Hi,
I"m currently investigating the possibility to add a rear tweeter to my speakers (they're open baffles with short wings, rather narrow baffle, 2 way, with a 7" B&W woofer and a Vifa XT25 tweeter, LR2 crossover at 2.9 kHz).
The main question is whether it is worth the money to use the exact same tweeter at the back. I've seen here:
http://www.diyaudio.com/forums/showthread.php?postid=1411798#post1411798
that the approach of using a different(cheaper) tweeter is feasible.
In my case, I was thinking about taking this here:
http://www.monacor.de/typo3/index.php?id=75&L=1&act=8&act_sub=25&artid=3573&spr=EN&typ=u
It's a decent neodymium soft dome (being small is an advantage, as I have to mount this on the back of my OB, which has about only 8cm wings).
Another question would be how to cross-over the rear tweeter, the choices being:
1. to use the exact same crossover as for front
2. to aim for a reasonably flat rear response (considering the woofer having a significantly different rear response)
So, what do you think ?
I"m currently investigating the possibility to add a rear tweeter to my speakers (they're open baffles with short wings, rather narrow baffle, 2 way, with a 7" B&W woofer and a Vifa XT25 tweeter, LR2 crossover at 2.9 kHz).
The main question is whether it is worth the money to use the exact same tweeter at the back. I've seen here:
http://www.diyaudio.com/forums/showthread.php?postid=1411798#post1411798
that the approach of using a different(cheaper) tweeter is feasible.
In my case, I was thinking about taking this here:
http://www.monacor.de/typo3/index.php?id=75&L=1&act=8&act_sub=25&artid=3573&spr=EN&typ=u
It's a decent neodymium soft dome (being small is an advantage, as I have to mount this on the back of my OB, which has about only 8cm wings).
Another question would be how to cross-over the rear tweeter, the choices being:
1. to use the exact same crossover as for front
2. to aim for a reasonably flat rear response (considering the woofer having a significantly different rear response)
So, what do you think ?
bzfcocon, These questions have been asked by the builders of dipole speakers designed by Siegfried Linkwitz and John Kreskovsky. I have the Linkwitz Orion system with the rear tweeters installed. Both systems use the Seas Millenium tweeter, but are crossed over at different frequencies. The Kreskovsky design is crossed over at 2.2kHz, fourth order, thus allowing a higher resonance (cheaper) rear tweeter, but the Linkwitz is crossed over at 1.4kHz putting excursion demands on the rear tweeter that can only be met with the Millenium or similar tweeters.
With your 2.9kHz XO you will find the rear radiation of your mid driver will start to degrade above about 500Hz due to the presence of the frame, spider and magnet. This means that the figure 8 pattern is going to be rather ill defined in the top two octaves of the mid driver. You may find that adding a rear tweeter with the same sensitivity as the front one will give you some high frequency "tizz" in the room reverberation rather than a replica of the front radiation. There are also issues of the different acoustic centres of the two tweeters leading to beam tilt problems that are worse for higher XO frequencies.
See www.linkwitzlab.com/ and www.musicanddesign.com/
Sorry to be so pessimistic. Perhaps you could "try before you buy" and remove one of your tweeters and install it on the rear of the other speaker and have a listen to it in mono.
Keith
With your 2.9kHz XO you will find the rear radiation of your mid driver will start to degrade above about 500Hz due to the presence of the frame, spider and magnet. This means that the figure 8 pattern is going to be rather ill defined in the top two octaves of the mid driver. You may find that adding a rear tweeter with the same sensitivity as the front one will give you some high frequency "tizz" in the room reverberation rather than a replica of the front radiation. There are also issues of the different acoustic centres of the two tweeters leading to beam tilt problems that are worse for higher XO frequencies.
See www.linkwitzlab.com/ and www.musicanddesign.com/
Sorry to be so pessimistic. Perhaps you could "try before you buy" and remove one of your tweeters and install it on the rear of the other speaker and have a listen to it in mono.
Keith
Keith Taylor said:
Thanks for the response !
While I understand and I am aware of the problems you exposed, I'm not really sure I understand your general point: do you actually mean that the rear tweeter is pointless anyway, since there's no way to get the rear radiation similar to the front one ?
Or should I understand that it takes a lower crossover point to obtain acceptable results, and implicitly a tweeter that can handle it ? (by avoiding the frequency domain where the woofer radiation is strongly asymmetrical )
Is it possible to partially correct the woofer rear radiation assymetry by tailoring the (rear)tweeter response accordingly, that is, letting the rear tweeter "fill in" the missing radiation from the woofer ? I know that polar response issues make that a complicated task, but maybe it is worth trying.
