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Old 17th November 2004, 03:32 AM   #1
bser is offline bser  United States
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Default diff between dipole and IB

I see many projects that are interesting that use a dipole bass driver. I have recently built an IB setup using a manifold mounted in the sealing and absolutly love the results. Is a dipole basically the same thing as an IB or am I missing something? It appears that a box is made to seperate the front and back waves instead of using a wall, floor or ceiling as in IB. The reason I ask is because I'd like to build a set of computer speakers but don't have the option for IB in that room but would still like an IB like sound. Thanks for helping
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Old 17th November 2004, 06:29 AM   #2
Wizard of Kelts
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An Infinite Baffle is not a dipole. An Infinite Baffle is a Closed Box.

Strictly speaking, an Infinite Baffle is a Closed Box so large, that it is several times the Vas of the driver mounted in it, and does not affect the Q and Fs of the driver placed in it.

The theoretical ideal is that the box is so large, it approximates an infinitely large board wraped around the speaker.

In a practical sense, an Infinite Baffle is a Closed Box which is about equal volume in volume to the driver's Vas, and which raises the driver's resonance point and Qts just a little.

Here is the formula for the rise in Qts and Fs when a speaker is placed in a Closed Box. For a Closed Box with a volume equal to it's Vas, both Fs and Qts are raised by 1.4. This kind of box would be termed an Infinite Baffle. However, a Closed Box with a cubic volume much smaller than the driver's Vas would NOT be called an Infinite Baffle.

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Old 17th November 2004, 08:10 AM   #3
jdybnis is offline jdybnis  United States
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KW,

I think the term "infinte baffle" is actually used in different contexts to mean several different things.

1. A driver mounted on an infinite plane (or approximation of such). Ex. "I measured the driver on an infinite baffle so there was no influence from edge diffraction."

2. A driver mounted on a box with infinite volume (or approximation of such). This is the sense used by the CultOfTheInfinitelyBaffled and others in subwoofer land.

3. A sealed box with a volume equal to or greater than the Vas of the driver. This is the sense used by Dickinson in the LDC.

Note that sense 1 does not imply sense 2 or vise versa. You can have an infinite baffe (1) that has a small sealed box on the back of the driver so it is not an infite baffle (2). Conversely you can have an infinite baffle (2) that sticks out into the room so it is not mounted on an infinite plane.

-Josh
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Old 18th November 2004, 12:26 AM   #4
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Quote:
An Infinite Baffle is not a dipole
Sure it is. The dipole just has an infinitely large baffle so the rear wave never meets the front wave. Picture a dipole sitting in the middle of the desert with the baffle extending for miles upward and to both sides.
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Old 18th November 2004, 12:45 AM   #5
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No one defined dipole so I'll give it a shot. The simple dipole application is a single driver mounted on a flat board. The wave from the front is out of phase with the wave from the back. The waves meet each other at the edge of the baffle and effectively cancel each other. This creates a sound field akin to a figure eight which has the benefit of direct sound reaching the listening point far ahead of reflected sound.
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Old 18th November 2004, 09:47 PM   #6
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Quote:
Originally posted by Timn8ter
No one defined dipole so I'll give it a shot. The simple dipole application is a single driver mounted on a flat board. The wave from the front is out of phase with the wave from the back. The waves meet each other at the edge of the baffle and effectively cancel each other. This creates a sound field akin to a figure eight which has the benefit of direct sound reaching the listening point far ahead of reflected sound.
A better way of looking at it is that the total power response is 4.8dB lower for a given on-axis SPL than with a monopole, and the rear wave is 180 degrees out of phase with the front.

Less reverberant energy and more direct sound reach the listener.

Polar response = cosine alpha
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Old 18th November 2004, 10:23 PM   #7
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A dipole sub and a 'true' IB sub (not acoustic suspension) have similar sound quality. But you'll need at least double the number of drivers, and more EQ to get a dipole sub to have the 'punch' of your IB sub
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