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Upgrading the RS Musical Instrument Cab to a Bass Cab

Discussion in 'Amps and Cabs [BG]' started by Thor, Apr 11, 2005.


  1. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    A good point, but there's a few things to consider about xmax predictions. The first is that your box program is assuming that the driver is receiving 150 watts pure sine at 82Hz, and if you were pushing 150w with an 82Hz fundamental- E2, almost right on the button- perhaps half of that wattage would actually be contained in the fundamental, the rest would be harmonics, so xmax wouldn't be approached. Dealing with musical program as opposed to pure sine waves you can expect that usable linear xmax predictions usually run about 3dB low. Also, xmax is based on the travel of the voice coil within the magnetic field of the motor, and is very different from xmech, the actual physical limit of the cone/coil travel range. While the xmax of the D12LF is 4.8mm, the xmech is 26.9mm. That allows for a lot of travel beyond xmax before coil slap can occur.

    Even more relevent is the xmax spec itself, which is almost arbitrarily arrived at. For instance, technically an Alpha 8 has a zero xmax. I regularly chat with a senior project engineer at Eminence who recently told me that they are now starting to use Klippel analysis of their drivers, which measures the driver in use and calculates the point at which a 10% distortion figure is reached, a far more useful spec than xmax in predicting driver performance. I wouldn't be surprised to see this spec become an industry wide standard that will eventually supplant xmax entirely.
     
  2. Thor

    Thor Gold Supporting Member In Memoriam

    I agree, but he sent me the program, which I can send on un-executed to another computer.

    If it has a problem, I can deal with it easily at home, but screwing up the work network can cost you a job; at home it costs 29.00 and a visit to the local Chinese fellow's computer tech store.

    Edit: Updates are below.

    Footnote: Nine years later, BFM's comment above is very interesting, I note that the new edition- Ed. 7 - of Vance Dickason's " Loudspeaker Design Cookbook" now has a section devoted to Klippel analysis. More home work to do!
     
    Last edited: Dec 5, 2014
  3. Thor

    Thor Gold Supporting Member In Memoriam

    I rechecked the ID measurements carefully and came up with 3.77 cu. ft ( x 28.32 conversion factor = 106.76 liters ).

    I noticed in the MadiSound catalog they have nicely sized sheets of acoustical foam for a reasonable cost. I will probably swap out the polyester filling and use this for a neater damping job.

    I have received 2 prices on the Delta 12lf's so far, they are $152 and $166 delivered. That would work in my budget. ( That is for 2 drivers.)

    One observation regarding the physical volume is that I can easily add volume to the box by adding cleats or sides to the back screw in panel, thus extending it out towards the back. ( Panel is shown in pic # 2) The panel dimensions are 16.25" x 34.25", deducting 3/4 inches for the width all around gives me 15.50 x 33.50.

    Each inch of extension adds 519.25 cu inches of volume, or .300 cu ft.

    Thus by adding four pieces of plywood 3.3 inches wide, I can make the box 1 cu ft bigger if required.

    This is an easy fix, so lets assume we can use that to method to optimize the box. I will wait to do the internal shelf brace to see how this affects the overall design. It would probably be beneficial to have the back screw right into the shelf for additional bracing.

    Given that, wouldn't an increase in the box volume be beneficial?

    Thanks Bill and Pete, that was a very good discussion before, and your points are duly noted.

    I am learning quite a bit plowing through this, and am optimistic now that this will turn into a pretty useful little mid size cab.

    One other construction question, is there any reason that when I remove the 2 angled piezo's, that I couldn't cleat up the angle, ie straighten out the top face and install the port(s) using the existing 3" hole or holes? Or do ports need to be located closer to the drivers?
     
  4. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Making the box bigger won't make all that big of a difference for you, I 'd do that as a last resort if you find it doesn't go deep enough. There's no reason not to put the ports up high, but I wouldn't go with less than a pair of 4" diameter for two twelves. Three inch could make a fair amount of chuffing noise. You'll need to make a pair of 4" ducts about 6 inches deep, no problem fitting those in the box. Foam is easy to work with; polyester batting an inch thick also works well and can be stapled in place, you can get it at Wally World where they sell pillow stuffing.
     
  5. Thor

    Thor Gold Supporting Member In Memoriam

    Would the angling make any difference in the ports?

