Ultra Light Weight Bass Cabinet

[Petebass]

Thanks Rob you did answer my question. But you also raised a further point of interest

Without any engineering training, my laymans way of deciding just how good or bad composite cabs are was to build an identical control cab from plywood (12mm and braced), then compare the sound both by listening and by measuring.

Initially the composite cab had an identical low freq output, and that was pleasing because it was in the sub 200Hz range where I would have expected any significant difference. But surprisingly the composite cab had a 3dB hump at 500Hz that was very audible and of course was evident on the measurements I took. I added brace to the composite cab between the back panel and the front baffle and the 500Hz peak disappeared.

I didnt really understand why this happened. Am I correct to conclude that the cab initially had a resonant freq at 500Hz and adding the brace raised it to a frequency that is less noticeable?

 

[PDGood]

Hi Rob and welcome to the forum with this great contribution. I wish I could give you the conversation you seek regarding the technological aspects, but I'm just not capable.
However, I would like to bring this whole thing back around to what's really important - the actual sound (as opposed to a description of the sound).

I've been following similar threads on the Thunderchild, Fearfuls, Acme Series III and Baer cabs with interest. But for me, if it makes music, descriptions will only go so far and I have to hear it for myself. TB member KJung has done some wonderful side by side video/audio comparisons, which is the next best thing to actually being there.

The frustration of course is that I would actually like to be there and try each product as I would in a store. Naturally, that's not possible with boutique builds. But given that it still takes some $$$ to build or purchase from a builder, it's important to know what you're getting into before you invest. So, I like to encourage builders to get the actual product out there where people can hear it for themselves whenever that is possible.

What I would suggest at a minimum, if you're up for it, is to loan yours to Ken for video clips as others have done and then we'll be able to hear the results of the build in direct comparison to similar items, and perhaps hear the contribution of the composite material in the process. Yeah, I'll still probably wish I could hear it first hand, but maybe you'll benefit by more feedback of the nature you prefer.

 

[BogeyBass]

So, therefore I think that my original conclusions are still correct. A composite build has much lower levels of resonance than a plywood build. Also, I posted the mechanical/structural properties of plywood, and from that, I think you can draw the same conclusions. Yes - you are correct, that the plywood does dampen out faster, but in fact, the composite starts from a much lower level, and never really needs to have all of that dampening... It's just low.

I'm just making my observations from 2 single graphs.

As far as i understand when something starts to resonate, it will continue to do so in higher harmonics

the higher harmonics exist only because of the original fundamental. they simply would not exist if it wasn't for the original fundamental frequency.

so when you see more higher harmonics, they only way they could be there is if there was more fundamental energy. hence when you see more harmonics, you have more fundamental.


and in this case with more harmonics it would just mean the material is vibrating/resonating more.

as you know its impossible to have something vibrate at the pure fundamental, because their will always be fixed points, and it is those fixed points that will bounce back the fundamental freq and create the higher harmonics.

thats why i think its vibrating more because, there is more upper harmonic content, showing more energy was there at the get go.

Ideally you would have to test every frequency, in a limited bandwidth say 20 to 300hz. and the sample length for each frequency would have to be exactly the same.

then show a spectrum analysis of all those samples combined to see the overall performance of that material.

and whichever had the less amount of peaks would be resonating less. or could at least see what frequencys certain materials are/aren't likely to resonate at. keeping in mind that whatever size the material is and whatever fixed points exist could skew that data.

so i guess it would be somewhat important that all the pieces are the same size and attached at the same points.
and like you said there is no perfect test, but it is only a comparison in similar conditions.

 

[robthebassplaye]

Thanks Rob you did answer my question. But you also raised a further point of interest

Without any engineering training, my laymans way of deciding just how good or bad composite cabs are was to build an identical control cab from plywood (12mm and braced), then compare the sound both by listening and by measuring.

Initially the composite cab had an identical low freq output, and that was pleasing because it was in the sub 200Hz range where I would have expected any significant difference. But surprisingly the composite cab had a 3dB hump at 500Hz that was very audible and of course was evident on the measurements I took. I added brace to the composite cab between the back panel and the front baffle and the 500Hz peak disappeared.

