'exotic' techniques for absorbing the mid backwave (and bass)

[terry j]

finalising the design/construction of my boxes, and am curious about this.

I wonder if this factor (sound coming from the box back thru the diaphragm) is one of the reasons for people liking the dipole sound??

Most boxes just get a bit of stuffing I guess, is the purpose of that stuffing to absorb the backwave? or some other factor/.

One of the things that has intrigued me is the 'reverse horn (?) of the B&Ws. From my poor understanding it somehow works by directing the backwave down a guide and absorb it there.

Has anyone experimented with this, if so what did you find?

Here is a link to someones practical implementation of it

http://home.wanadoo.nl/dezaire/ultiem/ultimate.htm, there is a diagram that you can download that shows how it was done. Basically by cutting holes in a succession of mdf plates you can sculpt the cone shaped cavity.

If part of the reason is to absorb the wave, I wondered why build it out of a solid material like mdf? Could you not substitute compressed f/glass, still have the sculpted shape yet provide better absorption?

What apart from the b&w trick are there? Is it worrying over nothing at the end of the day?

I have wondered why most cabinet stuffing (or internal foam) is placed on the walls? Would they not work (like room treatments I guess) by slowing the wave and converting it to heat?

If so, then like improving the bass performance of f/glass panels on the wall you space it off the wall, would not this stuff work better when placed in a zone of maximum velocity rather than max pressure? Ie, get the foam/fglass off the cabinet walls.

The arc tube traps work by forcing the equalization of internal and external pressure, how left field is the idea of mini tube traps within the box?? (just a silly curiosity question, doubt I will try it)

But a crazy idea I have thought of is instead of lining the walls with foam or glass, how about lining them with 'sand bags', a bit like the snake draught excluders under a door.

Fine loose sand (done in such a way that the sand does not escape) will certainly vibrate due to the bass, and unlike foam on the walls (if my theories above are somehow correct) will still provide the vibration/conversion to heat mechanism.

[Iain McNeill]

Originally posted by terry j
But a crazy idea I have thought of is instead of lining the walls with foam or glass, how about lining them with 'sand bags', a bit like the snake draught excluders under a door.

What, like glueing small bags of sand to the middle of large cabinet surfaces to dampen resonance?

Very interesting idea...

[planet10]

The B&W is a tapered 1/2 wave (sealed) transmission line. Like all "proper" transmission lines it sucks the backwave down the line and the damping material kills the backwave before it can reflect and return -- as Bud Fried was fond of saying -- it dramatically reduces (nothing is perfect) the time smear.

That little monitor really doesn't look large enuff to do what you want.

And probably easier to build like B&W's prototype.



I'm using similar techniques for some mid enclosures (Tysen and a Festrex project i'm working on). Being mid TLs, they are much similar.

dave

[terry j]

Originally posted by Iain McNeill



What, like glueing small bags of sand to the middle of large cabinet surfaces to dampen resonance?

Very interesting idea...

Kinda, but maybe not even gluing, but fixing top bottom and middle, heck even 'wire' them in with eyehooks, so the whole lot is fixed yet can vibrate away merrily.

Originally posted by planet10
[B]The B&W is a tapered 1/2 wave (sealed) transmission line. Like all "proper" transmission lines it sucks the backwave down the line and the damping material kills the backwave before it can reflect and return -- as Bud Fried was fond of saying -- it dramatically reduces (nothing is perfect) the time smear

That little monitor really doesn't look large enuff to do what you want.

thanks dave, I guess in the picture you have posted a lot of the size has also something to do with augmenting the bass??

Re that schematic, it does look kinda small, and if the box size is all about creating the curved chamber, then not a lot left to provide box size to provide deep bass.

That was the other question I forgot to ask, IF you did do the 1/2 wave transmission line, does the volume behind it still have to be that which unibox (say) says you need for you bass performance? (hope that made sense, kinda the earlier para re-worded)

Originally posted by planet10 And probably easier to build like B&W's prototype.

That is his 'long' prototype that is pictured?

How *critical* is it to have it a tapered 1/2 wave transmission line? What about straight sides as an example.

I would imagine that if my mid cutoff point is 200 hz, I would not need such a huge chamber?

Lastly, any thoughts about how the results reflect the work?

[planet10]

Originally posted by terry j
I guess in the picture you have posted a lot of the size has also something to do with augmenting the bass??

Bass extension in a half-wave TL is the same as sealed. Length needs to be 1/2 WL of the lowest frequency of interest (a heavy taper means it can be shorter). For a mid range TL it doesn't need to be too long and if a 1/4 wave line (like i'm doing -- i like the low pressure box), even shorter -- Tysen with ~300 Hz XO is 10:1, 8-9" long, The Featrex needs to be able to get closw to 100, so it is ~20" long, both are damped till aperiodic, stuffing density increasing towards the terminus.

That is his 'long' prototype that is pictured?

It is about the same length as the production fiberglass Nautilus woofer (but more space taken up by material). For easier construction you could just fold the TL more conventionally.

dave

[terry j]

ok, wavelength at 200 hz is 1.72m.

1/2 is 850mm, 1/4 is around 420mm, call it 500mm so I have some latitude of x-over point.

tysen, umm I spose that is a type of profile?

Current mid chamber is around 10 l, if I were to muck about with this do I need to keep the volume at ten l? To prevent too long a tube, then I can put it in a spiral.

do you have any suggested reading? Sounds like you have tried this, how do you feel the results are?

[phase_accurate]

The sandbag trick would not help against the backwave coming through the cone. But it would help against the vibrations of the panels.
A less messy and more modern method than the sandbag would be the following:

http://www.korff.ch/E/hawaphon.htm

Regards

Charles

[terry j]

I realise that charles, I changed the title of the thread to try and include that bass stuff.

thanks for the link, it's kinda what I was thinking, some sort of sack holding the loose material. I guess it is a way to retro fit the double skin build, that is filled with sand.

no-one has any opinion on whether it is useful to have the absorption on the walls themselves? or would it be better spaced off at least a few inches.

[planet10]

Originally posted by terry j
[B]ok, wavelength at 200 hz is 1.72m.

1/2 is 850mm, 1/4 is around 420mm, call it 500mm so I have some latitude of x-over point.

Current mid chamber is around 10 l, if I were to muck about with this do I need to keep the volume at ten l? To prevent too long a tube, then I can put it in a spiral.

do you have any suggested reading? Sounds like you have tried this, how do you feel the results are?

Attached is a mid-line that comes to 100-150 Hz once you consider end correction and the taper.

Yes i've tried this. Works well. And the guru i learned it from was using it long before i 1st did.

Tysen is the name of a speaker (and our godson), FF85KeN is in a 10"1 mid TL that exits out the back (no fold)



dave

[phase_accurate]

no-one has any opinion on whether it is useful to have the absorption on the walls themselves? or would it be better spaced off at least a few inches.

The absorbtion would work down to lower frequencies compared to placment directly on the wall.

Regards

Charles