Building my big sonosub...Sanity check

Discussion in 'Speakers' started by CobyH, Aug 18, 2004.

  1. CobyH

    CobyH Auditioning

    Joined:
    Aug 17, 2004
    Messages:
    9
    Likes Received:
    0
    ok...just want to run a sanity check. I've been running the numbers for days and I'm ready to start making dust. Here's what I'm proposing:

    Ported sonosub
    700 liters
    2 7" ports, about 4' long
    1 Tempest 15"
    50% stuffed

    this 'should' yeild a sub that's fairly flat down to 12hz or so. Concerns I have...the diameter of the ports is slightly larger than is 'recommended' but I'm not sure of the reason for the recommendation. (The recommendation I've seen is to keep the port 1/3 the diameter of the woofer). Whatcha think? The SAF isn't a problem; the sub will be over 8 feet tall at 24" diameter, but that's not a worry. Also, the ports will be on top; the woofer on bottom, so that I can give the ports a little room to breath. They won't be exactly 1 port diameter away from each other and from the walls....will that really cause a problem?

    coby
     
  2. Dustin B

    Dustin B Producer

    Joined:
    Mar 10, 2001
    Messages:
    3,126
    Likes Received:
    0
    This is going to sound a little harsh but it's what I think. Good thing you are doing this sanity check as I don't feel you will be happy with the results of your choosen allignment.

    1) That's more than twice the enclosure size needed for a single Tempest.
    2) There's no point tuning to 12hz.
    3) 4 feet is a bit long for a 7" diameter port.


    A single 6" port is more than sufficient for a Tempest. There's really no point going larger than 340L or so with the enclosure volume. There's also no point tuning much below 16hz.

    I'd recommend copying or slightly modifing one of these designs:

    http://www.adireaudio.com/Files/Vent...plications.PDF

    Possible modifications would be switching out dual flared ports for larger single straight ones. Modifing enclosure volume and tuning point a bit, but staying between 160L and 340L as well as tuning between 15hz and 18hz. Changing the dimensions of the enclosure but maintaining the internal volume.
     
  3. CobyH

    CobyH Auditioning

    Joined:
    Aug 17, 2004
    Messages:
    9
    Likes Received:
    0
    Well, your post wasn't harsh, it just wasn't very informative. Can you give me some 'whys' for the general suggestions you've posted? The advantages of going to a much larger sub is an ability to go deeper into the fr at a decent efficiency, while still maintaining low group delay and managing driver excursion. The reasons for going lower than what you recommend include reproducing those low frequencies found in current movie info(some of it down to 5hz, from what I've seen), and not driving the woofer below its tuning frequency and causing it to unload. I actually do listen to pipe organ music occasionally as well, since my wife is an opera singer that has recorded with pipe organs quite a few times, and there's nothing more interesting than those notes you can just feel instead of hear. So..point by point:

    1) That's more than twice the enclosure size needed for a single Tempest.

    Well, that depends on what you mean by 'needed'....

    2) There's no point tuning to 12hz.

    Sure there is. See above...unless those don't sound reasonable, and I'd like to know the whys of it if they're around.



    3) 4 feet is a bit long for a 7" diameter port.

    Why?

    4) A single 6" port is more than sufficient for a Tempest. There's really no point going larger than 340L or so with the enclosure volume. There's also no point tuning much below 16hz.

    Define 'sufficient'? What parameters are you looking at when you say sufficient? I'd say it'd be sufficient if you were running at a higher tuning frequency...And with the modeling, it's showing a good gain by going with as large an enclosure as I can squeeze into the wife's living room. Better efficiency down low, where I want it. What makes you say there's no point in tuning lower? I'm not sure if I'm missing something or not. But I would like to hear the reasoning behind your suppositions...

    Oh, one last question. You mention that I might not be happy with this alignment, but you never said anything that would be a detractor...what would sound bad about it? It's flat to about 12hz with in room response, and it'll model to well over 115db without pushing the excursion of the cone to it's limit...do you think it needs more spl to sound good? What exactly would be the downside to this alignment besides physical size?
     
  4. Kyle Richardson

    Kyle Richardson Screenwriter

    Joined:
    Jan 1, 1998
    Messages:
    1,073
    Likes Received:
    0
    Coby, I agree with Dustin. The EBS alignment will provide everything you are after but will do it better. You are sacrificing ouput where there is MUCH more information available (20-30Hz) by tuning to 12Hz. The EBS alignment (340 liters tuned to 15.8Hz) has very low group delay, is extrememly efficient, and isnt sacrificing the output in range I mentioned above as much as a 12Hz tune.

