S10 OB - Seas L21RNXP on Open Baffle
After completing my last iteration, I really don't feel the need to experiment with different speakers / OB topology. Any improvements I thought, would come from the drivers themselves which unfortunately is about $$$, not engineering. I should also start to make a PCB and perhaps paint the speakers or something like that :D
But when I saw someone is selling a pair of Seas L21RNXP really cheap on ebay I couldn't resist having a go at rigid piston driver.
The Seas drivers are beautiful indeed, and this is not even the magnesium series. They were old types of drivers but never used. The newer ones are L22RNXP.
What I would like to gain from this one are:
- Ability of the midrange to cross lower than P13WH. Let's say 120Hz, presumably better integration with the woofers.
- Transparency of rigid cone drivers. Is the claims true?
- Much higher SPL level
And some drawbacks I would expect/investigate:
- The need to cross the mid to tweeter lower, possibly straining the tweeter.
- Would the increase of baffle from 19cm to 24cm affect the polar response in a really bad way?
- Metal cone resonance
The size of L21RNXP also allows me to think about cloning the Orion. It's not hard to replicate the curves, but I really don't have the energy nor skills for the woodworking part :o
Metal cone resonance
Indeed .... :D
I mounted the drivers on 24cm baffles and perform full-range measurements. Unfortunately it's cold and windy here in Melbourne so I am too lazy to go outside to do polar response measurements. But the nearfield is as follows:
Very nasty resonance, which really sounds nasty. The first one at 4.5kHz and then more follows. Exactly like the measurements from Seas datasheet although they do it with a box. I listened for a while to learn the sound of these resonance. They won't be easy drivers to work with that's for sure.
I applied 1.5khz 24db xo, and this is the result. Again, ignore the absolute SPL.
First resonance is -35db down and clearly can be seen in the photo. How audible is it? In short: with pink noise it's very audible. I can't identify it with music. But this definitely need to be tamed with notch filter. Quick investigation this will be 4610Hz/-18db/11Q. Passive speaker designer will cry doing this.
I could see immediate reason why Zaph was so ecstatic about L18 as their resonance is almost an octave higher, and also his own driver with resonance at 9kHz.
Ideally the project would be an all-seas metal driver, 27TBFCG being my favourite, but I don't have $300 lying around so the current HiVi K1 will do.
I chose 1.5kHz xo point as above. K1 at this frequency range is okay, but not great. It's only great 2k and above.
Selecting Woofer-Midrange is quite easy. First the two poles are calculated, for L21 it's 66Hz and 11Hz.
Below 66Hz the natural driver rolloff is -6db/oct and becoming 12db/oct at 11Hz. I want the acoustic slope to reflect electrical function within 1 octave, giving 130Hz as good xo point. Only dipole shelving is required, no need to compensate the driver.
The woofer H-Panel is 24cm deep, giving theoretical peak at 700Hz. Very safe from 130Hz xo. The woofers are local products (Jaycar) which I acquired for $14 each. They have Qts of 0.7 so aside from shelving lowpass, a linkwitz transform circuit is added.
If I remember correctly 27TBFCG is only AU$75 each at Audiophile in Melb ie
Nice thread, I like where things are going. We are very much on the same path!
One thing to note about those metal cone resonances is they often show up as higher levels in the harmonic distortion, and are not filtered out by the XO. If they do show up, this can be audible as strain at louder volumes - but it may not be a problem at lower volumes. It is possible to measure and hear, if you'd like to find out at what volume it becomes a problem. Perhaps this is the extra-transparent-detail of metal cones? I don't know...
You recently asked if the Neo3 can go below 2k - I was doing distortion measurements today, and played loud, distortion is rising below 2k. At loud volumes the distortion levels were right at my threshold for okay/not okay. Playing at 96dBSPL/1m, things were 'clean' at 1.5k - as in 4th and 5th harmonics are about -70dB. So they still have a little head room.
My current search is a little backwards from yours - I've been using 8" drivers for a couple of years now, and am currently looking for 'the missing link' - I'd like to find a small driver that can cover from 700-2000Hz. The main issue with 8" drivers is that they start to beam around 700Hz, so if you want super smooth off axis behavior (esp. up to >2k), you need a smaller driver. I haven't found anything I like yet....
I am not sure that you are heading to the right direction. :(
The increased baffle width of 24 cm will be ok for the L21 and 1.5 kHz, but it is at the limit IMHO. The same is true for the onset of beaming, as cuibono already remarked.
What I can't see: Where are you going to get a tweeter that is capable of 1.5 kHz and still small enough to allow for a decent dipole radiation at something like 5 kHz? The dipole separation distance of the 27TBFC/G - mounted back to back (and without any further baffle) - would be 13 cm minimum. This is corresponding to a first dipole peak at 1.3 kHz. Beyond 1.3 kHz you would loose the CD effect of the dipole radiation pattern.
BTW: What was wrong with crossing the P13WH at 290 Hz (or even 350 Hz) to the H-frame? I see your area of possible improvement much more with smaller tweeters than with larger midrange drivers.
Resonance Notch and time alignment
You guys are 100% correct and I started the excercise with healthy amount of scepticism. I just have a brief listen now with proper xo point, notch etc, even time alignment. The 8" definitely beams at the frequency and they are running right at their upper limit.
However it's quite surprising the similiarity of sonic signature and soundstage that is projected by these speakers compared with S9. Yet one is poly cone, one is metal. I'm not sure whether it's that P13 is so great, or the Seas which are not good ???? ... or at a certain point they all sound the same???
Still I think I would pass a blind test. If I could recall the P13 was more "muddy" in the lower freq., if that's the appropriate word or valid observation at all. The Seas have a hint of shrill but this could be my mind just playing out or that it's the tweeters being strained. I really like the 300-800hz region of the Seas.
More investigation and understanding is needed.
As Cuibono pointed out, it's impossible to remove the resonance peak completely as harmonics from lower frequency would show up. I was able to reduce about 10db worth but that's it. Further notch depth strangely did not affect the acoustic transfer function:
The system is properly time-aligned, and I had to delay the tweeter much more which seems to point out that the acoustic center of the Seas is much further to the back proportionally compared to P13WH (and physically too of course).
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