Acme Full Range Series III 112

[DukeLeJeune]

Just to make sure I'm straight on this: Putting in both plugs raises the tuning. (And basically turns the Thunderchild into a sealed cab, does it not?) However, putting in one plug lowers the tuning? It seems to me like it should raise the tuning, just not so much... Assuming that it does lower the tuning and I just don't understand the mechanism by which this works (a likely enough scenario, and Duke's own post earlier in the thread says as much, unless I misread it), I guess you'd start to see some of the same compromises you see with the Acme cab.

A ported box is sort of like a beer bottle. Go get one out of the 'fridge and drink about half of it. Blow across the opening. Woooooo. That's the Helmholtz resonant frequency you're hearing.

The neck of the bottle is the port, the air space inside above the beerline is the enclosure's internal volume, and your breath is the woofer moving the air. Not an exact replica, but close enough.

There are three ways to lower the tuning frequency. The first is the increase the length of the port, or bottleneck. That's not practical.

The second is to decrease the cross-sectional area of the port, or bottleneck. In the case of the Thunderchild we accomplish that by plugging one of the two ports, maintaining the port length but halving the port cross-sectional area, and dropping the tuning frequency from 50 Hz to about 37 Hz.

The third way to lower the resonant frequency is to increase the volume of air inside the enclosure. That's not practical with a cab that's already been built, but it's very practical where beer is involved. So all in the name of science, you know what to do. Good. Now blow across the opening. Wuuuuuuu. See, the pitch went down!

Feel free to repeat the experiment as often as needed to educate those around you about Helmholtz resonances.

Now as Ric mentioned, I was getting a problem with cone-rock in my prototype cab, were certain notes played loud would cause the voice coil to click against the sides of the magnetic gap. That's a potential issue with very long-throw woofers in small boxes if the airload on the back of the cone is asymmetrical, which was the case in my prototype because both ports were up high on the back of the cab, behind the horn. On the advice of the Eminence engineer who designed my woofer, I moved the two ports so that they're now on a diagonal behind the woofer cone. I had been cautioning people that plugging one port could cause enough of an asymmetrical airload that cone rocking could take place and cause that clicking distortion, and suggested they try it out at home before trying it on a gig.

But recently one of my customers shared his one-port-plugged gigging experience, and he hasn't had any distortion, so looks like that's not an issue after all. So plugging one port is can be a net improvement in situations where you're getting too much low end with both ports open.

 

[KJung]

Hey Duke, here is where I am confused, since I have no background in the technical aspects of this. So, reading the above, you say that plugging one port lowers the tuning to 37hz, but should be used when you want to reduce low end on the gig (if I'm reading the last line of your post above correctly). I would think tuning a cab lower would result in more low end.

 

[steve_rolfeca]

Hey Duke, here is where I am confused, since I have no background in the technical aspects of this. So, reading the above, you say that plugging one power lowers the tuning to 37hz, but should be used when you want to reduce low end on the gig (if I'm reading the last line of your post above correctly). I would think tuning a cab lower would result in more low end.

Duke will undoubtedly say this better (love the beer bottle analogy!), but when you plug the port, you don't just change the resonant frequency of the box.

- Halving the total area of the porting, reduces the amount of low-end reinforcement you get from having ports in the first place.

- Lower isn't necessarily louder. The speaker has a naturally falling response at low frequencies, especially in such a small box. Moving the porting frequency lower, slides it down the curve, to a point where the boost from the remaining port is less audible.

- If you think of the box as being like an "air spring" that tightens and controls cone motion, reducing the port area will give you a stiffer spring, and more resistance to the long cone motions you get at low frequencies.

 

[KJung]

+1 That I understand. I guess I was just surprised that this also lowers the 'tuning of the cab' to 37hz, but yet results in less low end.

I guess that is my misunderstanding... that tuning a cab to a lower frequency actually doesn't necessarily result in more energy going to that lower frequency.

That is quite counterintuitive. I would have again assumed that plugging one port would give you 'half the effect' of plugging two, which I guess it does. It is the tuning drop that had me confused. I would have though dropping the cabinet tuning would have moved the Thunderchild to be a smidge more like the Acme..... deeper sounding, and a bit less loud in the midrange.

