Open baffle, the theory behind it and benefits.

[v-bro]

Hi,

can anyone point out or link to the basic theory of an open baffle?

I know the benefits myself like low distortion and great impulse behaviour, but one of my colleagues who has always built ported speakers is a total non believer and just doesn't understand how a system like that can work.

I'm looking for text about full wavelength and when chosen shorter why it needs correction. Why there's so much less distortion.

[Rudolf]

The fundamentals are explained by Linkwitz and Kreskovsky. That dipoles should have less distortion is highly disputable so ...

[cotdt]

Dipoles have HIGHER nonlinear distortion all else equal, especially at the lower frequencies since the woofer must work harder for the same SPL. You can compensate for this by using 4 woofers to match the distortion of a single in-box woofer.

[Raj1]

Don't discount OB's until you've subjectively heard them though. Imaging and soundstaging is par excellance with the right drivers.

[v-bro]

Thanks for your responses, I was under the impression that the distortion of OB was lower, but that probably is not right. Probably the lower distortion of many OB designs is due to the enormous amount of moving surface having to move only little. A direct comparison with the same moving surface in a boxed design shall probably have comparable distortion at the same level.

Originally posted by cotdt
Dipoles have HIGHER nonlinear distortion all else equal, especially at the lower frequencies since the woofer must work harder for the same SPL. You can compensate for this by using 4 woofers to match the distortion of a single in-box woofer.

Correct me if I'm wrong, but this only counts for a baffle that is less than max. wavelength and is made linear by correction. When you for instance build the speakers in a wall or really large full wavelength baffle the SPL should be the same at the same power, only difference perhaps is the hub of the driver will run to it's limits sooner.

Most baffles were built with compromises between performance and practicality, I'm merely looking for theory here...

I have found the sheets written by mr. Linkwitz exactly what I was looking for, so basically my questions are answered here.

Thanks again!

[cotdt]

at lower frequencies the woofer has to work harder, it's the same reason why adding baffle step increases distortion.

[v-bro]

For as far as I know (I'm no OB scientist, but I'm not entirely ignorant) this only counts for an OB with correction because you compensate the loss of low end output (due to acoustic short circuit at wavelengths larger than the baffle) by a bass-boost (the correction filter, similar to baffle step correction).

The bass boost certainly will introduce distortion, but a large enough baffle* needs no correction at all so should keep distortion at the same level or lower? Maybe due to the lack of air compliance you increase the excursion of the driver and at a certain point increase distortion. But as soon as you stay well within the mechanical capability of the driver I think the distortion should not increase.


*I've been told that ideally an open baffle should be at least as large as the max. wavelength and keep the frequency response straight and not decreasing towards the lower end like with the commonly used and more practical small baffles that need correction to compensate for this side effect.

[Moondog55]

Personal opinion follows so be aware.
OB's greatest benefits are, it's cheap, they are great fun to build, they are very forgiving and sound quality is good.
Greatest drawbacks are lack of suitable high-power bass drivers and low WAF if baffle is made big enough as they do take up a lot of floor space.

I think that OB's are a build it and see/hear type of speaker, even a poor implementation can sound rather good.

The theory is covered quite well by GA Briggs in several of his books ( as far as I know all out of print )
Try local bookshops or Amazon for G A Briggs "Loudspeakers"

[Lynn Olson]

The benefits and disadvantages of the dipole polar pattern are controversial - and separate arguments apply to different parts of the spectrum. A dipole in the 1 ~ 5 kHz region is not the same as a dipole in the 200 Hz ~ 1 kHz region. It also has to be remembered that nearly all "conventional" direct-radiator speakers are omnidirectional below 250 ~ 500 Hz, despite what happens at higher frequencies.

Some people notice box coloration more than others; I personally consider box coloration important because it falls in a part of the spectrum where direct-radiator woofers and midbass drivers are intrinsically flat (piston band), the 200 ~ 800 Hz region. Although the drivers are inherently well-behaved, the box is not - the dominant up-and-down, side-to-side, and front-and-back standing-wave modes are only partially damped by foam, fiberglass, synthetic fiber, wool, and other filling materials. These materials absorb much more effectively above 1 kHz than below 1 kHz.

If you want to hear this box coloration for yourself, remove the drivers, and if there's room, put your head inside the box and listen for the droning coloration that is added to the ambient environmental sounds. Be careful about doing this little test - once you identify the coloration, you'll start noticing it in all box speakers! This is similar to learning to see the "rainbows" in a color-shutter projection TVs - once you see them, you'll be noticing them all the time in RPTV's!

If you're in an experimental mood, compare and contrast the box coloration with and without different filling materials. Although they will modify the droning sound in different ways, you'll find that none of them remove it completely, and some filling materials add annoying colorations of their own. There is such a thing as too much filling - if the filling is too close to the cone of the midbass driver, you start to get mass-coupling effects, and efficiency and midrange response will be degraded. There's a reason that choice of filling materials is still controversial after fifty years - none of them works all that well. So the radical alternative is no box, or at least no structure that will support standing waves.

The biggest downside of dipole speakers is massive loss of efficiency below the dipole peak (typically in the 200 to 500 Hz region). If LF equalization is applied, the driver excursion and resulting distortion goes up many times. The only counter to this is selecting expensive low-distortion drivers (2~3X improvement at best) and/or increasing cone area (loss of WAF, your choice on that one).

[pelanj]

I have not still decided which speakers should I use for my new apartment (kind of space limited). The last path I would like to take is a pair of two way bass reflex minimonitors.

By any means I do not listen to high levels, more or less normal TV levels, which is quite low. Could Open Baffle be the candidate? I can not accept anything wider than Manzanitas, for serious listening, I will have to move the speakers accordingly, otherwise for background listening they will be placed along the longer sidewall of a 3,5x4,5x2,4 m room.

Could some of the disadvantages be eliminated by near field listening?
How about hybrids with more traditional bass below the dipole peak? Is it worth trying?

My target is a relatively small system - is that contradictory to OB even if I consider the low listening levels?