I don't buy this reasoning
From.the little I know, air under compression is far more linear than most suspensions.
There are of course exceptions/limits, and a high compression ratio throat chamber may increase HD.
But then, your compression ratio is very low, so I'd think the concern was unjustified
Not the compression in the throat chamber - in the rear chamber. The HD shows up when going from dipole to sealed bass so the front bandpass chamber is the same.
Air suspension is less linear when the ratio of stroke x Sd is *not* an insignificant fraction of the chamber volume. In my case it's probably not the air spring non linearity but the effect of high pressure on the diaphragm's resonance. In my case I noticed that a bad 4% HD peak at 104Hz present with the DC200 went away with the RS180P.
But for very small volume sealed subs like a 1 cu ft box fitted with a 15in driver - the max stroke displacement volume is not negligible compared to box volume. Sd for 15in driver is circa 800cm2 X 1cm stroke is 0.8liters. For 28.3 liters this is close to 3% volume fraction. Air non linearity is due to Boyle's law (ideal gas) where Pressure is inversely propertional to volume. Volume is proportional to stroke so linear position is inversely proportional to pressure or restoring force. The movement in has a different effect on pressure than movement out due to the curvature in the 1/x function (not linear).
Air suspension is less linear when the ratio of stroke x Sd is *not* an insignificant fraction of the chamber volume. In my case it's probably not the air spring non linearity but the effect of high pressure on the diaphragm's resonance. In my case I noticed that a bad 4% HD peak at 104Hz present with the DC200 went away with the RS180P.
But for very small volume sealed subs like a 1 cu ft box fitted with a 15in driver - the max stroke displacement volume is not negligible compared to box volume. Sd for 15in driver is circa 800cm2 X 1cm stroke is 0.8liters. For 28.3 liters this is close to 3% volume fraction. Air non linearity is due to Boyle's law (ideal gas) where Pressure is inversely propertional to volume. Volume is proportional to stroke so linear position is inversely proportional to pressure or restoring force. The movement in has a different effect on pressure than movement out due to the curvature in the 1/x function (not linear).
Did you do an impedance test on the woofers as mounted in the box?
We could compare it to bushmeisters impedance curve too if he's willing to make/share one. And it would be interesting to see one without the box, but woofers still mounted on the horn.
In a way I'm glad bushmeister is stepping away from the bookshelf constraint to see how far he can take this. I expect part of the trouble to be about the size of the back chamber. Personally I never liked the egg crate foams. It didn't bring any sonically advantages in lower frequencies to me. I'd rather use real wool felt, the more regular glass fibre insulation (don't have Rockwool or denim variants here) and Twaron's angel hair. Those were the ones making lasting impressions in measurements that I have done. Especially in combinations. Verifiable in impedance tests.
I did make the impedance measurments. Nothing anomalous - there is one main peak shifted up from driver fs of 34Hz to something around 60Hz, and Q was rather high at 1.15 for the DC200 as result of the small chamber volume. When I stuffed it with all the fiberglass it could take the Q went down a bit to about 1. There were no wierd ripples of anything other than a small blip at the bandpass vent overshoot frequency at 500Hz. But we aready eliminated the vent as the cause because in dipole config the vent is the same.
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So we are back at quality of the drivers used? And in the case of dipole vs sealed enclosure EQ-ed to the same FR curve, which one has lower distortion?
The more expensive RS180P-8 definitely has lower distortion when EQ'd to the same curve. The RS180 has a copper shorting ring, a longer xmax of 6mm vs 4.5mm, a larger magnet, a stiff cast aluminum frame vs stamped steel. I would have thought the lighter paper cone on the RS180P would not have done as well as the thick paper cone on the DC200 (nearly 3x higher Mms). The distortion went down about 5dB when compared at the same SPL at 80Hz.
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In addition to the differences you mentioned, the RS180P-8 has a higher Fs, and less than half the Vas of the DC200, and was used in a larger box than yours, so is more sensitive. Since it requires less power to achieve the same level, distortion is lowered.
Although the RS180P-8 has less Bl (7.16 compared to 9.5), with 1/3 moving mass the ratio of Bl to Mms is better, more "cone control".
You could cut a round hole in your cabinet and mount it on a sonotube (or PVC pipe) stand to increase the volume, lower the Fcb and reduce distortion. With a foam weather strip gasket between the two, the weight of the cabinet would probably be sufficient to eliminate air leaks. You would then have enough volume to make bass reflex tunings viable, increasing sensitivity and reducing distortion further.
Art
I don't buy this reasoning
From.the little I know, air under compression is far more linear than most suspensions.
There are of course exceptions/limits, and a high compression ratio throat chamber may increase HD.
But then, your compression ratio is very low, so I'd think the concern was unjustified
If depth is not a concern, you could easily fit one or two slot loaded 18" in a 15" wide cabinet, similar to the EV MTL2. This arrangement can also be done as push-pull to reduce even order HD, and gain some cabinet volume lost to the plenum (slot).
Another advantage of the dual driver set up is cancellation of mechanical force, the cabinet won't wobble or "walk" at high LF drive levels as a front loaded driver will.
Art
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Hehe I love the way you tall to folk like they were never educated
I would imagine in an extreme case (something like LS3 monitor) where volumetric displacement of the cone approaches (or exceeds) 10% system volume, or another arbitrarily defined figure, then an increase in THD is a fore gone conclusion.
I suspect however, that it is far far less of an issue with a large woofer, unless the combo is large woof with large xmax.
