Subwoofer Efficiency

I read through most of this thread and saw that it is now closed, but I still have some questions about efficiency.

https://www.diyaudio.com/community/threads/subwoofer-efficiency-is-mostly-irrelevant.358153/

My current application dictates that I have a limited number of watts available (1200RMS) and I am looking to use two 15" drivers. I understand a single driver might suffice, but I am likely going to use these in a home application also.

Anyway, looking at specs, which of the following might lend themselves to delivering the highest SPL yet maintaining decent SQL. If anybody has experience with any of these drivers that would be great. The reason these are on the list is either I have found used examples at a good price or the new units are very reasonably priced. Looking around $200 per 15" driver.

https://www.skaraudio.com/products/evl-15-inch-car-subwoofer?variant=EVL-15 D2

https://www.sonicelectronix.com/ite...xfnrIMS5NUujho2gVtlcGdw3KiZhV01hoCoKkQAvD_BwE

https://www.resilientsounds.com/product-page/gold-15

https://nvx.com/products/vcw154v3-3...JyiufwxvqqR2DCHmPlND4VCTZRk2canRoCug8QAvD_BwE

https://cdn.shopify.com/s/files/1/0655/3550/8703/files/Hawk_Subwoofer.pdf?v=1682346116
 
Woofer efficiency comes with large diaphragms, so limiting yourself to 15" is going to limit the efficiency. Look at the "sensitivity" spec. Big drivers are not cheap.
However, a big driver needs a big box, so you may have to settle for a smaller driver in order to get deep bass from a rational size box. Putting a big driver in too small a box results in very poor bass.
 
I get the whole thing about Hoffman's Iron Law, however I guess the question is really more about comparing with all parameters relatively constant except for the driver itself. Let's assume for simplicity a 3cu.ft. box for each 15" driver.
 
The specs page Sensitivity rating usually represents the average broadband direct radiating response, "broadband" in this case means a wide range of frequencies that are at least partially typical of how most people would use the driver. So that is a ballpark number that gives you some idea of the SPL that can be generated so higher is better, but as usual it depends on how low you want to go.

The box it is loaded in can heavily affect the resulting systems sensitivity...
  • a sealed box's sensitivy drops from the spec you see as frequencies drop,
  • a ported box can more or less produce the published sensitivity across it's whole operating range,
  • a bandpass box will have higher sensitivity than the raw driver across it's output range, but that range is narrower,
  • a horn loaded box can have much higher sensitivity than the driver in free air... like 10dB more, but the box is hugh.

So how exactly are you planning to use these drivers, what kind of cabinets will be loaded in?
 
As I understand it, sensitivity ratings are relatively useless for subwoofer applications as they are generally determined at 1k hz. A subwoofer rated at 86db vs one at 89db doesn't necessarily mean it will produce lower SPL at frequencies 100hz and below. Again, this is what I have gleaned from reading through other forum threads, so if that is not correct then somebody please let me know.
I have a general understanding that box size and style (horn, sealed etc.) also has an effect.
But my request is this...let's assume we keep everything exactly the same for a system aside from the driver itself. Same box, same power, same everything except for the driver. Let's assume 1200RMS and a 3cu.ft. sealed box.
Under those conditions, is there any way to tell which driver will be most efficient? Are there certain parameters to look for? Or would one generally have to just throw all of them into WinISD (or other software) and see what it spits out?
 
Not really, at least not since my mid '50s Altec woofers, i.e. what I consider 'quality' manufacturers; it has been its mid-band eff., which is right above its upper mass corner (Fhm) where its acceleration BW peters out. A 'gotcha' though is that the measured response might not match published when it's different from its 'official' 2.83V rating.
 
As I understand it, sensitivity ratings are relatively useless for subwoofer applications as they are generally determined at 1k hz. A subwoofer rated at 86db vs one at 89db doesn't necessarily mean it will produce lower SPL at frequencies 100hz and below.
No the transition is lower.. more like 50hz, so it really matters exactly what subwoofer application you are interested in. A pro audio sub typically operates from 35-40hz to 100-120hz where driver sensitivity is relevent for the majority of it's operating range, but a HT sub might operate from 15-20hz to 50hz and in that range only the true power efficiency is relevent, and unfortunately there is little difference in output between drivers of the same diameter/displacement over this operating range.
 
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Woofer efficiency comes with large diaphragms, so limiting yourself to 15" is going to limit the efficiency. Look at the "sensitivity" spec. Big drivers are not cheap.
However, a big driver needs a big box, so you may have to settle for a smaller driver in order to get deep bass from a rational size box. Putting a big driver in too small a box results in very poor bass.
QTS is the 'magical' parameter that determines how large a box is needed.
Very low QTS means a smaller box, and high QTS means a large box. This is related to the 'power of the motor'. (not coil)
It is true that quite a few Car Subwoofers are a low cost bargain regarding power handling, due to their coils
but rarely excel in efficiency > due to everything else. They are just not built to the same exacting design of expensive drivers.
However, if you've got heaps of reserve Amp Power, 2x 15" 4ohms can give you nice 8ohm functionality with high power handling.
 
but a HT sub might operate from 15-20hz to 50hz and in that range only the true power efficiency is relevent, and unfortunately there is little difference in output between drivers of the same diameter/displacement.
I think perhaps this is more to the point and I believe the main assertion of that thread from post #1. So maybe not below 100hz but more like below 50hz it seems there is likely very little difference between subs of 15" or greater diameter. Agreed that some subs will be more suited for sealed, horn, IB, or ported...but with box and power kept constant there will be little difference.
I guess a follow-up question would be...what is the difference in output at lower power? For instance, feed a very low 100w to any given 15" driver in that 3cu.ft. sealed box. Would their SPL levels from 30-60hz all look about the same?
 
