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-   -   Open baffles & baffle step (http://www.diyaudio.com/forums/multi-way/32262-open-baffles-baffle-step.html)

5th element 14th April 2004 11:46 PM

Open baffles & baffle step
 
Do open baffle exibit baffle step like a normal loudspeaker once the 6dB/octave dipole has been sorted out?

From theory I cant see that they would. As both the front and back are radiating sound then they will both do baffle step at the same frequency. Thus when the front would start to go omni so would the back and reinforce each other. As the back reflection above baffle step would not diffract to the front you would not get any reinforcement at frequencies above this. So thus at frequencies above bafflestep you would hear the front of the driver. But at lower freqs the rear wave would come into play cancelling out baffle step when the front wave starts to go omni.

That seems a little confusing to me and I know what im talking about! sorry for that its hard to explain.

Matt

goudey 15th April 2004 02:52 AM

The quick answer is no, baffle step for an open baffle is different.

Check out http://www.woodartistry.com/linkwitzlab/faq.htm#Q8


When the front wave and back wave meet at the edge, the pressures cancel but the velocities add (sort of). The result is low pressure and high velocity, and hence much diffraction in the midbass region. Higher frequencies are affected but to a lesser extent.

I recommend prototyping rather than simulation (I believe Linkwitz also makes this recommendation).

5th element 15th April 2004 10:47 AM

That didnt really help, I understood what he was saying but it didnt answer my question.

After the response irregularities, ie the 6dB roll off per octave as per dipole radiation, has been eq'd out, do you need to apply baffle step too?

I didnt think you did, but im not entirely sure because in the pheonix he does? cant be sure but I need to know.

cheers Matt

goudey 15th April 2004 12:15 PM

No, it is inappropriate to apply "baffle step compensation" to an open baffle system. And the Phoenix design applies just the opposite of baffle step compensation.

Process: Prototype, measure/listen, compensate/equalize, Prototype, ...

I have learned a bit from my prototyping. I do not have a simple model to predict performance. I follow the above process.

sreten 15th April 2004 12:29 PM

Quote:

After the response irregularities, ie the 6dB roll off per octave as per dipole radiation, has been eq'd out, do you need to apply baffle step too?
No because baffle step of 6dB is replaced by baffle roll-off of 6dB/octave.

Quote:

And the Phoenix design applies just the opposite of baffle step compensation.
Find it hard to believe a dipole applies 6dB of midrange boost.

:) sreten.

Stephen D 15th April 2004 03:07 PM

Quote:

Originally posted by sreten

No because baffle step of 6dB is replaced by baffle roll-off of 6dB/octave.

Wouldn't that mean that an open baffle has the same initial response roll off as a sealed enclosure? I find that hard to believe as it seems that a sealed merely looses pressure from going from 1/2 space radiation to full space, where as an open baffle has the loss from full out of phase cancellation of the combined front & rear wave to deal with. Also every equivalent comparison I've heard of unequalized open baffle vs. sealed has shown open baffle to be weaker in the bass... open back vs. seal guitar cabs as a simple comparison. What am I missing?

5th element 15th April 2004 04:47 PM

Yes that makes sense - talking to myself. :D

Right an open baffle would have a flat frequency response if it was not for the rear wave (its more complicated then that but bare with me). I took near field measurements of the W15CY001 in the open baffle and you get a flat smooth response in the mid and bass to about 60hz, then the response falls off rapidly, taking near field ignors the rear wave.

I think that what happens is this. The rear wave and front wave cancel each other out when they meet.

Imagine normal baffle step with its 6dB transition from 2pi into 4pi when bass is omni directional. Because all the sound is produced on the front panel and nowhere else when the sound reaches an appropriete frequency (lowish) is diffracts around the side of the cabinet and we get the loss of 6dB because its diffracting into open air from the baffle so sees the free air as a pressure change, the acoustic impedance decreases so the sound decreases in level.

Now an open baffle does NOT exhibit baffle step at all. Imagine for a second that the rear wave was infact in phase with the front wave. Above baffle step at high frequencies, when you are sitting, listening infront of the speaker you are going to hear only sound from the front wave. This is because the rear wave has not reached an appropriete frequency to diffract to the front.

As the frequency decreases and baffle step is reached normally in a closed baffle the sound creeps behind the cabinet and with it the 6dB fall off. With an open baffle when baffle step is reached or the frequency gets low enough to diffract, the front wave diffracts to the back as per sealed, but this time the rear wave also diffracts to the front. They will do this with roughly equal magnitude. So the loss of the front wave round the back will be equal to the loss of the rear wave round the front,the rear wave will compensate for the front wave loss, thus baffle step would not occur.

But in an open baffle the front wave is not in phase with the rear wave so distructive interference occurs. This gives the 6dB per octave roll off that an open baffle exhibits when baffle step would normally occur in a sealed box. So therefore if you compensate for the 6dB per octave roll off in an open baffle there is no baffle step to worry about.

With regards to the guitar boxes that are closed or open baffle, when open the rear wave at a certain frequency will cancel out the front wave as described so they will sound thinner and less bassey. Because where BS would normall occur in the sealed box and u get the 6db roll off, in an open baffle you get a further 6dB roll off every octave you decrease.

I hope that is clear enough and makes sense.

Stephen D 15th April 2004 05:04 PM

Quote:

Originally posted by 5th element

With regards to the guitar boxes that are closed or open baffle, when open the rear wave at a certain frequency will cancel out the front wave as described so they will sound thinner and less bassey. Because where BS would normall occur in the sealed box and u get the 6db roll off, in an open baffle you get a further 6dB roll off every octave you decrease.


That makes sense to me but doesn't that contradict your conclusion in your previous paragraph?

5th element 15th April 2004 05:23 PM

Quote:

Originally posted by 5th element
Imagine for a second that the rear wave was infact in phase with the front wave.
For the first half I was describing what would happen if the front wave and rear wave were in phase.

So the diffraction from the rear to the front would be the same as the loss from the front to the rear so no loss in level as frequency decreases would occur. So there is no baffle step.

But in real life they are not in phase they are the opposite so cancel each other out at a rate of 6dB per octave, instead of boost each other as per above.

Once the 6dB per oc is equalised out your back to flat again. Like with the in phase example. My original question was after you have accounted for this 6dB/oc loss do you need to further account for the usual baffle step, and the answer to that is no.

Does that help?

Stephen D 15th April 2004 05:34 PM

Quote:

Originally posted by 5th element

Because where BS would normall occur in the sealed box and u get the 6db roll off, in an open baffle you get a further 6dB roll off every octave you decrease.

And that would seem to imply a 12 dB/oct total compensation necessary, right?


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