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subwoofer project

Discussion in 'Electronic Design' started by Suraj, Aug 15, 2004.

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  1. Suraj

    Suraj Guest

    Hi

    I am trying to build an 8" subwoofer using a driver unit from "Dainty
    Speakers",Taiwan.The specifications written on it says that it can
    handle 200W RMS.There are no other datasheets or any Thiel-Small
    parameters accompanying the drive unit.Now that makes my effort of
    making a suitable enclosure more difficult.I am looking to bulid some
    sort of bandpass enclosure for use with my home theater system.I have
    designed my subwoofer active filter with a frequency bandwidth of 30Hz
    to 105Hz.I dont expect the 8" driver to handle frequencies below
    .....say 25Hz.

    Now,can somebody help me with making a suitable bandpass enclosure for
    this particular drive unit.I am planning to use LM3886 as the
    amplifier for this purpose.

    Thanks in advance

    Suraj
     
  2. Guy Macon

    Guy Macon Guest

    Chances are that the 200W RMS figure is a bald faced lie. It might be
    100W or it might be 50W.

    The LM3886 is a 68W @ 4 Ohm / 38W @ 8 Ohm part. Is that enough?
    Have you considered a National BPA200?

    How loud do you need it to be? At 30Hz, an 8" speaker can't move
    enough air to be very loud. Some speakers are a bit better by
    having a large excursion, but I doubt that yours is one on them,
    and they have a tendency to cause chuffing at the port at low
    frequencies.

    If you know nothing about the speaker, you must either figure out
    how to measure the parameters or make a test enclosure that is made
    in such a way that you can change it's properties, do a bunch of
    experiments, then make your final box.

    How much is your time worth? You can get an EIGHTEEN inch subwoofer
    in a well-designed enclosure that handles an honest 800 watts RMS
    for 399.99 (free shipping). See http://www.carvin.com for details.
     
  3. Tim Wescott

    Tim Wescott Guest

    200W RMS for one cycle only? Perhaps 200W RMS for a nice saturating
    square wave, no sine waves allowed?

    Or maybe they're just lying.
     
  4. Like you, I suspect the spec is worthless.

    But just for the sake of obsessiveness: wouldn't a 200W sine wave be easier
    on a loudspeaker (the OP's concern) than a square wave would? The other way
    around for an amplifier, of course.
     
  5. mook johnson

    mook johnson Guest

    You didn't by chance by that speaker from a guy in a white van did you? If
    so, toss it, it is not worth the time (search google for "White van
    speaker").

    Bandpass enclosures are very particular about which drivers are suitable for
    them and the box is a royal pain to build compared the a ported or sealed.

    If you have time to burn and a few bucks for a trip to radioshack, try this
    out.

    www.speakerworkshop.com/ Use this program to measure the T/S parameters
    for your driver.

    then go here for a free speaker box design webpage and see how you driver
    works in that box (BP4).

    http://www.linearteam.dk/default.aspx?pageid=isdonline


    If you want a possible real sub got to www.madisound.com and check out what
    they have. Good stuff at reasonable prices. There is a web forum there for
    help as well.
     
  6. justin

    justin Guest

    Great advices but someone should tell the OP that using 8" driver as a
    SUB woofer is meaningless. Regardles of the enclosure type, all it will
    likely do is induce a hump in the overall system response at 80Hz or
    so...


    j.
     
  7. Tim Wescott

    Tim Wescott Guest

    Well, yes, but I was just trying to think of how an honest marketing
    guy* would spec a 200W amp that's based on a 50W part.
     
  8. Ban

    Ban Guest

    Suraj,
    I'm not familiar with your 8", but since there are not even TSP available, I
    suspect it is a very cheap chassis. Those tend to have small magnets and
    therefore a high Q-factor. This rules already out a vented enclosure. If you
    make a big closed enclosure(60L), you might get an acceptable frequency
    response. If the enclosure is smaller, the things Justin says above apply.
    You get a Boom-box. :-(
    You can still use a smaller box, if you can modify the amplifier for a
    negative output impedance, but that requires some knowledge.
    A band-pass enclosure is difficult to build and requires to determine the
    TSP first. It will be very big too, if you want to reach 30Hz. It will be
    also difficult to reach the 105Hz as an upper frequency, usually the
    bandpass enclosures are only good for 1 octave.
    There are a lot of moderatly priced 12" out there, Usher is a very good
    brand, Monacor makes some very interesting subs Raptor line, also Peerless
    XL12.
    If you want to built something yourself I also recommend "The Loudspeaker
    Cookbook" by Vance Dickason.
     
