how to avoid transmission line resonance with front and rear firing dipole tweeters?

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thadman

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Joined 2005
Linkwitz said:
There is not much sense in building a dipole speaker with two small closed box speakers driven in opposite phase, when one of the objectives is to remove the sound character that boxes imparts. The two boxes a can be joined at their backs, though, and the connecting wall removed b.

Since the two cones move back and forth in unison, there is little air pressure inside the enclosure b at very low frequencies. When the internal length L becomes half wavelength, there is a sharp resonance of the transmission line between the cones, causing a severe dip and peak irregularity in the frequency response. The resonance occurs when L = lambda/4
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Does this phenomenon also apply to dipole (ie front and rear firing) midrange/tweeters? I'd like to maintain dipole radiation throughout the entire spectrum of audible noise and this could be very detrimental to my design. For simplicities sake, lets assume the ribbons are each <3.5" deep. This places the driven elements 8" apart from each other.

8=lambda/4.
lambda=32,
lambda=13,397/32,
lambda=418.65hz

How does Linkwitz avoid this phenomenon with his front and rear firing Milleniums?
 
J

Jaco Pastorius

Member
Joined 2006
Sorry for my awful english..

I’m from italy and I’m not able to write very well

I think that what Rudolf wrote is correct, the described phenomenon of resonance regards two speakers sharing the same volume. Millenium tws have separate volumes..

I’ve made a prototype of dipole speaker like the one suggested in a figure of the Linkwitz site.

Here are some photos..

An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.




I’ve investigated about the resonance described by Linnkwitz but I was not able to find this type of problem.. My prototype seems not to have this issue.

Any suggestion and comment is well accepted.. ;)

Thanks
 
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thadman

Member
Joined 2005
It appears you have front and rear firing cone midranges. How much more energy is required to drive them into xmax at high frequencies (say 160-200hz) as compared to a conventional open baffle?

From an aesthetic, diffraction, and perhaps cleaner rear radiation standpoint, having dedicated rear firing cone midranges (down to 160/200hz) would be very nice. I wouldn't have to transition (from an aesthetic and diffraction point of view) from the 8" deep midrange cabinet to the open baffle panel of the RS225s. If I decided to preserve the roundover down the entire length of the cabinet, the cone midranges would be within a 4" deep quasi u-frame creating a resonance. If I went with front and rear firing cone midranges, that resonance could be avoided by seperating the air volumes of the midrange drivers.

The Daytons aren't that expensive to begin with and justifying an extra $100-140 for rear radiating drivers would be very easy.
 
J

Jaco Pastorius

Member
Joined 2006
From an easthetic, diffraction...
Click to expand...

Sorry thatman but I don’t understand if u r talking to me :( :(

I’m not so able to understand English correctly

If u r talking to me I could answer that separating air volumes of midrange could be a good idea regarding resonance but I think that the important “no box sound” of the open baffle will be lost..

I have think a lot about the resonance problem due to the length between drivers..

My opinion is that sound wave stars from the back of each driver, go towards the other driver and meet the other loudspeaker wave that is reverse of phase.. In that point we probably have the waves deletion..

I “probably” could be in wrong..

Thanks
 
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