Very low QTS means a smaller box, and high QTS means a large box. This is related to the 'power of the motor'. (not coil)
From what I have read, the low Qts drivers not only need to have high power motors, but also generally have stiff suspensions and low Vas. These together lead to ability to use a small box size. However, is it not also true that they require tons of power to basically overcome the stiff suspension? This is where I see people saying stuff like "this sub wants tons of power". That to me means they just won't do well from an efficiency standpoint and we are now back to Hoffman's Iron Law.
 
Looking at the specs on that AE driver, the Vas is 467 liters which to me seems huge for a 15". I will concede the low Qts and high efficiency. Suggested 3-6 cu.ft. for sealed and 3-8 cu.ft. for ported seems reasonable. I see the Cms which looks like it is very compliant/loose which I think directly correlates to the giant Vas. Perhaps that is more where efficiency comes from...a loose suspension? In my simplistic understanding of things, a loose suspension means the motor doesn't have to push very hard to move the cone a certain distance. A stiff suspension requires more force to move the cone the same amount. But for this particular driver with the Vas so big, I'm assuming in the 3cu.ft. box, the motor will actually be meeting some resistance to that push from the sealed airspace in the box. So, the very loose suspension is negated somewhat by the air acting as a "spring". And since all the measurements for drivers are taken in free air, once we throw this driver in a box (say 3cu.ft. sealed) are we not back to Hoffman's Iron Law and efficiency that looks similar to other drivers in that 3ft^3 box? In other words, the free air efficiency/sensitivity measurements of the driver becomes somewhat irrelevant once we add a box.
Free-air measurements of sensitivity seem most useful for an IB application
 
You make some good & interesting points.
I think I gather that your interest/goals relate to very compact subwoofers.
Many small manufactured powered subwoofers incorporate active EQ to pull-up the lows you refer to.
I think it's probably time for me to get some simulation software. It would be so interesting to see 'variables results'.
 
Max box efficiency.........
 

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Besides the size of the diaphragm, speaker efficiency differences come from precision manufacturing, ie the voice coil air gap and the magnet strength. This is reflected in the Qes parameter. Also important is the magnet shorting ring, now a standard feature for ferrite magnet speakers. The current induced in the shorting ring makes the magnetic field ~rigid, causes the diaphragm to move instead of wasting energy heating the magnet and voice coil. Stiff surround speakers are generally more efficient since the surround returns energy to the cone instead of dissipating it. But, of course, a stiff surround raises the resonant frequency making it a poor choice for a sub-woofer.
 
Besides the size of the diaphragm, speaker efficiency differences come from precision manufacturing, ie the voice coil air gap and the magnet strength. This is reflected in the Qes parameter. Also important is the magnet shorting ring, now a standard feature for ferrite magnet speakers. The current induced in the shorting ring makes the magnetic field ~rigid, causes the diaphragm to move instead of wasting energy heating the magnet and voice coil. Stiff surround speakers are generally more efficient since the surround returns energy to the cone instead of dissipating it. But, of course, a stiff surround raises the resonant frequency making it a poor choice for a sub-woofer.
Very interesting.
Forgive my lack of knowledge, but would higher or lower Qes produce higher efficiency? I always thought Bl was the indicator of "motor strength", so maybe I am not considering magnet strength correctly.
I really don't understand shorting rings. I thought they were part of the construction to maintain more linearity and didn't really have anything to do with efficiency.

Now that you mentioned the "wasting energy heating magnet and VC", I suppose this might be part of the key to my essential question. With so many other tradeoffs, I'm thinking drivers that can convert the highest ratio of electrical energy to kinetic energy (cone movement) should have higher overall efficiency. This might be especially useful to measure for subwoofers seeing as compared to mids and highs they are using tons more power. Is there some way to measure how much electrical energy is used (or lost) when converting to cone movement?
 
This seems to be neverending topic, still foing the wrong way.

One thing is theory, one thing is staying on Earth with both legs.

If all else being equal, Higher Bl, lower Qes and Qts are always better. Low Qms ruins the efficiency though, although you want that somewhat too.

Basically, the more Newtons per watt you have, the more efficiency you have. But T&S parameters show low signal behavior. So better isbbetter is better. Box volume has nothing to do with "fitting" of the driver for it. Yes, driver response can get nonlinear, but it is a nonissue with current stage of DSPs. At no point is it better to have a driver with less motor and less magnetic field and less coil, and call it better. It is only better if you cannot drive it properly. Yes, Lower Qes drivers nerd higher voltage swing from the amplifier, because to get the power to the coil, you need to. That doesn't mean it eats more power. It is not eating the power and that is the reason why you need to turn it up.

F=B*I*L. More force translates into further cone movement, end of discussion theoretically.

In practice though, price, DSP, amp with high output voltage, linearity at high excursions and so on.

What you can do is to simulate very different drivers in an enclosure in hornresp, and look into maximum SPL graph limited at 1Watt. Then you will see true efficiency plot. And it gets wild.
Yet in pursuit of technology and betterness, we should get used to these new shapes. No flat response anymore. It goes against efficiency goal.
 
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