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Please comment on my speaker design assumptions

Discussion in 'Amps and Cabs [BG]' started by Mottlefeeder, Dec 15, 2005.

  1. I'm interested in designing a 1*12 cab, as much to hear how it sounds as anything else. Before I start, I would like to check that my assumptions are right.

    1. Several people have mentioned that the sound of a bass is mainly in octave 2 and upward. This suggests that for 5-string use, I would not hear a lot of difference between a cab tuned to 45 Hz and a cab tuned to 60 Hz.

    2. Tuning the cab to match the speaker Fs gives you good transient response. Is transient response related to punch, and related to group delay? In other words, should I design for minimal group delay, provided the cab -3dB point remains at or below 60Hz?

    3. Very few people in the audience will be listening on-axis, and a 12 inch speaker will beam high frequencies. To get better dispersion, I should cross-over to a smaller midrange unit, and use a horn tweeter above 3-4 KHz.

    From the above, I am heading towards a Delta 12LF in a 2.5 cu ft cab vented at 51 Hz, crossed over to an Alpha 6 (or similar) in a small closed box at 500 Hz, and an APT 200 (with an attenuator) from 3.5K upwards.

    My concern is that if this is an optimal design, I would expect to see several versions of it made commercially, but I haven't found any. Where is the flaw in my reasoning?
  2. No real flaws in your reasoning... just that a commercial cab like this would be very expensive. I would experiment with the crossover freq. between the delta 12 lf and the alpha 6 (maybe higher than 500); this can be done without great cost.
  3. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    True, but the tuning frequency doesn't necessarily have much to do with the f3 of the cab. Very often with pro drivers the fb is well below f3
    Group delay is one of those things that is often discussed, seldom understood, and probably best ignored.
    Ideally with a twelve you should cross to a six or eight no higher than 1.2kHz, and to a tweeter no higher than 3.5kHz, though with a decent midrange driver the tweeter is optional depending on your taste.
    That works.
    There's no flaw on your part. Many flaws in the engineering of the average bass cabinet.
  4. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    If you're going to cross over to the midrange driver so low, make sure it has sufficient power handling and sensivity to keep up with the woofer.

    Otherwise, all sounds good to me!

  5. Thanks for the comments.

    I'm thinking of making the 'mark 1' in MDF with an end that can be moved to change the volume, from say 2.5 cu ft to 4 cu ft, with a selection of cardboard tubes to change the tuning. Would it be of use to anyone if I recorded what the cab sounded like at different volumes and port tunings?

    I have access to Shure SM 57, SM 58 and AKG C1000 microphones, and a mixer that will bring them up to sound card line level. For the input I can play a Warwick Corvette fretless, or use the bass channel of the Ed Friedland Walking Bass CD. (The latter might be more consistent)

    Would this be of any interest to anyone out there, and can anyone confirm that the AKG would be the best microphone to do it with?
  6. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Don't. Don't use MDF and don't do those other things either, the effect on performance will be minimal and not worth the compexity involved. If you want the ability to change the tuning use multiple ports and plug them.
  7. This idea has rubbed me the wrong way for a long time, so I decided to investigate for myself.

    Link to Spectral Analysis of Low E String

    I recorded a studio quality sample of my MIM P-bass with DP146 pickup and EXL-220 broken-in roundwounds. The sample is open E measured over 4 seconds. The chart shows peak-hold values, which is crucial to the discussion.

    The 41 Hz fundamental has a higher magnitude dB (-11.8) than the 82 Hz (-14.1) 1st harmonic.

    I used peak-hold measuring techniques to prevent the program from averaging the response to a lower dB value. This is an important point. Measuring for peak (absolute) values, the fundamental is of shorter duration (the P "thump"), but it is very much alive in this signal. The peak fundamental is also stronger than the 2nd harmonic peak value.

    Averaged over 4 seconds, the 82 Hz harmonic shows a significantly higher dB value because it has a longer duration which increases its strength in the average. This produces a chart which shows a lesser fundamental and greater 1st harmonic.

    The audible proof is using a 31-band EQ to drop out everything from 63 Hz and lower. If your sound is unchanged, your bass (or cab) isn't providing much fundamental. However, if the bottom falls out, then building an 82 Hz cab is gonna cut the nuts off your sound.
  8. The 12LF is a good match for the Alpha 6. It has a bigger bottom than the Omega Pro 12 in SBB4. The 12LF is about the only 12" Eminence driver with an F3 below 50 Hz in a small cabinet size.

