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  • Port length question

    Hi,

    I am modelling a Faital 6FE100 in a 2 way, crossed around 2KHz.

    My idea is to eventually used high passed with a SW, but sometimes I would like to use them as "full range", so I am tuning them to 65hz in a 18 liters box. The thing is that the port, according to WinISD, is 4 centimeters long when using a 5cm diameter tube.

    Isn't that too short? I recall reading that a port under 6 inches long, behaves more like a leaky box and less than a ported speaker.

    If this is true, what are my alternatives? I can use a bigger tube but it does not seem correct.

    Here is the WinISD project:
    https://1drv.ms/u/s!AoNwvVYR7XiO2RKb...kDA19?e=omcAlr

    Thanks in advance for any guidance or recommendations.



  • #2
    Why not go 7.5cm dia. x 12cm long?
    Also, you might be going for the box/tuning adding some BSC (and it will), but that driver looks like it would love a bit larger box (1cf = 28L) w/the same port gives a tuning in the low 50s and an F3 in the mid 40s (as opposed to the high 50s).

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    • #3
      Chris, thanks for your reply. yes, the driver can hit mid 40s in a bigger box. WinISD recommends a 45 liters box, but I don't want a box that big now, so I traded size for low extension, and settled in a tuning of 65hz. Besides, at a 45hz tuning, it hits xmax at only 50w. so, you recommend just going with a higher diameter port? Sent from my Pixel 4a using Tapatalk

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      • #4
        Originally posted by mga2009 View Post
        I recall reading that a port under 6 inches long, behaves more like a leaky box and less than a ported speaker.
        If this is true
        It's not true. Once upon a time ports had no ducts, other than the thickness of the baffle. The port area was made the same as the cone area, and tuning was accomplished by changing the box volume. Ducting a port allows smaller port area, smaller box volume and tuning of Fb via the duct length. The area of the port should be made as large as required to keep port velocity less than 20m within the speaker pass band at maximum anticipated power input.
        Last edited by billfitzmaurice; 02-07-2021, 11:26 AM.
        www.billfitzmaurice.com
        www.billfitzmaurice.info/forum

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        • #5
          And here I thought I knew all the bizarre internet myths. That one is especially ironic considering the bad reputation that ducted ports had for many years.
          Francis

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          • #6
            I don't know about ducted ports having a bad reputation, but the reason for making port areas equal to or even larger than cone area was that it was thought by many that anything smaller would restrict the port air flow, causing response problems. By the 1950s it was generally realized that Bernoulli's Principle applied to ducted ports, and that opened the door for ported enclosures smaller than a steamer trunk.
            www.billfitzmaurice.com
            www.billfitzmaurice.info/forum

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            • #7
              Originally posted by billfitzmaurice View Post
              I don't know about ducted ports having a bad reputation, but the reason for making port areas equal to or even larger than cone area was that it was thought by many that anything smaller would restrict the port air flow, causing response problems. By the 1950s it was generally realized that Bernoulli's Principle applied to ducted ports, and that opened the door for ported enclosures smaller than a steamer trunk.
              Right away though, it got abused. In the search for smaller cabinets with WAF, some companies crammed inappropriate woofers into small boxes with ducted ports. They hooted and boomed and sounded bad.
              Francis

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              • #8
                I don't recall that from personal experience, but the last time I bought a speaker in a store it was 1973, and I only did so because at the time I was a touring musician and I didn't have access to a workshop.
                www.billfitzmaurice.com
                www.billfitzmaurice.info/forum

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                • #9
                  Originally posted by billfitzmaurice View Post
                  I don't recall that from personal experience, but the last time I bought a speaker in a store it was 1973, and I only did so because at the time I was a touring musician and I didn't have access to a workshop.
                  I was a young sprout listening to speakers in actual listening rooms. I guess those days are more or less gone. I mainly remember liking JBL woofers and Altec horns lol.
                  Francis

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                  • #10
                    Well, if the 6 inches length is only a myth, then I don't have to worry, and I will just use one 4cm long 5cm diameter flared port.

                    That gives me a 48 m/s rear port air velocity at 64W which is where I hit XMAX.

                    Would you recommend using 2 ports instead of the same diameter? That brings the air speed on the port to under 30 m/s but the first port resonance is around 1458hz, which is well under the crossover point (somewhere between 2KHz and 2.5KHz). Do you think this port resonance is manageable with some 1" lining inside the box?

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                    • #11
                      Why not go 7.5cm dia. x 12cm long?

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                      • #12
                        Originally posted by mga2009 View Post

                        That gives me a 48 m/s rear port air velocity at 64W which is where I hit XMAX.
                        That's twice what I'd settle for, unless I wasn't intending to drive it that hard.
                        first port resonance is around 1458hz, which is well under the crossover point
                        The crossover point is moot. All you care about is that port resonance output is at a minimum, which is very much related to port velocity. The lower the port velocity the lower the port resonance and the higher the resonance output, so you have to be looking at both to come up with the best overall result.
                        Do you think this port resonance is manageable with some 1" lining inside the box?
                        Lining the box doesn't affect the port resonance frequency. It must be lined anyway, to minimize internal reflections.

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

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                        • #13
                          Originally posted by Chris Roemer View Post
                          Why not go 7.5cm dia. x 12cm long?
                          I don't have a tube of that diameter at hand.

                          My other alternative is a slot port. 18cm x 3cm and 15cm long which gives an air speed of 17 m/s at Xmax (around 60W), well under the recommendation of BFM. The resonance of the port is 1.133Hz.

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                          • #14
                            A slot port that narrow brings up its own set of problems. My preference is for corner ports, as they also brace the panels.
                            www.billfitzmaurice.com
                            www.billfitzmaurice.info/forum

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                            • #15
                              In that case, a pair of 5cm x 12cm long will do.

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