    At 1.75" deep and 5.5" wide, a/b = tan A , it is angled off at 17 degrees. I wouldn't think it would matter, but I could be wrong. The 3 inch versus 4" chuffing is duly noted as well. Madisound in Madison WI has these items in stock for prompt delivery. I needed to order some reinforced
    rubber feet anyway.

    Note that the existing speakers are flush-mounted to the REAR of the front panel. Wouldn't a routed flush mount be more efficient, or is this a negligible performance physical upgrade?

    Given your responses, I am ready to start rebuilding this weekend.

    Time for a call to MadiSound as well.

    I will check in with progress pics as it moves forward.
     
    Last edited: Dec 5, 2014
  6. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Interesting. I didn't know that. I'd always assumed the fundamental was as strong as the sine wave, and the harmonic content made the signal even stronger. It was an assumption on my part and the reading on this matter didn't tell me anything to dismiss error. But your explaination makes sense.

    There's not much out there on large signal speaker analysis, and most of it points to Klippel. I've been meaning to get stuck into Klippel's stuff but haven't got around to it as yet.

    As for making the box bigger, I'd also wait and see how the current box fares. I also try to use 4" ports for any speaker used for live audio. I've got a 4" port on my Beyma 12 and at gig volume, you can feel a nice but gentle rush of air coming from it. I wouldn't want to go any smaller. If absolutely necessary, you can use 3" ports but I'd be using more of them, maybe 2 per driver (as I did on my 1x10 PA speakers). The problem here is that not only will they have to be deeper, they'll take up more baffle space than 1x 4" port per driver. Plus 4" ports are readilly available.
     
  7. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Not really! As Andy from Acme says, ports are a pressure device, not unlike a balloon. You can blow from any direction and the balloon will still fill up evenly in all directions. Ports are the same. My understanding of angled ports is that sometimes a port has to be so long that it hits the rear wall of the cab. If this is the case you have 2 options, either make a port "elbow", or use an angled port. It's all good.

    I've always thought routed mounts were purely cosmetic, but I'm not 100% sure.
     
  8. Check Avatarspeakers.com for prices on Delta 12lf speakers. They have outstanding prices on speakers they have in stock.
     
  9. Thor

    Thor Gold Supporting Member In Memoriam

    Avatar was the lowest quote. They came in at USD $67 per unit + $9 shipping = 152. Pi was 7 dollars cheaper, but their shipping was higher.

    So you are right on the money my friend!
     
  10. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    They are useful for limiting frame diffraction and are almost universally recommended in h-fi. Front mounting but not flush is not as good, and rear mounting is totally forbidden, but this is in the hi-fi realm where a half dB of diffraction sourced phase zits causes the average audiophile to turn prematurely gray. Remember, these are guys who spend $100 a foot for wire. Flush mounting in a bass cab wouldn't hurt but wouldn't make it hi-fi either.

    Try running a 2.83v sine wave into a speaker at a typical fundamental frequency; 100 Hz would do. Check the SPL. Then run pink noise in, turn it up enough so that you get the same base SPL reading at 100 Hz. Then see the difference when you read it broadband. It will be at least 3dB, and depending on the response of the speaker as much as 10dB.
    Pink noise doesn't duplicate a music signal, but in the power distribution is reasonably close.
    Even more revealing is if you take an RTA of the bass with a short enough time window so that you're catching the signal attack only, which is when power demand is at a peak. You'll be amazed how little of the signal lies in the fundamental, on average at least 6dB down from the broadband. That shifts once the signal goes into the hold phase, but at that point the power demand is also way down. The fundamental does dominate in release, as the signal fades, but by then power is insignificant.
     
  11. Thor

    Thor Gold Supporting Member In Memoriam

    Haha, that was good. Funny you should mention it, but as I was flipping through the last MadiSound Catalog, I ran into the section with the Boutique wiring and my jaw literally
    dropped. And you can pay quite a bit more than 100 bucks a foot! I thought bass players were nuts, but this is just obscene. Then I thought to myself, they wouldn't have 5 pages of this stuff if nobody was buying it. A few stops on some other forums, Altec-Lansing and DIYAudio confirmed
    my suspicions.

    I will route it if I can do is easily. If not, I won't worry about it, just bolt them on and take them for a spin. I am more concerned with producing sound than reproducing it accurately.