I didnt really understand why this happened. Am I correct to conclude that the cab initially had a resonant freq at 500Hz and adding the brace raised it to a frequency that is less noticeable?

Hi!

Very nice to have this discussion!

I think you are absolutely on the right track.

Yes, based on what you have said, adding a brace would definately raise any resonant frequency. The basic idea is that the best you can do is to keep raising the frequency until you cant hear it. So, as silly as it sounds, getting it out of the way of a normal bass players area of concern, and even out of the area that most folks can hear, will really make it sound a lot better. That is sort of simplistic, but I think it's accurate.


The way you describe it, a brace from the front to the back of the cabinet, would surely accomplish this, and it probably one of the best, easiest ways to do this.


So, if you think about panels moving, the best way to stop that (which really is the goal) is to design something that puts its "finger" right on that spot, and calms down that resonance.

To me, 500 Hz is farily high, and definately audible! So, you shoud definately want to tame that.

Based on my measurements, what you have encountered seems very plausable. The composite panels wont resonate at that frequency, but I could anticipate that the overall build (ie the constructino of the panels) might, if they were not really solid.

You can think about the resonant frequency of the panels, in a worst case scenario, as a function of their LxW, (ie thier apparent area, really the diagonal length is what defines it). So, presumabely, thats not whats doing on in your situation. Maybe, they are just not bolted down or affixed to the other structures enough? How well have you affixed them to the adjusent walls? To me, that seems like the issue, if you are hearing something going on...

I'd love to find out how this turns out, please keep me posted!

Rob

 

[billfitzmaurice]

So, anyway, back to resonant frequencies etc... Maybe it's the terminology, but bracing absolutely does raise the resonant frequency of any panel. By definition, making the area that can vibrate smaller will raise the fundamental node, and that is what bracing does.

True, but anecdotal. Panels don't vibrate because they're reacting in sympathy to the tones generated by the driver. They vibrate from the high internal pressures created by the driver cone movement. If stiff enough to withstand that pressure they don't vibrate, if not stiff enough they do, irrespective of the panel resonant frequency.

There are circumstances where sympathetic panel resonance is a significant factor. An acoustic guitar top is one example, as is a piano soundboard, or in the case of speakers those where significant internal air pressures are not created, like a typical table radio. But where most loudspeakers are concerned, especially the ones we use, resonance is such a minor concern compared to air pressure as to render it insignificant...unless the resonance is a loose bit of flotsam that drives you nuts when you can't figure out what it is that's making a racket.

 

[Petebass]

Maybe, they are just not bolted down or affixed to the other structures enough? How well have you affixed them to the adjusent walls? To me, that seems like the issue, if you are hearing something going on...

Trust me, that aint it! My construction was pretty much the same as yours, right down to the "monocoque" outer layers. It's solid!

I just thought that the panel most likely to vibrate would be the one with the most mass cut out of it i.e the front baffle. I thought that bracing that panel would be as good a place to start as any, and as it turns out I got lucky didn't have to go any further before the problem disappeared.

The cabs sound near identical now (except that I'm trialling different tweeters on them right now). There are some measured differences in the high mids but they are certainly less audible than the 500Hz hump was. I'm also measuring them in my garage, far from AES standards so I'm reluctant rely too heavily on these measurements. For now I'm happy, but as always, there's more to learn..........

The biggest difference between the 2 cabs is the 8.8lbs weight deficit, which is a lot for 1x12 cabs... even the lightest wood cabs all feel so darn heavy now

 

[dhomer]

Interesting read to this point.. An observation and a question. It looks like you used a closed-back midrange driver instead of using a small built-in enclosure. Other designers have determined (in similar designs) that the closed back drivers were not a good match for the system..

As it pertains to composite construction very similar to your technique, builders have had issues with properly securing the drivers on the baffle, i.e. fasteners coming out of the panels. What did you do to insure that wouldn't happen on your cab??

If I ever entertained the possibility of composite builds, I would probably favor a plywood baffle/port and a composite shell for this reason..

 

[robthebassplaye]

I'm just making my observations from 2 single graphs.

