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LM3875 and speaker impedance questions

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by whiz115, Feb 2, 2007.

  1. whiz115

    whiz115 Member

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    here they are...

    470mm Height
    245mm width
    205mm deep
    20mm thickness


    the cone is ~25mm deep it's difficult to measure it...the coil looks really huge.
    (my expiriance with speakers is kinda limited but i believe it's huge, it's almost 40cm i can't see it well from outside, because it's that cotton round thing in the center of the cone but it's big enough! that round thing it's almost ~50mm

    i have downloaded winISD...and i tried to put the data...but i didn't knew what is LE BL and Xmax are, the output was

    768mm Height
    480mm width
    288mm Deep
    106.1 liters

    i don't know if that is correct... :rolleyes:

    i haven't tried yet the zobel network with the 4,7 resistor..... i'm boring :rolleyes: :rolleyes: i'll try it later! :)
     
  2. hentai

    hentai New Member

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    well ur box is about 17l even less.
    In a closed box of 17l u'll have a huge Qtc therefore a boomy bass. In a vented box of the same sizes u'll still get a boomy bass a swallow around 100Hz in both cases.
    So for better bass u should either change the box to a bigger one preferably sealed (for good bass reflex u need even a bigger one) or try as Audioguru sugested an active egalisation, a practice very common.
    Yes the result from WinISD is correct and its one of the variants u have, u chould change the volume and see what happens.
     
  3. whiz115

    whiz115 Member

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    what is boomy? :D

    if it is not difficult can you explain the meaning of each one of the woofer parameters? these parameters refer only to woofers? not tweeters?

    i think my speakers are totally crap! :D numbers make them look even worst........... :rolleyes:

    Hentai thanks for all the things you're teaching me.
     
  4. dave

    Dave New Member

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  5. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Boomy is a big peak in the sound at 100Hz or higher in the case of your woofer in the tiny enclosure. It is called "one-note-bass" and drops off quickly at lower frequencies. Good expensive woofers also sound boomy when their enclosure is too small. Yours would sound much better if the enclosure is double the volume it has now.

    You should be able to use the speaker design software to show what happens to the frequency response when the enclosure volume is changed.
     
  6. Hero999

    Hero999 Banned

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    What actually causes it?

    An educated guess: resonance of the case?
     
  7. whiz115

    whiz115 Member

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    the one i already have? WinISD?
     
  8. hentai

    hentai New Member

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    Most speaker modeling software allows u to do that, WinISD included.
    About boomy bass The peak in response is cause by the way the woofer and the box match eachother. There is a parameter called EBP (u can see it in WinISD when it recomends a type of box) its efficiency band product and its the ratio between electrical Q (Qes) and the resonance frequency (Fs) and this tell u which box is better for the driver. If EBP<60 it whould work better in sealed systems if EBP>60 it will work better in bass reflex. Although an EBP between 50 and 70 could be used in both boxes. When u use a woofer with an EBP <50 in a bass reflex the sound will be boomy. when u use a driver with EBP>70 in a sealed system the sound will lose from bass like a low Q speaker.
    The boomyness is determined in sealed systems by Qtc if this is bigger than 1 then it will sound boomy. Most sealed system are designed for a Qtc 0.707.
    U can look at it this way. The box along with the woofer forms a high pass filter. Depending on the Q of this filter the speaker will sound boomy or shallow in bass.
    Some notes on TS parameters. Most of these parameters apply for the woofer for at lower freq things become problematic. Fs is the free air resonance frequency, this manifests as a big peak in the impedance caracteristic. Qes is the electrical Q, this has to do with the motor of the driver, the ensemble of magnet system and voice coil. Qms is the mechanical Q and it has to do with the mechanical parts the cone material the surround the spider net. Qts is the total Q and its Qts= Qms*Qes/(Qms+Qes)). Vas is the volume of air that has the same compliance as the driver's suspension. BL this shows how good the motor is. Even if the magnet is heavy if BL is low its no good. Alot of third rate companies sell iron. Re is the dc resistance of the voice coil. Le is the inductance of the voice coil, with this u can aproximate how it will handle high frequency if Le is very high than the woofer's response will be narrow. Sd is effective surface area of the driver, if u want a driver to go as low as posible than u need high Sd to push the air. There are also Cms (mechanical compliance), frictional force factor Rms, moving mass factor Mms,
    these parameters depend much on mechanical caracteristics cone type of suspension the coupling with the spider net and help determine the forces needed to drive the cone through air therefore SPL.
     

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