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  1. #1

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    I have begun to wonder about the concepts above, having built an open back cab for my speaker. I do not remember the measurements of my last speaker cab, but I have begun completion of a cab 400x400x300mm (10x10x7,5") with a back opening of 2.5". My wonderings center around this, at what point does an open back speaker become a closed back? The open back is just a stand for the speaker, producing sound front and back in an airy kind of way. The closed back has a more directed sound forward, as well as the frequency response of the speaker changes toward more bass reproduction as the speaker becomes closed. There is I understand, an additional avenue which involves the ported cab, and includes the variations on port diameter and length, but for the sake of this discussion lets let that go. Hypothetically I could slowly close the cab opening back, and at some point this would become a closed back. Is there a gradual variation in sound quality and frequency spectrum during this process? If so what? How does one decide when considering a build as to which way to go? Hoe much open the back should be? There must be a better way than individual experimentation with so many of us getting involved in the building side of guitar playing, even though there are individual preferences to be considered.

    Do any of you have experience in these matters? Do any of you know of websites that address this kind of consideration when pondering a build? Any advise is appreciated. Thank you!

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  3. #2

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    I'm not an engineer, but the new Barefaced guitar cab looks interesting!

  4. #3

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    One way to experiment would be to have an open back, with a cover wide enough to completely cover the opening, with slots for the mounting screws to ride in, so that the cover could be moved in infinite steps from open to closed, and see what the sound change is. I don't have the incentive to do that much work, though.

  5. #4

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    I've found Neodymium speakers and smaller speakers like an 8" work best in closed back designs. Aside from that I prefer open back cabs.

  6. #5

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    Closed cabinets have a much higher bass roll-off point than open back cabinets for most drivers. Speakers are called "drivers" when building them.

    So it depends ... and can vary from a little to dramatically.

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  7. #6

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    Quote Originally Posted by sgosnell View Post
    One way to experiment would be to have an open back, with a cover wide enough to completely cover the opening, with slots for the mounting screws to ride in, so that the cover could be moved in infinite steps from open to closed, and see what the sound change is. I don't have the incentive to do that much work, though.

    I should have been clearer in my origional post. I have made an open back cab. My pondering has to do with the effect of various degrees of closing the back and what effect to expect on the sound. Your idea of slots which will provide for gradual opening/closing so as to test results was interesting, but I do not see a realistic application.

  8. #7

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    I don't see a realistic application either, because I'm not seriously interested in the results. I have both open and closed back amps, and it's not something I lie awake nights worrrying about. But the only way to know the actual difference in sound is to do an experiment. Without that, it's all just speculation, by yourself, by random people on the internet, or both.

  9. #8

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    I had once a cab that could be open or closed. Closed increased bass response and the sound became more "forward" and focused. Less feedback. With the open back you had less bass and better dispersion. With ported cabs it's a whole other thing...

  10. #9

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    The link to Barefaced cabs discusses the advantages of vertical speaker placement IRT dispersion. Back in the '80s I was playing through a Music Man 112 RD (open back, with a slight backslant, for those unfamiliar with these terrific amps) fitted with a 12" JBL atop a 15" JBL-loaded 36" x 24" poplar closed back cabinet I made myself (dimensions approximate). The rig was exquisitely room filling, and I mean any size room, at any volume. I was a straight-up idiot to let either the amp or the cab go.

    Speaking of backslant, I have an original Brownface VibroVerb (2- 10" speakers, open back) with tilt back legs that was similarly room filling. Sweet!
    Best regards, k

  11. #10

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    If you are going to seal the back of the cab, consider putting in one of these: Scanspeak 290001 Aperiodic Vent (SSV) .

    An aperiodic vent lets out the air at a certain sound pressure. Below that level, it acts practically as a sealed cabinet.

    This is the inverse of what you are asking: at what point does a sealed box with an aperiodic vent act as a ported speaker?
    Great Deals with Great Folks: max52 (Guild-Benedetto Artist Award); prickards (Ribbecke GC Halfling); Cincy2 (Comins Concert)

  12. #11

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    Quote Originally Posted by rNeil View Post
    Closed cabinets have a much higher bass roll-off point than open back cabinets for most drivers. Speakers are called "drivers" when building them.
    I believe it's the inverse of that .....

    see below quote of post #8

    I had once a cab that could be open or closed. Closed increased bass response and the sound became more "forward" and focused. Less feedback. With the open back you had less bass and better dispersion. With ported cabs it's a whole other thing...