As far as i understand when something starts to resonate, it will continue to do so in higher harmonics

the higher harmonics exist only because of the original fundamental. they simply would not exist if it wasn't for the original fundamental frequency.

so when you see more higher harmonics, they only way they could be there is if there was more fundamental energy. hence when you see more harmonics, you have more fundamental.


and in this case with more harmonics it would just mean the material is vibrating/resonating more.

as you know its impossible to have something vibrate at the pure fundamental, because their will always be fixed points, and it is those fixed points that will bounce back the fundamental freq and create the higher harmonics.

thats why i think its vibrating more because, there is more upper harmonic content, showing more energy was there at the get go.

Ideally you would have to test every frequency, in a limited bandwidth say 20 to 300hz. and the sample length for each frequency would have to be exactly the same.

then show a spectrum analysis of all those samples combined to see the overall performance of that material.

and whichever had the less amount of peaks would be resonating less. or could at least see what frequencys certain materials are/aren't likely to resonate at. keeping in mind that whatever size the material is and whatever fixed points exist could skew that data.

so i guess it would be somewhat important that all the pieces are the same size and attached at the same points.
and like you said there is no perfect test, but it is only a comparison in similar conditions.

Awesome! Wow, excellent question, and a really good feeder for more discussions!

Yes, the first thing that is important is the fundamental resonant frequency. But, one thing that these Accelerometer tests can show is how much displacement exists at that fundamental frequency.

The testing I did was a swept sine measurement, so I did capture everything along the way, including the frequencies you mention. So, there perhaps is a discussiojn to be had about weather a swept sine frequency measurement can accurately test and represent a cabinet at those lower discrete frequencies. But, I think it can, but Id love to have the conversation!!! Doing discrete tests at certain points can definately be enlightening, based on my experinece, so I totllly agree!

So, think about it this way. No matter where that frequecny is (high , low , or somewhere else), it has a magnitude. Relatively big, or small.

And, then, based on that magnitude and frequency, it creates other harmonics (that is, resonant frequencies based on the original, or primay frequency that the material resonates at).

So, dampening is a property that defines how well any material can lower (or magnify, if it's not good at dampenining) those harmonic frequencies. And, if it makes those frequencies more apparent in the even or odd domains ( Even harmonics are 2f, 4f, 6f, ... Odd harmonics are f, 3f, 5f, ... ) Again, there is a lot of stuff on the internet about what is more pleasing, its subjective, but feel free to search and decide...

With these composite structes, the dampening is very small, (so that is, maybe not a high dampening factor, but there isn't a lot to dampen) but the initial resonance displacement is also exremely small (and at a much higer frequency).

Your comments about the panel size are right on target - and that was something that was a concern to me when I was doing this. The size of my test panes could totally skew the results. I definately acknowledge that. But, becasue they were the same size, I felt like I was doing an apples to apples comparison. But, you are absolutely correct, and in a perfect world, I should have done those measurements with more independant variables, as you describe.

Great comments - keep em comming! I love having these technical discussions!

Rob

 

[robthebassplaye]

True, but anecdotal. Panels don't vibrate because they're reacting in sympathy to the tones generated by the driver. They vibrate from the high internal pressures created by the driver cone movement. If stiff enough to withstand that pressure they don't vibrate, if not stiff enough they do, irrespective of the panel resonant frequency.

There are circumstances where sympathetic panel resonance is a significant factor. An acoustic guitar top is one example, as is a piano soundboard, or in the case of speakers those where significant internal air pressures are not created, like a typical table radio. But where most loudspeakers are concerned, especially the ones we use, resonance is such a minor concern compared to air pressure as to render it insignificant...unless the resonance is a loose bit of flotsam that drives you nuts when you can't figure out what it is that's making a racket.

Hello Mr Fitzmaurice,

You are, of course, absolutely correct. The internal pressure created by the motion of the driver pistons is what causes any energy to be transferred to the walls of the enclosure from the original electrial energy sent to the cabinet.

