The most efficient size for a sealed 18"?

[Viennatom_new]

Another thing to consider with sealed is that the impedance curve for the box has only one hump (which gets smaller the more damping you put in) while a reflex has two humps and a tapped horn has three. This is a good example of how damping (panel flex) affects the resonant peak sizes for a tapped horn:
The Subwoofer DIY Page - Projects : Using Impedance Graphs

The more peaks, the higher the average impedance is over the subs bandwidth and the less power is dissipated in the box for a given acoustic output. Speakers cannot be approximated as resistors apart from far below their first resonant frequency. This brings me to the IPAL drivers and other drivers with high Bl^2/Re (effective motor force), these drivers may have a low impedance magnitude at DC but they cause cabs to have massive peaks in the impedance increasing efficiency:
dB v2
but its worth remembering in a reflex or tapped horn you would get more peaks and more efficiency!

if you use a high motor force driver to move a cone back and forth a given distance (IE produce a set SPL at a set frequency from a sealed box) you will have less power dissipated in the motor of the driver in comparison to a cheap low motor force driver. The voltage sensitivity of such systems below resonance however is very poor so you will need big amps.

I know lambda labs sometimes installs sealed sub systems in clubs but they are huge arrays of drivers and amps. Very expensive!
YouTube

bass reflex has an impedance dip between the 2 peaks, where a) the cone almost doesn't move, so no air flow b) the current goes up quite high. That is where voice coils overheat first - driven heavily at the BR resonant frequency.
That lambda labs install is a demo what is possible. 7 Hz at insane SPL. I think of this install as an "costly advertisement" to get attention, as I do not know of any second club of that kind and they are selling (tapped) horns mainly (I happen to know some guys personally, since they are located a few kilometers away from where I live and there are sometimes free demo days).

 

[scholl]

I've been reading a ton about sealed subs, but the one thing that is never, EVER discussed, nor shown in the software (at least that I can find) is te efficiency of the final, resultant subwoofer. I guess this is because sealed subs are almost always used in home theater / music rooms and studio control rooms, so no one worries about having enough amperage.

I'm trying to figure out if I can make sealed 18"subs work in a live band / DJ situation, where efficiency is critical.
Note: I have actually already done this, for over ten years. The sound was lovely, but but the subs' inefficiency was a constant headache /nightmare:

I had been using four Bag End single 18's, with a single Lab Gruppen fp6400 driving them. With two subs on each side of the stage, I could do a medium sized wedding or rock club, but JUST barely. A high school dance was basically out of the question.

And I would constantly have to worry about tripping breakers. Those Labs were 1600 watts rms per side into each "two cabinet" 4 ohm load, and I owned THREE of these amps because I was constantly, literally, melting them & sending them off for repair.
Needless to say, I also got very good at reconing speakers. Literally several times per year, in-between gluing the surrounds.
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So a few years ago I retired this system, and got a pair of QSC KW181's. I can't stand the bloody things. No definition, and no "integration" with the tops. This year I decided to try something else, but that's not working either. The problem is, anything small enough to carry by myself is a trade-off.

Before I go any further looking at custom ported builds, I thought maybe I'd revisit the sealed sub idea, which brings me to my actual question:
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I only need maybe 3 more dB than I had originally with the Bag Ends.
Could a larger cabinet get me there?

I have a pretty good grasp now on the pluses & minuses of a larger sealed cabinet. For me, not needing lots of bass extension, it's all gravy.
I can estimate that the Bag Ends, stock, have a Qtc in the neighborhood of 7.0. That means I have quiet a bit of leeway in enclosure size, assuming I will like the sound of a lower Qtc. I calculated enclosure size for a fairly similar 18" driver, with known T/S numbers, and got this:

Qtc 0.707 ~ 3 ft/3 F3 51.7 Hz. (Max flat amplitude)
Qtc 0.577 ~ 5.69 ft/3 F3 51.9 Hz. (Max flat delay)
Qtc 0.5 ~ 9.6 ft/3 F3 55 Hz (Critically damped)

The Bag End cabs are about 3.5 ft/3
A 5.69 ft/3 cabinet would work great, ergonomically, so I'd certainly consider trying this. (Using the old drivers, at least to start.)

EXCEPT: I can't figure out HOW TO CALCULATE THE EFFICIENCY OF THE FINAL, FINISHED SUBWOOFER!
For my application, I obviously need this critical number. The stock Bag Ends are 96 dB SPL @ 80 Hz (1W @ 1m)
If I doubled the enclosure size but only gained 1 dB at useful frequencies, then the experiment is a bust. - But an extra 3 dB at around 60 Hz would be incredible.

Is there a way to calculate this, if you know the efficiency of the raw driver?

And note: I'm only concerned with efficiency above 40 Hz.

What do you really want? What do the fans really want? What is the size and cost budget? What is the music genre? How big are the venues?

The efficiency of the woofer system is going to be the rated efficiency of the woofer until F3 where it'll drop by 12 db an octave without electronic or acoustic reinforcement. Even with an 18 you'll be lucky to get 92db at 40hz sealed.

Start here: HiFi Loudspeaker Design

 

[kipman725]

yup and a tapped horn/6th order BP has two such dips. Good job music doesn't tend to be just sine waves at the tuning frequencies. Overall the situation is much better for the non sealed solutions though as pass band efficiency is higher so for a given SPL less power is dissipated in the motor.

Incidentally if you heavily stuff a bass reflex box you end up with something that measures like a sealed box but has some air exchange preventing the air in the box getting too hot. I came across some opus audio cabs that where like that (as far as I could tell the whole box was stuffed) and the impedance measured like a sealed box.

My personal opinion is that reflex or 6th order bandpass / tapped horn fits the majority of PA requirements and that sealed subs are for compact home audio boxes. When I used to run sealed PA subs for small events it sounded great for the first hour or so but then the air inside the boxes started to overheat which lowered the output, I was also having to run my amp hard and sometimes had driver failures. Already for small events (100 people) I was running 12*15" drivers. The last party I did I used only 8 of the same drivers in isobaric reflex boxes and had no bass fade, the drivers where almost room temperature after hours of heavy bass and my amp limiter was only very occasionally triggered.

 

[Viennatom_new]

Music is not just sine waves at tuning, right. But the temptation to mess up big time is there. "I am a stupid DJ ,I want it LOUD LOUDER and even MOAR LOUDER, oh did you notice the cone almost doesn t move down there at 40Hz, great! So why not apply a "gentle" DSP boost of 12dB at that frequency?"

 

[scholl]

Dayton Audio DCS450-4 18" Classic Subwoofer 4 Ohm

Dayton Audio RSS460HO-4 18" Reference HO Subwoofer 4 ohm

But they have power handling concerns

 

[chris661]

Already for small events (100 people) I was running 12*15" drivers. The last party I did I used only 8 of the same drivers in isobaric reflex boxes and had no bass fade, the drivers where almost room temperature after hours of heavy bass and my amp limiter was only very occasionally triggered.

This mirrors my experience in moving from sealed to ported for PA use.

Chris