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How do a ported cab and an open back guitar combo correlate, if any?

Discussion in 'Amps and Cabs [BG]' started by fishdreams, Sep 27, 2013.


  1. fishdreams

    fishdreams Supporting Member

    Sep 4, 2010
    Brooklyn, NY
    Endorsing: Arkham Vacuum Tube Amplification
    This is a bizarre idea, and if i am on the wrong track, fine. But here's the story: every week i have a session with a great pianist who has a Steinway in her loft which has very nice acoustics. I play solo or duos with her.

    usually I plug intothe GK + SWR 2x10 set (solid, regular bass guitar stuff) that is there, and don't think any further. But this week through circumstances I ended up playing on the Fender 25R (a $99 dollar open ended guitar combo)

    With my flatwound fretless the results were very interesting: a much more open tone, direct attack, harmonics that were ringing like bells, a tone that really came alive and off the floor with a teeny tiny dash of reverb. It worked great solo and in the duo setting with the grand paino; in this particular concept the regular bass set sounds very very dull in comparison.

    I loved it, except for the fact that I knew I couldn't really turn it up for fear of blowing up the speaker with the open back and all that. However, I think the open back of the combo had a lot to do with why the sound was so open and alive.

    I wonder if the port of a ported cab works on the same pirnciple, except that the goal is different (livening up the low end) . In this case I am looking for a live solo bass guitar tone that can hang with acoustic instruments and speak its own voice clearly in between them . A chamber music concept for bass guitar, solo or in small ensembles, not unlike of the characteristics of an amplified plucked cello.

    Are there any ideas if&how one could approach designing a, say, 1x10 inch cab that has a port/slat/or opening that would help bring out those characteristics (full range, dynamics, directness, loud and clear harmonics, openness of a full range tone) without being in danger of blowing up the speaker?
     
  2. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    Open back or a half open back is a large shallow port, tuned kind of high, and flawed in that midrange comes out of it. Ports are tuned to a resonant frequency, you can sort of calculate it, but with open backed caps, other factors dominate, it means the speakers tend to be uncontrolled by pressure in the cab and thus more liable to break through over excursion, guitar speakers are heavily damped themselves to deal with this. A closed back, but large guitar cab works fine for toppy guitar frequency bass, just don't expect much bottom. I use a second cab for bottom and a different amp (rig is a guitar rig, and a bass rig).
     
  3. basscooker

    basscooker Commercial User

    Apr 11, 2010
    cincy ky
    Owner, ChopShopAmps
    The "problem" may be the GK. You didn't mention the model, but GK does tend to have a bit of a rock tone baked in. There are lots of options in bass gear that totally meet what you're after, so no worries. I agree that at low volumes, lots of guitar amps sound great with bass, possibly due to the speaker voicing, or their own baked in sounds. From what I've read, the crazy 8 or crazy 88 might just fit the bill for you, paired with a newer hifi type micro amp.
     
  4. IntrepidCellist

    IntrepidCellist

    Sep 10, 2009
    Manhattan
    I think you are more hearing the differing EQ points and frequency response of the guitar amp and speaker, plus a little bit of mild, mild overdrive, rather than just the open-back cab itself.
     
  5. two fingers

    two fingers Opinionated blowhard. But not mad about it. Gold Supporting Member

    Feb 7, 2005
    Eastern NC USA
    Or you could just go for a larger open back cab with more speaker. Get your volume from increased cone area rather than physics of the box. Say, an old school 4-10 or 4-12 open back Fender cab. Or if they aren't that common (maybe more in combos) then you could TURN an older 4-10 or 4-12 closed back into an open back. Heck, you could even make it convertible. The panel you cut out, finish the sides and put velcro on them so you could run either or.

    With many of the vintage Fender combos, there wasn't much difference between a bass head and guitar head, electronically (if any in some cases). So, somethign like an old Super Reverb or Bassman 10 might just sound amazing for bass. And if the backs are closed, cut the rear panel to open it up. Then (as stated before) put velcro on it to be able to run it open or closed back.
     
  6. fishdreams

    fishdreams Supporting Member

    Sep 4, 2010
    Brooklyn, NY
    Endorsing: Arkham Vacuum Tube Amplification
    Thanks Mr. Foxen and basscooker. The GK is a backline 600 if I recall correctly. Nothing spectecular but not bad either. My own micro head is the TH500 which might a bit too rock voiced for this.
     
  7. fishdreams

    fishdreams Supporting Member

    Sep 4, 2010
    Brooklyn, NY
    Endorsing: Arkham Vacuum Tube Amplification
    Thanks - absolutely no overdrive. I was able to keep the volume on the amp very low and it still sounded quite present and alive (as I mentioned the acoustics in the loft are very good). The tone just sat amazingly with the Steinway.