My confusion continues a bit, since over on the Fearful threads, I THINK I remember reading that if you want more absolute low end coming out of a 15/6, you can block half the port.

So, still a bit confused by the impact of blocking one port versus totally sealing the cab.

 

[greenboy]

With SS setups if one has a sufficient EQ section that's probably a better way to go about this changing of low-end contour. But for tube rigs there's a couple reasons why that may not be practical. So plugging part of the port may work there, and can actually be better than a sealed cab. Here's the picture of FR changes.

Throws away a lot of free dB below 100 Hz.

Here's a related approach, extending the shelf (port length). It can be done in smaller increments. At the same time you lower the tuning you also lower the potential to unload the driver, so this one is mainly good for true subwoofer use when there is a lot of content way down low, like sub-bass synths that have a lot more energy at the fundamental than low-tuned electric bass. Also for EUB and upright the contour is nicer.

The disadvantage of this is that there will be times/venues or changes in tonal desires when one wants some of that mid-bass back. Then they have to EQ it in. Before the shelf extension, it was "free" SPL but now you would have to pay for it in increased demands on the amplifier.

 

[KJung]

Nice charts. I remember those when we were discussing the various tunings of the 3015 non LF small cabs. A smaller port/lower tuning resulted in not so much 'more low end', but rather a different nature to the low end which increased the ratio of deep bass to low midrange, at the expense of loudness and punch in the low mids and upper bass.

Is that correct?

Also, I would assume that blocking a single port on the Thunderchild (or your fEarful cabs) would in no way be similar to sealing up the cab (i.e., blocking both ports in the Thunderchild), and that the frequency response would look totally different, versus the single port block sitting in-between the 'full open and full blocked'. Is that correct?

From what I see here, there would be very little reason to block a single port in Duke's cab, assuming you had decent low end control on your preamp.

I guess we are getting a bit OT, but it is my thread, and all this ties in to how these 'Super12's' differ in performance IMO.

 

[alexclaber]

The lower you tune a cab, the more the woofer output rolls off before the port kicks in to support it. If you make the cab larger or use a heavier cone then the woofer won't roll off as much before the port kicks in and the port will be more strongly excited because the woofer will be proportionally louder - the downside is either a larger box or a lower sensitivity cab respectively.

One of my bugbears with small cabs is that they're often tuned too high to make the bass sound fat (because the port kicks in before the woofer has rolled off much, if at all) so you get a hump in the lows which is loud and boomy. Tuning too high means the woofer unloads badly on low notes (like it's working in free air) and starts distorting.

Lowering the port tuning won't change the midrange and you won't get a deeper sound, you'll just thin out the lows by giving the woofer less port assistance where it needs it. If you simultaneously made the cab twice as big then yes, you'd get that deeper sound, because the woofer wouldn't be more free to vibrate at low frequencies (less constrained by the internal back pressure) and thus be louder and thus the port would be more strongly excited too (that's how our Big Baby is configured - tuned lower than the Audiokinesis, higher than the Acme, larger than either).

Reducing the port area has zero effect on the response at lower levels. At high levels the port will start to become turbulent, compress, distort and eventually stop working entirely (phase reversal disaster!) - halving the port area reduces the SPL this will happen at by 6dB if all else remains equal. It's not quite as bad as that due to the port being less strongly excited but it's pretty complicated!

One of the standard tips when building DIY hi-fi cabs is to tune lower if you want a leaner quicker sound, tune higher if you want a fatter rounder sound. Typical tuning frequencies are between 40 and 50Hz. Hi-fi cabs tuned much higher tend to be faking the bass whilst ones tuned lower need to be very large or have unusual low sensitivity woofers (hi-fi / home cinema subwoofers tend to be like this).

 

[alexclaber]

Two pictures do a better job than five paragraphs!

If you tune a ported cab low enough it responds just like a sealed cab but you end up with stupidly long port wasting space or a stupidly small port chuffing and whistling.

 

[KJung]

Thanks Alex (as always). This is starting to make more sense to me.

From GB's charts and your explanation, there would seem to be very little reason or advantage to plugging one port on the Thunderchild. I've messed around with plugging both ports, and that does seem to have the impact of tightening up the low end (which is already tight/relatively constrained on the Thunderchild), so I guess except in really boomy, extreme rooms with a DB, that would not really get you much either.

