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  • Turtle
    started a topic How to model port resonance

    How to model port resonance

    I'm looking for recommendations/software/etc. for modeling port resonance. I have a calculation to determine what the frequency would be but I'm concerned it may be at an audible level at higher power. I've been using WinISD to model port velocity and such but I don't see an option to model port resonance.

    I'm currently working on a small enclosure that is forcing me to choose between a small diameter port(with velocity higher than I'd like) or a larger port which puts me at a lower port resonance than I'd like. I'd like to make an educated decision if I can.

    Any help or suggestions would be greatly appreciated.

  • fpitas
    replied
    Agree that elbows are fine where necessary. Use a radiused elbow, with a sweeping curve to minimize flow loss.

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  • Millstonemike
    replied
    Originally posted by Turtle View Post
    Thanks for the information and reassurance. With this particular build, the enclosure volume was predetermined, so I chose a woofer I thought would work well. Based on the dimensions of the enclosure, I could put the port on the top surface or on the rear(preferred) with an elbow. This is assuming a 1.75" port, which is what I'm planning. This port size should minimize any chuffing at full-tilt boogie(up to 50W); I modeled roughly 18 m/sec port velocity at 25W.

    I plan to do a build post in the appropriate sub-forum, to document my work on this one, and future builds as I progress. While I'm not exactly "new" to electronics or loudspeakers/sound reinforcement, I am just now starting to get serious about learning theory and so on. Previously, I've just done generic "slap a woofer/tweeter in a box" type of stuff. My long-term goal is to learn the theory, and then apply my personal style to building speakers/crossovers. For better or worse, I tend to learn by hands-on, DIY, type of projects.
    Elbows are fine. Use the center line distance for length calculations.

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  • Turtle
    replied
    Thanks for the information and reassurance. With this particular build, the enclosure volume was predetermined, so I chose a woofer I thought would work well. Based on the dimensions of the enclosure, I could put the port on the top surface or on the rear(preferred) with an elbow. This is assuming a 1.75" port, which is what I'm planning. This port size should minimize any chuffing at full-tilt boogie(up to 50W); I modeled roughly 18 m/sec port velocity at 25W.

    I plan to do a build post in the appropriate sub-forum, to document my work on this one, and future builds as I progress. While I'm not exactly "new" to electronics or loudspeakers/sound reinforcement, I am just now starting to get serious about learning theory and so on. Previously, I've just done generic "slap a woofer/tweeter in a box" type of stuff. My long-term goal is to learn the theory, and then apply my personal style to building speakers/crossovers. For better or worse, I tend to learn by hands-on, DIY, type of projects.

    Leave a comment:


  • jcandy
    replied
    Originally posted by Turtle View Post
    Thanks for checking that out. When I posted the picture of the plot, I may have had the port tuned anywhere from 52-58Hz, so I'm not surprised that we see a slight difference in resonant frequency. As I suspected, the amplitude is the major "discrepancy". It's quite possible that I have something setup incorrectly in WinISD but it's still useful to know where/what the resonant frequncy will be. At this point, I'm about 98% sure I'll go with tuning at 55Hz, with whatever length I calculate for a 1.75" port(8.5"ish). I was hoping to avoid an elbow in there but I don't really want the port on the top side of the enclosure.
    The port resonance is going to drop in amplitude rapidly as you decrease the port length. Fixing the port tuning frequency, you could achieve this reduction by increasing the box volume slightly, or reducing the port diameter. Or, you could decrease the length simply by raising the tuning frequency. However, in general (as others have suggested) the actual severity of the resonance is reduced because of laminar and turbulent viscosity which is not included in the simulation. Also, if you make the port rear-facing the resonance will not be part of the direct sound. I've seen a simulated resonance of this amplitude in a well-known, well-respected DIY design, so I don't think it's a show-stopper for you.

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  • Turtle
    replied
    Originally posted by jcandy View Post
    Here is my calculation using an undamped transmission line model for the port (with termination impedance using the usual low-frequency limit of a baffled piston). Slightly lower resonant frequency and significantly lower amplitude.
    Thanks for checking that out. When I posted the picture of the plot, I may have had the port tuned anywhere from 52-58Hz, so I'm not surprised that we see a slight difference in resonant frequency. As I suspected, the amplitude is the major "discrepancy". It's quite possible that I have something setup incorrectly in WinISD but it's still useful to know where/what the resonant frequncy will be. At this point, I'm about 98% sure I'll go with tuning at 55Hz, with whatever length I calculate for a 1.75" port(8.5"ish). I was hoping to avoid an elbow in there but I don't really want the port on the top side of the enclosure.

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  • jcandy
    replied
    Originally posted by Turtle View Post
    Thanks for the reply.

