Open baffles for dummies ;)

[Jennice]

Hi everybody,

I've come across a lot of threads which seem to like the open baffle aproach.
Thus, I got curious and started searching. I found a lot of pages (here and on Google), but the ones I found seemed to take the basics for granted. I really need some assistance on these basics to get me started. Any help appreciated...

Could someone please be so kind as to comment on me being right or wrong in the following?

1)
The way I understand it, an "open baffle" speaker has only the front panel (baffle) and maybe some small sides to give stiffness to it. No full sides, no rear to make a "real" enclosure.(?)

2)
What's so magical about these things? My guess would be that the drivers have "unlimited" air behind them, so they don't need to fight the preassure change in a cabinet - thus making the membranes move easier?

3)
This kind of speakers will have a bass problem. Low frequencies cause the Baffle Step Response in "ordinary" speakers. Won't this be the case with open baffles also? (sort of an accoutic short-circuit for low frequencies , if that term makes sense?)
If the lower frequency dependent on the baffle width, just like when calculating for baffle step?

Thanks-
Jennice

[HeatMiser]

I'm somewhat new here too, but I'm going to boldly jump in anyway with the expectation that some of the old-timers will appear to elaborate and correct me.

1) Yes.

2) A big part of the magic has to do with the unique dispersion pattern of a dipole; as you surmised, there is cancellation involved but the cancellation approaches 100% to the sides of the speakers (imagine a figure 8), greatly reducing radiation to the sides and thus lessening troublesome reflections from the walls etc.

Dipole bass in particular also benefits from being based on velocity rather than pressure; since there is no sealed area containing the backwave, the air in your room isn't alternately compressed and rarified, instead being simply energized and pushed toward the listening position. This combined with the lack of side radiation does much to avoid exciting room nodes, and the net result is a sound much less troubled by peaks and nulls scattered around the room, and with a well thought-out design can sound startlingly more natural than typical box speakers.

3) Yes there will be cancellation; dipole designs often deal with this by using somewhat larger (wider) baffles and sometimes with active EQ. There are other ways around it with careful driver choice and consideration of the physics involved, but those are for more experienced and clever speaker designers than me to discuss.

Hope some of that is helpful.

[Jennice]

Thanks for your answers

Yes, it sure is helpful. I'm mostly concerned about the cancellation issue of the sound. Is it comparable to the baffle step, so the baffle (in this case) should be fairly wide, compared to a normal box xpeaker, where a narrow baffle is desireable (as I understand it)?

Also, if these open baffle speakers become linear by the help of an equalizer, don't they end up being rather in-efficient?
Considering the wave-length of lof frequencies, I have a hard time imagining a deep bass produced from such speakers. Should they be supplemented by more ordinary designed sub's, or is there a similar "open" approach to sub-woofers, which complement these open baffle speakers?

Jennice

[454Casull]

Yes, dipole bass is inefficient, but you can't have your cake and eat it too.

What you _can_ do is use a dipole subwoofer to reproduce the frequencies below the ones which result in reverberant room behavior occurs (below 200Hz in many rooms, though the lower you cross the easier it is to integrate the subwoofer with your main speakers) - i.e. dipolar radiation in the frequencies that result in discrete room modes. Then, you can use a monopolar subwoofer starting below the frequencies whose wavelengths are twice as long as the longest dimension in the room (generally), as the modes disappear below this frequency.

Concerning one of your earlier questions, the small sidewalls are generally added to increase the effective path distance (to lessen the cancellation at lower frequencies). OBs don't need to be very stiff, as they store minimal amounts of energy. Unless you're talking about the pieces that brace the bottom of the baffle to the base, then yes, those are for added strength.

[Kittle]

Im in the same boat for open baffle too..

Living in an apartment I have VERY limited access to woodworking facilities, but I can easily cut round holes in boards.

My question is how do you fit a driver to your baffle ?

I found an app (forget the name) that allows you to simulate size & positioning of the driver, but no place to enter T/S parameters & thus see how they affect the overall performance.

does anyone have links to articles or open baffle simulation software?

And Jennice - Thanks for your Question!

[johninCR]

I discovered OB's, dipoles about 9 months ago. Just on a fluke I mounted a cheap driver on a piece of cardboard and was floored with the result. I would characterize the difference as being much more open and natural sounding. When was the last time you heard a singers voice coming out of a box? Never in real life.

