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Speaker buffs: more help please.

Discussion in 'Amps and Cabs [BG]' started by Rockin John, Sep 20, 2001.


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  1. Concerning the dimensions of a port in a ported enclosure.

    This thread follows on from a previous one pertaining to the Eminence Delta15 bass speaker.

    If the cabinet design calls for a circular port of 116.8mm diameter (area approx = 107 sq. cm), can the circle be replaced with a rectangular slot of the same area?

    Following from that, is there a proper ratio of the two sides to observe: is there a preferred place to position this slot on the baffle?

    Thanks guys.

    John
     
  2. The area of the port is the important part. Shape is less important, but a rectangular port rule of thumb is no higher than an 8:1 aspect ratio. I have neither tested nor researched if this ratio has a basis in fact. I've not measured the slot ports on the Eden XLT cabs, but I think they exceed this ratio.

    The principal issue with a port on the baffle board is not location, but weakening of the baffle itself. The number of cutouts on the baffle and their proximity weakens the board. The solution is to glue and screw a brace on the interior face between two cutouts.

    The single most important part is checking the tuning with a digital volt meter before you call it complete. Put a 100 ohm or higher 25-watt resistor in series with the speaker and the digital VOM leaders across the speaker terminals. Measure A/C voltage and look for the lowest voltage reading as that of the tuned frequency. There will be peak reading both below and above the tuning frequency. I have a freeware signal generator and loads of research information on my web site.

    http://www.ofgb.org
     
  3. OK. And thanks.

    When I made the original post about this driver a number of you guys kindly gave advice. The general feeling was that the cab size that Eminence's design guy suggested (I was in contact with him personally vai email) was way too small @ 107 L internal.

    However, counter feeling ran that if Eminence says that's what to do then that's what to do!

    It will be interesting, therefore, to test the cab as you say to see what happens. I'd appreciate advice as to exactly what to do with the figures the tests throw out: that is, how to decide whether or not the port area is correct and what to do about it if it's not.

    John
     
  4. The Eminence response could be several things.

    1) The guy is a minimum wage Nintendo-Boy and knows nothing about speakers other than what is on his dummy sheet.

    2) Eminence company politics requires him to not disclose the true size required by their off the shelf drivers because it makes them look bad.

    3) The T/S data published by Eminence is wrong, and only they know the right data, or he is looking at a different driver than you are.

    4) He works for Bin Laden. :D

    --------

    Some of our family friends are nurses. They say if they did 100% what the doctor directed for their patients, the doctors would kill their patients. So much for blind obedience.

    There are published methods for determining the Vas, Qt, Fs and other T/S parameters required for accurate tuning. The David Weems books have it, and most likely Vance Dickason's books do also. IMO you will be 99 44/100ths pure Ivory if you design to the published T/S specs, and verify the final tuning with a digital VOM.
     
  5. Hmmmmm....

    Part of the difficulty with this is that it's just not my field at all. Im sure you can understand my plight when a couple of authorities give completely different views on the same matter.

    The guy @ Eminence USA was Product Design Manager, Tom James. I'm not sure which of your catagories he might fit into.;) As for Mr Bin Laden, recent events have shown his influence probably extends far and wide. He might be a top speaker cab designer, too, but......:D

    It might be naive but I seem to be presented with a stark choice: I either build to the Eminence spec and get a sensible sized box or I build to those generated by such programs as WinISD and get a monster box. 'Thing is, I just can't accomodate a monster box.

    I sorta have to ask myself how flat are the cabs that you'd buy, anyway? And no system's completely flat for many reasons, not least of which is the fact that they've all got tone controls of some sort.

    What do you think might be the result of building a box to the Eminence spec? My experience with WinISD shows the response clearly won't be flat, but how much would that matter?

    I suppose, really, I'm, just hellish confused about the whole thing.:confused:

    Any further help gratefully received.

    Thanks.

    John
     
  6. The word "manager" pretty much gives it away.

    Those who can, do.
    Those who can't do, manage.
    Those who can't manage, teach.

    Next time, talk to a real engineer. Get armed with your T/S data and ask him point blank why he recommends 3 cubic feet and all the T/S calculations recommend 20+ cubic feet. If you get a song and dance, you've got a Nintendo-Boy and not an engineer.

