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Old 11th October 2003, 05:26 PM   #1
Vg is offline Vg  Sweden
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Default Baffle width vs dispersion

I am wondering whether wide or narrow baffels could be prefered to each other in certain applications when regarding dispersion, a small baffel will have its bafflestep higher up in frequency, and thus have a broader band of wide dispersion.

Also, when you compensate for the bafflestep loss at lower frequencies you will give the loudspeaker a raised energycurve at these frequencies, higher up in frequency for a narrow baffle.

What impact will these effects give, what is good and what is bad? What dispersion do you want?

Thanks in anvance and greetings from Sweden,
Viktor
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Old 11th October 2003, 05:38 PM   #2
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There are a lot of religions regarding dispersion. Pick your church Some like omni, some like narrow, some like dipolar, some like bipolar, some like flat on-axis with smooth HF rolloff at increasing angle.

Dispersion is a combined function of driver diameter, crossover, baffle size and shape, and driver placement on the baffle. There are some nice simulation packages for all these things- my favorite is the Excel-based one available at the FRD Consortium website.
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Old 12th October 2003, 02:22 AM   #3
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Default Baffle step and directionality

Hi Viktor!

It really depends on what your design goals are. A larger baffle means that the frequency where it behaves like half-space is lowered and a smaller baffle means this frequency where it behaves like half space is raised. No matter where you place the box, the baffle will form a half space condition above a certain frequency range.

These larger baffles means that the sound energy is more constrained, and that means there is increased directionality. With that, it means there is more focused energy because the same amount of energy is now spread over a smaller area - the target.

The baffle makes the system so that it is attenuated from 3db to 6db at all frequencies which are above the frequency represented by the half wavelength of the baffle. Baffle step correction is usually in the order of 3db for an in-room condition. This number has been found by manufactures to be the best, such as experiments done by Adire Audio which suggest that a full 6db correction is usually 2-4db "too hot", and no correction at all is conversely 2-4db "too cold". So the idea is to equalize this so that the bass is augmented by contrast.

As to weather or not you should incorporate baffle step at all - that really depends on the spatial condition in which you place the loudspeaker. For a 3db of correction, I'd suggest that its usually best to place the loudspeaker in a half space or free space condition only, otherwise the system may become too "bass heavy" and possibly peaky if placed in an even more contrained condition such as eigth space. Free space placement may be a bit "bass shy", unless the baffle step correction was increased.

So instead of using baffle step correction at all, one could just place the same loudspeaker with no correction at all into a quarter space or eigth space placement. That will do essentially the same thing as the baffle step equalization. Compared to free space, quarter space condition gains 6db and eigth space gains 9db. The "baffle extension" is weighted towards lower frequencies where the wavelengths are longer. What happens here is that the wall boundry forms part of the baffle, because the distance between the baffle and the wall becomes less and less in terms of wavelength, and so they "act as one". Thats why you could limit placement to quarter space or eigth space and use no baffle step correction and gain the same benifits as using a system with baffle step correction but placed in a free space or half space condition.

The dispersion characteristics are determined by a number of things, such as described by SY. What you actually "want" - depends on your application, and just that; what you want. Horns can be used instead to tailer exactly how you want your directionality/dispersion to be. I use horns myself in my system, which are wide-dispersion types because I want to fill the entire room with a small amount of sound sources. But if your looking at using many multiple horns in your system, such as in an array, then I'd aim for something like a constant directivity horn with narrow dispersion so that comb filtering issues between them do not expand themselves. This type of setup is typical of large stadium and theater installations.

Adrian
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Old 12th October 2003, 03:01 PM   #4
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Thanks for your answers.
I was thinking that whatever dispersion you want, you never want changes in the amount of dispersion, at any frequency. (Although it should matter less at frequencies where the ear cannot detect location of sound) Maybe it's better to have this change that has to take place at a lower frecuency, so more direct sound from the speaker reach the listener.

Regarding optimum response curves in the bass range, here's an interesting article:
http://www.sonicdesign.se/optimum.html

The excel spreadsheet Xlbox.xls on this page:
http://www.mhsoft.nl/SQL7/ViewDownloadFiles.asp
Seems to be a very good tool to include the room in the calculations, do anyone want to comment on how accurate it might be ???

/Viktor
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Old 13th October 2003, 05:43 AM   #5
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Hi Viktor!

> I was thinking that whatever dispersion you want, you never
> want changes in the amount of dispersion, at any frequency.

That is a fair goal. Typically though, the DI will change over the operating range, especially for speakers configured as a direct radiator. Horns on the other hand can achieve much more uniform dispersion patterns.

That "Optium Frequency Response Curves in the bass range" article only covers aspects of frequency response behaviour. Furthermore, it tends to say that frequency response behaviour is the only thing which matters in how a subwoofer sounds, when that isn't so. I personally believe that article is too simple, and its rambled on about them too. There's a few things in there which I disagree with too, but I won't get into that here.

The XLBox spreadsheet was not working on that site you linked, however I already had a copy of it from the melhuish horn website. Really, there's no way to predict how the room is going to respond, especially once we consider all the reflections off the items in the room. Absorbtion rates of those materials will also have an effect and the geometry of the room also. So will boundry distances. The point is that real predictions of how the room responds still remains undefined, even the best simulation packages are unable to give accurate predictions. I had a large discussion with another person on the diysubwoofers.org forum about room interaction and such, so for further reading I suggest you read the post "Room interaction with subwoofers and full range speakers" and also all of its 4 follow-up posts by Richard and myself, which appear at the bottom of that page.

Adrian
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Old 13th October 2003, 06:34 PM   #6
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Ok, seems like some good reading over there at diysubwoofers, I will throw myself over it as soon as possible

What would you say matters in how a subwoofer sounds? Apart from frequency response I can come to think of unlinear distorsion, transient response and possibly cone breakup if you use it to high, would you like to add anything?

/Viktor
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