[Luigi]

Having read the "Gravity well of a dark star" thread ad nauseam, I thought I had better check out some dipole bass.

I had a sealed cylindrical sub sitting around doing nothing much, so removed an end cap (damn that gorilla glue is tough stuff) and simply hooked it up to my plate amp.

It doesn't have a lot of oomph out in the room, so I tried it closer to a corner where it amped up considerably. Normally, my sealed bass booms up near to a wall or corner, but this doesn't.

Oh me oh my. Why did I not try this yonks ago?   The bass sounds more natural than previously when the sub was sealed, and seems to integrate with the mains better. Now I shall mess around with running it DVC or single voice coil.

Then I'll rip into the other sub, and have a pair of them running dipole.
All good. Finally, the Hempcones have some real solid bass foundation.
Luigi

[opnly bafld]

Hi Luigi,
Could you describe your sub more?
Length? diameter? I assume driver at one end, what size driver?
Do you know the T/S numbers? like qts, fs, vas
Good to know you like your stuff better now that you have torn it up.
Thanks,
Lin

[JohninCR]

Luigi,

Just so you know, you're probably getting a more cardoid response since it's a U-baffle, not a dipole.  It's still very directional, so you want it pointing at your listening position.  Most likely, you will like it even better with some resistance and damping inside the cavity, but don't overdo it with the polyfill.

How long is your tube?  Driver diameter?  Tube diameter?   Now that you've discovered the natural sound of OB bass, it's time to optimize what you have.

[Luigi]

Hi JohninCR

Just so you know, you're probably getting a more cardoid response since it's a U-baffle, not a dipole.  It's still very directional, so you want it pointing at your listening position.  Most likely, you will like it even better with some resistance and damping inside the cavity, but don't overdo it with the polyfill.

•••••••Actually, I took all the polyfill out as soon as I created the open baffle out of the infinite baffle. Will try again with a little fill; it does sound a touch hollow as is, though is much better, to my ears, than the sealed sub sound.

How long is your tube?  Driver diameter?  Tube diameter?   Now that you've discovered the natural sound of OB bass, it's time to optimize what you have.

••••••••Haven't got the exact dimensions on me, but it is a 12 inch driver from Lambda Acoustics when they were still up and running. I got it from Apex. So the cylinder, which is made of PVC, is around 15 inch in diameter and is about 24inches long.
What I was thinking of doing was to make an H type baffle out of the cylinder, ie, move the remaining end cap to the centre of the cylinder, so there's as much cylinder in front as behind.

Luigi

[Russell Dawkins]

"What I was thinking of doing was to make an H type baffle out of the cylinder, ie, move the remaining end cap to the centre of the cylinder, so there's as much cylinder in front as behind. "


if you do, you might want to check this out:

http://www.musicanddesign.com/u_frame.html

It addresses exactly this configuration.

[Luigi]

Hi Russell

if you do, you might want to check this out:

http://www.musicanddesign.com/u_frame.html


Thanks for the information. I will check this out over the weekend.
It means I may not have to do much to my other cylinder sub, which has a 15" Tempest at one end, and is sealed.


I will remove the end cap and take it from there!

Luigi

[JohninCR]

H or U is a no-brainer for me.  H (dipole) bass drops rapidly below the on axis room mode, plus it's 6db/oct cancellation starts at 58/(cab depth in meters) vs 29/(cab depth) for U-baffles.

[Luigi]

Hi Guys

I had a stab at reading the stuff by John on Musiciandesign, but whoa. Frankly it might as well have been written in another language

Could you paraphrase for me? What is it I actually need to know practically in terms of position placement and critical sub dimensions?

The cylinder sub I have taken the end cap from and therefore turned into a form of U frame is 16 inches wide and 14 inches long. My listening room is about 18 feet long and 15 feet wide.

I have another cylinder sub with a 15 inch Tempest driver (Adire) which is 35 inches long and 17 inches wide.

I think both of the drivers have plenty of Xmax but am unsure of other critical data starting with a Q and F etc.

TIA for any relevant info
Luigi

[JohninCR]

Luigi,

Welcome to OB's where rules are only guidelines and they're meant to be broken anyway.  The width of your cabs isn't really important, but depth is.  What you have is:

Short U  -6db/octave rolloff starts at 82hz.  Above 82hz response will
     increase up to about 240hz and above that there will be a sharp
     null at almost 500hz.  Based on the dimensions of the cavity it won't
     exhibit 1/4 wave resonances until well above 200hz.

Long U  -6db/octave rolloff starts at 32hz.  Above 32hz response will
     increase up to about 94hz and above that there will be a sharp
     null centered at almost 190hz.  Based on the dimensions of the cavity
     it will exhibit 1/4 wave resonances starting above 90hz.

Tempest driver-  This woofer appears to be well suited for this use.
    It has dual voice coils, so you can use resistive dampening across one
    of the voice coils and effectively raise the Qts to almost .80 .  With a
    small variable resistor across that VC, sound will be adjustable.

