosures. You can model with a typical program using a Ql of 2.5 If you have a program that allows you to enter Ql, Qp, Qa, try Ql=Qa=10 and Qp=3.

That should give a reasonable simulation of what an aperiodic box will do for you. You will find that it does not give extension, it taketh away as compared to the same size sealed box.... The losses act like a notch filter at resonance to reduce output and "effective Qtc", while raising F3.

If your box constraints are that tight, there would be nothing wrong with jamming them in a 3 or 4 liter enclosure, stuffing it well and only resorting to aperiodic if you think they are a little too rich in the bass - doubtful, though.

my $0.02

[konut]

Thanks for the response Ron E. As I have no program what so ever it would be difficult for me to get any starting point for this project. In a thread in the full range section, someone recommended a 14 liter enclosure for this type of design, so I'm at a loss as to any realistic starting point. I had hoped some one would take pity on my miserable soul and do it for me. Anyway ,I appreciate your input.

[Ron E]

I told you what I would probably do. IF you want a full rationale - here you go - classic aperiodic requires you to design a sealed box for a Qtc of perhaps 1.0-1.3, then put the hole in it. with the fibrous material. There are perhaps rules of thumb for how big the hole has to be, but it would be just as easy to determine through experiment since the more resistive the material, the larger the hole must be and vice versa.

I think you will find, once you download some software - (although I did the calculation in my head), that a box size of 3-4 liters will give you a Q in the 1.x ish range.

Since I don't think a Qtc of 1 sounds bad in a speaker with a cutoff of this area - I suggested you forgo the aperiodic fussiness and experimentation and leave it sealed with a Q of 1 or so and only try the aperiodic if you are unhappy with it as it stood.

If I were you I'd worry a lot more about the crossover and baffle step compensation etc than the bass alignment - it will affect the results an order of magnitude more.

[konut]

Thanks for giving me realistic perspective on this. I have no qualms about using graphic equalisation to compensate for baffle step. I also will be using an electronic crossover to mate with a top mounted Morel tweeter so I can adjust crossover freq to taste. Thanks again!