    Thanks Bill, both Pete and I have agreed we both learned a lot from your posts here.
     
  12. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    If P.T. Barnum was alive today he'd probably be selling wire. Have you heard about the $150 wall outlet? This is not a joke, it really exists and people are buying them. When I first saw them I assumed the price was for a box of ten. It isn't.
    http://www.parts-express.com/wattgate-381-audio-grade-duplex-receptacle-outlet--110-439



    Mod Edit 2014: Link fixed. Product discontinued. Amazon link here. $239 bucks now.
    That's right. No typo!

    The customer reviews deserve a thread of their own. Can't decide whether it should be in Misc, Amps or Bass Humor.
     
    Last edited by a moderator: Dec 5, 2014
  13. Thor

    Thor Gold Supporting Member In Memoriam

    Holy Guacomole, Batman!

    'This way to the egress ...' P.T. Barnum.

    I have had another small issue present itself.

    The shelf will not accept a 4" ID port as the height ID is just shy of 4" itself. Nor can it be located on the front face of the speaker panel, too big.

    Can I sub 3" ID ports with the following assumptions? 2 pcs of 4" ports @ 6" long has a cubic volume of 150.8 cu. in. In order to provide the same venting volume, I could locate 4 pcs 3" ports at 5.33 inches long, that would equate to the same cubic volume:

    ( 3.1414 x 1.5^2 x 5.33 x4 = 150.6 cu inches )

    Is this a legitimate port substitution, or am I barking up the wrong tree as far as the calculation? I can fit 2 ports up top and 2 ports on the speaker face.

    I could locate 4" ports on the side or back as an alternate, but this would not be as aesthetically pleasing, imho. The upper back panel would probably be my second choice.
     
  14. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    You can use three inch ducts if you use enough of them; 3 five inches long would do.
     
  15. Thor

    Thor Gold Supporting Member In Memoriam

    How did you determine that number and length?
     
  16. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    WinISD, with two D12LFs, 3.0 net cu ft.50 Hz tuning.
     
  17. Thor

    Thor Gold Supporting Member In Memoriam

    Ok, 3 it is.

    That I can make room for.

    Hopefully there will be some work in progress pics after the weekend to show.
     
  18. Thor

    Thor Gold Supporting Member In Memoriam

    Ok, now I have WinISD set up on my computer at home. That is a great little program for designing, in this case, redesigning the box.

    Little tweaks in Port sizes sure change the plots!

    Updates:

    This weekend I built an internal shelf brace and cut some supporting cleats for the box. These are fitted, but I will hold off gluing them in until they are checked for fit against the final speaker selection. I will get a pic up tomorrow of the work.

    I removed two of the piezo horns in the horizontal array. When I checked the holes, I realized that the holes will only take a 2.75" port, so I am going to have to use 3 of those, I have a source for the ports of that size, as I needed one for my Moskowitz cab project.

    I then ran WinISD again for the port length. Is there any reason I shouldn't tune the box to 45 hz
    instead of 50 hz?

    I ran the specs of the Delta 12LF's, for this calculation.

    At Funkengroven's suggestion, I tried to acquire JBL 2206 H drivers used, but they seem to be selling for about 150 plus $25 shipping each on Ebay. While it is a fine driver, $350 is way out of the budget I allowed, so I am back to considering the Delta 12LF's.

    I am also considering the Peerless CC line 831857 12" for this project. Peerless makes some nice stuff.
     
  19. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    I agree, 45Hz looks better than 50Hz in that cab. For bass, lower is better if you can manage it without destroying your frequency response. Don't forget that a ported cab's speaker excursion can get pretty out of control at frequencies below Fb. An open E has a fundamental of 41Hz so where possible I try to keep the tuning in the 40's rather than the 50's. In this case, 45Hz looks good.


    Nice speaker, but not for Bass guitar. Sensitivity of 89dB means it'll need a gazillion watts to get to gig volume, and if only has a 220 watt rating.

    Still, for a -3dB point of 24Hz, I could almost live with a 185 litre home theatre sub........hhmmmmmm
     
  20. Thor

    Thor Gold Supporting Member In Memoriam

    Ok, forget the Peerless, I hadn't looked that closely at the specs yet. The one that specs out is a little out of my price range. :bawl:
     

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