Build Log. Compact Home Cinema Speakers and Sub. Semi-Pro-Fi. "From The Ashes"

[lorenmjones]

Re: Build Log. Compact Home Cinema Speakers and Sub. Semi-Pro-Fi. "From The Ashes"

Looking good. It is so nice to have a high efficiency speaker with decent power handling. Good buy power compression at high (for the home) listening levels.

Can you explain in dumb dumb terms where you get that much efficiency gain with your bass cabinet?

[mdocod]

Re: Build Log. Compact Home Cinema Speakers and Sub. Semi-Pro-Fi. "From The Ashes"

Hi Loren Jones,

Lungs: Amplification
Lips: UM10-22
Tuba: folded bass horn

The tuba provides modification to the acoustic coupling, and a number of natural resonances. The result is that the small, low displacement lips, can produce BIG sound. This is an extreme example, and requires that the tuba be designed to resonate very aggressively at specific frequencies (has very high Q). A bass horn speaker, is more of a balancing act of overlapping resonant and damped response from either "end" of the horn, as well as the use of some destructive coupling, to achieve a usable passband (rather than only distinguished, highly resonant, notes).

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For a given box volume and power input, the maximum output and efficiency can actually be the same for almost any driver. The catch is bandwidth and distortion. Yes, a 2" full range driver could excite a low frequency resonance in a big horn, and achieve the same efficiency at that frequency, as an 18" pro-sound driver in an equal size "box," but the usefulness would be limited to such a narrow bandwidth that it would not be practical.

Regards,
Eric

[lorenmjones]

Re: Build Log. Compact Home Cinema Speakers and Sub. Semi-Pro-Fi. "From The Ashes"

Ok. I can understand that I guess I had always just heard that bass horns had to be physically large re: length and mouth area to be effective. For example the somewhat legendary LAB sub is 45" x 45" x 22.5" IIRC and reaches down to below 40 hz in a single but 4 with the mouths coupled reaches down around 30 before rolling off---indicating that a single horn with a larger mouth would reach similar freq before rolling off.

I know that is a Front loaded horn, but how does that little folded horn play so low?

Hi Loren Jones,

Lungs: Amplification
Lips: UM10-22
Tuba: folded bass horn

The tuba provides modification to the acoustic coupling, and a number of natural resonances. The result is that the small, low displacement lips, can produce BIG sound. This is an extreme example, and requires that the tuba be designed to resonate very aggressively at specific frequencies (has very high Q). A bass horn speaker, is more of a balancing act of overlapping resonant and damped response from either "end" of the horn, as well as the use of some destructive coupling, to achieve a usable passband (rather than only distinguished, highly resonant, notes).

----------

For a given box volume and power input, the maximum output and efficiency can actually be the same for almost any driver. The catch is bandwidth and distortion. Yes, a 2" full range driver could excite a low frequency resonance in a big horn, and achieve the same efficiency at that frequency, as an 18" pro-sound driver in an equal size "box," but the usefulness would be limited to such a narrow bandwidth that it would not be practical.

Regards,
Eric

[mdocod]

Re: Build Log. Compact Home Cinema Speakers and Sub. Semi-Pro-Fi. "From The Ashes"

While compact, It's still long internally, so it is "tuned" low. Another way to describe what I have built, is a tapered transmission line.

It will suffer from more compression than the "enormous" designs out there. Once I get the interior all finished, it should still have less compression than a reflex box, so it's a nice compromise IMO.

[lorenmjones]

Re: Build Log. Compact Home Cinema Speakers and Sub. Semi-Pro-Fi. "From The Ashes"

Makes sense. I have built a fairly similar design for live sound using a B&C 18SW115. I didn't design it though as you could guess. It doesn't have nearly as much efficiency gain. More like 4db. Works very well though, and with that particular sub we were worried about port compression if we had used a standard reflex box.

While compact, It's still long internally, so it is "tuned" low. Another way to describe what I have built, is a tapered transmission line.

It will suffer from more compression than the "enormous" designs out there. Once I get the interior all finished, it should still have less compression than a reflex box, so it's a nice compromise IMO.

[mdocod]

Re: Build Log. Compact Home Cinema Speakers and Sub. Semi-Pro-Fi. "From The Ashes"

Don't compare the amount of gain over the driver's characteristic sensitivity, compare the final efficiency, extension, and maximum SPL of the working system for a given box volume. Treat the box volume as the constant, and treat how the box volume is used as the variable. The variable includes the driver selection and all of the different ways to utilize that available box space, with that driver.

