Speaker enclosure calculations

Discussion in 'The Projects Forum' started by cumesoftware, May 17, 2008.

  1. cumesoftware

    Thread Starter Senior Member

    Apr 27, 2007
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    I'm calculating a speaker enclosure for my Quatour project. However I want to do this in a decent manner. The speakers I'm using are Visaton models, the FR10 4OHM models, and have the following characteristics:
    - Nominal impedance (Z): 4Ohm
    - Frequency response: 80–20000Hz
    - Maximum cone displacement: 8mm
    - Resonance frequency (fs): 92Hz
    - Diameter: 100mm
    - Mechanical Q factor (Qms): 2,29
    - Electrical Q factor (Qes): 0,7
    - Total Q factor (Qts): 0,54
    - Equivalent volume (Vas): 2,3l
    - Effective piston area (Sd): 50cm²

    I concluded that a ported enclosure would be better suited. Using the calculator (from ajdesigner.com) I came to the volume of 6.31 litres for a SC4 alignment. This alignment gave me the flattest frequency response and a reasonable enclosure volume. Indeed it enhances the bass, but only to the 0bD level (exacly what I want). The box should be suitable for all kinds of music, and therefore should have the flattest response possible. Besides SBB4 gave me a value of 3.14 litres, too small and QB3 gave me a value way too large. So far, SC4 is the most adequate alignment.

    I double checked this values using the basic car audio electronics website. I need someone too see if anything is wrong, or if I'm choosing the wrong design.

    Also, I don't know what SBB4, QB3 or SC4 means in terms of port alignment. Is there any website with examples on port alignments for ported designs?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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  3. cumesoftware

    Thread Starter Senior Member

    Apr 27, 2007
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    Thanks!

    I've been checking the website you gave, and I've found it a very useful resource. Indeed, the calculators there also confirm the values I gave, although with some rounding errors. It seems that the calculator for ported enclosures assumes a SC4 port alignment, although not specified.

    I wonder what SBB4, QB3 and SC4 means. I've searched google, but found no answer.
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    It is a little 4" speaker similar to a little RadioShack speaker that was in a cast metal enclosure. No bass.
    Frequencies above only 6kHz are all over the place.
    Its efficiency is low so its max output level is low.
    It is like my little computer speakers.

    I lost my speaker enclosure design software on the broken hard drive of my old computer.
     
  5. cumesoftware

    Thread Starter Senior Member

    Apr 27, 2007
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    It is the speaker I can use. It is the most suitable for my project and sounds better than depicted on the chart. I simply can't use a bigger speaker, due to space issues. Remember that these speakers are meant to be multimedia speakers.

    About the efficiency, you are right. 86dB SPL is very low, but quite audible still. Gives about 92dB at full volume. Of course, if you buy a cheap speaker like the one RS has you get about 80dB (or even less) in real measurements, quite normal for cheap speakers.

    Besides, the TDA2003 has considerable distortion. Thus it is not worth investing on expensive speakers.

    By the way, is the port diameter related to frequency tuning? The software gave me a bunch of lengths for different diameters. Is there any specific diameter I should use?
     
    Last edited: May 17, 2008
  6. Audioguru

    New Member

    Dec 20, 2007
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    The diameter of a port and its length determines the frequency it resonates at and its air speed at full power. If the air speed is too high then you hear it SHHOOSHING.
    For such a low power I don't think you will hear the air noises from the port.

    Will you add a sub-woofer to fill in the missing two low octaves of music?
     
  7. cumesoftware

    Thread Starter Senior Member

    Apr 27, 2007
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    Well, the software didn't admitted ports below 20mm diameter (due to the noise). For each diameter I've obtained the following values:
    - 20mm diam: 17mm length
    - 25mm " " : 31mm " "
    - 30mm " " : 50mm " "
    - 35mm " " : 72mm " "
    - 40mm " " : 98mm " "
    - 45mm " " : 128mm " "
    - 50mm " " : 162mm " "
    My question is, is there a diameter and length I should use in particular? Or any of the former will do?

    I'm not planning to add a subwoofer, as this would change the whole project (including having to build a supply that doesn't exist on the market due to the increased load) and the whole purpose of this too (acceptable sound for low price). But don't worry. The F3 with the enclosure will be about 60Hz. Better, no? Plus, the TDA2003 doesn't handle frequencies below 40Hz, so I wouldn't stress too much on this.

    I'm planning to make a better project that uses a subwoofer.
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    A high power woofer needs a big port diameter to reduce the speed of the air in the port and its resulting noises. The power in your speaker is low enough to use the 20mm diameter which could be even smaller.

    The port and cabinet resonances are adding about 1 octave to the bass which makes it sound boomy and not "tight".
     
  9. cumesoftware

    Thread Starter Senior Member

    Apr 27, 2007
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    What do you mean by boomy and not tight? Is that good or bad?

    Does the length of the port counts with the enclosure thickness, or do I have to add the thickness to the calculated length?

    And what SC4 means in terms of alignment?
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    A boomy sounding woofer does not have enough damping so it slowley starts building up loudness at the system's resonant frequency then it continues resonating after it is supposed to stop.

    A tight sounding woofer is well damped and plays the low frequencies the way they should be heard. It starts and stops quickly when it should.

    The length of a port is its inside length.

    A ported enclosure is a 4th order filter (SC4?) with a rolloff at 24dB per octave.
    A sealed enclosure is a 2nd order filter with a rolloff at 12dB per octave.

    A properly designed ported speaker system can have its low frequencies extended if its port is tuned too low and a peaked 2nd order highpass filter is added. Then the combination is a 6th order filter with a cutoff at 36dB per octave.
     
  11. cumesoftware

    Thread Starter Senior Member

    Apr 27, 2007
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    Indeed. Tightness is the advantage of a sealed system, since the air prevents the excessive vibration of the driver. But this speaker isn't suitable for use in such systems.

    Someone at diyAudio confirmed that the calculations I've made were for a C4 alignment (the software is wrong). Nevertheless, this alignment is the one that gives the flattest response system.

    About the port, that someone told that I should use the largest port possible, as long as the space between the port and the back has at least one port diametre.

    Here are some images of the simulations:
    SBB4.png
    QB3.png
    C4.png
     
    Last edited: May 18, 2008
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