Tuning cabinets lower than speaker Fs...

Discussion in 'Amps and Cabs [BG]' started by Magman, Sep 26, 2004.

  1. Is this an effective way of protecting speakers from over excursion? My current research and going over the archives here suggests....yes.

    In winISD, simulating this shows that bass response suffers and group delay goes up, but if you are throwing a lot of say 30-50 hz sound through a 210 enclosure tuned to 60hz, then maybe this trade off is acceptable?

    Even if the Fs of the driver is 60hz, if the cab is tuned to 40, there will be minimal driver movement with a 40hz test tone (correct?) whereas if the cab is tuned to 60hz then the driver will be flapping around with nothing to 'hold it back'.

    I realise that the best solution is to have a cab tuned lower to begin with, with appropriate drivers.

    Just trying to understand cab tuning better.

  2. The Clap

    The Clap

    Jan 5, 2004
    Scottsdale, AZ
    I'd like to hear more informed opinions on this, but what you're describing sounds correct mostly. I think it would be better to defeat the 40-60Hz signal with a crossover or EQ rather than lower porting, both for issues of 'tightness' (Group delay), and overall bass delivery efficiency. A 12db second order lowpass crossover at 60Hz would already halve the power at 60Hz (3db) and cut signal by a factor of ~15 at 31Hz. That's a good reduction in excursion at those frequencies.
  3. Yes, I believe you are correct.
    Thing is, I havn;t actually seen any 60hz high pass filters/cross overs for sale in the parts online stores - otherwise simple solution would be, as you say, is to stick a highpass filter in the cab. I suppose you can make them from electical components yourself, but havn;t got as far as to research how to do that yet. The high pass filters I have seen are all above 100hz, which is a bit too high if your talking about midbass 10's.

  4. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Tuning too low will rob sensitivity in the passband and cause you to put more power into the box, which could be counterproductive. If you're concerned about overexcursion, which is quite audible anyway and is not a 'silent enemy', use a high slope (18dB/octave minimum) high pass filter at the box frequency, or simply go to a sealed box. As for how much power you're driving with at 30-50 Hz, a lot less than you think, on average 20dB less than at 80-100 Hz.
  5. I think that you lose power handling above the tuning frequency as you drop it down. There is always a catch when tuning cabs and you always have to respect the drivers capabilities. You really can't get it to do more than it was designed for.
  6. IvanMike

    IvanMike Player Characters fear me... Staff Member Supporting Member

    Nov 10, 2002
    Middletown CT, USA
    lately i've been using a dbx 262 compressor between my preamp and poweramp
    usually when it gets activated the low end hits the threshold 1st. This seems to help tremendously.
  7. Petebass


    Dec 22, 2002
    QLD Australia
    It also raises the -3dB point significantly, robbing the cab of usable bottom end. You conpensate by adding bottom's via EQ and guess what, you've just reintroduced the excursion problem.

    The problem with 18dB filters at such low frequencies if that the inductors required are often large, heavy, and expensive. Don't be tempted to overcome this by using inductors with a metal core. They're smaller, but they're only good for 100w or so before they distort.

    I've had some success in the past with 12dB and even 6dB filters. Of course, the shallower the slope, the higher your cutt-off frequency has to be. With some experimentation, you can achieve a good balance between reducing speaker movement without robbing the speaker of audible bottom end.
  8. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Sorry not to be specific- passive filters below 100 Hz are way to expensive and for the price of quality components you can buy a decent 31 band or parametric EQ, which is what I should have said. Better yet, spend $200 and build a real
    sub like this one:

  9. I have also played with WinISD, and I like the way it works, but I also use 'Perfect Box' ( downloaded from B Gavin's site I think), because it plots power handling as well as dBs out.
    My recollection was that if you tune a cab too low, you get a dip in the power handling at about the speaker resonance.
    BTW Perfect Box is a Dos based program, so it may not run under WinXP.
  10. Petebass


    Dec 22, 2002
    QLD Australia
    This is true, you do get a dip in the power handling, meaning the speaker requires less power to hit the limits of it's suspension. The same thing happens when you make the cab too big. In the case of using a lower Fb, it also reduces the frequency at which this will occur, but it is usually somewhere above the Fb, meaning it will probably be a problem.

    For the record, the new version of WinISD does power handling graphs. It's in Alpha testing stage but it seems to work well enough.

    Do you have a specific speaker in mind, or are we just talking hypothetically at this stage?
  11. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    No. What happens with a properly tuned box is that at the Fb the driver is operating quite close to its Fs, where it normally is working very efficiently with a very small amount of excursion. One way to see this is by using a sweep generator; at the Fb the driver cone is barely moving. If you move the Fb far enough either above or below the Fs you lose this natural efficiency and response around Fs is drastically compromised. In other words the scenario is the opposite of what you postulated.

    When properly tuned the response of a sealed or TL box drops at 12dB/octave, while with a ported box the phase of the port output shifts below Fb and that results in a 24dB octave SPL drop, so if you must run below Fb make sure it's with a sealed or TL box.