Model the 18SW115 in the same size box that I am using. Note that because the 18SW already has much higher acoustic coupling of it's own, it's not possible to use a horn design for such a driver in only 3ft^3 (at least not if you are after a useful response profile). The driver characteristics force the use of a reflex box as the highest level of sophistication available within this box volume to match the passband I am using. Tune to 30hz and it will have almost the exact same efficiency and extension profile as my bass horn does (including a "smashed" displacement departure angle below tuning, and useful acoustic coupling below tuning due the damped nature), except, because it will have achieved it's acoustic coupling primarily from a driver, rather than from a less agile resonant-acoustic-impedance modifying box, the 18SW has a more friendly bandwidth up top. I have not broken Hoffman's iron law about what can be done with this box size. Note that this size box actually suppresses sensitivity from the characteristic sensitivity of the 18SW in the passband I am shooting for. This is not unusual. The 18SW, actually has more output headroom in this box size than my horn (by a few dB), but at the cost of more power and the more expensive driver. For a given input power, both will do about the same thing from 30-80hz. All I have done, is arrived at the same destination, but took a different road ;)

When you compare your sub to my sub, don't even look at what was gained or lost over a particular driver, it is irrelevant in this application. The only case where driver efficiency begins to take over again is ABOVE EBP. In this case, looking for a fairly flat response to 40hz, and extension to 30hz and below (if possible), any driver with an EBP over ~40 will be able to achieve a similar efficiency/extension for a given box volume. In other-words, a pro-sound 18" midbass driver with a characteristic sensitivity of [email protected], with an EBP of like 200+, will have the same efficiency at 40hz as a driver with an EBP of 50 with 20dB less characteristic sensitivity, as long as they are compared in the same box volume.

I'd venture to guess, that if you compare the efficiency of your bass horn to mine, if the bandwidth is similar, then you should have about 6dB for every doubling of box volume over 3ft^3 that your horn is.

Regards,
Eric

[SirNickity]

Re: Build Log. Compact Home Cinema Speakers and Sub. Semi-Pro-Fi. "From The Ashes"

Mmhmm, yes. I understand some of these words. :rolleyes:

This thread goes into my "read this again in a year" file. My future self thanks you for taking the time to post some excellent information.

[lorenmjones]

Re: Build Log. Compact Home Cinema Speakers and Sub. Semi-Pro-Fi. "From The Ashes"

Thanks Eric,

So one take home that you are I think trying to point out is that Hoffman's Iron Law really means that for a given enclosure volume and bandwidth there is only a certain efficiency available. But I get confused by how the driver interacts with that, because we are comparing different enclosure designs but holding only the Vb constant. So the SW115 in 90L vented will be in the range of 90 db efficient over 30 to 70 hz but the UM10 in a vented box that size will be only around 83 db efficient (with the exception of a huge peak at Fb of 30). So to get the same efficiency over the given bandwidth and in the given enclosure size using the UM 10 a more sophisticated box is needed than for the SW115. Is there a theoretical maximum sensitivity of a given enclosure volume and given bandwidth? What effects do the driver have on this Hoffman's Iron law situation; motor strength?[/B] I guess the question more directly for me would be that if we further restrict the discussion to the same cabinet type (lets say bass reflex) within the bandwidth and enclosure size constraint method we are talking about, to what extent do the driver's characteristics bend Hoffman's Law?

Great info. OK, go gently on me. I am not an engineer or a mathemetician. So, that perspective on thinking about how drivers and boxes interact is very new to me. Most people (at least I think so) tend to look perhaps at a target bandwidth (or LF extension if we want to put it in those terms) and look at various drivers that might work. They then look at what sort of enclosure (volume and most frequently tuning frequency) that driver will "require" and then compare different drivers in what is essentially the most suitable reflex enclosure for a given driver.

If I read what you are saying correctly you are using the reverse perspective. You have a given box volume to work with and are saying that in that box volume with a given bandwidth in mind you will end up with similar efficiency from a wide variety of drivers? Indeed when I put 18SW115 in a 90L vented box with Fb of 35 it comes out to be pretty similar to the sensitivity of your UM10 tapered line over the bandwidth of 30 to 70 or so. But again, your driver requires a more sophisticated box to get the same output in the same size box. The SW115 at 60 hz will be around 128 compared to 121 for the UM10 at 3400w and 1000w respectively (but watts are cheap right;) ).

Thanks for sharing your knowledge. What is your day job?

Loren

Don't compare the amount of gain over the driver's characteristic sensitivity, compare the final efficiency, extension, and maximum SPL of the working system for a given box volume. Treat the box volume as the constant, and treat how the box volume is used as the variable. The variable includes the driver selection and all of the different ways to utilize that available box space, with that driver.

