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How do you choose the .5 frequency for a 2.5 way?

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  • How do you choose the .5 frequency for a 2.5 way?

    I'm plamming on building a mmttmm center channel speaker from left over parts I have. I want this to be a 2.5 way to try to offset comb filtering and such. I understand that in a verticle 2.5 way the frequency for the .5 is largely determined by the frequency of what would normally be used for bsc.

    My question is this:

    Since the speaker is going to be horisontal instead of verticle I'm not sure that the conventional .5=bsc is a valid approach. How do I choose the inductor for the .5 woofers?

    The speaker will be quite wide as I'll be using four 6.5"s. It will serve as a base for a 50" TV and the baffle will be aproximately 22" from the back wall and the TV screen is 16" from the back wall.

    Thanks in advance.

  • #2
    Re: How do you choose the .5 frequency for a 2.5 way?

    Originally posted by hooligan72 View Post

    Since the speaker is going to be horisontal instead of verticle I'm not sure that the conventional .5=bsc is a valid approach..
    It isn't. Your main issue is not having comb filtering, so CTC is the primary consideration.
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    • #3
      Re: How do you choose the .5 frequency for a 2.5 way?

      CTC will be as tight as possible. I'm curious about how to choose the .5 inductor.

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      • #4
        Re: How do you choose the .5 frequency for a 2.5 way?

        Originally posted by hooligan72 View Post
        I'm plamming on building a mmttmm center channel speaker
        Is that a typo?
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        • #5
          Re: How do you choose the .5 frequency for a 2.5 way?

          Originally posted by hooligan72 View Post
          CTC will be as tight as possible. I'm curious about how to choose the .5 inductor.
          The short answer . . . by ear.

          The amount of BSC you want is dependent on placement. If you have the speaker under a screen that essentially extends the baffle, then you may not want any BSC at all.
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          • #6
            Re: How do you choose the .5 frequency for a 2.5 way?

            I'd probably sim the drivers as being 1/4 less than full BSC for the design and then tweak by measurements/ear to dial the crossover in.

            I noticed the same thing that Ryan mentioned on the two tweeters. I'd avoid using 2 tweeters in the design if you're worried about comb filtering.

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            • #7
              Re: How do you choose the .5 frequency for a 2.5 way?

              Four woofers in a horizontal configuration...

              Draw a ceiling view, looking down at your center channel and seating locations. With multiple woofers arranged horizontally, the path length of each woofer to seating position "A" will vary. Easy to see and understand when you draw the acoustic "rays" from each woofer to the seating position. Because each woofer's path length varies, that causes constructive and destructive interference patterns (comb filtering) due to some of the wavelengths being 1/2 wavelength out of phase with others.

              (Ray acoustics isn't valid for bass freqs but that's ok, in this design the comb filtering problems are located higher up in the frequencies where Ray acoustics is valid)

              As you move from seating position "A" to "B" and "C" etc etc... continue to draw the paths (rays) from each woofer to the seating position of interest. It will soon become clear to you that having multiple paths from multiple woofers is tough to deal with as you will always have some of the woofers 180 degrees out phase with other woofers. This isn't too bad when using a vertical array of drivers, however it becomes a serious issue when you are using a horizontal array.

              My recomendation might not sit well with everybody on this forum but here it is:

              1. Roll off the two "outer woofers" that are furthest away from the tweeter at a low frequency, like 125 Hz. That way the outer woofers will contribute to the bass, (+1) but not to the midrange problems (+1) that arise because of path length issues... and shorter wavelengths associated with midrange frequencies.(+1)


              2. I would not suggest using two tweeters in a horizontal design such as this. There is too much potential for them to interfere (comb filtering issues) with each other at higher frequencies. (shorter wavelengths)


              3. Furthermore, crossover the tweeter as low as you can safely do it without stressing the tweeter. Some of the more robust tweeter designs have low Fs... like 600Hz, so a 1400 Hz XO point is do-able. This will help with some of the problems. You want to keep the shorter wavelengths (high frequencies) out of the woofers passband as much as possible... specifically because there are multiple woofers in a horz arrangement and the higher frequencies will interfere (comb filtering) with each other. The higher the freq, the shorter the wavelength... so keep the shorter wavelengths on the tweeter and your comb filtering problems will be significantly better.
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              • #8
                Re: How do you choose the .5 frequency for a 2.5 way?

                Sound Advice!

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                • #9
                  Re: How do you choose the .5 frequency for a 2.5 way?

                  Originally posted by ReissM View Post
                  My recomendation might not sit well with everybody on this forum but here it is:
                  -Unless:
                  Curt's Speaker Design Works

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                  • #10
                    Re: How do you choose the .5 frequency for a 2.5 way?

                    Originally posted by ryanbouma View Post
                    Is that a typo?
                    Not a typo. The only reason I was thinking of using both tweeters is for efficiency. I don't have measurements on the tweeters so I can't be sure that they will keep up with the four woofers. I would certainly rather use one instead of two.

                    My plan was to have the tweeters vertically aligned. Would that help?

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                    • #11
                      Re: How do you choose the .5 frequency for a 2.5 way?

                      Thanks Curt. I think we both hit submit at the same time:o

                      I would just hate to put the whole thing together and find one tweeter doesn't cut it.

                      So a large inductor for the outside .5 woofers is the concensus I'm getting, right?

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                      • #12
                        Re: How do you choose the .5 frequency for a 2.5 way?

                        A larger inductor will be necessary in order to roll off the outer woofers. What that inductor will be is determined by the drivers and the crossover point. Take a look into the crossover for the stentorians to get an idea of what the crossover will look like for the bass section.
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                        • #13
                          Re: How do you choose the .5 frequency for a 2.5 way?

                          Originally posted by isaeagle4031 View Post
                          A larger inductor will be necessary in order to roll off the outer woofers. What that inductor will be is determined by the drivers and the crossover point. Take a look into the crossover for the stentorians to get an idea of what the crossover will look like for the bass section.
                          I just looked up the Stentorians and that is exactly what I have in mind, but with two .5 woofers instead of four. Series parallel for the woofers with the large inductor on one of the series pairs.

                          125hz sounds kinda low, but I guess with that portion of the crossover being 6db slope it should blend well. I don't want to end up with something that's over boomy.

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                          • #14
                            Re: How do you choose the .5 frequency for a 2.5 way?

                            Generally speaking, most of the 2.5s I have seen roll off around the 200-300 mark. The lower you roll it off the more expensive the inductor is going to get. The stentorians roll of the lower section I think around 300
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                            • #15
                              Re: How do you choose the .5 frequency for a 2.5 way?

                              Originally posted by hooligan72 View Post
                              Thanks Curt. I think we both hit submit at the same time:o

                              I would just hate to put the whole thing together and find one tweeter doesn't cut it.

                              So a large inductor for the outside .5 woofers is the concensus I'm getting, right?
                              When doing the tweeters think something like the ND16 or the face-plat-less vifas. They will allow the closest C2C possible.
                              .

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