    My bass is passive with bartolini humbuckers, with d'addario chromes - and is by its self quite an alive and loud instrument, compared to, say, my jazz basses.
     
  8. fishdreams

    fishdreams Supporting Member

    Sep 4, 2010
    Brooklyn, NY
    Endorsing: Arkham Vacuum Tube Amplification
    cool idea - but with living in a 4th floor walkup, taking the train and mostly performing in small listening rooms I do prefer something small sized...
     
  9. will33

    will33

    May 22, 2006
    austin,tx
    You could think of an openback cab as a very highly tuned port (200-300hz??) with most of the sound escaping through the "port".

    It still amounts to an uncontrolled speaker who's rear wave is bouncing off the back wall, then mixing with the front wave and/or reverberating/reflecting around the room.
     
  10. fishdreams

    fishdreams Supporting Member

    Sep 4, 2010
    Brooklyn, NY
    Endorsing: Arkham Vacuum Tube Amplification
    That makes perfect sense - but would it be possible for this acoustic chamber music application to think of a cabinet as the soundbox of an acoustic instrument and the bass/speaker/amp combi as the plucked string/sound source?

    I mean: a cabinet that goes for maximunm resonance instead of maximum stiffnes, needing/requiring a tiny amp and speaker, I guess.
     
  11. will33

    will33

    May 22, 2006
    austin,tx
    It's possible at low enough volume....meaning whatever as long as the speakers don't go to flapping too far/distorting.

    If it sounds good...it is good, applies in all cases. If you were accompanying an acoustic piano being played tastefully in a nice room...it may've been "just right".
     
  12. DukeLeJeune

    DukeLeJeune rational romantic mystic cynical idealist Supporting Member Commercial User

    Nov 24, 2008
    Princeton, Texas
    Owner & designer, AudioKinesis; Auth. mfg, Big E (Home Audio only)
    I take the minority position that midrange coming out of the ports can be beneficial. And when I say "minority", well I can't think of any other speaker designers that really embrace this idea, so I may be in a minority of one.

    Four sources of coloration affect the midrange energy that can escape through a port:

    1. Port tube resonance (think of talking through a cardboard tube)

    2. Diffraction at the sharp edges of the port (less intuitive, but such diffraction can add harshness to the sound)

    3. Undamped resonances in the cab color the midrange energy that's present inside the cab

    4. Shorter-wavelength upper midrange energy doesn't round the bends as well and so is reduced in level relative to lower midrange energy

    That being said, the open-back guitar cab proves that, absent port-induced colorations, the midrange energy that comes off the back of the cone can be beneficial. In my opinion, and in my experience, flared ports positioned right behind the woofer cone offer us a window of opportunity:

    1. Flared ports have sort of a "fuzzy" effective length, and so their tube resonance isn't as pronounced.

    2. Flared ports eliminate diffraction.

    3. Flared ports positioned right behind the cone tend to funnel the midrange energy out before it gets significantly colored by inside-the-cab resonances

    4. And by positioning the flared ports right behind the cone, the shorter-wavelength upper midrange energy has a straight shot to the outside world.

    Here's what it looks like (obviously one of the cabs has some of its ports plugged, which is a different topic):

    gi.mpl?u=2112&f=ThreeCabsRear.

    The midrange energy that comes out of the ports sounds decent, but it is a) reduced in level relative to the midrange energy off the front of the cone; and b) would need some additional high frequency energy to approximate the full-spectrum energy we get from the front of the cab (which has a horn). So in the cabs you see, there is a rear-firing tweeter, which is not as loud as the front-firing horn, such that it matches the SPL of the midrange energy that we get through the ports. I call this addition of a reduced-level rear-firing tweeter the "acoustic friendly" version of my cabs.

    The net result is still not as open-sounding as an open-back guitar cab, but it's a step in that direction. There are other, more aggressive techniques that more closely approximate the effect of an open-back guitar cab, which I have experimented with but have not yet gone into production with.

    But I do like the idea of a bass cab that has the same sort of "vibe" as a good open-back guitar cab.
     
  13. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    Right behind the cone means magnet is there, does that act kind of like extending the port into a right angle, but all the way round.
     
  14. DukeLeJeune

    DukeLeJeune rational romantic mystic cynical idealist Supporting Member Commercial User

    Nov 24, 2008
    Princeton, Texas
    Owner & designer, AudioKinesis; Auth. mfg, Big E (Home Audio only)
    The magnet is nearby, but not directly blocking the line of fire from cone to port - see the photo, and imagine a relatively small-diameter magnet (neodymium).