So, how does Andy's tuning fit into this, since he has SO much deep low end coming out of that little box. I assume he has, for better or worse, the optimal porting to support the deepest low end that driver is capable of (i.e., in his relatively large box (compared to the typical 112). Is this correct?

 

[greenboy]

The lower you tune a cab, the more the woofer output rolls off before the port kicks in to support it. If you make the cab larger or use a heavier cone then the woofer won't roll off as much before the port kicks in and the port will be more strongly excited because the woofer will be proportionally louder - the downside is either a larger box or a lower sensitivity cab respectively.

One of my bugbears with small cabs is that they're often tuned too high to make the bass sound fat (because the port kicks in before the woofer has rolled off much, if at all) so you get a hump in the lows which is loud and boomy. Tuning too high means the woofer unloads badly on low notes (like it's working in free air) and starts distorting.

Lowering the port tuning won't change the midrange and you won't get a deeper sound, you'll just thin out the lows by giving the woofer less port assistance where it needs it. If you simultaneously made the cab twice as big then yes, you'd get that deeper sound, because the woofer wouldn't be more free to vibrate at low frequencies (less constrained by the internal back pressure) and thus be louder and thus the port would be more strongly excited too (that's how our Big Baby is configured - tuned lower than the Audiokinesis, higher than the Acme, larger than either).

Same as the fEARful approach. The entire schmegola. Ultimately, if you look at pack space/weight to output ratio it's hard not to go there unless you have instruments that actually have a different tonal profile in the lower regions.

Reducing the port area has zero effect on the response at lower levels. At high levels the port will start to become turbulent, compress, distort and eventually stop working entirely (phase reversal disaster!) - halving the port area reduces the SPL this will happen at by 6dB if all else remains equal. It's not quite as bad as that due to the port being less strongly excited but it's pretty complicated!

Which is why it's unsuited to those using high-output amplifiers who are at least part of the time going after bigger SPL. But it can be a good option for some tube heads.

 

[alexclaber]

Based on my discussions with Andy he believes that a cab made for 5-string bass has to be tuned to 31Hz (I disagree rather strongly!) With the standard 3012LF you could just about get away with that tuning if you made the cab huge (a 1x12" the size of a fairly big 2x12") but there would be a lack of fatness in the mid-bass region. So to make that work you need to increase the cone mass, tweak the suspension stiffness and reduce the magnet strength so the woofer resonates more easily at lower frequencies so its output doesn't drop off before the port comes in to help out.

Increasing the cone mass means you have more energy wasted as the cone moves back and forth (it's moving air that matters, not moving paper cones) and decreasing the magnet strength reduces the motor's efficiency, hence it's a low sensitivity design.

 

[greenboy]

So, how does Andy's tuning fit into this, since he has SO much deep low end coming out of that little box. I assume he has, for better or worse, the optimal porting to support the deepest low end that driver is capable of (i.e., in his relatively large box (compared to the typical 112). Is this correct?

It'll be interesting to see independent measurements without room skew.

 

[KJung]

It'll be interesting to see independent measurements without room skew.

Interesting stuff. And your comment of how Andy's approach could make some sense with some all tube amps MIGHT explain why his design sounds so darn good to me with the Genz Streamliner, which is voiced quite similar to some of the all tube amp executions I've played (and also why it is the first cab that has ever brought my beloved Markbass F500 to its knees).

I'm with you and Alex in general.... a bit less low end extension pays off in many ways in a live situation.... efficiency, usable low end, mid response, etc. However, as you know from your experience with the old Acme's, there is something very intoxicating about that unusually low end extension, and the 12's seem to me to be a significant improvement in the midrange over the original three way designs (the low mids and mid mids anyway.

One thing for sure, the FullRange doesn't sound like anything else out there, for better or worse.

 

[greenboy]

Based on my discussions with Andy he believes that a cab made for 5-string bass has to be tuned to 31Hz (I disagree rather strongly!)

Likewise. After the WATERFALL PLOTS: WHAT THEY MEAN TO BASS RIGS that's the conclusion anyone who understands the material would arrive at probably.

