Nelson Pass: The Slot Loaded Open Baffle Project

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To follow up my previous post (#119). The implication is that the driver diaphragm actually will move less for a particular SPL than the same driver in free space must move. I submit that this is the source of most of the disagreement on the subject.

Jeremy

No, it is slightly more complicated than this.

The driver diaphram will move less, because of increased acoustic loading. The intertwined result is a lowering of back EMF, causing the impedance of the driver to go down, and therefore I to go up. More energy enters the driver and gets transferred into acoustic energy, but with less cone excursion. Like with horns and correctly tuned BR's.

vac
 
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To follow up my previous post (#119). The implication is that the driver diaphragm actually will move less for a particular SPL than the same driver in free space must move. I submit that this is the source of most of the disagreement on the subject.

Actually, my point is fundamentally that the response is asymmetric. I don't
actually care whether it is more efficient on the whole.

If you want to decide that the front loading is causing less excursion of the
diaphragm and thus less output on the rear, that would be OK with me.
 
Let's come back to "DIY" portion of things.

This is what I've done during the weekend (when you guys were busy on debating...)

DSCF0617.jpg

They'd be lying on floor anyway, so i skipped one of the walls. The white stuff are foam strips for better sealing with floor. Simple, quick and dirty, my style :D



DSCF0619.jpg

The front(upper) baffle is not completed yet. That'll be provided by another section of the system. Despite short of baffle area, it's still workable.

The woofer is 18" (Sd=1140cm^2), the slot is 91cm wide by 5.5cm high (500cm^2), which is about 44% of Sd. With such aspect ratio, I guess the effect of 'compression' would be higher than the number suggests.

.....
 
Hi

I believe you will find that if one made a simple sealed box using the same size baffle and same number of drivers, one with and one without slot loading, that they would measure identically at the low frequency end as the “velocity transformation” is accompanied by a corresponding reduction in the area. The result is "slot loading" has essentially no effect on the radiation efficiency at low frequencies.

As sreten suggests, it will alter the directivity up high and higher up (at frequencies where the woofer spacing would produce a narrower pattern) , the depth of the port will produce a cancellation notch at frequencies where path length from the driver radiation to the closed end is N quarter wavelengths (the reflection returns 180 degrees later).
Also, the mass of air at the exit and compliance of the air in the slot form a resonator which the drivers are couple to and this is like the low pass filter in a band pass system. This adds a small extra mass to the total moving mass too.
This can provide a gain at the higher end of the spectrum as well as an acoustic low pass filter. Now your entering horn territory.
Best,
Tom Danley
Danley Sound Labs
 
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Those of you who know some of the backstory can imagine how that made my day.
And, no, I haven't had a change of heart regarding some of the members here.

Grey

Hi Grey, I'd like to say Welcome back! I hope this won't be your only post. :)

There are always people who rub us the wrong way, I find it best to just avoid them if possible, and where not possible to allow time to cool down before posting ;) I don't know the back story, but I think you know as well as I do, why certain things said privately need to remain private. There have at times in the past been threads for actively discussing moderation of the site, at the moment that is not the case. Feel free to raise any issues via the report post button though.

I'm sure you know what a balancing act effective moderation is. I hope you can put behind you any history and become an asset to diyAudio once more. I know I'm mentioning moderation in this reply, but if you wish to discuss it further please PM me as this thread is not an appropriate place for that discussion :)

Tony.
 
Actually, my point is fundamentally that the response is asymmetric. I don't
actually care whether it is more efficient on the whole.

If you want to decide that the front loading is causing less excursion of the
diaphragm and thus less output on the rear, that would be OK with me.

The asymmetry is the most important feature, agreed, but improved efficiency wouldn't be bad.

It took me two days to convince myself it was worth entering the fray. After posting, it took about 20 minutes with a saber saw to bring into existence a small, single driver example. The SPL meter and my ears confirm it is a very effective mechanical crossover at about 800 Hz. It sounds just okay, but without the baffle to hide the rear of the driver it is a full range speaker.

Jeremy
 
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my brain hurts....
slot volume vs driver area* Frequency range and I'm assuming ~1 to 400hz* avg excursion. BTW I like the single sided driver slot as you could use a wall or floor as one side of the slot. I can see dollars flying out the window for drivers as I visualize a slot sub woofer/open baffle mid-low tweet/ with some kind of super tweeter... can you say 6 channel amp with active crossover.

It's also nice to have some color commentary even if the color is grey...


regards, E
 
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