    The proximity of the magnet may indeed have an effect on the port tuning frequency, as the measured tuning frequency is about 5% Hz lower than the calculated tuning frequency. In this case the effect is beneficial, reducing the required port length a bit. I haven't had any reports of chuffing even with half of the ports plugged (to reduce the tuning frequency) so I think the inner flares are effective even with the magnet nearby.
     
  15. DukeLeJeune

    DukeLeJeune rational romantic mystic cynical idealist Supporting Member Commercial User

    Nov 24, 2008
    Princeton, Texas
    Owner & designer, AudioKinesis; Auth. mfg, Big E (Home Audio only)
    As long as the spectrum of that extra energy bouncing off the rear walls supports, rather than degrades, what's coming off the front of the cab, it can be quite beneficial. Done right, the extra reverberant energy takes us that much closer to approximating the behavior of an actual acoustic instrument... in my opinion, anyway.

    Over in the high-end home audio world, there's a lot of love for fullrange dipole speakers, which put out half of their energy to the rear. Unfortunately their size and inefficiency at generating bass energy make fullrange dipoles impractical for bass cabs, but open-back guitar cabs are a variation on the theme of sorts.
     
  16. fishdreams

    fishdreams Supporting Member

    Sep 4, 2010
    Brooklyn, NY
    Endorsing: Arkham Vacuum Tube Amplification
    wow so many great reactions. Mr DukeLeJeune thank you so much for going with the idea.

    As this is really chamber music I am thinking, something should be possible with a small super clean tube amp (10W or so) and a matching cab ported out of the back.
     
  17. DukeLeJeune

    DukeLeJeune rational romantic mystic cynical idealist Supporting Member Commercial User

    Nov 24, 2008
    Princeton, Texas
    Owner & designer, AudioKinesis; Auth. mfg, Big E (Home Audio only)
    With that kind of modest power handling requirement, an open-back bass cab may be possible. I'm thinking of an open-back cab maybe 28" wide by 24" tall by 10" deep, sporting a single Eminence Alpha 15A woofer. Offset the woofer toward the bottom of the front baffle, down close to the floor, as this a) maximizes boundary reinforcement from the floor, and b) imposes the longest wrap-around path length, which we want to be as long as is reasonably feasible in the interest of getting good low end out of the cab. The shorter the wrap-around path length from front of woofer to back of woofer, the less low end we get. The Alpha 15A's weak magnet gives it characteristics that help it to theoretically put out some low end in a large enough open-back cab.

    I've never built anything like this and don't seriously expect you to try it, but that's how I think it could be done. If you think you might actually build it, shoot me an e-mail and I'll calculate suggested dimensions for the opening in the back panel.
     
  18. Arjank

    Arjank

    Oct 9, 2007
    Above Amsterdam
    +1

    Back in my weird experimenting days I ones made an open mid cabinet with two 6" paper mids to sit on top the 115 cabs I had.
    Both drivers were also horizontally aligned and each was pointing in a different direction.
    Very spacious sound, the bass sounded way more three dimensional that way. The only thing that such a system looses is focus/accuracy. It's no way it could be compared with my 210 with coaxial mid/high in that respect. The 210 has very good dispersion to compensate for the 3D sound.
     
  19. will33

    will33

    May 22, 2006
    austin,tx
    I would agree, the rear wave/reverberation thing is not necessarily a bad thing in all cases. It sure sounds nice with plenty of guitars. In a low power/volume demand situation where you don't have to worry about "controlling" cone excursion, I could indeed envision it behaving, like the OP hears it....more as an actual acoustic instrument...or as part of, or an extension of the instrument.......as opposed to an "amplification system" designed to make a tiny little twang over a pickup turn into something really loud.
     
  20. seamonkey

    seamonkey

    Aug 6, 2004
    You could add a active crossover
    It can go into the effects loop of the guitar amp or bass amp.
    Send the lows to the bass amp, and the mids/highs to the guitar amp.
    You'll be able to safely play "bass" at levels you can with your bass amp.
    The mids/highs will still retain of the artifacts you've noticed from the guitar amp.

    Open back just doesn't scale well. A lot depends on reflected sound in the room. It's not very controllable.

    There's really no perfect single driver. It's best to break them up into separate frequency bands with a crossover. Greenboy designs are a prime example.
    With the right EQ you'll get characteristics of what you're hearing with the guitar amp, only much more controllable, and working at different volume levels. Less amplifiers to carry around. Not all multiway cabinets have this ability, so be careful with the hype.
     

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