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Old 3rd December 2004, 05:18 PM   #21
tiroth is offline tiroth  United States
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Can someone say authoritatively if BSD "reflections" are the same as room reflections in their mathematical interaction with the acoustic wave? If the mental model of the pressure wave hitting a discontinuity (edge of baffle) and "bouncing back" is true, than it would seem intuitive that in a non-round baffle, BSD reflection effects would occur with non-constant delay; e.g. non minimum phase.

Simply being able to say "yes" or "no" to this question would seem to resolve the matter.

That does seem to imply something interesting about round baffles, doesn't it?
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Old 3rd December 2004, 06:10 PM   #22
Pan is offline Pan  Sweden
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"If what you say was true, then an equalizer could be used to correct all response anomalies caused by the room."

The room is so complex, but looking at one room mode in isolation a regular EQ can counteract it IF the room mode is measured carefully and the EQ circuit is tailored to exactly mimik the Q of the mode.

Also a loudspeaker is a resonant device in itself, but still considered a minimum phase system. A in band resonance in a driver does not change the driver into a non minimum phase device..? It just change the Fr. and phase according to the Q of the resonance. With a simple LC or RCL circuit the response can be flat and corrected in phase as well, resulting in a transient perfect system as far as a driver can be that.



"The strong implication would be that true BSC (phase as well as FR) would require FIR filters."

I do not think so.

I think BSC brings back frequency response and phase to normal.

Lets take a step repsonse, the leading edge leaves the driver and travels towards listener and baffle edges. After 0.5ms or so the wave reach the edge and suddenly experience a pressure drop due to some energy diffract around the cabinet.

What we need is a circuit that is ready to push some extra current into the driver at this point in order to counteract the baffle step, since we can not have a transient perfect system that is not flat in Fr. The delay in the inductor is mathced in time to the time it takes the signal to reach the baffle edge.

If this simple baffle step circuit would change the system into a non minimum phase system, we would see a phase turn similar to a high order (non 1st order) crossover.

For now Im convinced I was right from the beginning but I am ready to change my mind instantly if someone comes with a better idea.

/Peter
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Old 3rd December 2004, 06:19 PM   #23
tiroth is offline tiroth  United States
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Well Pan, I respect what you are saying, but I think you are oversimplifying in assuming the steady state. In a listening room 20' long with the listening position in the middle, the back wall reflection is arriving 20ms after the start of the tone. There is no way you can correct this reflection!

Now, you may well be right that BSD is a different beast than this. But I haven't seen an argument that persuades me yet.

It's an interesting discussion, I just have a feeling that someone like MarkMck already knows the answer.
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Old 3rd December 2004, 07:39 PM   #24
Pan is offline Pan  Sweden
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What the speaker do and what the room do are two separate things. Of course they affect eachother (im not stupid ) but you must realize that you do not only have the back wall reflection, you have lots of reflections that come and go, adds and subtracts.

You cant blame a speaker or a BSC circuit for the mess that the room contributes with. Even though youd maybe like to balance the amount of BSC against the contribution from the surfaces of the room.


"but I think you are oversimplifying in assuming the steady state"

Actually I did talk about transient behaviour, not steady state. Allthough as far as the speaker goes you will get the steady state from the impulse or step response (but not the other way around).

/Peter
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Old 3rd December 2004, 07:57 PM   #25
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Baffle diffraction -- of which Baffle Step is just a component -- is a continuum. At high frequencies, when the wave hits the discontinuity of the cabinet edge we get a reflection/reradiation of the wave. At low frequencies the wave does not even see the cabinet. At HF we have radiation into 2 pi steriradians, at LF 4 pi. In between is a continuum transitioning from one to the other.

In the last years my preference is to deal with the LF phenomenom by eliminating any baffle step from my boxes at the source -- a bi-pole (or alternatively a 0.5 speaker on the back if i'm not building with the FRs i prefer). As long as the box is symetrical back-to-front this perfectly compensates for Baffle Step (at the cost of an extra driver)

dave
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Old 3rd December 2004, 09:57 PM   #26
Pan is offline Pan  Sweden
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Dave,

sure it must be easier to get good results from a 0.5 driver mounted on the front? Youll always end up with the delay from the back driver otherwise... no?

/Peter
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Old 3rd December 2004, 10:24 PM   #27
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Quote:
Originally posted by Pan
sure it must be easier to get good results from a 0.5 driver mounted on the front? Youll always end up with the delay from the back driver otherwise... no?
Absolutly not... at the frequencies of concern the delay is close to insignificant and the 90 degree phase roll is hidden in the shadow of the box instead of mucking up the midtrange.

If you are really concerned make the box wider than it is deep.

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Old 3rd December 2004, 10:34 PM   #28
Pan is offline Pan  Sweden
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Ive never build such a design but what you say makes sense of course. I understand that there is no problem in low frequencies, but can you really get a smoth transition without ripples or a dip at/around the "step"?

/Peter
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Old 3rd December 2004, 10:56 PM   #29
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Quote:
Originally posted by Pan
but can you really get a smoth transition without ripples or a dip at/around the "step"?
The beauty of the driver on the back is that -- as long as the cab is syymetrical back-to-front -- the bafflestep on the back driver perfectly fills in the baffle step on the front driver & visa-versa.... purely by the nature of the acoustical geometry of the system.

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Old 4th December 2004, 12:04 AM   #30
Pan is offline Pan  Sweden
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Have you accurately simulated that or measured outdoors/anechoic chamber?

Mirage seems to have problems with getting a 100% result if my memory serves me.

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. :-)

I understand the basics and the function of this. Its just that I have not spent much time on calculating exact what happens at what frequency so to speak.

/Peter
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