With the standard 3012LF you could just about get away with that tuning if you made the cab huge (a 1x12" the size of a fairly big 2x12") but there would be a lack of fatness in the mid-bass region. So to make that work you need to increase the cone mass, tweak the suspension stiffness and reduce the magnet strength so the woofer resonates more easily at lower frequencies so its output doesn't drop off before the port comes in to help out.

Increasing the cone mass means you have more energy wasted as the cone moves back and forth (it's moving air that matters, not moving paper cones) and decreasing the magnet strength reduces the motor's efficiency, hence it's a low sensitivity design.

That isobaric subwoofer I built a long time ago had added mass. I epoxied fender washers in back of the dust cap, if I recall it was 60 or 80 grams. Fine sub for the purpose, but lousy for pack space and SR, and wasted on electric bass.

 

[greenboy]

Interesting stuff. And your comment of how Andy's approach could make some sense with some all tube amps MIGHT explain why his design sounds so darn good to me with the Genz Streamline -----

Really what I said/showed about lowering the box tuning to resemble a sealed cab in many respects really had nothing to do with that piece or SS heads in general - or the the Acme cab either.

I was merely filling in the picture after seeing a lot of confusion as people started talking about plugging ports and box tunings, and then mentioned some tradeoffs and special considerations.

Specifically, the comment I made about all-tube heads and the plugged port image next to it is because of the amp topography of an all-tube head and the way some designs are implemented. It doesn't really apply to SS at all.

 

[steve_rolfeca]

...a bit less low end extension pays off in many ways in a live situation.... efficiency, usable low end, mid response, etc. However, as you know from your experience with the old Acme's, there is something very intoxicating about that unusually low end extension...

+1. I guess I agree with Alex AND with Andy on this. I love the sound (and the idea) of super-extended low end, but it's not always practical in a world of compact cars, non-existent roadies, and bad room acoustics.

When I originally started figuring out how to amplify a 5-string back in the mid-'80's, I built a single 15 that was supposedly flat down to 31Hz (with the help of an external processor). It was absolutely huge, and didn't play very loud, but low C's and D's sounded fabulous through that box. There was no flub down low, and it worked fine even in really boomy halls.

However, I didn't use the extra low end extension all that often. Meanwhile, the cab weighed a ton, and I'd have needed at least 4 of them to keep up with an aggressive drummer. For reasons of practicality, I'm quite happy to live with a few Hz lopped off the low end in most real-world situations.

The new Acme 12's certainly look interesting to me, in terms of price as well as weight and performance. I won't know 'till I try, but I'm guessing that I'll be very happy with the smoother mid/treble response of my soon-to-arrive Thunderchild 112. If not, the Full-range is probably going to be next on my GAS list.

 

[KJung]

Really what I said/showed about lowering the box tuning to resemble a sealed cab in many respects really had nothing to do with that piece or SS heads in general - or the the Acme cab either.

I was merely filling in the picture after seeing a lot of confusion as people started talking about plugging ports and box tunings, and then mentioned some tradeoffs and special considerations.

Specifically, the comment I made about all-tube heads and the plugged port image next to it is because of the amp topography of an all-tube head and the way some designs are implemented. It doesn't really apply to SS at all.

+1 My comment was about voicing, and what seems like a HUGE low end coming out of some all tube designs versus most modern solid state designs. You get the same sort of feel from the Streamliner, and that voicing seems to really work will with the Acme112's... really well! Of course, it won't bap or burp or do extreme midrange punch, but there are other heads and cabs that do that great.

 

[3rdcurve]

+1 My comment was about voicing, and what seems like a HUGE low end coming out of some all tube designs versus most modern solid state designs. You get the same sort of feel from the Streamliner, and that voicing seems to really work will with the Acme112's... really well! Of course, it won't bap or burp or do extreme midrange punch, but there are other heads and cabs that do that great.

+1 Is the the acme the best all around cab for every player?

Nope

Does it sound absolutely incredible in certain contexts? Yup (At least in mine with the streamliner and a pbass with flats).

Just like any other cab on the planet, Andy's design will have drawbacks and strong points. If you try to make it into something it is not, you can't enjoy what it is.

