Interesting article re: port length

Discussion in 'Archived Threads 2001-2004' started by Jack Gilvey, Mar 3, 2001.

  1. Jack Gilvey

    Jack Gilvey Producer

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    ...over on the Adire site . Among other things,the author points out that (I'm paraphrasing) at Fb,where port velocity is of concern at high levels, nonlinearity of even the best drivers is worse than any non-linearity of the all but the smallest ports.
     
  2. Greg Monfort

    Greg Monfort Supporting Actor

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    Correct, as few diaphragm designs are sufficiently rigid to handle the pressure at Fb and have much usuable BW. The Contrabass driven cones are a good example. Since the goal was extremely high SPL at Fb (14Hz), and low distortion at rated output, they had to resort to dipping them in epoxy. Tom says he can support his weight on one that's laying face down on the floor without deflecting it and I believe it, as its bending resistance is like ~14ga steel. The tradeoff is no useable BW above ~125Hz, though it's academic since the motor can't take the abuse.
    GM
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  3. Tom Vodhanel

    Tom Vodhanel Cinematographer

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    I think it's practically useless for high output DIY woofing.
    The piece doesn't even appear to be about subwoofers at all, at least not *High performance* subwoofers.
    One of those topics where 12 guys will give you 10 different opinions...but no one wants to take the time to give you the slightest bit of A/B data. Dickason suggests anything less than the driver area is a compromise for HO woofs.
    The driver *non linear* stuff is particularly misleading. Since the driver is going to be practically motionless...any motor/suspension NLs will be reduced,right? In almost all cases, aren't driver Nls related to approaching/exceeding the clean Xsus,or Xmax of the driver? Again, the piece seems CLEARLY non-*HP* subwoofer related(imo).
    Anyone want to put a non flared 3-4" port against a couple flared 4" ports in a head-head match to see who has more clean output down low?
    TV
     
  4. Jack Gilvey

    Jack Gilvey Producer

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    I didn't think it applied well to subs either,at least not the types discussed here, if we include the phenomenon of high-power port compression (seemingly readily identifiable)under the heading of "non-linear behaviour".
     
  5. Dave Campbell

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    I would like to see some tests on subwoofer losses at high SPL too. For example:
    4in vs 4in flared ports
    One vs multiple ports
    Port vs PR
    Then we can make better judgements on design tradeoffs.
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  6. Jack Gilvey

    Jack Gilvey Producer

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    I'd be very interested in comparisons along those lines also,Dave. Particularly on the use of multiple ports vs. a single port to achieve the same area, and on the effect of flares on the ends of ports,such as Adire's/Madisounds 4" kit.
    ------------------
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  7. Greg Monfort

    Greg Monfort Supporting Actor

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    Drat! now you've made me actually read it instead of just accepting JG's paraphrasing. [​IMG] Anyway, DP's assertions and my comments are valid IMO.
    When you measure a driver/vent near/at Fb, what all do you think you're measuring? The vent can't keep any upper harmonics generated by cone flex, etc., in the cab, so unless the damping material absorbs it down to below the noise floor, it will come out.
    Remember, the driver is working very hard at/around Fb, just look at its current draw, and the subsequent Re/Qes/Qts rise, lowering max output several dB. JD can tell you all about it, though I'm sure you've measured it many times too. Not to mention that its greatest excursions will be in the BW just above Fb. It may look like it's just sitting there at Fb, but unless it's extremely rigid, it's flexing at higher frequencies, which is a much higher order of non-linearity.
    WRT to Dickason's comment, as I noted in one of our first threads IIRC, this is valid for a simple reflex, but not a high mass loaded vent, which is what a large Sd T/S vent would be. Since virtually no one uses reflex designs anymore, this needs to be stricken from future printings IMO.
    Also, as DP notes, and I stated (implied? can't remember) in that same thread, is there's a point where the vent becomes a stopped (organ) pipe. If you doubt this, take it to Dickason's extreme as I did once, and build a design that requires a 12"dia x 96"L piece of concrete tube for a vent. You won't be impressed with its performance by any measurement, unless ~one note bass with lots of upper harmonics is all you're expecting.
    This isn't to say that too small a vent doesn't make noise of its own or limit output, they knew this millennia ago, but there's a point of diminishing returns. I don't agree with Klipsch's findings, especially since tests by qualified others decades ago fall in line with the results of my many prototypes done to satisfy my own curiosity, but since I'm in no position to conduct an exhaustive study, it remains "IMNSHO". [​IMG]
    Why you think that this article doesn't apply to HO DIY systems is beyond me, as the > the output, the > it compounds side effects. If you want to test the true measure of a driver's ability under load, just attach a high compression horn to it. Excursion at 30Hz/120+dB will only be a few mm p-p, but if it can't handle the compression ratio, the distortion due to non-linearities will be unbearable (at least it is to me).
    GM
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    Loud is beautiful, if it's clean
     