The important questions are:  At what point do you want to cross over to your mains, and what XO slope you will use.  Also, what are your performance goals.  Is it for music only or for HT too.

I haven't tried a 36" deep U-baffle.  My knee jerk reaction is that it's too deep, but if you cross it very low and damp the cavity well, it might work fine.

[diy_freak]

Tempest driver- This woofer appears to be well suited for this use.
It has dual voice coils, so you can use resistive dampening across one
of the voice coils and effectively raise the Qts to almost .80 . With a
small variable resistor across that VC, sound will be adjustable.

Why go for passive correction if you are driving the dipole sub with an active filter? Though it's elegant from a simplicity point of view, it would be a waste of power and a bit of damping factor too.

[JohninCR]

Tempest driver- This woofer appears to be well suited for this use.
It has dual voice coils, so you can use resistive dampening across one
of the voice coils and effectively raise the Qts to almost .80 . With a
small variable resistor across that VC, sound will be adjustable.

Why go for passive correction if you are driving the dipole sub with an active filter? Though it's elegant from a simplicity point of view, it would be a waste of power and a bit of damping factor too.

Something is wasted only if it's not used.  In free air operation of OB, pushing a driver to full excursion takes little power, so power handling isn't a factor as long as it's used as a sub (Tempests are useful only as a sub).   For damping, RDO gives you the full range of damping permitted my the driver, from the same as if both VC's are used up to double that.  Somewhere in that range should be the perfect damping for OB operation in any room or baffle.

The only DVC driver I wouldn't do this with is my JBL 1500gti.  That's because it's 2 VC's are at adjacent points along the former, but wound in opposite directions.  Once the cone reaches full excursion the adjacent coil will enter the opposite coil's gap and turn into an electro-magnetic brake to help prevent over-excursion.  I wouldn't want to give up that protection, especially if used in OB.

[Luigi]

A bit of an update.
I have now sawed all the end caps off my cylinder subs!  

Wish I had done this ages ago. The OB bass doesn't seem to load the room like the sealed subs did, and I far prefer the more natural sounding bass that is now bolstering the bottom end of the Hempcones.

The Tempest bass works fine for theatre but isn't as well defined as the smaller cylinder for music. Fortunately, I had two of the smaller cylinders so am now using stereo pair of OB subs, which sound glorious with music. Lambda Acoustics really did know what they were doing. A shame they are no longer.

Interestingly, the sound seems punchier when both voice coils are in use. And I have gone back to fully stuffed mode, as when the subs were sealed. It seems to overcome that slightly hollow cavity sound.

Now for the tricky bit. Placement in my rectangular room I guess this is a matter of trial and error and a healthy dose of wife acceptance factor. (Says she: I don't mind where they go so long as I cant see them -doh)

At present, both are behind the open equipment rack; one is quite near the left corner, and the other is three feet to the right and both are about a foot from the front wall.

I have found in the past that you can avoid some unwanted room modes by raising subs up off the floor. I have both raised to about seat level at present.

Even as is, this is by far the best bass I have ever experienced.

Luigi

[JohninCR]

Luigi,

You'll most likely find the only modal problem is with the on axis mode.  Output is drastically reduced in the other directions and I've yet to notice anything at those frequencies.  It sounds like you're on the right track and having great success already.

I hope you have those Hemps freed from the confines of a box.  If not, you're in for a real treat.

Take care,

John

[diy_freak]

Something is wasted only if it's not used.  In free air operation of OB, pushing a driver to full excursion takes little power, so power handling isn't a factor as long as it's used as a sub (Tempests are useful only as a sub).   For damping, RDO gives you the full range of damping permitted my the driver, from the same as if both VC's are used up to double that.  

I agree that since dipole subwoofers don't need a lot of power to reach Xmax, you can't waste that much in the first place. Still, since active filtering normally is a requirement for dipole subs, why not add a linkwitz transform or any other circuit that will do the trick in the working range, with the benefit of more flexibility than a passive solution?

What does RDO stand for? I'm not familiar with that abbreviation. How does it avoid effecting the damping factor?

[Dmason]

I am listening to the Hemp cones on simple baffles right now, and I can say, they sound very special indeed. Not at all surprised.

John

Could you PM me your email address, I have something to share with you.

thanks.

Luigi

Enjoy!!

[JohninCR]

What does RDO stand for? I'm not familiar with that abbreviation. How does it avoid effecting the damping factor?

RDO, stands for Resistive Damping Operation.  It may have been coined by Adire.  It's used with a dual voice coil driver.  The Qes of a DVC driver is typically quoted using both voice coils.  Leaving one of the coils open gives the driver only half the usual electro-magnetic damping, therefore essentially double the Qts.  This normally puts the driver up into the range where it is useful for open air operation.  The RDO part, is putting a variable resistor across the open terminals.  Lower resistance increases damping and increasing resistance decreases it.  This results in an adjustable Q for the driver.  

Regarding corrective circuitry, I'm sure active is more flexible, but I go to great lengths to avoid all circuitry.  Starting out of necessity, I am a true minimalist, and use mechanical means to dial in the response of my OB's.