cabinet design - story and questions

Discussion in 'Amps and Cabs [BG]' started by Matthias, Aug 3, 2001.

  1. Matthias


    May 30, 2000
    Vienna, Austria
    I’m sorry that this one’s a bit long, but here we go:

    About 10 years ago I built a 1x12“ cab (200W/8Ohms driver in a small box, ported design) not knowing anything about cab calculation and design.
    It was powered by a Hartke 2000 (120W@8Ohms ) in the first years, now I use it as an extension cab for my Gallien-Krueger 150MB 112 combo (seeing max. 75W, as the G-K is rated 150W@4Ohms).
    The result: a fairly loud and bright sounding cab with a kind of built in low mid boost and more than enough low end for small venues and rehearsals, also a great stage monitor. And I never had any problems with ‚farting‘ although I play a 5 string bass...
    In fact I’m so happy with my '2x12 mini stack' that I’m considering a 2x12 cab for my main amp too.

    But man, reading all those great threads on cab design, I investigated the T/S parameters and did some calculations (using WinISD and the formula posted by Joris).
    The result is disastrous:
    Celestion K12-200e (turned out to be a PA driver...)
    Fs=64.7, Vas=80.7, Qts=0.651, Qes=0.694, „average sensitivity“ 98dB (magnet weight: 36oz., voice coil dia.: 2.5“, xmax not specified, comparable Celestion drivers have 2.35mm/0.09“!?)
    cab: Vb=17.5l/0.62 cu.ft., 2 ports with dia=5.3cm/2.09“, length=9cm/3.54“
    this results in Fb=77Hz, F3=105Hz and a 6db peak at around 170Hz....
    Now I have 3 questions:

    1. I hear you should never use a cab below Fb, because port unloading can lead to excessive driver excursion which can destroy the driver. Why did I never have any problems? Maybe because I always used „underpowered“ amps which I never run at max. power? Do you think the speaker will pop out when I hook up my H&K (260W@8Ohms) and push that baby a little? :D

    2. After removing excessive damping material, Vb is now 26l/0.92cu.ft. and now I want to improve the specs a little without changing anything but the ports. According to WinISD best results are achieved with Fb between 45 and 60Hz, this gives me a slight LF boost below approx. 50Hz compared to sealed design.
    Do you recommend ported or sealed design for this particular driver? (Avoiding port unloading!?)

    3. Is ported design louder than sealed design?
    What I mean is: If both cabs have the same frequency response, will the ported version be louder?
    Thank you for your patience to read all this!

  2. MikeyD


    Sep 9, 2000
    Hi - I don't have time to address your whole post today, and you'll probably get some better ideas from others. I have never blown a speaker from this phenomenon. I haven't worked out the math yet, but I believe that unloading of the speaker diaphragm is a gradual process whose rate is probably dependent on the system's damping ratio. So it isn't as black&white as "don't ever go below Fb". As you start to go below Fb, you have to be ever more careful. It may be possible, for example, to destroy a 500-watt woofer with a 100-watt amp if you max. it on a 10-Hz. signal. I have a lot of speakers that have Fb well above 31 Hz. of my 5 string, and they do not fail. However, when I crank up the volume and bass EQ, I do pay a lot of attention to what the cones are doing. If they look/sound like they are reaching their limits, I back off.

    A couple of years ago I did some experiments with a pair of JBL stereo speakers (15" woofers), using a synthesizer. I ran the frequency WAY down to see what they would sound/feel like. It also gave me an experiential understanding of psychoacoustics - how we hear extreme lows. I ran this all the way down to about 4 Hz.(!) The cones were moving a lot, but I made sure the amp output was such that excursion limits weren't reached. If you pay attention, you can avoid the problem as I have.

    I may be confused about your post relative to damping material. This stuff tends to increase the effective volume of the enclosure because the speed of sound through it is reduced.

    Ported enclosures whose frequency response is similar to that of closed boxes are typically more efficient.

    - Mike
  3. "Why" is a philosophy question, and beyond the scope of this class. :D

    Ported enclosures roll off at 36 dB/octave below the tuning frequency, Fob. As it rolls off at this steep rate, the cone acoustic loading decreases. In English this means the cone performs the same is if it were in open air, hanging from a coat hanger off your closet door. You didn't blow it up because you didin't apply enough power to it.