Your references to Guitar tops, or Piano soundboards are 100% appropriate. As are the way we think about their sounds. Bracing, as you know, in guitar tops and Pianos, is highly significant and valued, as it plays a very important role in the overall sound quality of the acoustic instruement - due to controlling resonant frequencies of those panels. What does is amplify, and what does it negate? Thats what determines tone.

But, it's not sufficient, in my opinion, to state that either the walls vibrate, or they dont. That would be fantastic in a perfect world, but for me, that is precisely the difference between a cabinet constructed of plywood, or a cabinet constructed of an inelastic material such as composite foam/fiberglass.

However, in my opinion, the resonances (ie the fundamental nodes, and the resulting resonances - higher, lower, odd or even) of the cabinet will be driven by that initial pressure input, and therefore play an important part in the overall soud quality of the system. To me, thats why folks have a preference for one cabinet design over another - it's really quantifying the "SOUND" of the box.

Excelent discussion, lets keep this going! I'm very open minded about any of this!

Rob

 

[robthebassplaye]

Interesting read to this point.. An observation and a question. It looks like you used a closed-back midrange driver instead of using a small built-in enclosure. Other designers have determined (in similar designs) that the closed back drivers were not a good match for the system..

As it pertains to composite construction very similar to your technique, builders have had issues with properly securing the drivers on the baffle, i.e. fasteners coming out of the panels. What did you do to insure that wouldn't happen on your cab??

If I ever entertained the possibility of composite builds, I would probably favor a plywood baffle/port and a composite shell for this reason..

You know, thats an interesting observation. Actually, I had bought both - the closed back and the open back midrange. I listened to them both, and I measured them both, and ultimately decided that I would use the open back and put an enclosure within an enclosure. So that is, in fact the way I built it.

If I was doing it again, I think I would use the closed back, it's a LOT EASIER for building, and I'm not sure that the audio quality was that different...

Not very scientific, I know, but next time, I'm taking the easy way out.

Fantastic catch - really nice to know that folks are actually reading this stuff that I put my blood, sweat and tears into!

I've attached some more photos of the mid driver design and the enclosure that I used for it...

Rob

 

[robthebassplaye]

Interesting read to this point.. An observation and a question. It looks like you used a closed-back midrange driver instead of using a small built-in enclosure. Other designers have determined (in similar designs) that the closed back drivers were not a good match for the system..

As it pertains to composite construction very similar to your technique, builders have had issues with properly securing the drivers on the baffle, i.e. fasteners coming out of the panels. What did you do to insure that wouldn't happen on your cab??

If I ever entertained the possibility of composite builds, I would probably favor a plywood baffle/port and a composite shell for this reason..

Sorry, I sort of missed the primary point of your comment.

Yup, I think if I did it again, I would also use a plywood front baffle. Working the the T-Nuts, and seeing how well they fasten (or don't) to the fiberglass/foam buildup is a bit concerning. So, if I ever do this again, I'll probably go with a wooden baffle just for those reasons.

But, its unfortunate. Beacuase I have really high expectations that the composite structure really contribues to a fantastic, pur sound, and my guess is that a baffle made from plywood would significantly affect that sound.

But, again, it's a matter of pragmatisim. This thing needs to hold up and to work, and it also needs to sound good. So, maybe thats a good compromise. And, you have to consider cost (and weight in this design).

But, I'd probably go with a wooden baffle!

Rob

 

[robthebassplaye]

Trust me, that aint it! My construction was pretty much the same as yours, right down to the "monocoque" outer layers. It's solid!

I just thought that the panel most likely to vibrate would be the one with the most mass cut out of it i.e the front baffle. I thought that bracing that panel would be as good a place to start as any, and as it turns out I got lucky didn't have to go any further before the problem disappeared.

The cabs sound near identical now (except that I'm trialling different tweeters on them right now). There are some measured differences in the high mids but they are certainly less audible than the 500Hz hump was. I'm also measuring them in my garage, far from AES standards so I'm reluctant rely too heavily on these measurements. For now I'm happy, but as always, there's more to learn..........

The biggest difference between the 2 cabs is the 8.8lbs weight deficit, which is a lot for 1x12 cabs... even the lightest wood cabs all feel so darn heavy now

Hey PeteBass - Wow, thats cool.