Model the 18SW115 in the same size box that I am using. Note that because the 18SW already has much higher acoustic coupling of it's own, it's not possible to use a horn design for such a driver in only 3ft^3 (at least not if you are after a useful response profile). The driver characteristics force the use of a reflex box as the highest level of sophistication available within this box volume to match the passband I am using. Tune to 30hz and it will have almost the exact same efficiency and extension profile as my bass horn does (including a "smashed" displacement departure angle below tuning, and useful acoustic coupling below tuning due the damped nature), except, because it will have achieved it's acoustic coupling primarily from a driver, rather than from a less agile resonant-acoustic-impedance modifying box, the 18SW has a more friendly bandwidth up top. I have not broken Hoffman's iron law about what can be done with this box size. Note that this size box actually suppresses sensitivity from the characteristic sensitivity of the 18SW in the passband I am shooting for. This is not unusual. The 18SW, actually has more output headroom in this box size than my horn (by a few dB), but at the cost of more power and the more expensive driver. For a given input power, both will do about the same thing from 30-80hz. All I have done, is arrived at the same destination, but took a different road ;)

When you compare your sub to my sub, don't even look at what was gained or lost over a particular driver, it is irrelevant in this application. The only case where driver efficiency begins to take over again is ABOVE EBP. In this case, looking for a fairly flat response to 40hz, and extension to 30hz and below (if possible), any driver with an EBP over ~40 will be able to achieve a similar efficiency/extension for a given box volume. In other-words, a pro-sound 18" midbass driver with a characteristic sensitivity of [email protected], with an EBP of like 200+, will have the same efficiency at 40hz as a driver with an EBP of 50 with 20dB less characteristic sensitivity, as long as they are compared in the same box volume.

I'd venture to guess, that if you compare the efficiency of your bass horn to mine, if the bandwidth is similar, then you should have about 6dB for every doubling of box volume over 3ft^3 that your horn is.

Regards,
Eric

[lorenmjones]

Re: Build Log. Compact Home Cinema Speakers and Sub. Semi-Pro-Fi. "From The Ashes"

Here is a pic of my 18SW115 cabinet before the finish was on it...just for grins. I'm not free to show the internals really since it is someone elses work, but it is very similar to yours with a small chamber behind the driver then folded tapered lines. In my case there is a mirror image folded tapered line on each side.

[mdocod]

Re: Build Log. Compact Home Cinema Speakers and Sub. Semi-Pro-Fi. "From The Ashes"

Hi Loren,

I love the look of that box. That has a sort of vintage bass horn look to it from the outside that just stirs up a sheer raw sort of excitement. I bet your neighbors just love that thing too!

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EBP and motor strength are intertwined. So motor strength DOES play into the efficiency of the system, but only to the extent that the driver is still operating as a resonant system. Below EBP, excess motor strength becomes a brake, or dampener. So, as I was saying there previously, as long as the motor strength results in an EBP that is not below the intended passband, then there isn't any "lost" sensitivity. Granted, if the EBP is above or near the top of the passband, (like that 18" monster you've got), then that will often be represented by a "sloped" maximum efficiency, which is easiest to see emerge in simple box designs like undersized reflex box you simulated while "following along" before, and noted that in the upper part of the passband, can play louder than the UM10 by a larger margin. That "slope" represents the transition from electronic brake to resonant behavior. The B&C unit there is "on the slope" while the UM10 in this case, is sort of straddling the transition.

At a more complex level, electrical dampening at the driver is often a desirable trait when designing high gain boxes. The reduced Q of the driver, allows the box design to be built with lower compression ratios, and bigger "ends" without causing large peaks in response, since those peaks can be subdued by the electronic dampening. For pro-sound applications that must play loud all the time, reducing power compression and eliminating any sort of "wind noise" from the system is very important. Horns that do this must have wide throats and low compression ratios.

The "sophistication" part of a box, has to be done with a great deal of optimization in order to maintain the gains as the box size increases beyond that of the basic sealed/reflex designs. Those horns don't fold themselves!!! Obviously one can't just throw more box volume at it and automatically get the desired result. Each increase in sophistication comes with a greater demand for precision in the design. In the mathematical construct world, there's a steady line on a chart that says "bigger=louder or deeper." At the application level, it is not always possible to actually build what would need to happen to stay on that line. The real world implementations often fall below that "ideal" line. Any undersized reflex box for a given driver (sloped response is a dead giveaway) is the easiest to "jam" up against the bottom of that Hoffman's iron law "line." When we start to deviate from simple reflex boxes, the designer should ALWAYS be simulating a reflex design with more driver in it of the same volume to the "side" to check that the box volume in the sophisticated design is "holding the line" so to speak. If we restrict the box sophistication, then there is a point at which increasing the box size no longer provides a worthy return on investment.