    Damping material added to the box makes the apparent volume LARGER from the driver's point of view. It is theoretically possible in our nitrogen/oxygen based atmosphere to increase the apparent volume by 40%. Typically an apparent increase of 20% is realized.

    My recommendation is to understand your driver, then use it within the constraints of its limitations. The driver parameters will dictate its use. If this is unsatisfactory, get another driver. These days, it is MUCH easier to scope out the driver using T/S data and apply it to your needs, than trying to retro fit a driver that doesn't meet your needs.

    IMO, this driver is better suited as a door stop because the specs are unacceptable for a bass driver.


    Ported cabinets have three data points to understand. Fol= low resonance, Fob=port tuning frequency, and Foh=high resonance. All three data points can be identified with a tone generator and DVOM.

    Fol: occurs below the tuning frequency. No useful port radiation. Cabinet air volume acts like a solid connecting rod: speaker moves in, air in the port moves out and the phasing is 180 degrees from the cone. Sound is additive until approximately 120 degrees anti-phase, so it is not an immediate loss below Fob. This introduces non-linear distoration and tends to cancel output from the cone. There is no acoustic loading on the cone here.

    Fob: port tuning frequency. Nearly all the sound radiation comes from the port. The cone itself is at a virtual stand still and the port radiation is in phase with the cone. This is when the box resonates like a Coke bottle when you blow across the open neck. Same exact principle. The impedance presented to the amplifier is at its lowest at Fob.

    Foh: upper resonance. Depending on the parameters, output at Foh can be small or large, and is in phase with the cone. A poorly designed enclosure (read: too small) will have a big output at Foh which is heard as a "boom".

    It is possible to put a big driver in a teenie cabinet, and tune it with a long duct. When HiFi ported cabs first came out, they sounded AWFUL because of this. What happens is, as the cabinet volume decreases, port output at Fob decreases, and output at the higher Foh increases dramatically. This occurs in the mid-bass range, and gives a real nasty boom. I suspect many of the commercial cabs that stuff an 18" driver into a small cabinet have engineered them where the "boom" is preceived by the unwashed as "solid" bass.
  4. Matthias


    May 30, 2000
    Vienna, Austria
    Mike, Bruce, thanks a lot for your detailed replies!
    Ahh, I didn't know that!
    Is there a way to estimate or calculate the influence of damping material? Otherwise all those calculations would be useless...
    I know your point of view on that topic, but for my needs, this driver is ok. :)

  5. Calculate for the empty box without damping material. All the design formulas work this way. If you want to add damping, put on 5 of 6 sides, leaving the speaker board uncovered. One layer is fine, don't worry about adjusting the volume. Making a cabinet acoustically larger with damping material doesn't hurt your calculations or performance. You'd have to completly stuff the cabinet to realize the 20% gain.

    Be sure to NOT obstruct the port in any way with damping material. The port should always be >= 1 diameter away from any wall (or damping material).

    For a bass cab, damping is a waste of time IMO. It's only valid use is cranking that extra 20% of volume out of a too-small cab.
  6. Matthias


    May 30, 2000
    Vienna, Austria
    Thanks, Bruce.
    One last question:
    What should I use as damping material? I know that some kind of woolen stuff (don't know the English word) is being used, as well as mineral wool (English?) which is bad for the lungs though.
    Until now I used rubber foam, but I'm not sure if that will do it.

  7. Fiber fill... stuff bought at fabric stores and used in pillow filling or quilts. The sheet stuff comes in a size and thickness that is just right. Non irritating also.
  8. Matthias


    May 30, 2000
    Vienna, Austria
    BTW, your site is very interesting - the musician's reference must have been a *lot* of work...
    And the sound samples of famous bassists and their basses are a great idea.
  9. The bassists' sound files are not just restricted to big name players. I accept MP3 from any bassist who is willing to contribute. I need as much information as possible: bass, pickups, strings, control settings, DI or amp, etc etc.

    This lets the community at large hear the differences between types of basses, pickups, tone, etc.

    The spread sheet is a constant work in progress, and updates almost daily. I recently added a bunch more data on commercial cabinets.