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Need port calculation.

Discussion in 'Amps and Cabs [BG]' started by kurosawa, Nov 9, 2012.

  1. 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)
    If you tune to low-E, and know the cab will never be asked to handle a low-B at high power, then I suggest a 54 Hz tuning frequency. Three inch internal diameter port, 5" long. If you use a flared port, make it 6" long. A flared port will give you a little bit more low-end impact and is definitely not going to chuff. I don't think a straight-edged port is likely to chuff either in this application, but your safety margin is reduced.

    If you tune to low-B (or like a tighter low end), then I suggest a 48 Hz tuning frequency. A 3" internal diameter port, 7" long, will get you there. If you use a flared port, make it 8" overall length.

    Neither of these are "absolutes". Choosing a tuning frequency is a juggling of tradeoffs, so if there is a particular low-end characteristic that you're looking for, let me know. Also, if you will definitely be using a protective high-pass filter, let me know. As a "pro" I can't assume that a protective high-pass filter will be used, so I have to juggle "low end power and authority" against "fartout resistance".

    Your providing us with accurate internal dimensions along with a link to the spec sheet is payment enough. Thank you for taking the time to do so.
  2. Mike Arnopol

    Mike Arnopol Supporting Member Commercial User

    Jan 4, 2005
    Owner of MAS Soundworks
    I like the 310 better than the 300 Better QTS.
  3. Wow, thanks for the port info, Duke! All I have to do is cover over these horn holes and put 'em in. I do have the fdeck v3 filter. Might need to play the 5 through the cabs, never know.

    Yeah, Mike, this might be my most economical out with some assurance it'll sound good. I could always get the Ashdown VS115 and stick the 15PR400 in it but that's a crapshoot.

    I trust the tiny size of the cabs will keep the MDF from returning to its component sawdust particles.
  4. 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)
    In that case, I suggest a 51 Hz ballpark tuning, which you'd get with a 3" diameter straight port 6" long, or a 3" diameter flared port 7" long. By "flared", I mean flared on both ends. The inside flare is actually the more important one.
  5. I was cruising the speakers at US Speaker this morning when suddenly it hit me like a F-16 sucking a goose: I know what the magic sound of bass is. Really. The Money Sound (TM, pat. pend.)

    I just don't know how to measure or replicate it.

    The years I was nailed to the radio, bass had a specific shape. It wasn't, as I'd previously thought, a P bass with stainless flats and foam rubber in the ashtray through a B15N (I mean, we put that B15N thing to rest long ago with the Motown direct box, right?)

    No, THE precise sound of bass was a P bass with stainless flats and foam rubber in the ashtray through an AM radio through Detroit EQ through a 6x9 under the dash of a 1967 Falcon. Somewhere under the dash, I guess. I never looked.

    Youngsters are gonna have to take it on faith, but this country was united, once upon a time, even in the midst of a war it was divided over, and it was Top 40 AM radio that did it. Everyone didn't just know what everyone else was listening to, EVERYONE KNEW THE FREAKIN' WORDS to "everyone else's" music.

    Hendrix, Four Seasons, Roger Miller, Stones, Sly, Johnny Cash, Byrds, Four Tops, Beatles, and on and on, all known to everyone. Some social researcher very perceptively said this society fell apart when it stopped singing together.

    Really! Every Christmas party was a sing-along. Everyone knew the Anthem, whenever it was sung it was a stadium full of singing. Church. Birthdays. Have you read the articles lately about how people can't sell pianos, they're filling dumps with them these days? I read those articles from the US and UK. Why? Only one reason people bought pianos, for the sing-along, and now that's dead.

    And the AM car radio, that was the other sing-along device. The real one, the one that counted for teens because it was where life happened, in your car. Or at the job site. In the workplace.

    And young cabinet designers can't for the life of 'em figure out why old farts like me have NO use, in fact negative use, for anything above 5kHz. Where the hell do you think AM radio cuts out? I mean the absolute limit, no broadcasting possible above this frequency?

    Yah, now back at the beginning of this post, why did I say I know a particular bass sound that is The Money Sound? Because, all you youngsters know this already but you never put 2 and 2 together, because OLD FARTS HAVE ALL THE MONEY.

    Now you're gonna argue with me on that one. Well there's a little money in this or that micro-fragment of the music market because that's all there is that's left of Music, that once-upon-a-time thing that existed before you. Yeah sure, there's young money scattered over a hundred markets. But all the old farts are sitting with their money in ONE place.

    So now my big research question. We already know what the magic sound of bass ISN'T. It isn't anything over 5kHz. But what's the bottom look like? See, this is the crucial question for bass players. We know it rolled off somewhere higher than the fundamentals of our lower octave, maybe even our lower two. Now right there, things are way different.

    And the bass, such as it was, was boosted a BUNCH. So we need some intrepid scientist to figure out how recording engineers EQ'd the Fender bass back then, then how AM altered that, then how the radio itself altered that, then how the original Jensen 6x9 filtered THAT.

    I will say that if ALL that can be replicated without adding to or subtracting from it, then I will have a bass sound that NOBODY will find emotionally lacking, that is to say BUTTS WILL TWITCH.

    Could it be put in a stomp box? Maybe. But what speaker and amp are you running into, see the problem?

    No, all I want to know is how all that EQ adds up. Just that profile. Then I or anyone else here can figure out the rest.
  6. 1958Bassman


    Oct 20, 2007
    GOOD MDF won't be affected by the vibrations but the cheap stuff from a big box store can. Bad glue joints can be destroyed, too.

    Does your amp have a high pass filter? Use it- the Fs of this driver is a bit on the high side and with a small box, it's easy to send signal below the port tuning frequency, especially when playing double stops. I suspect double stops is the reason I read so many threads about blown speakers- Not only does the instrument produce the two notes, it also produces sum and difference frequencies- If you play the E and A strings at the same time, you're sending 55 Hz and 41.2 Hz fundamentals through the amp and speaker. The sum is 96.2 Hz, which isn't a problem. The difference frequency is 13.8 Hz and you'll never hear it, but it can kill your speaker(s).
  7. Yeah, I had a DS with bad caps and playing pedal steel, it was weird hearing the sum and difference freqs moving around as I squeezed the pedals, but I heard what you're saying with my own ears. Thanks for explaining! Well really I guess I should set the adjustable freq of the HPF to the lowest fundamental of a 4 or 5 string, whichever I'm playing? I mean that stuff does count, I read somewhere that a double bass produces only 1/4 acoustic watt of fundamental in its lowest octave, the box is too small even on 4/4 basses, but still somehow I think we sense that.
  8. Thanks, well the cab being only 11" deep on the inside, would that put the inner end of the tube too close to the back wall?
  9. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    I suggest putting the port in the side of the cab, or in whichever wall gives you the most inside clearance.
  10. OK thanks, is there anything wrong with having it point at the cone from inside?
  11. Yes! You do not want midrange to be able to come out of the port directly. It will cause phasing issues.
  12. Aha. Thank you.
  13. 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 think most people would agree with you, but that's an area where there is some difference of opinion.

    Well, okay, there's my opinion and then there's pretty much everyone else's... but I like having some midrange energy come out my rear-facing ports. I also think the midrange energy that comes out a flared port sounds better than the midrange energy that comes out a straight-sided port (if anyone is interested, I'll explain why I think that is the case).

    In practice, the midrange energy that emerges from the port is a lot lower in SPL than what comes off the front of the cone, so it's probably not going to either harm or help very much anyway.

    Imo, ime, ymmv, etc.