cab unloading

Discussion in 'Amps and Cabs [BG]' started by mikemulcahy, Nov 8, 2001.

  1. mikemulcahy


    Jun 13, 2000
    The Abyss
    Bruce (bgavin) once mentioned that a cab "unloaded" at a certain frequency. What exactly is that and how does it relate to cab/speaker size?

  2. Loading is the air cushion applied to the driver cone. With a speaker cabinet, the air inside the cabinet provides an additional stiffness (load) to the rear of the speaker cone. This effects some control over the motion of the cone and prevents it from flopping wildly as it would do in free air or with an open back cabinet.

    A sealed (non-resonant) cabinet provides a constant load where a resonant cabinet such as ported, labyrinth, bandpass, etc provides a variable load on the speaker cone. In a ported cabinet, the maximum loading is applied at Fob, the box tuning frequency. At this frequency, the cone is critically damped and has almost no movement. Sound radiation at this frequency is almost entirely from the port.

    Below the tuning frequency, the box behaves as if it were an open back cabinet and the acoustic loading falls away very rapidly. The driver is then "unloaded" (no acoustic loading) and subject to physical damage the same as operating in open air. All ported box designers acknowledge the cabinet should not be used below the tuning frequency for this reason.

    Guitar players don't have loading problems with their open back cabinets because their lowest frequency is so much higher than those of a bass. Cone movement increases 4x with every 1 octave lower reproduction. A driver moving .125" at middle E has to move .500" at low E to maintain the same volume.
  3. mikemulcahy


    Jun 13, 2000
    The Abyss
    Most informative as always, thanks Bruce.

  4. leper


    Jun 21, 2001 an overclocker?

    i see that lapping directory on your page and it makes me feel like im back at my other home
  5. [ Off Topic ]

    No, not at all. OC'ing isn't for me, as I build business machines for my clients, and couldn't stand all the grief and support issues associated with OC'ing.

    But... there is a lot to be learned from these guys. I'm now running on a 1.4 GHz T-Bird cooled with an Alpha PAL8045 + Panaflo H1A. Had to modify the HSF by cutting off a corner so it would fit my EpoX 8KTA3Pro board.
  6. leper


    Jun 21, 2001
    bruce you big sissy just go for it :p

    just kidding, but i had to ask, as i thought i might just have found nirvana: the mixing of bass and computers


  7. bgavin, that's great info! Is there some way
    to calculate the tuning frequency of a cabinet?
    I play a 5 string through a Hartke ported 4x10
    and my volume does drop off in the lower register.
    I don't want to damage my cab.

  8. 1) Digital volt meter
    2) 100 ohm 25 watt resistor
    3) Computer with sound card
    4) Tone generator (get one on my site)
    5) Power amp

    Put the 100 ohm resistor in series between the power amp and the cabinet. Hook the DVOM across the speakers, or between the speakers and the resistor. You want to measure the voltage across the speakers, not across the resistor.

    Install the signal generator and have it drive your sound card. Hook the sound card Line Out to the power amp Line In. Loudness is not necessary to get a good measurement.

    Measure and record the A/C voltage at a series of frequencies from 31 Hz and higher. Do enough until you see a pattern like a smiley face. The lowest A/C voltage is the tuning frequency. If you measure a wide enough range, you will see a voltage peak both lower and higher than the tuning frequency. This is nice to know also, but mostly tourist information.

    Note the cone movement at the lowest voltage reading. It should be nearly motionless. If not, you have air leaks. The most cone movement will be about half an octave up from the tuning frequency. It would be interesting to see if this coincides with the upper voltage peak. Again, more tourist information.
  9. I have an analog volt meter. Is that not accurate enough?
  10. No.

    The voltages across the speaker will be well under 1 volt, so accurate digital measurement is required.

    The purpose of the 100 ohm series resistor is to keep the amplifier as a constant-voltage output. The resistor will make the circuit into a 12:1 voltage divider with an 8 ohm speaker, and most of the load is across the resistor itself. Since most of the load is purely resistive, the voltage will remain constant. There is probably a voltage variation in the sound card output as well, but correcting for that is getting pretty anal.