    The 4' long ports will start to get port resonances withing the playing range of your subwoofer, thus, adding output to certain frequencies which you dont want. By changing the enclosure size/port diameter to shorten this length you avoid such problems. Twin 7" port is overkill for a single Tempest and all you are doing at that point is taking up usable enclosure volume.

    I'd really recommend the EBS Alignment as outlined by Adire.
     
  5. Edward J M

    Edward J M Cinematographer

    Joined:
    Sep 22, 2002
    Messages:
    2,031
    Likes Received:
    0
    Coby:

    I'll echo what Dustin/Kyle said, and also suggest (if you haven't done so already) that you post over in the Advanced Projects and DIY section. There are lots of DIY gurus over there with experience in building Temepst based subwoofers who will be more than happy to further discuss these points.

    Also consider that (depending on the size of the room) you will probably begin to experience room gain below a certain frequency, and this will augment the deep bass response of the subwoofer. In a moderate size room, the EBS Tempest might actually display a rising response to 15 Hz, and retain strong output (above baseline level) down to 12-13 Hz.

    While the trend toward deeper content in DVDs is certainly apparent, I personally view the presence of (for example) the hot 5 Hz spikes in the BHD Irene scene to be more of a novelty than something the average enthusiasts should expect to reproduce in-home at realistic levels with low distortion.

    If a sub can get down to 15-16 Hz clean (
     
  6. CobyH

    CobyH Auditioning

    Joined:
    Aug 17, 2004
    Messages:
    9
    Likes Received:
    0
    So, if I go with a 13hz tune on the 700l box, I still maintain the output I need, and I don't get port resonance problems in the playable range (1/4 wavelengths below 80 hz are avoided; 80hz is 42" long at 1/4 wavelength). And the extra size doesn't seem to hurt anything...just help. I've run the numbers and this alignment increases output from 20-30hz, not decreases....I don't see where you're getting that from? If the point is to increase the spl at the 20-30hz range, then the alignment I'm going after is exactly what's needed, not the smaller one that squashes that band more. That's part of what I'm doing...is trying to get a flatter response. Running the numbers on the alignment you just gave (the EBS at 340l) shows a decrease at 20hz of 2db, a slightly worse group delay, and twice as much airspeed in the port. I still don't see the advantage of going to a smaller port and a smaller enclosure? The output at 150w is modeling at 116db at 20hz and 117 at 30hz for the EBS alignment, and with this larger enclosure it is 118 at 20hz and 117 at 30hz. Not a huge difference either way, but it is better at the larger alignment.

    Is there a way of attaching a file here so I can show the comparison?
     
  7. CobyH

    CobyH Auditioning

    Joined:
    Aug 17, 2004
    Messages:
    9
    Likes Received:
    0
    Thanks, Ed. I'm actually doing this based on the in room response using lspCad, not the halfspace one; trying to factor in the room gain. I'm interested in taking measurements afterwards to see how well the sim predicts the performance as well. So far I can't see any reasons not to go with a larger enclosure, but it does make sense (I think?) to keep the port length down to below 1/4 wavelength of what I'll be using it for.
     
  8. Edward J M

    Edward J M Cinematographer

    Joined:
    Sep 22, 2002
    Messages:
    2,031
    Likes Received:
    0

    Certainly a good idea. Measure it at 2 meters ground plane and then inside the house and see what changes.

    While Dustin or Jack Gilvey or Kyle might chime in, I don't think there's anything wrong with the additional enclosure volume per se; it should incrementally increase the efficiency of the system. I do think the EBS alignment probably provides a very good balance of efficiency at the tuning point and reasonable enclosure size. There is likely a break point (as you approach an infinite baffle) where increasing enclosure size results in a diminishing rate of return with respect to improving efficiency.

    With respect to GD, you won't hear any difference between the two alignments in that respect; the human ear is quite insensitive to even moderate changes in GD in the bandwidth you are discussing.

    Also, are you modeling the EBS with two 4" flared ports (you referred to port in the singular).

    I also assume you are modeling output vs. cone excursion vs. frequency. Maxing out cone excursion (and bottoming the driver) above the tune point is typically the limiting factor in output with subs tuned really deep (like 12-13 Hz).
     
  9. CobyH

    CobyH Auditioning

    Joined:
    Aug 17, 2004
    Messages:
    9
    Likes Received:
    0
    Certainly a good idea. Measure it at 2 meters ground plane and then inside the house and see what changes.