    Both the 12LF and Alpha 6 are similar in sensitivity and work well with your chosen crossover. The SPT is hotter, and requires an attenuator in the crossover.

    The BB4 tuning frequency is well within your playing range, so you will need at least a 4.5" diameter port, or equivalent, to avoid chuffing at maximum power. Experience tells me the upper vented resonance in this design will be stronger (louder) than the larger cabinet alignments, so expect it to have a strong mid-bass hump.

    There is a direct trade-off between bass extension and cabinet size in your project. The 2.5 cubic feet size is the BB4 alignment with an F3 near 49 Hz. The SQB3 at 4.8 cubic feet will give you an F3 of 39 Hz, at the expense of a much larger cab, but with less hump in the mid-bass.
  9. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Try again with a shorter sample time, like 100ms. Then try it at 10 ms. As the note fades the balance between the fundamental and the harmonics shifts drastically. In the initial attack the harmonics are much higher than the fundamental, but beyond that the harmonics fade rapidly while the fundamentals retain their amplitude for a far longer period.
  10. Geoff St. Germaine

    Geoff St. Germaine Commercial User

    Use a wavelet transform if you have access to a program that can perform it (for instance MatLab Wavelet Toolbox). Then you can see the frequency spectrum more as a function of time. Actually, I should just do it.
  11. I'm working with WinISD Pro, which shows a flat response to 500 Hz, and I think I can tune the port to get the two impedance peaks to be the same. Is that the area I need to work on so I don't get the hump? :)

    Also, with WinISD Pro, I get an Xmax power limit of 120 W at 80 Hz, where the speaker Z is 14 ohms. Is it reasonable to think of that in ampifier 'voltage output' terms, i.e. 14/8 * 120W = a 200 watt into 8 ohm amplifier
  12. Is this a comment for me on speaker box design, or a comment for the 1st octave/2nd octave discussion? If the former, you've lost me!
  13. Xmax is important if you design a hifi box; in musical instrument amplification Xmech is much more important. As long as you do not exeed Xmech there is no real problem. You should keep the powerdip is small as possible but also consider other requirements wich might make for a bigger powerdip. As allways you have to compromise.
  14. Dont worry; its the octave discussion.
  15. That is not what I see in my testing.

    I notice the fundamental is strong at the onset, and fades quickly. The 1st harmonic sustains, and builds the graph. I'm using Spectra Plus and it wants a longer sample time to provide high differentiation in the low frequencies.

    Doing the 31-band EQ thing, I hear the whole bottom fall out when I drop the sliders from 63 Hz and lower.
  16. The upper resonance amplitude is a function of box volume.

    It has been demonstrated you can take a too-small box and tune it anywhere, but the upper resonance is quite pronounced.
  17. This isn't my field, so I may be way off beam, but it seems counter-intuitive that a slider will only operate on frequencies =/- half the range to the next slider. If it did, and you boost/cut it by up to 15dB, surely your phase relationships will be destroyed? Is it possible that the more extreme your eq, the broader its base will be, so using the 63 Hz slider and those below as a hi-pass filter will take out much more than just sub-63Hz.

    Have you tried the same test with a 18 or 24dB/octave filter?
  18. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Absolutely. How much more it will take out depends on the Q of the filter, of course. I personally leave the 63Hz slider either flat or boosted a bit, and pull the 32 Hz slider down 6dB or so on a 4 string.
    Bruce, I don't remember where I saw it but there is a spectral analysis of all the various orchestral intruments floating around on the web. Fender bass isn't there but the double bass is, results about the same.
  19. Can you clarify how you define a 'too small' box. Is that what WinISD Pro pushes you towards when it recommends an alignment, or is there some rule of thumb that I need to apply to steer WinISD in the right direction.

    Also, if I make the box bigger, the Xmax power handling gets worse. Is this a constraint due to the design decisions that Eminence made with their off-the-shelf offerings, or will I find it with all bass speakers?
  20. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    A too small box will give a response hump, but that's too small compared to the optimally flat alignment, and not necessarily too small if you want a response hump. WinISD goes to the maximally flat alignment by default. A smaller box gives a stiffer air load and does reduce excursion, but that's not necessarily a good thing if in so doing it also limits response. In any event as far as pro sound is concerned xmech is more important than xmax; going past xmax means output is no longer linear with respect to input, but it doesn't hurt the driver. Staying within xmax is only critical in hi-fi applications. My preference with VBs is to tune to around 50Hz so that the driver won't unload and then go as big as you need to for the desired response. Generally a hump below 100Hz is OK, even desireable, but above 100Hz not so as then you'll get boom.