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Impact of different Qtc for closed box woofer performance

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  • Impact of different Qtc for closed box woofer performance

    Hello,

    I have now measured the parameters for my woofers (Dayton RS270-8, very consistent values but differ slightly from the data sheet) and the next step is to decide on what to do with my existing boxes:

    With the chosen driver, I get a net volume of 21 litres. My simulation SW show the following possible designpoints:

    21L Qtc= 0,885 F3=53,5Hz
    27,5L Qtc=0,8 F3=50,6Hz
    38L Qtc=0,707 F3=49,6Hz

    The 21L-point is ”easy and comparably for-free” in that I can use the boxes more or less as-is. The 27,5L point means adding volume in the back of the box, a bit tricky but doable. The 38L option is really no option in reality.

    Now, what do you suggest? Is lowering F3 with 3Hz worth the woodwork effort of enlarging the box? Does the different Qtc values really show itself in performance?


    Best regards from your never-tiring-swedish-friend…

    //lasse
    Perry Mason talking to his dentist:

    "Do you swear to take the tooth, the whole tooth and nothing but the tooth, so help you God?"

  • #2
    Is that bare, lined, stuffed or stuffed and compressed, Qa 100,50, 10 and 5 respectively?
    www.billfitzmaurice.com
    www.billfitzmaurice.info/forum

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    • #3
      Originally posted by billfitzmaurice View Post
      Is that bare, lined, stuffed or stuffed and compressed, Qa 100,50, 10 and 5 respectively?
      Hi,

      I am not that familiar with the term Qa but the simulation choises are: "Walls covered" and "no leaks".

      Does this make sense??

      Regards//lasse
      Perry Mason talking to his dentist:

      "Do you swear to take the tooth, the whole tooth and nothing but the tooth, so help you God?"

      Comment


      • #4
        .707 is defined as maximum flat delay w 0 db ripple
        .8 will give approx .213 peak db ripple
        .885 will be close to .687 peak db ripple

        re LDC Table 1.0
        "Not a Speaker Designer - Not even on the Internet"
        “Pride is your greatest enemy, humility is your greatest friend.”
        "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

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        • #5
          I think you would be hard pressed to hear the difference between 21 L and 27.5 L. You can always stuff some poly or wool in the box to slightly lower the Qtc.
          Craig

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          • #6
            I would look at the 30-40 hz range, and what the room is doing to that range. I recently reduced a box size as an experiment of sorts, to see how small of a box I could get away with. The bass in my room changed a lot! I will be going back to a low Q alignment when I build the final boxes.

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            • #7
              I typically go with sealed Qts of .707 or if that is unreasonably large go as big as I can.`I have tried lower q designs and they're just not punchy enough for me.
              I would try the 21L boxes and see what you think. Try it fully stuffed also.

              Comment


              • #8
                Originally posted by PWR RYD View Post
                I think you would be hard pressed to hear the difference between 21 L and 27.5 L. You can always stuff some poly or wool in the box to slightly lower the Qtc.
                I agree with this. When I first started building speakers 25 years ago, I always aimed for a Qtc of 0.707 because flattest and most extended sounds best right? Since then though, I've realized I like the added thump and power handling a higher Qtc provides. Now I aim between 0.8 - 0.9 for larger woofers and between 0.9 - 1.0 for smaller drivers. Try the 21L and add some polyfill if it's a little too boomy for you.

                Dan
                _____________________________
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                • #9
                  Originally posted by DanP View Post

                  I agree with this. When I first started building speakers 25 years ago, I always aimed for a Qtc of 0.707 because flattest and most extended sounds best right? Since then though, I've realized I like the added thump and power handling a higher Qtc provides. Now I aim between 0.8 - 0.9 for larger woofers and between 0.9 - 1.0 for smaller drivers. Try the 21L and add some polyfill if it's a little too boomy for you.

                  Dan
                  The higher power handling is due to lower excursion at low frequencies. Lower excursion = less bass output. However, your opinion is still reasonable for a speaker with a low f3. My speakers f3 is higher than the speaker being discussed by the OP. With a higher Q, I loose some output in the 40hz range.

                  Comment


                  • #10
                    Originally posted by lasse View Post
                    I am not that familiar with the term Qa but the simulation choises are: "Walls covered" and "no leaks".
                    Whatever software that is it's not exactly precise. The effects of Qa for absorption losses, Ql for leakage losses and Qp for port losses can all be seen in WinISD 0.7.

                    www.billfitzmaurice.com
                    www.billfitzmaurice.info/forum

                    Comment


                    • #11
                      My (1973) Large ADVE\TS ended up w/a Q near 0.90, but - their F3 was a lot closer to 40Hz than 50. I'd find that bass fairly lacking (but 49 wouldn't be noticeably different IMO).

                      If your measured Qts, Fs, and Vas were 0.47, 30, and 3.4cf - you could duplicate the ADVENT air-suspension response by using a 1.5cf vented box, tuned to 22Hz.

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                      • #12
                        I like a Qtc factor between 0.8 and 0.9 also. I have found that this results in full sounding bass with good power handling. My most recent build had a Qtc of 0.88.

                        I would suggest trying the existing 21L boxes. If your woofers are new, they may sound thin at first. Give them some time to break in before you consider any changes.
                        Last edited by Billet; 10-25-2018, 02:32 AM.

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                        • #13
                          The lower the bass, the tighter and leaner I like it = Qtc 0.5 = big subwoofer boxes. The smaller the driver - the higher the Qtc. I'd target ~ Q of 0.6 - 0.707 for a 10"

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                          • #14
                            Hi all,



                            And thanks for all kind support in this thread.



                            I just returned from the timberyard (is this a valid word??) with the fiberboard needed for the front baffle assembly. (Unfortunately, one had to buy a full 2 square-meter board so now I have some spare board in the workshop…)



                            I decided to follow what most of you suggest, and that is to try the 21L variant first. I plan to let the removable back baffle be removable for now and for the first listening period, but tightly fastened. If I detect something that make me think that 9 extra litres would make my life more complete, then I will fix that then. If it sound good as-is, then I will glue the back in place, of course.



                            Next challenge in this build will be gluing pieces of fiberboard together ”face-to-face” and get an even layer of glue over the whole area of contact.



                            Again, thanks for your kind support and advice (Sometimes, I feel like I´m the last speakerbuilder left in Sweden…)



                            Best regards//lasse
                            Perry Mason talking to his dentist:

                            "Do you swear to take the tooth, the whole tooth and nothing but the tooth, so help you God?"

                            Comment


                            • #15
                              Hi all,



                              And thanks for all kind support in this thread.



                              I just returned from the timberyard (is this a valid word??) with the fiberboard needed for the front baffle assembly. (Unfortunately, one had to buy a full 2 square-meter board so now I have some spare board in the workshop…)



                              I decided to follow what most of you suggest, and that is to try the 21L variant first. I plan to let the removable back baffle be removable for now and for the first listening period, but tightly fastened. If I detect something that make me think that 9 extra litres would make my life more complete, then I will fix that then. If it sound good as-is, then I will glue the back in place, of course.



                              Next challenge in this build will be gluing pieces of fiberboard together ”face-to-face” and get an even layer of glue over the whole area of contact.



                              Again, thanks for your kind support and advice (Sometimes, I feel like I´m the last speakerbuilder left in Sweden…)



                              Best regards//lasse
                              Perry Mason talking to his dentist:

                              "Do you swear to take the tooth, the whole tooth and nothing but the tooth, so help you God?"

                              Comment

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