I believe the open sound is created by the reflections and their timing from the back side of the cone and the natural sound results from the free air movement of the cone combined with the lack of reflected energy from inside the box back out to the cone.

The bass roll off resulting from wave cancellation is less than you'd think at only 6db per octave until you get down to a driver's Fs. Below Fs it combines with the driver's natural roll off and becomes an 18db roll off. With narrow baffles and/or low Qts drivers (which won't make OB bass without EQ), I characterize the sound as thin and it just takes a little help to attain a rich full sound for music.

One of the things I love about OB is that it's easy to experiment. Just put a driver on a piece of cardboard and you have an OB speaker. If you are lucky enough to have a relatively high Qts driver, then the improved sound is immediately apparent. If you have a sub, that helps with first impressions. Just run one channel with a boxed speaker and the other with your test OB. It's a real eye opener.

Then, once you can do bass in dipole at least down to 40-50hz to combine with the sweet dipole midrange and crystal clear vocals, you'll be able to compare dipole bass to muddy, in comparison, boxed bass. Instead of just a bass beat, you here the different notes and instruments creating that beat.

Fully dipole for music isn't too difficult and doesn't have to be big. If you want flat extension and high output down in the 20's that's a whole other ball game.

I already have the drivers for my next project. $200 worth of $12.50 12" drivers (yes 16) and a pair of Selenium 15" coaxials that I got in day before yesterday (another $300 with shipping and duties). The bottom will consist of 8 12" drivers per side in a W baffle 28"H X 21"W X 22"D . The Seleniums will go on top with a small baffle.

Why so many drivers in the bottom? Because I want these speakers to have the same output at 20-25hz that the Seleniums have a 100hz. Fully dipole, no subwoofer necessary, even for HT.

[wunhuanglo]

is right here, free.

http://www.woodartistry.com/linkwitzlab/index.htm

[Sjef]

It's hard to talk about the real efficiency of open baffle bass. Because of the low roll off the efficiency degrades towards the lower frequencys downwards 20 hz, but in the higher range at say 80 to 100 hz they are as efficient as a closed box speaker. And that's the range where a lot more of the music power is going to. (I presume you listen to music and not to test tones all day).

My open baffle woofers produce a sound pressure of 99dB/1w/1m at 100Hz while thy only produce about 80dB at 20 Hz. Are they only 80dB efficient then ? well no, the amplifier has to deliver only 1watt to achieve 99db output at 100Hz, about 10 watt for 99dB at 50Hz and about 20 watt for 99dB at 20Hz.

I would not worry about the efficency to much. When you are gonna make open baffle bass the only way to do it is going active anyway and powerfull amps are very common these days. I would worry more about the max SPl out of it. If you just be sure to use about four times as much moving area compared to what you would use for a close or vented box then you are playing safe in that area as well. Simply put, if you had a 10"driver in mind for a closed box, just use four of the same drivers for open baffle.

I am using two 15"drivers in a w-baffle wich can produce as much SPL as one 10"driver in a vented box. The w-baffle with the two 15"driver however is smaller in size then the vented box for the 10"and the open bafle goes down to 20Hz with ease while the 10"doesn't go any lower as 35Hz.

Don't expect to get deep bass from a 8"driver on a open baffle. It's just a little bit more than just removing the enclosure.

[rcavictim]

Originally posted by johninCR
<snippage>

I already have the drivers for my next project. $200 worth of $12.50 12" drivers (yes 16) and a pair of Selenium 15" coaxials that I got in day before yesterday (another $300 with shipping and duties). The bottom will consist of 8 12" drivers per side in a W baffle 28"H X 21"W X 22"D . The Seleniums will go on top with a small baffle.

Why so many drivers in the bottom? Because I want these speakers to have the same output at 20-25hz that the Seleniums have a 100hz. Fully dipole, no subwoofer necessary, even for HT.

John,

Is there a website describing the 12" drivers you are buying for so cheap? I'd like to check them out.

I have a pair of the 12CO1P Selenium coaxials. I think these are an outstanding quality driver for this amount of money. I used them in a large front loading horn for my HT as well as for music. I wanted very high efficiency. One thing I'd like to do is pull one and try it just to hear in a standard BR box of around 4 cubic feet.

[Mats J]

Sjef) what 15" are you using?
John) so are you going to share with us what those $12.50 wonders are? Any pics yet?