I am a real newbie at this, hopefully I don't come across as some know-it-all.

What you have described makes perfect sense. Thats where I would have started to tame the sound as well...

Here is a real word example that can put me in my place...When I put my cabinet together, it sounded like CRAP <sp?> - really bad. Then i figured out that I had only installed 4 of the 8 screws that held the 12" driver in, and the air that was comming out of those 4 holes TOTALLY messed things up. So, thats a pretty beginner mistake, I think. I have a large dynamic range of knowledge and experience (that means lots of knowledge in certain areas, lots of experience in certain areas, and little or no experience or knowledge in others), and I'm definately not too humble to acknowledge what I dont know, or what I haven't expereienced.

So, the point of that is, I can (and want to) learn a lot.

Thanks for sharing your experineces!

Rob

 

[will33]

Hi Will33!

Very interesting question.

I spent a lot of time considering where the ports shoud be, since there are tradeoffs relative to baffle size and porting (once you get into the details about where things go, these seemingly minor issues become a big deal when you are trying to keep size to a minimum)..

Anyway, I was hesitant about putting one port on the front, and another on the back, as I did in this design.

But, as long as the cab is positioned a fair distance from a back wall, there are no negative effects from this. If it is positioned close to the wall (as you point out, a quarter wavelength is significant), then there would be effects, and depending on the room, they might sound good, or they might sound bad...

As far as the crossover design, and the driver placement, I was not that concerned about the alignment. The HF horn is elliptical, with a primary pattern of horizontal, which seemed appropriate for this application. It is really smooth, the transition from the mid to the horn is allmost unnoticable.

But, for a bass cabinet, the transition from the LF driver to the mid is really what it's about, and because of the wavelengths involved, I felt that alignment and phasing issues were at best a second or third order effect, after the significant effect of the stiffness of the walls, compared to traditional plywood cabinets.

Excellent comment, I'd love to get more feedback from you about this, I'm sure there are things that can be imporved in this design, and I'd really like to be able to pass those things along to other builders.

Rob

Port frequencies are omni-directional, you can put them pretty much wherever although I've never thought about what may or may not happen with phase by putting them on different or opposing panels. That's interesting.

Was asking about keeping the center to center distance of the components less than a wavelength apart (sorry, not 1/4 wave like I said earlier) at the crossover frequency. Exceeding that distance can give wierd lobe patterns, cancellation, uneven response, etc. Was trying to find a link to a short, concise example or chart but all are pretty lengthy tech papers etc.

Not being nitpicky, some nice work you've got there. But you may find it interesting as you seem to be very "detail oriented" to say the least.. It may or may not affect your mid/tweet spacing depending on your measurements. It would be much more noticable in a hifi type system and may not be noticable at all on stage, at gig volume with guitars playing, cymbals crashing, etc.

 

[tjh]

Sorry, I sort of missed the primary point of your comment.

Yup, I think if I did it again, I would also use a plywood front baffle. Working the the T-Nuts, and seeing how well they fasten (or don't) to the fiberglass/foam buildup is a bit concerning. So, if I ever do this again, I'll probably go with a wooden baffle just for those reasons.

But, its unfortunate. Beacuase I have really high expectations that the composite structure really contribues to a fantastic, pur sound, and my guess is that a baffle made from plywood would significantly affect that sound.

But, again, it's a matter of pragmatisim. This thing needs to hold up and to work, and it also needs to sound good. So, maybe thats a good compromise. And, you have to consider cost (and weight in this design).

But, I'd probably go with a wooden baffle!

Rob

... or, perhaps a thin plywood veneer secured to the front baffle would accomodate both issues ??

 

[robthebassplaye]

So, Ive seen a number of your replies, implying that I copied something from someone else. I take that as a significant insult, just to get that out there. But,I dont really want to turn that into a big discussion...

I really hesitated, and thought about it a lot, before I replied to your coments. Perhaps I have made a poor decision in replying, but oh well, it wont have been the last poor decison I have ever made...