  9. Ken Smith

    Ken Smith Guest

    If you design the enclosure so that it is basically a horn with the 8"
    speaker as a driver at the base, the bottom end of the frequency responce
    can be way lower than the speaker's natural frequency. The downside is
    size.

    The horn presents a high impedance to the cone of the speaker and thus
    lowers the resonant frequency.

    At subwoofer frequencies, base reflex still works too. Again the downside
    is size. The box gets quite big by time you fold up the back end path.
     
  10. Suraj

    Suraj Guest

    Hi,

    Thank you all for the help extended to me. Now,I am from India and it
    is really difficult to get good quality audio stuff here.Luckily we
    have distributors for National :) Even I feel that the 200W RMS
    sticker is just a lie.It can hardly handle a single LM3886.So I think
    that there is no need in looking for the BPA200 design.I am
    downloading the "Speaker Workshop"software.Let me give it a try...A
    special thanks for Mook Johnson for those links.

    I feel that this would eventually turn out to be a time consuming
    process and I will finish it in my spare time.:)

    Suraj
     
  11. Rich Grise

    Rich Grise Guest

    Peak Envelope Power ;-)

    Cheers!
    Rich
     
  12. justin

    justin Guest

    I agree with you - given that Suraj wants to make a sub for a HOME
    theater system - this is a bad choice. Mainly in terms of the size of
    the enclosure that would end up being the size of a fridge.

    Efficiency, distortion and phase shift would end up being really bad
    and overall this project would end up as a disappointment and a waste
    of time.

    Suraj, if you can't get better driver, as a last resort try using 4 of
    the same in one box. This will result in an equivalent of a single 50"
    driver. With an active x-over this may actually end up sounding quite
    good. There is a lot of info at Linkwitz site and also here
    http://www.trueaudio.com/st_lkxfm.htm

    Amp design will become easier too.

    j.
     
  13. Guy Macon

    Guy Macon Guest

    four 8" drivers are the equivalent of a 16" driver, not a 50" driver.
     
  14. Ken Smith

    Ken Smith Guest


    Not even that. A 16" driver will usually have a larger travel as well as
    area.
     
  15. Nico Coesel

    Nico Coesel Guest

    The WAF rating (Womens Acceptance Factor) of horn speakers is very
    low. But the sound is excellent. A few months ago I've visited
    somebody who build his own horn speakers. He had a 6W class A
    amplifier. That was enough to move the stomage...
     
  16. justin

    justin Guest

    Yes, that line was supposed to read

    "...this will result in an equivalent of a 50 sq in per driver..."

    and then I accidentaly left out the obvious.

    Thanks for the clarification.


    j.
     
  17. Guy Macon

    Guy Macon Guest

    Emphasis on t*per* diver. :)

    8" driver = 50 square inches.
    4ea 8" drivers = 200 square inches.
    16" driver = 200 square inches.

    I once saw a design using a 16x16 matrix of small speakers, with
    clever series/parallel wiring to give the ones on the edges less
    power. I don't remember the details but it had an effect on
    dispersion IIRC.
     
  18. Bessel array.
     
  19. Guy Macon

    Guy Macon Guest

    That's the term that I was trying to remember. It's been over 20
    years since I last read up on them. Lots of stuff on the web now
    so I can reeducate myself. Thanks!

    http://citeseer.ist.psu.edu/rd/8534450,484862,1,0.25,Download/http://c
    iteseer.ist.psu.edu/cache/papers/cs/24096/http:zSzzSztyche.mat.univie.ac.atzSz
    JanssenzSzp100.pdf/on-analytic-design-of.pdf
    http://www.pesupport.com/cgi-bin/config.pl?read=180824
    http://www.audioasylum.com/forums/hug/messages/74240.html
    http://ldsg.snippets.org/ALSR/taper.html
    http://www.livejournal.com/users/crowinsnow/398585.html?mode=reply
    http://www.google.com/search?"Bessel+array"
     
  20. Ban

    Ban Guest

    But for a subwoofer you have already a perfect dispersion, the wavelenght is
    very long. The Bessel-array was used and patented by Philips around 1983. It
    uses only 1 amplifier and the speakers are wired normal(+1) invers(-1) and
    in series (+/- 1/2). For a seven weighting factor array we get:
    1/2 1 1 0 -1 1 -1/2 since the middle value is 0 this
    speaker can be omitted. the speakers have a distance of "d" from each other,
    the omitted speaker also has its place.
    The Bessel panel avoids the lobing and forward concentration of the radiated
    acoustic energy.
     
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