Folded baffles/ U frames – why not?

[rick57]

I guess many of you know John K’s NaO, with dipole mids and bass, and bass “U frame”. The advantage of the folded baffle’s ‘wings’ - effectively a wider baffle, is more extension at the lower end.

I asked John K (on another forum) why he didn’t employ a U frame on the mids, and he simply replied “It's not practical to make a U frame mid.”

Any suggestions why not – I can’t see a practical problem –
does it cause a honking sound – a resonance that needs taming??

Thanks

[Variac]

Well, I have always heard that it starts to sound "boxy" as you add more and more walls to the baffle. It makes sense, when you have added 5 walls it IS a box.


A flat midrange baffle isn't that big anyway. If you make it a lot bigger, you theorectically extend the low end, but usually you
end up exceeding the optimum cone excursion and the sound suffers for this reason also.

[rick57]

In simple terms more walls may well be more "boxy". (In slightly more technical terms I think it’s adding resonances). But if they are only resonances, assuming the right value parts can be found, they can be dealt with by notch filters.

But why would cone excursion increase? If the lower end is extended by effectively a larger baffle, I thought excursion should *decrease.

> A flat midrange baffle isn't that big anyway
Depends probably on the trade-off between (is it) Fequal, vs SAF.

Some in earlier dipole threads have suggested that the effective baffle width is really closer to the width, ie excluding the wings folded back. This flies in the face of science.

Any other opinions? Kuei Yang Wang are you there?

[Variac]

-Perhaps resonances, but if there are a lot of them then one or two notch filters won't fix 'em all. Generally, its a lot better not to add problems and then try to remove them later

-Perhaps time delays as sound bounces off surfaces at different distances from the back of the cone. This produces smearing of the sound and sounds to me like less precise imaging.

I believe that a baffle extending to the ground is only required to be about 18" wide to handle under 200hz.

I guess my point is that most mid drivers can't handle the increase in excursion of lower notes anyway.
Also, open baffles also don't benefit from the bump in low response that bass reflex boxes have

[rick57]

Who's tried one? Nuuk??

[rick57]

Any experienced opinion?

Thanks

[rick57]

JK later replied on another forum; for the benefit of diyAudio-ers:
To create a true H or U frame response, the driver must be omnidirectional over the entire useful frequency range. No problem with woofer.
But for a mid you can think of it this way. Even on a flat baffle, the mid is not a true dipole. It is only a dipole below the baffle step. Above the baffle step the front and back radiation don't really interact because the driver is directional.

If you think of a mid on an 11" wide baffle in a sealed box, the response will show a baffle step effect. Mount the driver on a flat baffle with no enclosure and the basic difference in the on axis response, is that below the baffle step region the response continues to roll off at 6 dB/octave rather that flatten out at a –6 dB level.

If you completely damped the back wave of the U-frame, what you would end up with is a sealed box like response. If you built a U-frame for the mid it would need to be suitable damped and that means enough to really kill any cavity resonance since, (unlike the U-frame woofer where the cavity resonance is above the crossover point) the resonance will sit right in the middle of the passband of the mid.

And if you damp the backwave that much, you will effectively eliminate any rear radiation, defeating the intent of an open baffle system, unless you want an open baffle system that sound fairly similar to a sealed box system. There are reasons you might want to do this. ;-) But it's not a dipole (like) speaker.

[ThorstenL]

Konnichiwa,

Originally posted by rick57
Kuei Yang Wang are you there?

Of course.

Originally posted by rick57
Some in earlier dipole threads have suggested that the effective baffle width is really closer to the width, ie excluding the wings folded back.

That is so and for a specific and good reason....

Don't think pressure, think waves. we are not dealing with a pressure transducer in a dipole, we are dealing with a velocity transducer, so a lot of thinking developed in the pressure domain does no longer work. I have attempted to explain this previously as well as I can, without providing extensive mathematical proofs and in some ways completely different approaches to to modeling.

In the most simple terms, the actual surface of the baffle relates to the wavelength it can radiate. Fold it back and you loose surface. Now this effect happens at frequencies different to those where the pressure cancellation sets in where the folded back baffles can help.

This relates of copurse to my common quote that "A well designed speaker requires a baffle in excess of 20" Wide and 32" high, this is just basic acoustics".... This rule does not get disbanded because we suddenly deal with a dipole.

To put it differently, in a folded dipole you first experience a loss in SPL at the point where the wave folds around the front baffle. This will ultimatly cost you 6db in SPL. Below the point where cancellation sets in due to the fact that pressuresappear in the same place but with opposite signs you FURTHER loose output.

Now there are some mechanisms which have been descibed in various publications as well as by me that "help us out" in order to get a better LF behaviour than seems possible, but in effect folding back the baffle changes the way it interacts with soundwaves compared to a flat bflle and it does so in ways that are not beneficial.

Unless you the folded back wings are at least as deep as the baffle is wide or more (which makes it more of a box, both sonically and objectively (and no, notch filters cannot ELIMINATE resonances!!!!) the first and second loss term combine smoothly almsot as if the baffle "wings" are not there.

Once the wings are deep enough to make a marked difference the whole acoustic system no longer behaves as dipole and requires a more complex analysis and is riddeled with resonances at higher frequencies, making it particulary unsuitable for midrange systems....

Originally posted by rick57
This flies in the face of science.

On the contrary, it is simple and reliable science. Those who think they have all the answers and require reality to conform to their inaccurate understanding fly in the face of science. In other words, read the tag line.... ;-)

Sayonara

[Nuuk]

Who's tried one? Nuuk??

Also here sir!

Yes, I tried folded back wings on this project and it worked quite well. Bit if I put a cloth over the back, or filled the cavity with stuffing, the sound went 'boxy'.

While experimenting with the Goodmans 201 OB's I found that wings had little or no effect!

[miguel2]

I guess that there is always a mid term, like this design . The rear suports do some wave cancelation. Note the back upper plate too.

Nuuk, what do you mean about wings having no effect? Wings with some piano hinges can be placed in plane with the main baffle, making it bigger.

Miguel