There is no financial reason for me to have copied anything. I dont sell, license or benefit financially from anything on this web site. In fact, it's sort of my first public appearance relating to basses or pro audio. So, I have nothing to gain by anything I post here.

I do want to get this design out into the public domain, that all I want to do is share my learning, my experiments, and my measurements with anyone with the technical aptitude to understand it.

It's a bit altruistic, which might be hard to fathom, but thats the truth.

GreenBoy called me, and I am pretty sure we are on the same page. I'm not taking any money out of his pockets, nor of anyones pockets that I konw about... This is not a business for me, and I'm not sure how to make that more clear.

So, really, it's not some kind of diabolical scheme. Just info sharing, on my part. I love technology, I love science, I love experiments and data. Really, thats what I love. So, if you could keep that in the back of your mind as you respond to these posts, I think I would sleep better at night. Maybe you would too.

Thanks,
Rob

 

[robthebassplaye]

... or, perhaps a thin plywood veneer secured to the front baffle would accomodate both issues ??

Yup - I think thats what I would do. But, those hurricane nuts are particularily nasty. If you didn't care about the removal process, there might be a better way..

The defects seem to mostly be realted to the install/extraction process.


Rob

 

[robthebassplaye]

Port frequencies are omni-directional, you can put them pretty much wherever although I've never thought about what may or may not happen with phase by putting them on different or opposing panels. That's interesting.

Was asking about keeping the center to center distance of the components less than a wavelength apart (sorry, not 1/4 wave like I said earlier) at the crossover frequency. Exceeding that distance can give wierd lobe patterns, cancellation, uneven response, etc. Was trying to find a link to a short, concise example or chart but all are pretty lengthy tech papers etc.

Not being nitpicky, some nice work you've got there. But you may find it interesting as you seem to be very "detail oriented" to say the least.. It may or may not affect your mid/tweet spacing depending on your measurements. It would be much more noticable in a hifi type system and may not be noticable at all on stage, at gig volume with guitars playing, cymbals crashing, etc.

Hi - those are really good things to take into consideration. I am "detailed Oriented", as you may have discovered... (My wife has a BIG problem with that, but anyway,..., I'm sure I drive her nuts)

I did not do a lot of research into HiFi, but no doubt, that should play into this thread. I'm going to research it ! But, I didnt take any of that into account in this build. So, I'll let you know what I find out, and what is pertinant.

Thanks!
Rob

 

[robthebassplaye]

So, Ive seen a number of your replies, implying that I copied something from someone else. I take that as a significant insult, just to get that out there. But,I dont really want to turn that into a big discussion...

I really hesitated, and thought about it a lot, before I replied to your coments. Perhaps I have made a poor decision in replying, but oh well, it wont have been the last poor decison I have ever made...

There is no financial reason for me to have copied anything. I dont sell, license or benefit financially from anything on this web site. In fact, it's sort of my first public appearance relating to basses or pro audio. So, I have nothing to gain by anything I post here.

I do want to get this design out into the public domain, that all I want to do is share my learning, my experiments, and my measurements with anyone with the technical aptitude to understand it.

It's a bit altruistic, which might be hard to fathom, but thats the truth.

GreenBoy called me, and I am pretty sure we are on the same page. I'm not taking any money out of his pockets, nor of anyones pockets that I konw about... This is not a business for me, and I'm not sure how to make that more clear.

So, really, it's not some kind of diabolical scheme. Just info sharing, on my part. I love technology, I love science, I love experiments and data. Really, thats what I love. So, if you could keep that in the back of your mind as you respond to these posts, I think I would sleep better at night. Maybe you would too.

Thanks,
Rob

And - GreenBoy, if you want to weigh in on this, I think I'd appreciate it. I'm not out to harm anyone.

Rob

 

[Petebass]

The Kappalites are amazing drivers. I think even Greenboy would acknowledge that once you decide to use them, and if you follow a logical path, the rest of the cab kinda designs itself. Good design is good design no matter where it came from! So lets move on eh!!!

 

[Passinwind]

And - GreenBoy, if you want to weigh in on this, I think I'd appreciate it. I'm not out to harm anyone.

He's not too roughed up about it, judging from his posts in his own forum.