  13. #12

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    It depends on the driver data for volume. When building closed cabs you have to pay close attention to VAS or you'll have a pretty high and potentially odd shape to your bass.

    Which is why so many bass or mids drivers are more suited for ported cabs. And then you have to carefully plot the port diameter vs. length and proper flare for ends of the tube.

    Ported cabs can frequently get a lower bass cutoff frequency for the same driver in a much smaller box, but it will also be a much steeper rolloff curve. And may very significantly by power output.

    Open back is an interesting case ... not actually a "free air" response (such as driver mounted on a board with no sides or enclosure) but not exactly following closed or ported models either.

    At times is closer to folded response, but ... not really.

    So when I've talked with experienced enclosures design folk, and mentioned open back ... they tend to roll their eyes and say just go play with it until you're happy but it will never sound the same in any two environments. So don't expect much.

    But guitars can sound pretty nice with open backs of the right sort for the guitar in the right room and placement ...

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  14. #13

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    [QUOTE=rNeil;961044]

    So when I've talked with experienced enclosures design folk, and mentioned open back ... they tend to roll their eyes and say just go play with it until you're happy but it will never sound the same in any two environments. So don't expect much.

    Hi rNeil, thanks for the input. I am led down two avenues here myself.

    -On the one hand, yes I am experimenting with my own build, to insulate or not, and how much. To which degree the back should be opened ei percentage of the backs area (total, one third, one twentieth), which could be expanded to whether or not the closed part of the back should be one piece or two, or three? What is the general effect of the cab volume in relation to the speaker size?

    -On the other hand, were open back cabs so iffy so as to make experienced cab designers/builders shrink and shake their heads, why do the likes of Fender, Mesa, Peavey, Roland, Bogner etc still produce them, and guitarist buy them?

    My conclusion is at this point that there must be enough collected knowledge about the reactions within open cabs, and how to construct them/how not to construct them, and which should be available to people who enjoy making them like myself. As you well point out there seems to be a wealth of info on the considerations to be made when designing a ported cab that is available. Where is the counter part info mass concerning open back cabs and how they perform within the parameters of total cab volume, cab volume distribution in height, width, depth, back openness, speaker size? It would seem that this forum, or another would have people versed, not just opinionated or shooting from the hip. Best regards 0zoro

  15. #14

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    I would love to find some wealth of knowledge on open back construction. I've got a 15" Eminence Big Ben driver and the wood to make my own open back cab.

    This is to use with an old Ampeg SB 12 tube amp, that I want to transport sitting in the bottom of the cabinet. And use sitting on top.

    So the amp dimensions make for a minimum interior width, and of course set a minimum height for the driver so that the amp fits under the cone. And naturally I need space on top to park the head while playing.

    I'm so used to working with exact calculations when building towers, subs, and general speakers that I keep looking for The Rules.

    What I've been told is that for any general volume of the cabinet assume somewhere between 1/2 and 2/3 open will work ok.

    Um ... yeah ... not exactly a Thiele-Small formula calculation.

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  16. #15
    I worked quite a bit with Rich Raezer when he was designing his cabs. One thing he realized early on was that for jazz guitar cabs, you couldn't use the standard formulas for cabinet design. For example, Mesa followed the design parameters of the Thiele cab and (IMO), the result is a dull, lifeless thumpy cabinet that sounds awful for jazz, is heavy and probably better suited for chunking out distorted heavy metal chords.

    Rich built his cabs utilizing standard formulas *BUT* then he started altering port sizes and positions *TO TASTE*. I think this is important because much of what we find pleasing is based on psycho-acoustics and our preferences based on what we grew up on.

  17. #16

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    And what we each "hear" for the sound we want from our guitar and amp varies so much.

    Two people can like the same sound but for very different parts of it. So when they try to recreate "that sound" as they appreciated it, their results can be strikingly different.

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  18. #17

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    I think that open back cabinets - like many combos - will depend on a reflecting wall behind it. My drummer says the he prefers me using an open back cabinet, because then he hears the guitar parts better. I think the difference in perception is how the speaker fills the room. If we talk small units, closed cabinets is the only way to enable some bass but at reduced efficiency. Flat frequency response only rules in HiFi, i believe. I only have one sealed enclosure amp, my AC4 Mini. It has no bass, really, but for some purposes it is a really nice sounding amp.

