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Adding mutiple drivers for sensitivity
I understand that every doubling of the amount of drivers adds 3 db but what about impedance?
Say I have two 4 ohm drivers with a sensitivity of 87 db. If I wire them in series, do I still get the 3 dbs? What if you have the same driver but 8 ohms ( probably lower sensitivity ) but wire them in parallel. Do I get 3 db or 6 db? I am thinking 3 db but this part always confuses me. I am thinking impedance has nothing to do with it but then what advantage would it be to go to 4 ohms instead of 8 ohms? Just more power out of the amp?
Comments
If you add two 4 ohm drivers in series you gain 3db for to doubling surface are and lose 3db due to higher impedance (now 8 Ohm's) for a net zero. You do have more power handling so you can reach a louder SPL level but it will take more power. [See correction by dcibel below]
If you add two 8 ohm drivers in parallel you gain 3db for doubling surface area and gain another 3db due to lower impedance (now 4 ohms) for a net +6db.
Yeah, what he said.
Except to split hairs about "more power" for 8ohm. That is where efficiency and sensitivity diverge. "more voltage" I think would be more appropriate. But that wanders outside the scope of the question so I'll stop there.
Regardless of how you wire them, the maximum SPL the speaker can produce is increased by 3dB, and the pair of speakers will require the same power to produce a given SPL. Change in impedance from different wiring arrangements affects voltage sensitivity, not power. With this in mind, just pick the driver impedance and wiring arrangement that makes the most sense to you.
This is what I thought but I guess I didn't ask the right question, although both it clears it all up for me. I guess SPL is spl and sensitivity is something different.
Thanks for clearing that up for me. I thought that this was the case at one point but then I was second guessing it.
Max SPL increase may be 6dB, I should have worded better, 3dB for the same power applied is what I meant. A4E's crossed out statement isn't wrong, it will take 3dB more power to produce that last 3dB.
I guess I should have explained more of what I was trying to figure out but all of the info here really cleared things up for me.
Mainly, winISD it seems, doesn't factor in the impedance difference in the sensitivity. That is why I originally started the topic. But, when it comes to SPL, I am guessing it doesn't matter since, They will only take so much power and produce so much SPL.
Anyway the sensitivity thing I think I had right in my head but the way winISD does it, or not, seems wrong, hence the confusion on my part.
I had thought it assumed parallel wiring, but now that I look again it appears to not factor in impedance changes at all.
I wonder if that is the reason the isobaric configuration gives a weird resulting spl.
To get into specifics, ( this may help) I was looking at using two DCS205-4 in series for an 8 ohm load. WinISD says 89 db with 1 watt. Specs for one driver is 88.6 db which seems a bit high looking at the frequency response chart. I would say closer to 86 or 87 db. But, WinISD does not seem to factor in impedance. For max spl it seems, though, that it wouldn't matter. I think it was saying 114 db for two of them.
I don't remember the specifics but I have also found that WinISD doesn't calculate something correctly. (I think you are both right about it being the impedance.) I eventually want to switch my box modelling to VituixCAD but I really like how you can display many different models at the same time in WinISD.
Yes, me too. I have downloaded it but haven't spent the time to get to know it. It does seem quite nice though. I really need to find some time to work with it.
Exactly. I thought the Isobaric sensitivity was way low. I know you lose 3 db with the design itself but it just takes 6 db right off the top.
Isobaric should remain same sensitivity over a single driver, but require twice the power- with the drivers in parallel. In series wiring, more losses happen without the halved impedance to recover the loss.
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. . . Plus you have the inductive reactance of the series speaker coil changing the response of both drivers . . .
Yep, halve or double, take your pick.
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Yes. This is what I understand.
All good stuff my friends.
I won't touch many of the questions and answers so far, but I wanted to point out that with 2 drivers vs 1, you have 2X the surface area and so can achieve louder SPL before distorting. I once built a 6 driver tower that could comfortably play stupidly loud. Max SPL depends a lot on swept volume, no?
Yeah, certainly the more drivers you add, the better. It only becomes a problem when the box gets to big.
You are right though. The more cone area and xmax, the better.
Lower distortion comes with mutiples.
Back in 2003, when Bob Cordell was building his active 3way towers and brought them to Dayton finished in 2004 (with a single unfinished open unit in 2003) he had a very nice mid and tweeter, and a quad of 5" or 6" no name woofers. He had measured lower HD on the pair vs a single, and was shocked how low the HD dropped going from a pair to the quad. Since he was using them as true woofers, the HD never went up with regards to a bad or cheap bread and butter motor design. IIRC, this project was also using driver feedback in his own proprietary software to lower HD even further. He had shelf braces installed in the slender towers that had matching PCBs mounted to each brace to power the respective drivers. It really was impressive to see and hear.
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Wow, still a blurb on the web. It was called the Athena. https://www.cordellaudio.com/loudspeakers/
It used his EQSS technology.
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And now we finally understand why.
For decades audiophiles and engineers and speaker building enthusiasts would mostly agree on one thing- big woofers are better than smaller woofers for bass. Or you may have heard “if you can see the woofers moving- it’s distorting!”
The idea is that given the same displaced volume of air, using a large cone to move air gently (larger Sd, lower excursion) sounds (and measures) better than using a small cone to move air furiously (lower surface area, higher x-max drivers).
Yet we didn’t understand why.
Well it was under our noses the whole time!
Most of the harmonic distortion comes from that flabby bit of rubber that connects to cone to the frame. Whilst the cone acted as a piston moving in and out, the rubber strip was flapping in the wind!
