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How to model port resonance

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  • billfitzmaurice
    replied
    Huge in terms of amplitude, but hardly there in terms of band width. Artifacts of that sort tend to be greatly exaggerated by the software and seldom approach the prediction in practice. It may not be audible at all. I've never actually measured a port resonance anywhere near as severe as predicted.

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  • Turtle
    replied
    Originally posted by billfitzmaurice View Post
    WinISD does as well, check the 'Use transmission line model for port simulation' in the Advanced tab.
    It seems this is the answer for me. I'm getting comfortable with WinISD but didn't know about that option. Thanks for that.

    Assuming I'm reading it correctly, I'll have a HUGE spike at the port resonance. However, it seems that any port diameter I tried, results in a huge spike at port resonance. See the pic for visual reference.

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  • billfitzmaurice
    replied
    WinISD does as well, check the 'Use transmission line model for port simulation' in the Advanced tab.

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  • PWR RYD
    replied
    I like to use Unibox for my low frequency enclosure design and it will display the port resonance on the frequency response graph, the impedance graph, and the vent velocity graph. You only need to check mark the box at the bottom of the "vented box" data entry area.

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  • Turtle
    replied
    Originally posted by 4thtry View Post
    Do you have a measurement mic, such as OmniMic?
    No, not yet but will be purchasing one soon; I'm just starting to get semi-serious about building speakers. I've been lurking on the forums here, for a while though. The Plumber's Delight design is definitely interesting and is partly what got me thinking about the port resonance in the first place. I was just hoping I could model it somehow, before I start cutting holes. In this enclosure, I'd rather have the ports on the back of the box(small ports/high velocity) but going to a larger/longer port will require it to be on the top.

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  • 4thtry
    replied
    Do you have a measurement mic, such as OmniMic? If so, you could take a near field (NF) measurement of the port's output and see for yourself just how bad the 1st and 2nd port resonances look in relation to the main port resonance. This NF measurement technique will also show you how internal box resonant modes are competing with the main box resonance. Internal box resonant modes can be just as bad, if not worse, than port resonances.

    On my Plumber's Delight speakers (see signature link below), I used a very long port and then tuned out the 1st and 2nd port resonances with a quarter wave trap. This was done by trial and error, shortening the trap, bit by bit, until the resonances disappeared.


    Bill

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  • wogg
    replied
    Originally posted by Turtle View Post
    ...I might be over-engineering all this but hey, that's what makes it fun...
    Indeed! You'll fit in here nicely

    Personally I haven't done a long enough port on a full ranger to notice any resonance. Lookup "Plumber's Delight" for the 1/4 wave trap idea, it was really slick.

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  • Turtle
    replied
    Maybe I'm just worrying too much about the port resonance and should just go for the longer port anyway to avoid chuffing with a small port. I might be over-engineering all this but hey, that's what makes it fun...

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  • fpitas
    replied
    Originally posted by Turtle View Post
    In this instance, the enclosure size was predetermined. I have to work with what I have.


    The port length, physically, isn't an issue. I can make it fit without an elbow. I'm just concerned that the resonance may be audible, which is why I'm looking for an option to model it before building. The trap is an interesting idea though. You mentioned WinISD may have the option to model resonance, so I'll have to take a closer look when I get the chance.
    It tells you the frequency of resonance, but whether it's audible or not is a different question. Rear or bottom ports do tend to minimize that sort of problem.

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  • Turtle
    replied
    Originally posted by billfitzmaurice View Post
    Use a larger box and you won't need as long a port.
    In this instance, the enclosure size was predetermined. I have to work with what I have.

    Originally posted by fpitas View Post
    Understood that PRs aren't cheap. You can of course use elbows to get a longer port, but like you mentioned the port resonance might become a problem. I recall that somebody here designed a quarter wave trap that hung off the side of the port to squash the resonance.
    The port length, physically, isn't an issue. I can make it fit without an elbow. I'm just concerned that the resonance may be audible, which is why I'm looking for an option to model it before building. The trap is an interesting idea though. You mentioned WinISD may have the option to model resonance, so I'll have to take a closer look when I get the chance.

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  • fpitas
    replied
    Understood that PRs aren't cheap. You can of course use elbows to get a longer port, but like you mentioned the port resonance might become a problem. I recall that somebody here designed a quarter wave trap that hung off the side of the port to squash the resonance.

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  • billfitzmaurice
    replied
    Use a larger box and you won't need as long a port.

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  • Turtle
    replied
    I'm trying to keep my budget reasonable but if it makes more sense to go with PR's, that's what I'll do. I'm just not super comfortable with PR's and need to get a better understanding of the pros/cons associated and how to properly implement them in a design.

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  • fpitas
    replied
    I'm not a PR expert, but you should be able to add weight to the PR and get the F3 down where you need it.

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  • Turtle
    replied
    Originally posted by fpitas View Post
    As I recall, the Pro version of WinISD shows the port resonant frequency. If the correct diameter port is just too long, can you use a passive radiator?
    I've considered using a passive radiator but there are a couple of reasons I don't want to - Cost and higher F3 with a PR. If I remember correctly(not currently able to verify), my modeled F3 with the port is 53Hz and with a PR, it puts me in the neighborhood of 80Hz.

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