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Old 4th December 2004, 02:55 AM   #31
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Quote:
Originally posted by Pan
Have you accurately simulated that or measured outdoors/anechoic chamber?
Nope... just a really good thot experiment and listening experience

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Mirage seems to have problems with getting a 100% result if my memory serves me.
They are multiway bipoles with a single bass driver if memory serves me right.

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My mental model of this is that the enclosure needs to be thin as a credit card in order to make this work perfect... but on the other hand, what is perfection. :-)
As long as the 2 bass drivers are within a half-wavelength below the baffle step they should be fine.

dave
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Old 4th December 2004, 08:11 PM   #32
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Quote:
Originally posted by rgrayton

Russ, This started out as a question to Mark McKenzie, but he seems to have abandoned the discussion.
Since you were quoting Mark in the first post in this thread I figured that it must have came from another thread.

Russ
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Old 4th December 2004, 10:25 PM   #33
Svante is offline Svante  Sweden
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I saw this thread a bit late, but I'll share my view on the subject:

A driver and the diffraction produced at the edges is not a minimum-phase system, but as we shall see, it can be seen as one in some cases anyway.

In the simplest case with the circular baffle, the diffraction from all locations along the baffle edge will coincide and form a secondary impulse in the impulse response, and such a system is not minimum phase. The circular baffle results in a baffle step with large oscillations (in the frequency domain) due to the sharp secondary impulse.

This means that a circular baffle with the driver in the centre is a bad idea, in fact it is about the worst I can imagine. It is better to smear the secondary impulse from the edge by assuring that the driver-to-edge distance varies in different directions. Parts of the edge should be really close to the driver, other parts should be far away. In that way, a clever driver positioning can smear the secondary pulse in such a way that the amplitude and phase response becomes very close to that of a minimum phase system at low and mid frequencies. At high frequencies, the driver directivity reduces the radiation toward the edge, which makes the edge reflections and thus the minimum phase issue less important. All in all, this can make the resulting response close to a simple first order (one pole, one zero) system, and this can be compensated by means of a simple coil and resistor.

The error that remains is mostly small compared to other errors in the design, and can even be utilised to compensate other errors in some cases.

You can try this with my little "Edge" hack, and see that with a clever positioning of the driver, and with a simple compensating network, both phase and amplitude can become reasonably flat.

One final thing: The result of the diffraction at the edge is principally similar to a reflection at a wall in the sense that it produces an"echo" in the impulse response. However, the time that elapses between the first pulse and the reflection is typically larger in the wall reflection case due to a longer distance. The two cases are quite different in psychoacoustical terms. If the delay is short (as in the baffle diffraction case) the reflection merges psychoacoustically with the original sound and results in a colouration of the sound. In the wall case we will hear the reflection as reverberation or an echo if the delay is really long. The colouration can be compensated by a simple passive network, on the other hand the wall reflections probably need digital means to be compensated to inaudibility.
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Old 4th December 2004, 11:16 PM   #34
Pan is offline Pan  Sweden
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IŽd say Svante sums it up very nicely here.

/Peter
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