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As far as where I "start" when it comes to a design. Well, in this case, I started at "I want to build compact, dedicated passband cinema speakers, and a compact bass horn." This entire cinema project, was a chance to force myself to test all that I have been learning in the last couple years about acoustics, and a chance to exercise the "simulation" side of things, to determine if my simulation method (both for the horn, and the crossovers) is sound. I could have slapped a big woofer in a 3ft^3 box, but the idea of that approach has lost my interest for quite some time now (there's nothing wrong with it, it's just boring for me). This is a chance to perform a "proof of concept" and see if this approach can work. Like you, I've seen plenty of discussion about bass horns, and seen plenty of fridge size boxes, and heard a lot of "drivers with these characteristics would be good" type threads. I wanted to challenge the norm a little and see where a different approach would lead me. I also didn't want to be staring down a hole at the exposed rear of a driver.

I figured, the best way to leave "room" on the table for box related gain in a compact size (forcing myself to fold up something complicated), would be to start with a driver with very poor acoustic coupling on purpose! The worse the acoustic coupling at the driver, the more gain there is to be had from the box side of the equation. Again, as long as the EBP wasn't BELOW the passband I was going to use, then the driver efficiency would not penalize me for a given box volume. I also knew, that actual driver displacement would still set a limit on maximum output, so while the rest of the world is talking about a tapped horn and looking for low Qts drivers (because they are easier to tame in high gain boxes), I put Qts on the back burner (it's still on the stove, don't freak out!) and went looking for high Vd/Vas ratios instead. I've been tinkering with variations of this approach in simulation for at least a year now with many different drivers. I was just recently afforded the opportunity and excuse to actually move forward, and build it.

When I am trying to help someone solve an acoustic problem (involving low frequency reproduction), I do tend to think in terms of Box volume first, followed by bandwidth requirements, then budget, then driver/amp options to fit the needs inside of the size/bandwidth/budget constraints. On the other hand, sometimes problems do start with the driver. I am (eventually) going to be building a pair of similar horn subs for a friend of mine. Budget and aesthetic preferences led us to those cheap infinity car subs (reference 860w I believe). He was intrigued by the idea of tiny inexpensive drivers doing semi-pro-sound level stuff in his living room. it makes an interesting conversation piece at bare minimum and they will look pretty nice in an all-metal-cone build.

There is no rule about where to start, but knowing that you can start almost anywhere, and arrive at a similar destination is a concept that most people will refuse, regardless of how true it is.

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Day job? dang, I knew I was forgetting to do something. crap.
I have not been employed for awhile. I'm tinkering with the idea to go ahead and lease some shop space to setup a place to go commercial with speakers and sound reinforcement. I'd like to justify going the rest of the way and getting set up with measurement equipment (and maybe a little padded room?) and the whole 9 yards for this "hobby" but I can't come up with a way to get to play with this stuff all the time unless someone else is paying me to do it. For the moment, I have some time to figure out what to do on that front, as we are in the process of rebuilding after a natural disaster, so I have plenty of work to do that can effectively displace what would be an outlay of costs to contracted labor anyway. My "job" right now, in simplest terms, is to prevent unnecessary expenditures related to the rebuild by doing whatever I can myself. The universe (or, as some may prefer, God) has an interesting way of solving problems. For me, a natural disaster has given me a fresh start, a fresh set of problems, and lots of good hard work to do that pulled me out of a depression.

Regards,
Eric

[lorenmjones]

Re: Build Log. Compact Home Cinema Speakers and Sub. Semi-Pro-Fi. "From The Ashes"

Eric,

That's a serious reply!! Thanks for sharing all that. It would seem you have realized the level of understanding I have and tailored your info to match. That all makes perfect sense and is very informative.

So to be sure I'm understanding correctly a little, a way to see the "line" of Hoffman's law is to throw a big efficient driver in a small reflex box and that downward sloping line below the driver's ebp essentially defines the max sensitivity in that Passband from that box volume?

That 18sw115 isn't mine. It was built for our church. Anyone sane looking to devote that kind of box volume to a home sub would hopefully get extension much below 30hz. It does very well for its intended use though. That driver is a beast. A bridged MA2400 won't even start to make it show any distress in this box.

I hope your rebuild and job hunt go smoothly and end well.

Thanks again and take care,