Help needed making good cabinet

Discussion in 'Home Theater Projects' started by Jordan_Brulotte, May 25, 2004.

  1. Jordan_Brulotte

    Jordan_Brulotte Stunt Coordinator

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    Hey everyone! I've been browsing these forums for awhile now getting as much info as I can, but now it's time for me to start asking some questions.

    I'm currently building a tempest sub enclosure out of 3/4" MDF, as many people are. I can afford the space to make a larger enclosure, as my current design is a 276.5L with 2x 3.5" ports tuned to 20HZ. I'm using the SUB250R amp from Creative Sound Canada to power it. My use for it will be 50/50 music and HT.

    1: I recently read though that building a sub of this size for a tempest driver will cause it to lack "punch". Is this true? I wanted to try to steer clear of the Adire plans, because I wanted to make a unique looking cabinet, and I didn't know what kind of effect the changes would have on Adire's alignments. Should I make it smaller?

    2: should I use polyfill in the enclosure? I get the impression that it is best used in small cabinets. Would there be any advantages to using it in a box this size?

    3: I may make my own port flares, as store bought ones may not look good with this box. Flares confuse me a little though. If WinISD tells me I need 2x 12.12" ports, does that include the flare or not? what kind of radius should the flares have compared to port diameter? And will the flare length and radius affect the tuning or just vent noise? Basically I don't know the math involved in making a great vent.

    I may have a few questions later, but I really appreciate any help I can get. Thanks guys!
     
  2. minhG

    minhG Stunt Coordinator

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    i'll give your questions a go.

    1) The way i understand the 'lack of punch' comment is this. Different alignments (box designs) will give you different frequency response. Using the Adire stuff, their SBB4 alignments gives extra output in the 30-50hz range, so you get a noticeable 'punch'. Their EBS alignment is almost flat from 20hz-60hz so it will sound flatter. People like different things, some people like extra punch others like the smooth flat response.
    I just built an EBS Shiva and have no problems with the output. It isn't as punchy as my 8" sub in my hatchback (auto), but that's to be expected. But the bass is so clean that it's well worth it. With a tempest, i wouldn't be worried about output.
    If you wanted to change the response, you can change the box size and tuning. If you're going to build a customer enclosure--you would need to do these calculations yourself.

    2)Polyfil is meant to trick the driver. It makes a small box 'seem' larger.

    can't help with the flare question.
     
  3. Aaron Gilbert

    Aaron Gilbert Second Unit

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    Unless it's WinISD Pro, I believe the program does not compensate for port flares. As I understand it (someone correct me if I am wrong), a large flare tends to make the port appear longer than it really is. For example, say you have a 10" port that includes 2" of flare on each end, thus 6" straight section. It may behave as a 12" straight port would. This is just an example, I don't know the actual math behind it, either. The flare length and radius should indeed affect the tuning, but likely not by much. The length plays a much greater part in the tuning frequency. The thing to remember is that in a very large box like you have, it requires a huge change in port length to affect a very small change in tuning frequency. For instance, in my enclosure, an 11Hz tuning requires two 3" (non-flared) ports 28" long. To tune to 20Hz, the port length drops to just over 7"!

    Hope this helps,


    Aaron Gilbert
     
  4. ThomasW

    ThomasW Cinematographer

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    A DIY flare made with a router bit can't compare to one of the molded plastic flared ports. So don't bother trying to calculate into your model.

    True flared ports allow a small port to flow as much air as the next larger diameter port. For example a 3" flared port basically flows as much air as a 4" straight port.

    Yes DO line the insides of your vented box. Use eggcrate foam or bonded polyfill. Using it will improve the overall sound quality. Also you might experiment putting a wad or plug of regular polyfill directly behind the driver (like Adire does). This allows 'fine-tuning' of the sub
     
  5. Aaron Gilbert

    Aaron Gilbert Second Unit

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    Thomas,

    I'm curious, which sound characteristics are improved by the lining? Dickason shows only very minor changes between 0% lining/fill and 50% lining (five walls covered with 3" thick fiberglass), yet he says the 50% is a good choice because it has 'decreased response changes due to box standing wave modes'. Well that's great, but I have no idea how to listen for that. What I'd like to know is subjectively, what is the difference in sound between non-lined and lined cabinets, assuming midrange output is not an issue?


    Aaron
     
  6. ThomasW

    ThomasW Cinematographer

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    Vance does a really good job writing books......:wink:

    First I'll tell you what we do, then I'll give a lower cost options.

    We line all the interior surfaces with 3/8" high wool content, felt gasket material. We even glue a 'donut' on the magnets of the drivers, (obviously don't block the vent hole). Note that this material starts around $8.00+/sq ft.

    A lower cost option is to use something like the 1-1/2" acoustic foam from PE, use egg crate mattress foam or multiple layers of "bonded poly". This is the stuff used to make quilts

    Then we make a pad of from layers of the bonded poly. We start with 2'-3" thick. This is placed directly behind the woofer. The amount is increased or decrease as needed to dial-in the level of tightness and detail.

    We final tune by ear, increasing the amount of the material in the pad as long as the bass contines to tighten. When the bass no longer tightens and the only effect of added material is decreased output, drop back to the last amount used........
     
  7. Aaron Gilbert

    Aaron Gilbert Second Unit

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    Thomas,

    Thanks for the detailed reply, much appreciated! Ok, so tighter bass, that's something I can relate to. I'll have to give it a try, I'm pretty sure I have plenty of foam and/or bonded polyester lying around here somewhere. $8 a square foot would definitely be out of my frugal budget, seeing how my subwoofer has about 25 square feet of interior wall space! That would make it more than twice the price of the woofer. [​IMG]


    Aaron Gilbert
     
  8. Cam McFarland

    Cam McFarland Supporting Actor

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    Thomas,


    Pardon my ignorance, but what is meant by "directly behind the woofer"??


    Also, what about this...."The amount is increased or decrease as needed to dial-in the level of tightness and detail."

    Obviously my ears are shot from too many high speed runs on my bikes & no ear plugs, but what does this mean...."tightness & detail"....I presume you are refering to listening to music??
     
  9. ThomasW

    ThomasW Cinematographer

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    Cam,

    Directly behind the woofer means immediately behind the magnet. It can touch the magnet, but should not touch the cone.

    Yes tight and detailed refer to the sound characteristics of the bass. Tight is the opposite of boomy. Detailed is just as the word itself implies one can hear 'details' about the instrument being played.

    Poor quality subs tend to blur detail. One hears the bass notes, but can't tell what instrument created the note.

    Good quality subs allow the listener to easily distingush whether a bass note was created by a bass guitar vs a bass note played on say an organ.
     
  10. Cam McFarland

    Cam McFarland Supporting Actor

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    Thanks Thomas......sad to say, but I probably could
    not tell the difference regardless.....:b
     

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