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Old 2nd December 2004, 10:21 PM   #1
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Default At what point is the baffle "infinite"?

I am considering an infinite baffle sub project, but will not be able to have the rear fire onto a separate room for that perfect install.

About the best I could achieve would be to mount it in a ceiling cavity, which has a finite volume, or a big closet, also a finite volume. There would be some leakage of the rear output to the front in both cases. For the closet it would be past the edges of the closet door, and for the ceiling cavity it would through a section of suspended ceiling tiles.

At what point does the baffle approximate infinity, or if this is a better way to put it, at what point does the effect of the rear energy become negligible?
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Old 2nd December 2004, 11:01 PM   #2
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Tough question. IMO it's when the free air resonance of the driver is not raised by more than, say, 2%?
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Old 2nd December 2004, 11:51 PM   #3
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Infinite baffle is when the front and rear waves never meet, irrespective of cabinet size. But if they do meet past 1/2 wavelength distance from the front of the cone to the rear it matters not anyway.
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Old 3rd December 2004, 12:43 AM   #4
cjd is offline cjd  United States
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The bigger component is the volume as a multiple of driver VAS. Which probably has the result that Bill mentioned. It also ends up being a (technically) infinite baffle in that there is no way for the soundwaves to "wrap" around it much.

Have you checked out the Cult (of the Infinitely Baffled) FAQ? It answers most if not all questions and should certainly get you going.

Leakage does need to be controlled. For example, the area above a suspended ceiling isn't going to work as the "enclosure".

Very few IB installs have infinite rear-chamber volume (which can really only be achieved by mounting outside).

C
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Old 3rd December 2004, 01:17 AM   #5
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Quote:
Originally posted by BillFitzmaurice
Infinite baffle is when the front and rear waves never meet, irrespective of cabinet size.
I disagree because the smaller the enclosure the closer it comes to "acoustic suspension."
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Old 3rd December 2004, 01:22 AM   #6
jdybnis is offline jdybnis  United States
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Check this thread: diff between dipole and IB
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Old 3rd December 2004, 03:12 AM   #7
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The two Fitzes are talking about two different things.

Let us start from the beginning.

A baffle is the board the speaker is mounted on. It might be merely the front board of a box, or the board might be standing alone, unattached to any thing else, with just a speaker mounted in it.

If you have a lone board with a speaker mounted in it, the speaker will produce little bass, becasue the back wave of the speaker will simply move aroung the side of the board and cancel the front wave. Remember, the wave being produced by the back of the speaker cone will be exactly opposite to what is coming out of the front.

If you make the board of infinite width and height, though, the speaker's back wave cannot make it around the board to the fron and cancelling the soundwave coming from the front of the speaker. Such a board will produce good bass.

A board of infinite length and width will be termed an Infinite Baffle.

In the real world, of course, we cannot make the height and width infinite. But if we make the height and width onehalf wavelength of the lowest note we plan to play on the speaker, we have a board that will act not too much differently than an Infinite Baffle. that one half wavelength is enough to keep the back wave of the speaker from coming around and cancelling the front wave.

Hence, Bill Fitzmaurice's answer about half a wavelength.

However, since we live in a finite world, there is another way to produce something that is similar to a true Infinite Baffle. that is a Closed box Woofer with a volume equal to, or greater than, the speaker's Vas.

Plese note the equation below. This gives the rise in Qts and Fs as a result of smaller box size.
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File Type: gif formula for closed box speaker.gif (4.4 KB, 209 views)
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Old 3rd December 2004, 11:15 AM   #8
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As the equation above illustrates, if we wrap the baffle around the speaker to make a Closed Box, we will completely cut off the back wave, but we will also raise the Q and Fs.

If the volume of the Closed Box we end up with is equal to the speaker's Vas, then both the Qts and the Fs will be raised by a factor of 1.4, as the above equation illustrates. Plug the numbers into the equation above, and that is the answer you arrive at. That is fairly significant, but not near what most Closed Boxes do. Most Closed Boxes are built with the box volume only a small fraction of the Vas, with Qts and Fs being raised higher than 1.4.

If, however, you make a Closed Box with a box volume 25 times the Vas, then your Qts and Fs will be raised only by a factor of 1.02. That is a fairly insignificant factor. An Fs of 30 Hz will be raised only to 30.6 Hz, A Qts of 0.40 will become a Qts of 0.408.

When you put a speaker into a door or wall between rooms, or even in the ceiling, you have a chance to have a volume behind it that is twenty-five times the Vas or more, hence the Fs and Qts are raised only by 1.02.

This is the situation Bill Fitzpatrick was referring to-when the baffle is wrapped around the speaker, cutting off all interference from the back wave. He chose the the figure 1.02 as the factor where the raise in Fs and Qts could be considered insignificant.

If you plan to build a box for your speaker, instead of putting it into a door or wall between rooms, or in the ceiling with a significant volume behind it, you probably do not have a realistic shot at having a box volume 25 times greater than the Vas.

As a result, compromises must be made.

Therefore, it has become to be regarded that an enclosure can be termed an Infinite Baffle if the box volume is equal to or greater than the speaker's Vas. This will raise the Qts and Fs of the speaker in it a factor of 1.4 or less. The greater the box volume compared to the Vas, the less the Qts and Fs get raised.

And again, if you want to simply mount the speaker in a flat board, that flat board will prevent the rear wave from slippping around and cancelling the front wave if it's height and width are one half wavelength the lowest frequency you want to play. Under that, you get gradual cancellation from the back wave, the lower you go the more the cancellation.

For a speaker you wish to use down to 30 Hz, a board of 225 inches high by 225 inches wide will be necessary. For a speaker you only want to run down to 50 Hz, a board 135 in by 135 in will be necessary for the board to act like a true Infinite Baffle.
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Old 4th December 2004, 04:32 PM   #9
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Thanks for the thorough explanation!

I think my opportunity for an IB has sailed (gas fitter was here yesterday), but I can see that something like a Stryke IB15 with an Fs=16Hz, Vas=467L, Qts=0.44 can potentially give pretty good results in a large(!) enclosure.
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