Baffle Difraction &

[ralph-bway]

Hi All,

I built a set of arrays using the PE 4.5" 49 cent drivers very similar to these:

http://ceramicsubs.web1000.com/4x4x42/index.htm
(sorry this is the best pic available to show the cabinet)

Now as you can see the front baffle is just wide enough to house the drivers. How does such a narrow front baffle affect the imaging and soundstage of the speaker.

Also I'm interested in doing an open dipole design for front mains to operate from aproximately 250HZ - 4KHZ using six of these drivers I have on hand for each side:

http://www.partsexpress.com/pe/pshow...number=299-434

From all the reading & searching I've done it seems the wings should be at least as wide as the front baffle.

I have 80 of the Onkyo tweeters to cover above 4KHZ and thought if I mounted them on a seperate baffle and if angled properly the Audax & Onkyos would be on axis to my listening chair. Does this seem reasonable or am I missing something.

I also have 4 Peerless drivers for the subs (right & left).

[cc00541]

The narrow baffle is an aid to imaging as the time delay between the direct and reflected sound from the baffle edges is very small.

To have a dipole accommodate a flat response down to 250 Hz, each of your wings would have to be around 24” wide, although you could actively equalize the 6 dB / octave drop off characteristic of dipoles and get by with a smaller baffle. Another option would be to angle the baffles back to minimize the footprint of the array.

SL is the guru, and much more eloquent than I, on the subject of dipoles. Here’s a link to his dipole discussion:

http://www.linkwitzlab.com/models.htm#A

C

[johninCR]

I don't know how C came up with the 24" wing figure. The wavlength of 250hz is 54", so a 54" wide flat baffle would give you a peak in response at 250hz, not flat response. To get only down to 250hz requires minimal baffle and much depends on the shape and placement with the sides swept back. Most likely just cutting off the back of the array cab in your pic will get you down to 250hz. Having the baffle that is big enough to be stable should be enough. You don't want 90 degree sides due to cavity resonances and make your front baffle big enough to have the tweeter array on the same front baffle. Having the tweeter array on a different angle of baffle is asking for trouble.

If great imaging is a goal, then you should consider the tapered array configuration mentioned in Dr. Griffins paper. Also with regard to imaging, I find that narrow front OB arrays image better than wide fronts.

[BillFitzmaurice]

The CTC from the midbasses to the tweeters should be as close as possible, so by having them in separate boxes you've got problems from the get go. Do the best you can to get them as tight as possible. If you want to run the Onkyo's that low you'll need to run at least a 4th order HP filter.

[cc00541]

I don't know how C came up with the 24" wing figure. The wavlength of 250hz is 54", so a 54" wide flat baffle would give you a peak in response at 250hz, not flat response.

Yes, you are correct about the peak, but below that point the response will roll off at 6 dB/octave summed with the natural response of the driver. Above that point would be a dip @ 500 Hz, etc.

IMO, the peak is not as significant as the 6 dB roll off below it, and I assumed Ralph would read SL's documentation, so I did not mention it. I also assumed a 6" baffle width for the drivers and 24" wings. A 6" wide baffle with no wings would start the characteristic 6 dB roll off at around 1130 Hz.

C

[ThorstenL]

Konnichiwa,

Originally posted by cc00541
Yes, you are correct about the peak, but below that point the response will roll off at 6 dB/octave summed with the natural response of the driver. Above that point would be a dip @ 500 Hz, etc.

IMO, the peak is not as significant as the 6 dB roll off below it, and I assumed Ralph would read SL's documentation, so I did not mention it. I also assumed a 6" baffle width for the drivers and 24" wings. A 6" wide baffle with no wings would start the characteristic 6 dB roll off at around 1130 Hz.

You know what. You are eloquent in quoting theory.

Now here is the practice. I have a driver with the following parameters:

Fs = 55Hz
Qts = 0.7

It is placed on a baffle which is around 2' (24") wide and thus, based on your "milkmaid" calculation rolls off at 500Hz. Now my problem is that I get in room a measured response that places -6db @ 50Hz and is free from drastic ripples.

Explain.

Sayonara

[johninCR]

Here's another example. My 15" Seleniums, Qts .62 , start their dipole bass roll off at 300hz WITH NO BAFFLE, just propped up using some paper egg crates.

[ThorstenL]

Konnichiwa,

Originally posted by ralph-bway
Now as you can see the front baffle is just wide enough to house the drivers. How does such a narrow front baffle affect the imaging and soundstage of the speaker.

It affects it very badly. Assuming the wavelength is such that the driver (array) does not show significant beaming anything having a wavlength longer than 2 X the width of your baffl will in eaffect "wrap around" the baffle and will creat a very wide dispersion, putting ound anywhere and everywhere in addition to directing it at listening position.

Further, such narrow faffles invariably louse up instrument tonality. Evaluation of wavelength and formants of instruments dictates that a speaker should have a baffle with the smallest dimnsion no les sthan around 10", preferably more.

Originally posted by ralph-bway
From all the reading & searching I've done it seems the wings should be at least as wide as the front baffle.

Wings? I have a simple rule that indicates the EFFECTIVE baffle width for use. That is that the baffle is as wide as it is wide visually. Sweep back "wings" from a plan baffle and you losse progressively the actual "surface extension". Once you fold back much past around 60 degrees the acoustic system does not operatle really any longer as classic dipole and other approaches must be taken to get predictions to work well.

I would suggest that if you make your baffles at 10" Wide (visually) and your drivers have a flat response loaded into "open air" (eg Qt = 0.7 or higher) and sufficiently low resonance you will be fine, for anything but serious fundamental bass in the lower notes, or wherever the self resonance of the driver places the rolloff.

Sayonara

[johninCR]

Originally posted by Kuei Yang Wang

Wings? I have a simple rule that indicates the EFFECTIVE baffle width for use. That is that the baffle is as wide as it is wide visually. Sweep back "wings" from a plan baffle and you losse progressively the actual "surface extension".

I know that we have discussed this before, but I still don't understand this "visual width" idea. While I understand that other factors come into play with swept back wings, I only want to talk about bass extension. Simple geometry tells me that when you bend back the sides of a flat baffle, the rear wave must travel farther to reach your ears than with the baffle being flat. Bass extension from H & W baffle shapes also appears to contradict this "visual width rule".

Can you please explain "surface extension", maybe that's the part I'm missing.

[ThorstenL]

Konnichiwa,

Originally posted by johninCR
I know that we have discussed this before, but I still don't understand this "visual width" idea.

It is simply a ballpark idea. Seems to work. I explained before that you need to start to think in terms of waves, not pressure. I can't help anymore.

Originally posted by johninCR
Bass extension from H & W baffle shapes also appears to contradict this "visual width rule".

Actually, they don't, because they are much more copmplex than the simple dipole the "visusal width" rule applies to. And due to their resonances they are invariably operates strictly below the actual "baffle cutoff" and not around it.

Originally posted by johninCR
Can you please explain "surface extension", maybe that's the part I'm missing.

To understand the way an open baffle (not any more complex things, a plain baffle with or without wings) works you need to get the point that you are attempting to "launch" a soundwave into the room, into a specific direction.

Then take stones to a quiet pond and place some 2-dimensional "baffles" in the way of the waves your pebble causes, preferably you drop it extremely close in front of the "baffle".

Sayonara