    Ok; will do. A friend is building a sonosub as well with the same driver but he's going for the Adire alignment; we were planning on doing some comparisons in the same listening room.


    While Dustin or Jack Gilvey or Kyle might chime in, I don't think there's anything wrong with the additional enclosure volume per se; it should incrementally increase the efficiency of the system. I do think the EBS alignment probably provides a very good balance of efficiency at the tuning point and reasonable enclosure size. There is likely a break point (as you approach an infinite baffle) where increasing enclosure size results in a diminishing rate of return with respect to improving efficiency.

    That models up; much above 1500l or so and the returns diminish more and more. One difference though, that I've been looking at (and hence the move to 7" ports); is the modeled effect of port nonlinearity. The 4" ports tend to show much more effect as the power level is increased; moving to 7" ports decreases that, preserving the flatter frequency response from mild to loud.


    With respect to GD, you won't hear any difference between the two alignments in that respect; the human ear is quite insensitive to even moderate changes in GD in the bandwidth you are discussing.

    Also, are you modeling the EBS with two 4" flared ports (you referred to port in the singular).

    Actually, yes, I was using 2 ports, though not flared. lsp cad demands flares of a radii at least half the length, and that wasn't practical to build (over 9" flare radius on both ends...I can't make that economically nor do I think that's available for purchase?)

    I also assume you are modeling output vs. cone excursion vs. frequency. Maxing out cone excursion (and bottoming the driver) above the tune point is typically the limiting factor in output with subs tuned really deep (like 12-13 Hz).

    Using the larger box and going with 7" ports puts the cone excursion up; there's less power handling, but less chance of an odd low frequency bottoming out the sub. With the 4" ports, I get the same peak excursion, but it's below the tuning freq. With the larger box, there's two peaks; one below and one above the tuning point, both at about the same excursion as the single peak with the 4" ports. BTW, the second peak in the 4" box is only about 2.6mm less than the larger box's second peak.
     
  10. Dustin B

    Dustin B Producer

    Joined:
    Mar 10, 2001
    Messages:
    3,126
    Likes Received:
    0
    Here's what I get in LspCAD for the two designs without adding in room parameters.

    EBS Allignment (200W applied)

    Suggested Allignment (120W applied)


    As has been noted group delay will be a non issue in a comparison between the two.

    For port air speed the rule of thumb is under 17m/s for straight ports and 25m/s for flared ports. Both designs are easily under this measurement. A single 6" straight port will be as well.

    Your suggested allignment will hit Xmax at just under 12hz and the EBS at just under 14hz. Little difference here. Both have very sharp drop offs in output level below these two frequencies.

    Both can do 16hz just fine which is as low as you need to go for pipe organ stuff. They will be less than a dB appart at this frequency.

    As for material under 16hz you won't be able to tell the two designs appart. The only thing these frequencies can do is shake things, provided they are loud enough. Telling the 16hz shaking appart from the 5hz shaking will be impossible, especially since neither design can produce enough output at 5hz to shake anything. I doubt either would shake much at 10hz either. Don't forget both go well over the mechanical excursion limits by 10hz with full wattage applied without rumble filters. So expect much lower output levels at the very low frequencies than LspCAD is predicting.

    I may screw up this last part as I'm not fully up to speed on it, but here it goes. Finally your alignment would be called underdamped. That rise in response right around tuning is considered undesireable. It indicates a resonance of the cone as well as the port which will likely result in some unwanted distortion, bloat and possibly ringing.

    Contact Adire about your design, they can be more detailed than just about any of us on this forum. I'm confident they won't recommend you go with an enclosure much larger than their recommended EBS alignment.
     
  11. steve nn

    steve nn Cinematographer

    Joined:
    Jan 12, 2002
    Messages:
    2,418
    Likes Received:
    0
    >>Don't forget both go well over the mechanical excursion limits by 10hz with full wattage applied without rumble filters. So expect much lower output levels at the very low frequencies than LspCAD is predicting.
     
  12. Jack Gilvey

    Jack Gilvey Producer

    Joined:
    Mar 13, 1999
    Messages:
    4,948
    Likes Received:
    0
    I remember what fun I had when I first started playing with software simulations years ago..the stuff I came up with! [​IMG] I'd agree with Dustin, but it might be fun to build this if just to learn. I want to know what kind of wife is cool with a 700l box, and if she has a sister.


    The proposed design seems geared toward getting better/more response (actually, a hump) between 12Hz and 16Hz than the EBS alignment. Tuning the 340L cab to 12Hz would likely result in a flatter in-room response and no hump.
     

Share This Page