    HiFi folks found out years ago if you want to reproduce a wide range of frequencies, specialized drivers are required. The typical bass cabinet has a couple 10s and a tweeter, and no mid range. The 10s are expected to carry the weight high enough for the tweeter to cut in. Many tweeters are piezo types with awful distribution and response curves.

    To prove this, hook your bass cab up to your stereo system and compare it against a good set of floor standing HiFi speakers. The coloration in tone, and lack of clear highs and crappy lows produced by the bass cab is exactly the same quality as when your bass amp drives it. If your stereo receiver has a 1/4" TS standard AUX jack, try to run your bass into your receiver. Play the bass through your HiFi speakers and see if you like the tone any better. Then plug in the bass cabinet and compare it when driven through the receiver.

    For the record, my JBL E110s significantly color the sound compared to my JBL floor standing HiFi speakers. VERY noticeable. The mental picture I get of the E110s is Bright Yellow as compared to Clear Glass of the HiFi speakers. YMMV.

    The box has to be usable to the lowest frequency you require. This is 31 Hz for a 5-string player. "Usable" is not interchangeable with "flat". First, this means the porting has to be close to 31 Hz or lower. I strive for 31 Hz exactly, as this is the least cone movement at the lowest usable frequency. Cone movement increases significantly about a half-octave above tuning frequency, then reduces again. The most cone movement is about a half-octave higher than the tuning frequency.

    The box could be easily -6 dB at 31 Hz and can still be corrected (kinda) with tone controls. Remember you are boosting the lows, and increasing cone movement at the low range. Hence the port tuning at 31 Hz. There is a practical limit here, so don't expect to turn up the bass EQ on the amp until it is perfect. It will get better, but still subject to the limitations of the driver itself.

    The box has to meet your size requirements. 20 cubic feet is ridiculous to consider for a portable rig. You can trade off any two of the three design parameters, so this leaves Efficiency as the one beyond your control. Low + Small = Inefficient. Break out the watts. You cannot have it all.

    http://www.trueaudio.com/st_trade.htm

    This is a nice clear explanation of why you can't have a speaker that is small, loud, and goes low.
     
  7. Ken Nahora

    Ken Nahora

    Jun 1, 2001
    bgavin wrote:

    "Those who can, do.
    Those who can't do, manage.
    Those who can't manage, teach."

    To that, let me add:
    Those who can't do anything, spend way too much time on the internet.

    To bgavin:
    I know Tom James; you're no Tom James.
     
  8. Luis Fabara

    Luis Fabara

    Aug 13, 2000
    Ecuador (South America)
    Audio Pro - Ecuador
    Tell that to Unix Administrators, Web Master´s , networking designers, programmers, computer related people, etc etc.

    Im a little offended with that comment, specially because I work around the internet and for the internet.
    Besides being a Music Instrument´s distributor, I do configs of Linux Servers, Intranets, ISP , etc etc.

    Also, the Internet is a great resource of Information, you can get as much knowledge you need or want, etc etc.

    Bsh..t..
     
  9. You are a newbie member here, so you deserve to be cut some slack, once, about flaming people and making personal attacks against them. I also see that your profile is completely empty, including your age, which means you are likely just a little fella hiding behind an alias so you can talk trash. Give it a rest please... nobody here is the slightest bit interested in your personal attacks.

    As for time spent on the Internet, I own a computer consulting firm that builds networks, servers, and database application development that is Internet-centric. All of us are connected to the Internet at all times, so it is easy to take a few moments to hit TalkBass to get some relief from computers.

    As for not being able to do anything, please put a stop to your flaming insults. You know nothing about me nor my abilities, nor about any of the other folks here that you insulted. There is an regular group of daily participants on TalkBass, and I'm sure none of them appreciate you calling them do-nothings.

    I'm glad you know Tom James; it's good for you to have a friend. The next time Tom has you over to dinner, please send him my thanks for publishing the complete Thiele-Small data for his line of speakers. Those of us who actively engineer and build speaker cabinets very much appreciate how much information he has made available.

    You win the power-of-observation prize: I am no Tom James. Tom is a manager, and I am an engineer, and almost never can one do the others' job well.
     
  10. yawnsie

    yawnsie

    Apr 11, 2000
    London
    It looks like Rockin John's question has been answered, so I'm shutting this thread. If anyone has a problem, PM me.
     



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