Maybe I didnt know where to look, but, on both Linkwitz and JohnK's sites I couldnt find a precise statement regarding the rear tweeter response.
For the NaO, I understand that the rear tweeter is simply an "inverted" copy of the front, which, as you said, does not guarantee a consistent rear response.
For the NaO, I understand that the rear tweeter is simply an "inverted" copy of the front, which, as you said, does not guarantee a consistent rear response.
The NaO's rear tweeter is attenuated if you look at the schematic. John K has mentioned that due to the short wave length whether it is inverted or not does not make much difference, though in the schematic it is inverted.
bzfcocon, I think you understand the situation with the mid rear radiation falling off at higher frequencies causing a hole in the response. To fill in this hole would require a non dipole rear driver, representing quite a challenge in XO design. It is difficult because you have no control of the lowpass response of the mid driver. What you do will effect front AND rear radiation equally. I think the Orion has a bit of a hole in its rear response in the octave below the XO, but the overall effect is beneficial
I think John K has some measurements of polar response in one of his studies that shows a modest ammount of dipole null can be achieved between the two tweeters in the first octave or so. Both he and Linkwitz advocate connecting the tweeters in antiphase to simulate dipole operation.
Power response of loudspeakers is a much debated subject, and it is mentioned that sound sources in nature have many and varied directivities. What is not in debate is that changes in directivity with frequency need to be brought under control; something that dipole sources take care of. While crossing over as low as possible avoids mid driver breakup modes and impaired figure 8 pattern the resultant low pass filtered rear radiation is far from "natural". This explains why the Orion design benefits from the rear tweeter.
In an ideal world an inherently dipole tweeter such as a ribbon or a pleated (Heil) would seem to be a solution, but both of these have limitations. The former lacks excursion and both have a narrow vertical pattern. I am not suggesting you give up, just take on board as many considerations as possible.
Keith
I think John K has some measurements of polar response in one of his studies that shows a modest ammount of dipole null can be achieved between the two tweeters in the first octave or so. Both he and Linkwitz advocate connecting the tweeters in antiphase to simulate dipole operation.
Power response of loudspeakers is a much debated subject, and it is mentioned that sound sources in nature have many and varied directivities. What is not in debate is that changes in directivity with frequency need to be brought under control; something that dipole sources take care of. While crossing over as low as possible avoids mid driver breakup modes and impaired figure 8 pattern the resultant low pass filtered rear radiation is far from "natural". This explains why the Orion design benefits from the rear tweeter.
In an ideal world an inherently dipole tweeter such as a ribbon or a pleated (Heil) would seem to be a solution, but both of these have limitations. The former lacks excursion and both have a narrow vertical pattern. I am not suggesting you give up, just take on board as many considerations as possible.
Keith
Thanks Keith !
I have seen Jonh K's measurements, he shows both an improvement of the dipole null at 90 deg AND a very reasonable rear response (almost flat). He admits though that this is obtained by carefully positioning the microphone (on axis).
On the other hand, the NaO design seems to use a simple flat baffle (while the Orion has wings) which also impoves front/rear simmetry.
Overall, I am also not attempting to obtain the perfect (polar) rear response, but, as far as understand, restoring at least SOME of the front/rear symmetry using a rear tweeter should be beneficial. So I will surely give it a go.
Since the back radiation is rather "ambient" only, it seems that it"s not worth spending very much on the rear tweeter - JohnK also suggests Seas Millenium for front and 27TDFC rear (sure, they're somewhat similar). Of course, not any garbage tweeter will do.
Keith Taylor said:
I have seen Jonh K's measurements, he shows both an improvement of the dipole null at 90 deg AND a very reasonable rear response (almost flat). He admits though that this is obtained by carefully positioning the microphone (on axis).
On the other hand, the NaO design seems to use a simple flat baffle (while the Orion has wings) which also impoves front/rear simmetry.
Overall, I am also not attempting to obtain the perfect (polar) rear response, but, as far as understand, restoring at least SOME of the front/rear symmetry using a rear tweeter should be beneficial. So I will surely give it a go.
Since the back radiation is rather "ambient" only, it seems that it"s not worth spending very much on the rear tweeter - JohnK also suggests Seas Millenium for front and 27TDFC rear (sure, they're somewhat similar). Of course, not any garbage tweeter will do.
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