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Port Size and Stuffing

I was modelling a couple of driver and like the response and box size, but ran into an issue with port chuffing. At full tilt, port chuffing is pushing 80 m/s. I am planning to have the port on the bottom of the cabinet, but assume this will still be audible. Would a heavy carpet or a pad of stuffing under the speaker help muffle some of the noise? Any other suggestion? Increasing the port size i tun into too long of a port... 

This is with 100W per driver for a max response of 106db SPL

Comments

  • Modeling for more sane power and response, at the other end of the spectrum with 2.5W per driver for 90Db SPL at 1M, I get this...

     
  • at 10 W per driver, 96DB SPL, this is the result


  • Going a bit over at 20W per driver at 99DB SPL, this is the response and port noise.... This is good enough i suppose for 99% of all listening.... even for the occasional party? and XMAX is not exceeded at any frequency...

     


  • In WBCD I always hit the "MAX SPL" button to see where the xmax limited SPL is, this gives you a good idea of the peak SPL you can generate before you exceed xmax, then drop 6-12dB crest factor for average SPL. Design the port around the power input that generates max SPL.
    rjj45
    I'm not deaf, I'm just not listening.
  • Good is unibox , xmax is exceeded at f3 and lower at around 106db. Modelling at xmax requires a 3.5 inch port. A 3 inch port needs to be 20 inch in length, which doesn't fit inside the box. 

    The 2" port required around 8inches in length, much more manageable. Usually I model around xmax, but expecting a 5 incher to hit 31hz at 106 db 1m.... It does have 9" for xmax
  • Port compression is also a big concern, even before audible port chuffing happens.
    rjj45
  • Ports can be bent and folded to make it fit.
    I'm not deaf, I'm just not listening.
  • What is port compression and can it be simulated or measured? Is it also depends on power? At low power, there should be any issues? Only when pushing it I expect things to go haywire...
  • You could run an SD215-PR per W5, right?
  • dcibel said:
    Ports can be bent and folded to make it fit.
    Yes, definitely, but not in this case (box), hence asking how bad it would be in real world under realistic conditions and if there is anything that can be done to reduce it.

    Would love to overbuild, but the design choice asks for a compromise, would like to know how bad of a compromise it is.
  • ani_101 said:
    dcibel said:
    Ports can be bent and folded to make it fit.
    Yes, definitely, but not in this case (box), hence asking how bad it would be in real world under realistic conditions and if there is anything that can be done to reduce it.

    Would love to overbuild, but the design choice asks for a compromise, would like to know how bad of a compromise it is.
    Well, at 20W you're doing okay, beyond that you may start to be disappointed. So the question is, will you be satisfied with the SPL at 20W.
    ani_101 said:
    What is port compression and can it be simulated or measured? Is it also depends on power? At low power, there should be any issues? Only when pushing it I expect things to go haywire...
    "Port compression is a reduction in port effectiveness as sound pressure levels increase. As a ported system plays louder, the efficiency of the port reduces, and distortion emitted by the port increases" - Wikipedia

    Basically, the greater amount of air needed to squeeze through the port opening, the higher the back pressure generated by the port. Just like any fan system, a higher amount of restriction (pressure) requires a greater amount of force to overcome it to create flow, so the port becomes less effective. It's part of that whole "chuffing" issue, but when a speaker is chuffing it's not just wind velocity as a result, but loss of bass efficiency and increase in distortion. The ROT to keep velocities below 25m/s or 17m/s depending on who you talk to, is there to get at the happy result of a port that is "good enough" with reasonable dimensions. You just have to pick the power level (SPL) where that velocity occurs at.

    Simulations of any speaker performance at high SPL lack accuracy. The simulation is based on T/S parameters which are small signal by definition. The simulation doesn't take into account changes in suspension stiffness and BL over the driver excursion. For example, below 20Hz you probably show excursion sky high even at low power, yet I'm sure if you play 20Hz signal the cone won't fly across the room.


    ani_101
    I'm not deaf, I'm just not listening.
  • Might consider port out of the bottom and using the bottom plate to extend the port. A slot port along the interior can also help. I personally think the lower rear port that have a flare have less noticeable port noise.

    Also consider checking the port resonance box if that is unibox so you can see where the port resonance shows up.
     John H, thanks to JP I did get that email
  • A folded slot port can usually have enough area and length to minimize air speed at a given (highish) SPL. You inevitably start to add distortion as you really push a speaker, the bass starts to sound funny, and finally you can actually hear chuffing. Not a "sound of death", but pretty high distortion due the factors that others have detailed.
    Don, Donno, or "Hey you" all work for me, But never "Mr Johnson"
  • What does your cabinet shape and internal volume look like?
  • 20 liters, 20x6.5x10 internal dimensions, the woofers are on the front 6.5x20 and the port is on the bottom 6.5x10.
  • ani_101 said:
    20 liters, 20x6.5x10 internal dimensions, the woofers are on the front 6.5x20 and the port is on the bottom 6.5x10.
    Is the box already built? If you added another 1" of depth you could fit an APR10 or the 10" RSS passive.
  • Yes, box is built, had not simulated at xmax. Should be ok at normal volume me thinks, but let's see what it does at elevated spl.
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