“…years ago we finished the smallest perfect motor, and shortly before Christmas we were measuring it and we thought it was time for champagne and everything. But guess what. Measurements, they sucked. And they barely budged from an ordinary driver. A lot of disappointment so it almost spoiled the Christmas. So, again, we had to carefully analyse and find what is the source of the problem. Well it turns out that a a very old problem is that err... (Lars Risbo showing a Peerless woofer to the camera) we have the surround which typically is something like a hemispherical profile piece of rubber here. If I push the cone in, then you can see that the crest, the top point of the row, that moves upwards, which means when my cone moves in the negative position, then the effective radiating area is higher than the resting position. Conversely if I do the opposite you can see that the crest of the row is having a smaller radius, actually the radiating area, it now shrinks. It turns out actually that the area of the surround, that can be 10, 20 even 30% of the total radiating area. And you have all this focus on pristine cones and geometry and everybody is using the same flabby piece of rubber. And this thing has been going under the radar. So after discovering this problem err, umm my partner Carsten [Tinggaard] said “Oh wait a minute, there was a paper in the 90s.” Turns out there were some researchers at the Danish Technical University who discovered this problem, wrote a paper, err. that basically no-one had referenced and.. they were so disappointed that no one had picked it up. But uh, we accidentally discovered that because we now had the perfect motor and we said "Why is the distortion still so high". Turned out that was simply the problem."
Reference:
https://www.youtube.com/live/o90AYyceaMI?si=rXdp4j1X1ibMYotM&t=2802
And so it goes that using 4 woofers to move air gently is better the using two woofers
(or a single woofer), to move twice (or 4 times) as far.
Or why a higher sensitivity woofer with 10mm Xmax sounds (and measures) better than a lower sensitivity woofer 14mm Xmax woofer.
So it seems low Qts and Vas woofers would be good to combine multiples in a smaller enclosure. What properties allow this while maintaining a low f3? High motor force and cone mass?
Hence the concept / goal of the M&K push-pull sealed subs to address (mitigate to some extent) these issues.
What about the surround?
Seems Lars and co figured out a solution while no other manufacturer has followed suit.
Qts and VAS aren't going to change that are they?
I typically agree that there is no replacement for displacement. Having worked with the Purifi woofer I can vouch it is flat out the best midbass on the market - but it is still displacement limited, manufacturer claims notwithstanding.
The bass from my twin $60 GRS subwoofers blows the Purifi out of the water. It really is a question of physics.
I think few are debating the physics of the big cone vs the small woofer when it comes to bass. Like how the Purifi 10" midwoofer outperforms the 4" Purifi midwoofers. Or the how the 15” GRS will outperform the 8” GRS.
I was referring to Bob Cordell’s Athena where he used a quad 5.25” midwoofers. I think if we have multiple woofers with Vd of “a“ and compare that to a single woofer of the same Vd of “a”, then the quad will be better. Based on viewing dozens of Klippel measurements, most woofers seem to have a suspension that is most symmetrical and varies the least in the first few millimetres of stroke. Things get wacky as they approach their excursion as defined by air gap / voice coil height geometry
For the moment let’s disregard the price equation. A quad of MAC-06 6” drivers would most likely outperform ( when it comes to how cleanly it can play in the bass, in terms of THD or IMD ) a single Wavecor 8” midwoofer when both setups are playing at the same matched SPL.. Each MAC-06 has an x-max of 4 mm, but the combined Sd would be larger than a single 8”. As you say, there’s no replacement for displacement, so to match the SPL at any given frequency, the 8” (or larger driver) with a smaller Sd, would need to have a larger x-max.
To be clear, the max SPL at any bass frequency would be determined by:
4 x Sd(MAC-06) x 4mm
= 1 x Sd(larger woofer) x larger x-max
But whilst the larger woofer is flapping at its full stroke, the 4 MAC-06 will be flapping at 4mm.
If it’s true that the half roll surround is a main contributor of distortion, as suggested by Lars, then it follows that the less the surround moves, the cleaner the bass is.
The problem is that we’ve probably all gotten used to the sound of distortion. What does the sound of 4 MAC-06 per side sound like? I certainly haven’t tried it, but I certainly would like to try.
On the other hand, I’ve heard plenty of single subs. Many of us remember (or are still into) car audio, and they have some subwoofers with CRAZY amounts of excursion. Play loud? Yes. Play clean… err…
But if we’re all used to the sound of flabby surrounds flapping away, perhaps the sound of low distortion bass (8, 12 or 16 units of MAC-06, with an equal amount of air volume displacement, might feel wrong.
Can you drill into why you consider the Purifi mid to be the best?
The thing that I notice on low distortion bass is it seems lean at first, and you question the modeled extension. But then along comes a legit 30hz signal and you realize how much distortion most little drivers really exhibit. Up to about 90db my pair of GRS exhibit that phenomenon before distortion starts rising slightly. That is well outside of where I listen, and I think they are barely hitting 2-3mm of excursion at those levels.
I've been just recently playing around with a related 'equivalence' calculation.
Being a long-time maggie fan (had old MG-1s back in the day), looking at the smaller LRS+ 's mid-bass panel section (rough guestimating @ approx 36 x 7in ~ 1.75cu ft, and a minimal excursion- I don't have the actual numbers) I'm getting close to Vd of a couple 10in woofers. I really like the bass from my older open back dual Beyma 12s, but at 14 x 42in it's not a crowd / living room pleaser... Expecting that a smaller open back would need a supporting sub, I'd consider going that route but would like to get the rolloff / cross to the sub in the 60Hz region (I've always felt the 80Hz level targeted by theatre systems was a tad high) with a bit of eq w/o extending the woofers too far into excursion / distortion range (maybe wishful thinking).
Does anyone have the LRS's / measurements, and/or decent guess at the their mid-woofer's Vd?
Thx