    For anyone who might be interested - the woofer I'm working with is a DSA135-8, in a .21ft.3 enclosure, attempting to tune near 55Hz.. In the plot I posted earlier, I believe I was using a 1.75" port, which works out to roughly 8.5" long when tuned to 56Hz. I'll double check those numbers when I get a chance.

    I used the "transmission line model" advanced option in WinISD, as suggested by billfitzmaurice. I'm just not confident that I'm using the software correctly, to model resonance, or perhaps the software is exaggerating the level at resonance. Either way, I feel like this is a "good" design, I'm just trying to get a better understanding of port resonance and how to model/mitigate the effects.
    Here is my calculation using an undamped transmission line model for the port (with termination impedance using the usual low-frequency limit of a baffled piston). Slightly lower resonant frequency and significantly lower amplitude.

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  • Turtle
    replied
    Originally posted by jcandy View Post
    That resonance looks abnormally large. Attached are results for a SB17NAC35-8 (Vas=33l, Fs=32Hz) in a 28l box tuned to 25Hz. This requires a 13" port (2" ID). So already for this case the box is a bit small and tuned much too low but the resonance is smaller than your case.
    Thanks for the reply.

    For anyone who might be interested - the woofer I'm working with is a DSA135-8, in a .21ft.3 enclosure, attempting to tune near 55Hz.. In the plot I posted earlier, I believe I was using a 1.75" port, which works out to roughly 8.5" long when tuned to 56Hz. I'll double check those numbers when I get a chance.

    I used the "transmission line model" advanced option in WinISD, as suggested by billfitzmaurice. I'm just not confident that I'm using the software correctly, to model resonance, or perhaps the software is exaggerating the level at resonance. Either way, I feel like this is a "good" design, I'm just trying to get a better understanding of port resonance and how to model/mitigate the effects.
    Last edited by Turtle; 07-09-2019, 02:49 PM.

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  • jcandy
    replied
    Originally posted by Turtle View Post
    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.
    That resonance looks abnormally large. Attached are results for a SB17NAC35-8 (Vas=33l, Fs=32Hz) in a 28l box tuned to 25Hz. This requires a 13" port (2" ID). So already for this case the box is a bit small and tuned much too low but the resonance is smaller than your case.

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  • devnull
    replied
    Originally posted by Brian Steele View Post
    The out-of-band stuff that emerges from a vent depends not only on on the length and cross-sectional area of the vent, but also the size of the box, the box's dimensions, the location of the vent in the box, the location of the driver in the box and if the box is lined, stuffed, etc.

    The only easily available software I know that comes fairly close to taking all of that into consideration, and even then it only does so under specific circumstances, is Hornresp.
    Boxsim will also show those effects and also the Reynolds number of the port

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  • Brian Steele
    replied
    The out-of-band stuff that emerges from a vent depends not only on on the length and cross-sectional area of the vent, but also the size of the box, the box's dimensions, the location of the vent in the box, the location of the driver in the box and if the box is lined, stuffed, etc.

    The only easily available software I know that comes fairly close to taking all of that into consideration, and even then it only does so under specific circumstances, is Hornresp.

    Leave a comment:


  • Turtle
    replied
    I really appreciate all in the input here; I'm getting a lot of good information and learning a lot. I think I'll be going with the larger/longer port for this build and I'll get some measurements, to see what the effect really is, in this enclosure.

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  • guitar maestro
    replied
    Originally posted by Turtle View Post
    So am I just worrying about a problem that isn't really much of a big deal? I'm starting to get the feeling that most people here aren't worried about this at all.


    This is pretty in-depth info, thanks for sharing. It's a bit much for me but I'm trying to wrap my head around it.
    Box leakage and mainly absorption losses tend to mitigate the magnitude many of those kinds of resonant peaks in the response.

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  • andy19191
    replied
    Originally posted by Turtle View Post
    So am I just worrying about a problem that isn't really much of a big deal? I'm starting to get the feeling that most people here aren't worried about this at all.
    If you look at the some of the measurements in Stereophile you will see examples with port resonances only a few dB below the output from the cone. So if you are looking to build a front ported 2 way then yes it may be an issue. Perhaps not a big one but something to quantify with measurement and listening and improve in the next build.

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  • Turtle
    replied
    Originally posted by billfitzmaurice View Post
    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.
    So am I just worrying about a problem that isn't really much of a big deal? I'm starting to get the feeling that most people here aren't worried about this at all.

    Originally posted by bohdan View Post
    Port resonances are covered in Chapter 4.2 of SoundEasy User Manual.
    http://www.bodziosoftware.com.au/Chapter_4_2.zip
    This is pretty in-depth info, thanks for sharing. It's a bit much for me but I'm trying to wrap my head around it.

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