  19. #18

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    Bass response is a combination of "free air response" data of the driver, the power of that driver's magnet to move cone mass for a specific energy input, the cone mass of the driver, the size of the box relative to all the earlier points, and whether or not the enclosure is sealed, ported, or some form of open.

    Pretty complex other than any smallish driver/box combination is NOT going to have serious bass response without great electrical mucking about.




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  20. #19

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    Quote Originally Posted by rNeil View Post
    ... smallish driver/box combination is NOT going to have serious bass response without great electrical mucking about. ...
    And acoustic mucking as well. Most sealed HiFi speakers are quite heavily dampened. This is part of the filtering. I never use my AC4 Mini for anything but guitar but I think that if I tried to use it for full range music - or even for voice - it would sound "boxy" and nasal.

  21. #20

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    The inspiration for the original Fender Tweed Deluxe openback combo amp was the tweed fabric covered...suitcase. Yeah, cut a hole in the back for tube ventilation. The suitcase determined the electric guitar amp tone that came to be heard on so many records it became the archetype of amplified guitar tone. All other Fender openback combo amps and cabs that came after it were elongations of that suitcase size. Why a suitcase? It was a most familiar portable shape that had a handle.

    There is really no science in openback cabs. The suitcase form determined the tone and that became THE TONE.

    The Oval Back 24"x18"x12" cabinet that became popular with Dumble style amps came about because 24" was the width of the Dumble Overdrive Special head, 4:3 was the familiar ratio of a CRT TV, and 12" because of the depth of the huge EV12 driver and a 4:3:2 cab just looks right. Round is boring so let me, meaning HAD, try oval. Pass it out to Larry, Robben, Stevie, and lately John, and man, don't veer from the form because that is the Dumble ODS tone.

    Guitarists are a superstitious lot; don't apply science when superstition will do.

    That's my story and I am sticking with it.

    How do we design an openback cab? First, assume a suitcase...
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  22. #21

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    PS If not for the guitar industry guitar cab drivers would have been consigned to the dustbin of history a long time ago. Guitar cab drivers are not high fidelity and the very distortions that make them well-loved by guitarists for "tone" are decried for high fidelity reproduction of sound.

    So, there is no science involved except for the preservation of ancient manufacturing techniques and materials for making these drivers to reproduce the distortions today as guitarists still want them.
    Great Deals with Great Folks: max52 (Guild-Benedetto Artist Award); prickards (Ribbecke GC Halfling); Cincy2 (Comins Concert)

  23. #22

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    I thought I would enclose the reply (below) I received from a design engineer who works at one of the speaker manufacturers on the subject of the properties of an open back speaker and the science that is applied to their building. While it brought clarity to my questions I find few of them answered as i had hoped for. I thought you may enjoy.

    "For pro audio and bass applications, there’s a lot of science involved in designing and building the cabinet. If you understand the science, it’s pretty straightforward, and you’re basically trying to accurately reproduce the sound with the speakers and cabinet. For guitar, everything is very subjective and guitar speakers have rather odd parameters, like low Vas, high resonant frequencies, and high Q values. It’s an art, and what the science determines is not what always is practical or what sounds good. You’re using the speaker and cabinet to create tone, and not necessarily just accurately reproducing the signal. So, you can use the science to know what affects certain changes will make to the sound, but it’s not as straightforward nor scientific to come to specific conclusions. You have to experiment, or go with something known and don’t try to reinvent the wheel.

    The volume affects the low frequency range below 200Hz or so. Everything above 200Hz is produced from the cone area of the speaker. A larger cabinet will enhance bass response, up to a point of becoming “boomy” or “muddy”. A smaller cabinet will tighten up bass response, but could sound more “boxy”, if too small. What’s too big or too small? That’s some of the subjectivity. For the open back panel(s), the more the back is open, the more the sound will fill the room. This is at the expense of bass response. Too much removed, might sound thin and harsh. The more the back is sealed, the more focused it will sound. Bass will be enhanced. It will sound more full."

  24. #23

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    Yeah, the engineer guy pretty much said as I said, assume a suitcase...
    Great Deals with Great Folks: max52 (Guild-Benedetto Artist Award); prickards (Ribbecke GC Halfling); Cincy2 (Comins Concert)