    I still wouldn't sweat the 30-50 Hz requirements, but if that is problematic then a true sub is the solution.
  12. There is another issue with extended bass shelf tuning (EBS), which is what you are asking about.


    An EBS system is really sloppy because group delay time increases significantly approaching Fb.

    All the reasons mentioned above are good reason to use active crossovers and bi-amping.
  13. I'm quite tempted to attempt to build this, now I have the saws&clamps and a bit more confidence after some domestic DIY work.... funnily enough I came across your site Bill while researching horn loaded enclosures and asking myself "why arn't more people using this approach" - and now you post on this thread.

    Interesting that one of the most efficient speaker design approaches goes back into the 1940's....


  14. Jim Ingraham

    Jim Ingraham

    Nov 14, 2002
    I want to chime in here.. i built one of Bills Tubas a few weeks ago and finally got to try it out at an outdoor gig over the weekend. All i can say is WOW.... this thing is the real deal! We used it for PA support powered by just one side of my plx 1602. there seemed to be no end to the amount of bottom i could dial in to our sound. it wasn't boomy bottom either but full, rich, gut churning clean lows.
    I was going to build a pair of these but i really think the one will be sufficient for the size places we play. Instead i ordered plans for DR250a and plan on building a pair of these to replace my EV mains, which im going to gut and rebuild into monitors. thanks for putting these great plans out there Bill! :D
  15. How was construction? I have a trim-saw, mitre saw, jigsaw, cabinet clamps and would hope that would do me. Would probably get some nicer pine for the outer cabinet and give it a stained oil and wax finish: sticking on carpet I fear the most as I'm really bad at wrapping xmas gifts ;-), but staining etc I've done before and like the look of.


  16. Jim Ingraham

    Jim Ingraham

    Nov 14, 2002
    Well, for me construction was not hard at all, but... Im a cabinet builder with a well equipped shop. I think the most important tool to have is a good table saw with a 24" min capacity. If you can get nice straight, consistent rips, this cab goes very easily. All your width rips must be done at the same setting. If you dont have access to a big table saw, most of the big home centers will rip plywood for you.
    Other than that a hole saw kit comes in very handy as does a screw gun. It takes just over one sheet of 1/2" ply.
    Hers a pic of the Tuba about half way thru the process:
    As you can see its a little more complicated on the inside than a traditional direct radiator box. Bills plans are easy to follow however and he provides good diagrams and photos of the process.
    as far as finish goes, i dont like carpeting boxes either (although I have). So in this case i sprayed it black. If you got some nice looking birch or oak ply, I imagine that would look really sharp. Good luck :D
  17. Ericman197


    Feb 23, 2004
    Are any of Bill's cabs available finished? Does anyone build them and sell them? I'd be interested in experimenting with one of them for various purposes, but I have no building skills whatsoever.
  18. Yes, there are a lot of holes! I'll investigate hole saw kits...

    Thanks for the pic, this gives a great idea of what the project is like. Like a sea shell.

    so as regards 'ripping'... you mean using handheld circular saws and cutting against a clamped guide could be problematic?

    Tablesaws have always seem to have been the devices next to woodshop technicians put up the nasty mangled thumb pictures. :eek:

  19. The tuba has an air column length of some 13 feet, 9 inches. Frequency analysis of the tuba response shows the highest output at the 3rd, 4th and 6th harmonics. The 3rd harmonic and higher wavelengths are entirely encompassed within those "60 year old formulas and pre-conceptions." The tuba is working as designed, and operating above the calculated cutoff point for this air column length. BTW, the formulas for shaping the violin were devised hundreds of years ago, and those rules still stand today.

    Consistent with your logic stated above, commercial bass cabs are not flat below 80Hz because they don't need to be. Manufacturers continue to stuff large Vas drivers into a small box and get that big mid-bass hump in the 80 ~ 120 Hz range, which makes them saleable. As you state above, your design operates from 80 Hz and up. This is the lowest frequency where the concentration of energy from a typical Fender bass begins.

    Please understand I'm not in any way disputing your design, or how efficiently it operates. My heartburn is with the self-annointed Galileo who claims or infers a horn operating flat to 30 or 40 Hz in a small box. It just doesn't happen. The proof is in the response plots, which they will never publish.
  20. Analzing DIed input of B input from my bass guitars on the computer, there really isn't very much fundamental output. Its no organ.

    heres a review of a rather esoteric down tuned very bass heavy band:

    A friend of mine saw them locally, and said the whole place was shaking "like being a in spaceship or something - man, you should have been there, it was like a religious event".
    (An idiotic event to some I'm sure :)

    Yet - I get the impression they are using 'standard' amp stacks which probably don't go down much further than 50hz. I could be wrong, but this suggests to me to shake the place to bits and come across as 'really really heavy and bassy', with the floor buckling you don't need good sub 40-50hz response in your enclosures.
    They tune their guitars to Aflat I believe.

    Perhaps listeners are not used to really deep bass except in some dance clubs or at a church with a big organ.
    The aethetics of those low tones are perhaps rather undiscovered.