 

[monsterthompson]

Not being certain of the specifics of cab building or note vibrations in hz, but understanding a bit about about ressonant frequency, wouldn't changing the tuned frequency of the cab alter the audible characteristics of every octave and strong overtone related interval (3rds, 5ths, etc.) of that tuning? For example, if you had a cello with a res freq of A, then every A note would jump out as a wolf tone. It would seem there is a need to tune to a res freq that doesn't create an imbalance in note production across the range of the instrument, nor a beating effect. In that regard, I can see the "less isn't more" angle of trying to get more "bass" from the port block just because of the 50 to 37 shift.

 

[alexclaber]

+1. I guess I agree with Alex AND with Andy on this. I love the sound (and the idea) of super-extended low end, but it's not always practical in a world of compact cars, non-existent roadies, and bad room acoustics.

When I originally started figuring out how to amplify a 5-string back in the mid-'80's, I built a single 15 that was supposedly flat down to 31Hz (with the help of an external processor). It was absolutely huge, and didn't play very loud, but low C's and D's sounded fabulous through that box. There was no flub down low, and it worked fine even in really boomy halls.

However, I didn't use the extra low end extension all that often. Meanwhile, the cab weighed a ton, and I'd have needed at least 4 of them to keep up with an aggressive drummer. For reasons of practicality, I'm quite happy to live with a few Hz lopped off the low end in most real-world situations.

For years I was a huge advocate of the Acme bottom and I still love it. I don't believe in-and-of-themselves very deep lows ever cause problems in bad acoustics etc but they can cause other issues to occur (due to lower sensitivity and power handling and so on) but with a bit of cunning you can design them out to the most part.

The compromise which I wasn't happy with was that I've been known to play very loud and also slap aggressively and the Acmes would struggle when both those elements converged. I have to say it's pretty awesome that we've got three production 1x12"s plus the fEarfuls that can do so much more than anyone dreamt a 1x12" could do when they emerged for bass at the turn of the century. I should add that if Andy had brought out the Low-B112 about 5 years ago I'd probably never have started Barefaced - but then I'd have run into some of the same issues as my trusty old Low-B2s had. Anyway...

If you compare the:
Thunderchild (F3@56Hz, F6@47Hz, F10@40Hz) 50Hz tuning
Big Baby (F3@44Hz, F6@38Hz, F10@32Hz) 41Hz tuning
Low-B112 (F3@41Hz, F6@31Hz, F10@26Hz) 31Hz tuning

You see that the Big Baby and Low-B112 are much closer in bass response than the Thunderchild. The Thunderchild has much more similar bass response to our Compact, which is lovely and fat and far better than most small cabs, but a totally different vibe to the very extended lows of the Big Baby.

The main difference between the Acme and Big Baby in the lows is that the Big Baby has much more port support in what I believe is that critical region for bass guitar bottom, especially the percussives, and unloads in the mid 30s whilst the Acme woofer has to work harder, port comes in very low down (which is why it gets away with a smaller port area) and then doesn't unload until the mid 20s. However I don't see the port unloading being a problem in the least because not only do bass guitars (even super long scale monsters like mine) put out relatively little energy below 40Hz (however low the fundamental of the note you're playing), the highpass filtering or low frequency roll-off inherent in so many bass amps further reduces the energy down there. With my PLX3002 power amp the 30Hz filter would change the response to:

Big Baby (F3.5@44Hz, F7@38Hz, F13@32Hz) 41Hz tuning
Low-B112 (F4@41Hz, F9@31Hz, F15@26Hz) 31Hz tuning

Whilst with a typical micro-amp the filtering on them makes the response more like:

Big Baby (F5@44Hz, F10@38Hz, F20@32Hz) 41Hz tuning
Low-B112 (F6@41Hz, F15@31Hz, F25@26Hz) 31Hz tuning

So although the Acme has slightly deeper response it's quite hard to detect with the extra roll-off built into most amps and that same roll-off protects the woofer with the higher tuning from over-excursion due to unloading. So you get such similarity in the depth of bass response on the gig that it's almost impossible to tell apart but with higher sensitivity and power handling from the higher tuned cab.

On the flipside the Low-B112 is a gloriously minimalist design which keeps the price way down (no massive crossover or expensive midrange driver) and if you're not so obnoxiously loud the output point is moot anyway! And although you'd need two to match a Big Baby at full tilt (well they'd beat it but not by as much as you'd think) if you're Stateside that would cost about the same...