First Boombox project, need advices!

[MickB]

... one I have a lot of experience with and have always exceeded my expectations is the Monacor SP60. I buy these little woofers for less than €10 a piece and they are super efficient, have quite low resonant frequency, are super linear and reach up to very high frequency without nasty behaviour. Added to that the mechanical power handling of them is unequalled by any driver I know, even at ten times the cost....

V-bro, do you happen to know how much those SP-60s weigh? The specs on the Monacor site don't give a weight for that driver.

Sorry to barge in on your thread, toxicious, but it is relevant (sort of)

BTW, I agree about plywood being a good material for the box. I'm obsessed about keeping the weight to a minimum for the portable systems I build and they have to be able to take some big knocks. I use 9mm exterior (WBP) plywood. It's very light and extremely tough if you construct it carefully. As it's not very thick I use quite a lot of internal bracing (made from 4 mm ply with holes cut in it for lightness) so that the larger panels can't flex in and out from the internal pressure changes. Thicker ply is stiffer and needs less bracing but it's also heavier so it depends what your priorities are.

[Saturnus]

If I may inject a small comment in the discussion here. I'd go with a bipolar design for these driver and this use. It's vastly superior in every way. The normal caveat against bipolars are that they have lower sensitivity from the front side than a system where all drivers are front facing. That is incorrect. If the design of the front facing drivers is to be correctly implemented the resulting baffle step correction needed will take away the sensitivity advantage of the front facing driver design completely. And you're left with a system that has all the drawbacks of a front facing system, and no advantages over a bipolar design.

The reason for this are the small cabinet dimensions and the relative short distance between back and front which makes pretty much makes a bipolar an omnipolar speaker.

EDIT: I'd also not chose the Monacor D28 (or D25) tweeters. They're vastly too expensive compared to their performance. This tiny little tweeter that snap-in on a 9mm front plate only costs $8 each and are widely regarded for excellent performance.

http://www.parts-express.com/pdf/275-025s.pdf

[Saturnus]

Btw, for those fans of calculations regarding the Boominator bipole. Here's the rough calculations involved.

Since we will have a baffle step which is a fixed +6dB rise above a certain corner frequency equal to 115/m, where m is the baffle width in meters. So for the Boominator the BSF is 383Hz.

When we have 2 speakers connected to the same signal these will summarize, ie. give a fixed +6dB rise below a certain frequency equal the speed of sound divided with the distance between their acoustic center. With bipoles the acoustic center are directly between the drivers, so if we have a rectangular shaped box the distance between the acoustic center is equal to the baffle width times 3. And since the speed of sound is 343Hz the resulting summarizing is equal to 381Hz.

Let's recap, the BSF is 383Hz, and the SumF is 381Hz. So in every way, perfect. Theoretically, at least. You'll need to run a mass of measurements to assure you have the correct location of the woofer, seen from the end, and the optimum location of your used tweeter.

This theoretical perfection applies to every rectangular bipole speaker, no matter what the actual width is the baffle step frequency and the speaker summation frequency will always be identical.

[toxicious]

My english isn't the best, or I don't know all the terms yet. But I have read all the posts and tried to understand them.
Bipolar is the shit what I have read now, but if I understand Saturnus, I need to make a lot of measurments and calculating to find the perfect position for the woofers?

The tweeter that you suggested, Saturnus was indeed cheaper. But if I look on it's picture I don't understand how to fasten it to my box?

...that snap-in on a 9mm front plate...

Are you supposed to place them in a whole and glue them?

Sure, bipolar seems great if I don't have to make all those calculations, but there is one thing that makes it bad for me:
I suppose the speakers are sitting back-to-back in the box? (or like 1 cm between)
Then the depth of the box will need to be about 13cm, kind of small (think about the battery). And for example, to really use the speaker, it would need a placement in the middle of the party. You don't want to place it against a wall or outside of the group of people that wants to listen. You can't aim the sound.
Correct me if I am wrong.

I will go with a 6 ampere battery I guess then, that will be enough

If I choose the front facing setup, are these measures okay: 70x20x20cm (70 is width)?

