A lot of comments on the Bd15 cabinet (BD-Design)

by GC, Sunday, February 11, 2007, 07:50 (6292 days ago) @ angeloitacare
edited by GC, Monday, February 12, 2007, 19:56

Dear Angelo

The cabinet:

I think your'e on the track with something that many will find nice. Wifes as well.
The "aura" of this speaker will have to be sustained by outstanding craftmanship and mouth watering finish. That would appeal to the customers you are aiming at, or the customers buying from the manufactorers your are inspired of. It is those customers you have to "steal". Customers for this kind of stuff are a relative constant sum of people. It's hard nowadays to grow new customers, like we do when culturing potatoes or salad in our gardens.

I have noticed your'e a bit unsure about how big/small the cabinet should be, considering the visual dimensions vs. accoustical ideals.
So from a popular point of view, you would have most succes in having them as small as possible (Paying respect to right visual porportions though). That is likeable for most people. Wifes as well again.
You would solve a "big" problem following the advice from Bert incorporating two basses in the P-P-P-P config.
Rarely it is the pricetag scarying the customers away if they like what they see. And you would not sell these speakers for less than.....????? Would you?

A complete different topic is

The X-over

I will use the term "accoustical phase" here as electrical phase has nothing to do with what we hear. It is the phase from the trancducers pistons we are inetersted in, not the the electrical phase happening in the filter.

You are up against Mother Nature here with a displacement between the DB 15's and the Orpheans of, was it, 60 cm's?
That requires a bit experience to make that turn out positively.
What are we talking about here? The phase my friend. The accoustical phase.
I would claim that correct accoustical phase is the key to good sound over that of a flat response.
From an ideal point of view the travelling distance between the involved voice coils and your two ears should be exactly the same. Further the travelling time for all audible frequencies should also be the same; meaning that only certain x-over philosophies would fit here. (And forget about any ideas of designing an x-over according to mathematic formulas written in any book.)

Now your design does not exactly make life easy for you here. The only "luck" you have is that the x-over point is relatively low , approx 250 Hz. The wavelengt here is around 1,32 m. So with your 60 cm displacement, your basses will be displaced appr. 180 degrees accoustical, equal to some very audible msec's.
Aha...just reverse the electrical phase 180 degrees right? Nope.
But would that really be audible. You bet it will. The bass will, if you don't take your precautions, arrive with an earlier train at the station than your mid/highs.
That blurrs everything and affect the whole transparency of the speaker in a negative way.
And I am not talking about summarizing the system to liniarity. That's an easy thing to do, but an unhelpful thing to do.

So if you are not an expert in these matters I will advice you to use a very steep accoustical filter between the basses and the the mid/highs.
The steeper the better. This will help you a little, as the drivers should not summarize the freq. curve and the accoustical phase over a wide freq. speactar, but merely only at the x-over point and a little on each side of the x-over point, leading to the fact that they should either summarize the total accoustical phase so much. Remember at e.g. - 3 db down, the phase shift is 45 degrees!!!
As the x-over point is as low as 250 Hz, (which is actually not so low, not far away from the real midrange) you might have some luck to make the two driver systems communicate. But...not perfectly.
There are some hokus-pokus you can do with the filter as letting the bass cut less steep just arround the x-over, but not far away from that point, that creates a sort of over-shoot on the freq. curve. That would make your bass act "faster" where the over-shoot appears. Contrary you could let the Mid have an under-shoot at the same point making it slower. And a possible handshake between the two comes closer.
But that has also a lot of unwanted side effects on the liniarisation point of view.
Incorporating the two basses in the PPPP config will also help you a little in this respect, as the accoustical center of the bass then will be exactly in the middle between them, meaning some 10 cm deeper into the cab. Now you only have 50 cm's displacement. Better than before, but still not ideal.

You are helped a little by the fact the the mid/highs are considerably faster reacting (pulse response) compared to the basses. This is due to the diaphragme/mass/magnetical power ratio.

Now the above troubles you could solve by throwing in a digi device and an extra amp correcting time traveling problem, but that was not your intention I guess?

Now I did not point out the sad story by using steep filters. Just to make you completely unhappy
You combine a "fast" horn with "slow" bass, and exactly by using steep filters you also seperate the sound caracter of the two. From and ideal point of view they should overlap each other broadly to optimize the sound mixture. They don't sound the same way the two systems, right?
But you don't have that option as I see it.
Bert took these matters into consideration in the SWINGs by placing a horn in front of the bass, speeding up things at the x-over point and optimizing caracter between the two systems. And he has even far less diplacement between the two driver systems. These facts he already thought out in the design phase. (Good boy)

I just want to tell you that you will have to spent some time to maximize the sound here.

Angelo, I don't know anything about your skills in this department. If you are completely "green" here, you would need an experts help.
If Bert is willing to help you out I don't know.
But if I should help you out, it would cost you quite a sum. This takes time, a lot of time. My hourly fee is a humble 150 EURO/hour. :scared1:
A joke of course, as I don't have the time, neither the wish.

Choice of materials:

Let me just here express my hate to MDF and alikes. MDF sounds very dull and swallow all harmonics from the unavoidable resonating cabinet. Add it's own dust/glue caracter, which is awefull.

GC:wink:

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