    The setup above with a DVOM works very well for me.
  11. Hey Bgavin...
    I went to test the tuning on my cabs, just for fun, and downloaded the sweepgen.exe , but it isn't working. I hit start, no sound. I can pause/unpause, but it freezes if I try to hit stop. I'm running WIN 2K pro on an HP omnibook XE3 - any ideas?
  12. Ty McNeely

    Ty McNeely

    Mar 27, 2000

    HP's-The source of every problem:D
  13. Wxp4759cb


    Nov 23, 2000
    Columbia, MO
    Windows :eek:
    thats your problem!
    get Linux!!!
  14. Engineering/test tone

    This is the tone generator I use all the time. I use it in single frequency mode, and manually step it through a range. This lets it have enough time for the voltage and DVOM to stabilize.

    Call Bill Gates at home and tell him your sh*t doesn't work with his XP!


    Sorry, a bit off off-topic amusement. There is a BOATLOAD of stuff that doesn't work under XP. MS has a very long history of early-release grief with every product they produce, so I don't expect XP to be any different. What I don't understand is, why do folks always jump on the MS bandwagon right away? One should always wait 6 months or until Service Pack 1 + Hotfixes X,Y, & Z before putting a new MS product into service.

    As for H-P, Auntie Carly has boned that company as hard as she can. They will fire her after the Compaq thing flops, and just as well. Morale out at H-P (Roseville, my area) is in the sh*tter, the product quality sux hard, and the support is just awful. They seem to be turning their backs on their printer business, which is stupid, cuz their computer products are awful. Printers make them money. Now, they're piddling away their cycles putting web servers into printers so they can order their own supplies. Printer drivers suck and crash, but the machine can order its own toner.

  15. Thanks bgavin! that one works!
  16. Ty McNeely

    Ty McNeely

    Mar 27, 2000

    Well, all I have to say is that HP will never make money off of me again. We got a DeskJet 870C when we first bought our computer. Gave us problems from the get-go. It finally just completed screwed up, so we sent it in and they sent us a new one, since the old one was still under warranty. Well, the new one is crap too. Paper doesn't feed into it no matter how hard we try. We end up having to manually feed about every 2nd piece into the stupid thing. So, I've vowed to never buy another HP product.

    *Edit: Sorry ya'll, I know this is really bad.....a moderator takin' things WAAAYYY off topic, but thats ok:D
  17. MikeyD


    Sep 9, 2000
    That makes (at least) two of us! I had the same experience with an OfficeJet 600 I bought a few years ago. I got so frustrated with it - and the fact that HP wouldn't support it for Win NT - I finally picked it up and threw it on the floor. It exploded into hundreds of pieces! It was a relief to have that thing out of my life. I wrote to HP asking for my money back, but they wouldn't hear of it. I will never buy any of their products again. I have a '95 HP Laserjet 5P which I absolutely love, but this company has turned sour, IMO, so I will look elsewhere for computer products from now on.
    - Mike
  18. [ off topic ]

    Wow! But ain't that satisfying?

    ScanJet 5370: Put (3) into a client site until I got one that actually worked out of the box.

    DJ952c: My personal printer: mangles envelopes 100% of the time. They have a moving guide that drops down into place during envelope printing to make sure the envelope hits the guide and jams.

    Netserver: South American client. Contacted HP tech support, needed special (read: proprietary) part for their down Netserver. Arranged to buy a seat on an airplane, hand delivery and expedited customs inspection. HP sent the wrong part.

    DeskScan: Didn't work when Win98 came out. Web site sez "you have to buy the upgrade" so I paid $29.95 for the damn upgrade.

    JetAdmin: No longer available on the web site. You must pay for upgrades (read: bug fixes)

    LJ8000: (3) major releases required before it would work correctly with WordPerfect (the law office staple). Attempt to install this working version on Win2000, it exits without any kind of message. Hmmm... must not work, would be nice to get a message such as "Jerk, this ain't for Win2000".

    But.. HP is merrily coding away to put automated web servers inside their printers so they can communicate with the soda machine and the goodies machine and collaborate to place their own supplies order.

    Methinks HP seriously has their priorities all wrong. Jill Barad = Mattel, SNAFU. Carly Fiorina = HP, SNAFU. Perhaps both would be better off taking it all off and posing for Hustler. They'd make more money that way...