Got my amp today btw, will solder it on Wednesday :P

[Saturnus]

You pressume incorrectly. The bipolar Boominator like design is actually a lot easier to calculate for correct linearity. The roughly estimate is that you get the perfect sound if the center of your woofer is in the middle of the baffle width, and 1.5 times the baffle width for the ends. The optimum tweeter position is similarly centered on a line 0.25 times the baffle width from the end. Position on the width will depend on the tweeter used but optimum is between in the middle of the baffle and 0.25 the baffle width from the top.

The strange thing is that such a design doesn't care where it's placed. You can place it practically anywhere in relation to yourself and any walls, and it'll sound the same, and be just as loud, as a front facing system.

As I've explained. A properly calculate bipolar system have no disadvantages compared to a correctly calculated front facing one. Only advantages (besides from the double cost naturally).

As I also explained. As long it's a rectangular tube, actual width is irrelevant. The speakers don't have to be mounted magnet to magnet but you'll lose the mechanical stability benefits if they aren't.

[MickB]

I suppose the speakers are sitting back-to-back in the box? (or like 1 cm between)
Then the depth of the box will need to be about 13cm, kind of small (think about the battery)

The speakers don't have to be mounted magnet to magnet but you'll lose the mechanical stability benefits if they aren't.

Could you mount a block of wood between the opposite speaker magnets so that they are mechanically connected but you won't have to use a narrow box? You could use double sided adhesive tape between the wood and the magnets which would be enough to hold it on place when there's a bit of pressure on it but you'd still be able to dismantle it if needed.

[Saturnus]

In principle yes but you won't get the mechanical stability even then. You'd have to glue it all together, just like you have to glue the magnets together. And it works best if the speakers are mounted on the backside of the front which basically mean that once assembled, there's no way to dismantle them again.

Anyways, there's actually 3 advantages of mounting the magnets together. Mechanical stability is only one of them:

1) better mechanical stability by the front and backplate being mechanically coupled to eachother where stresses would normally be greatest.
2) the magnetic fields are centered more in the voice coil resulting in slightly higher sensitivity, up to +1dB depending on speaker construction quality. The worse construction quality, the more sensitivity is gained.
3) stray magnetic fields are cancelled, making the same effect as if the magnets where shielded (but without the sensitivity loss shielding usually involves).

[V-bro]

All this bipolar theory holds up well for the lower end of the spectrum, but for the treble frequencies then you'd better use another tweeter in the back. With the price of that Dayton tweeter this is not a problem though...

So magnet to magnet mounting, how about the drilled pole cores? SP60 has a very nice magnet system with drilled pole core...

I wouldn't worry about the size, a battery will always fit in there... And it WILL sound BIGGGG!!

The weight of the SP60 is 900 grams, not the lightest.. And yes I would certainly brace the magnets, the baskets are relatively thin steel plate (no wonder for that price) and can bend when the thing drops accidentally.

I wouldn't worry too much about not being able to remove the speaker drivers, they are rock solid so won't be blown up that easy and they don't cost that much, so your next project might as well get a new pair of them.... Then do tie wrap the speaker cables down to the basket at a point so you get some strain relief. Or fix some of those speaker connection clips in the amplifier cabinet so you can work on the wiring without breaking the connection on the speaker drivers.

[Saturnus]

The pole piece vent could be a problem. On the HP10Ws it doesn't exist in that even though the hole is there. It's also blocked from the factory because they have instead implemented a mesh center dome. You could in principle do the same by cutting a (small) round hole in the center dome and glueing some speaker cloth over the hole to avoid dust coming in.

I'd estimate that the lost and added weight will equal itself out. And it really wouldn't affect cone stability as long as the hole you cut is (near perfectly) round.

[V-bro]

I think then the best way to do that is burn the hole with a soldering iron and then glue something in that breathes yet blocks dust from entering. I was thinking of that plastic mesh from those modern triangular tea bags.... A tiny hint of glue could then put a small cut round piece of that in place....



Still I don't really think it matters that much to block it, although I have never tried.

I also don't think the performance has to be enhanced that extra 1db because they are already astonishing even in a non bipolar design and on their own....

And one more thing, the DT28N is quite a bit more efficient than that Dayton, but that's on-axis off course. The Dayton probably has better dispersion.