I.E. I can't see a purpose in using a 15" in an app like this, with xmax of more than 4-5 mm. It's just a sub otherwise, and I think we all know what issues subwoofers present in matching to mid.
I like slot loaded opposed push pull subs too. The few that I have made have performed well. In a way, it is similar to how the woofers are working in the horn bandpass slots. When open back as a dipole they can get surprisingly low. dual 12's might be a good option too.
If you think giving you an example with real numbers is patronizing - well I guess you prefer people who use hand waving and hyperbole to explain themselves.
Nice idea Art. However I would like to keep the sub box to the same depth and width of the synergy horn cab to help them look purpose built as a couple. So thats 18" wide and 17" deep.
Probably not enough room to slot load, but to be honest a sealed well braced 80l cab with something like the faital pro 15hp1030 and 500w, is more than enough for my needs. 🙂
Not at all.
I follow the logic (and agree it is logical).
I simply (and without knowing the full detail of the build), think that the peak in HD is due to an extreme case, where your Fb is far too high, and a situation that would likely be avoided if one were not trying to squeeze a driver into an app it isn't optimal for.
What was the system Q?
(I have a few drivers that are on paper suitable for a small enclosure, relatively speaking, but do not like it one bit, preferring instead a sealed volume much larger than initially calculated for. I.E. system Q under 0.7 performed much better)
That is to say heavy cone sealed boxes, or high Vas sealed boxes have always seemed to work out better for me. Guessing this is down to your hypothesis, or also the distortion of lightweight cones.
Whether the latter applies in your case, I couldn't say, but am.interested in your thoughts on that
xrk971,
Not sure if the example you are talking about is the sealed vs open baffle distortion comparison, but I asked for examples at the same SPL level (not voltage) with the closed box compared to an open baffle.
Although in this post you fairly consistently have been showing HD plots with (HF) drivers equalized to a target at a specific level, the comparison you posted for the CB to the OB was not, so does not give an apples to apples result.
Also, the CB is quite undersized for the Vas of the speaker, while the OB at 24" x 36" in a corner (judging from the picture, the post did not specify test conditions or distance for either) would have a different response than free standing.
Without an even SPL and room placement playing field, no definitive conclusions about distortion can be drawn from that comparison, other than at 2.83 drive voltage the CB was around 10 dB louder, and exhibited higher HD over a small portion (about 10%) of the bandwidth below F3.
Art
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So no one but me thinks that taking a speaker that measure with a certain amount of HD as an OB and sticking it in a sealed box, which automatically raises the HD (even if the SPL went up - all that is due to not having dipole cancelation) is not strange? I guess I was used to that concept as I had not observed it before when I put my drivers into a Dagger or Nautaloss sealed cabinet. I think it's them all volume at play. If the box were big (by inference, an infinite box is an IB or half of an OB.
I don't think OB has inherently lower HD at all.
Below resonance I'd expect CB to exhibit lower HD than OB, of course depending on excursion in both cases.
Much like I'd expect a vented cab to show more HD below resonance due to unloading.
Since im not an OB fan and as such haven't messed with them much, I'd think that HD due to lack of loadING to be higher, for a given input.
It's hard comparing SPL CB vs OB due to the large cancellation loss in the latter case. (I.e. I'd also expect ripole/slob/n baffle to have high HD at bass freqs- ripole probably the worst- but the causes aren't quite the same -excursion generated HD vs 'pressure box' effect (non linear excursion generating HD)
What point I was making (badly) is perhaps that woofer is simply in a far too small a box (Vb/Vas far too low) and that if you were using this woofer and designing for a different app, then you would be unlikely to push it into such a condition. (You CAN design after all).
anecdotal cr@p and hand waving alert!
I once wanted to build a small CB using Mivoc 18". Specs showed the CB would be tiny relatively, Fb circa 90hz. Vb was something like 1-2 ft^3. But I dislike the sound of a large woofer and Fb around 80-100 Hz - even if it has a system Q of 0.7 it still sounds far too chesty, undamped etc. I have found I was check prefer Q 0.5 thereabouts when I have a large CB system. Unscientific I know
Below resonance I'd expect CB to exhibit lower HD than OB, of course depending on excursion in both cases.
Much like I'd expect a vented cab to show more HD below resonance due to unloading.
Since im not an OB fan and as such haven't messed with them much, I'd think that HD due to lack of loadING to be higher, for a given input.
It's hard comparing SPL CB vs OB due to the large cancellation loss in the latter case. (I.e. I'd also expect ripole/slob/n baffle to have high HD at bass freqs- ripole probably the worst- but the causes aren't quite the same -excursion generated HD vs 'pressure box' effect (non linear excursion generating HD)
What point I was making (badly) is perhaps that woofer is simply in a far too small a box (Vb/Vas far too low) and that if you were using this woofer and designing for a different app, then you would be unlikely to push it into such a condition. (You CAN design after all).
anecdotal cr@p and hand waving alert!
I once wanted to build a small CB using Mivoc 18". Specs showed the CB would be tiny relatively, Fb circa 90hz. Vb was something like 1-2 ft^3. But I dislike the sound of a large woofer and Fb around 80-100 Hz - even if it has a system Q of 0.7 it still sounds far too chesty, undamped etc. I have found I was check prefer Q 0.5 thereabouts when I have a large CB system. Unscientific I know
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Thanks, so far it sounds as extraordinary good performer, look forward see results when its a finished 3-way system. Have a little feeling down the road would have to get hands on XT1464 building a bushmeister clone : )