  8. DanWiggins

    DanWiggins Second Unit

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    Greg,
    Excellent analysis! I think you hit it on the head. Just because the driver "is not moving" does NOT mean the system isn't undergoing tremendous forces! I've seen thin surrounds actually tear themselves apart from suckback pressures at Fb. Cone appeared to be standing still, surround was fluttering like a flag in a breeze... [​IMG] Not to mention what the cone will be doing as well (this can also happen in small sealed boxes with high excursion drivers with thin surrounds, too).
    Additionally, there's issues with excessive motor heating at Fb. As you point out, impedance minimizes, which means current maximizes. Max power delivered, but the driver doesn't move, means excessive heating. The number one defense a driver has against overheating is cone motion to provide air movement (some estimate as high as 90% of the cooling is from air movement). At Fb, the driver is stalled, so there's very little cooling. The difference between a driver stalled and one moving at half of Xmax can easily - and rapidly - add 100+ deg F to the core temperature (empirically measured). This changes the Qes of the driver rapidly, meaning more distortion from the system mistuning itself and power compression. Not to mention softening the glues at the triple joint, which changes the way the motor couples to the cone and spider.
    Lastly, I believe both Harwood and Gander studied nonlinearities of drivers in vented systems, and concluded that vents can be too big to be useful. In fact, I believe Harwood explicitly states that his measurements show that as one exceeds a length twice that of the diameter of the vent, air flow in the vent is no longer symmetrical. Air flows into the box easier than it flows out. You start to get box pressurization, which displaces the rest position of the driver, leading to yet more nonlinearities.
    I think that, overall, Dick Pierce's guidelines are quite sound. You don't want your vent to be too small, or you run the risk of excessive port compression. However, one can definitely create a vent that's too big. Port compression may no longer be an issue, but a whole host of other issue emerge (pipe resonances, box pump-up, loss of vent Q, etc.). You can take vent sizes to the extreme in both directions, either too large or too small.
    Best bet: do a PR system. Eliminate most of the issues in both cases. But I suspect that's what you'd recommend anyway, Greg, what with your CBs doing bass duty and all! [​IMG]
    Dan Wiggins
    Adire Audio
     
  9. Tom Vodhanel

    Tom Vodhanel Cinematographer

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    >>>When you measure a driver/vent near/at Fb, what all do you think you're measuring? The vent can't keep any upper harmonics generated by cone flex, etc., in the cab, so unless the damping material absorbs it down to below the noise floor, it will come out. >Remember, the driver is working very hard at/around Fb, just look at its current draw, and the subsequent Re/Qes/Qts rise, lowering max output several dB. JD can tell you all about it, though I'm sure you've measured it many times too. Not to mention that its greatest excursions will be in the BW just above Fb. It may look like it's just sitting there at Fb, but unless it's extremely rigid, it's flexing at higher frequencies, which is a much higher order of non-linearity. Also, as DP notes, and I stated (implied? can't remember) in that same thread, is there's a point where the vent becomes a stopped (organ) pipe. If you doubt this, take it to Dickason's extreme as I did once, and build a design that requires a 12"dia x 96"L piece of concrete tube for a vent. You won't be impressed with its performance by any measurement, unless ~one note bass with lots of upper harmonics is all you're expecting.
     
  10. John E Janowitz

    John E Janowitz Second Unit

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    This article does talk about some important issues with long vents. In addition to what it says, for quite some time now I've mentioned losses in long vents compared to short vents. Will a pair of 4" diameter, 20" long vents allow for more max SPL than a single 4" diameter 10" long vent? Most likely yes, as long as the driver is not at it's limits. However the pair of longer vents will have a higher QL due to 4 times the internal surface area and 4 times the friction, so at any given input power the single shorter port should have slightly more output, up until it begins to compress. Adding two more ports, doubling the length to 40" would only make these effects worse, and more obvious. The longer the ports, the more surface area inside the port, the greater the friction, the higher the losses, and the less output you get.
    I always try to keep my vent length to less than 4 times the diameter. As Dan mentioned, there have been tests to show that as your lenght is more than twice the diameter you run into problems with non-linear air flow. I have noticed that as your length gets around 4 times the diameter that the ports are very lossy and you end up losing several dB of output on the lowest frequencies. On an additional note, a single larger diameter port is almost always better than mustiple smaller diameter ports. A 6" diameter vent and pair of 4" diameter vents are roughly equivalent in total cross-sectional area. However the 6" vent will have much less internal surface area, and the ratio of lenght to diameter will be much smaller.
    John
     
  11. Jack Gilvey

    Jack Gilvey Producer

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    You guys are a great resource, thanks fer bein' here. (I didn't paraphrase wrong,did I, Greg?)
     
  12. DanWiggins

    DanWiggins Second Unit

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    John,
    Exactly! I believe Gander's paper even shows that doubling port area does NOT gain you double the output. One test he did was with a high efficiency 18" driver (roughly 96 dB @ 1W, 1m) tuned around 40 Hz. He did this so that he could test at medium SPL levels (96 dB with 1W delivered) and high SPL levels (116 dB SPL with 100W delivered) without the driver compressing or distorting. The results are completely transferrable to other tuning frequencies, since the driver was completely linear for the two tests.
    He tested with a single 110mm diameter port, and with twin heavily flared 110mm diameter ports (I think it was a 19mm flare radius on each end, but I'll have to go back and check the original paper). The single port did show less output than the twin ports; however, the twin ports had roughly 2.5 dB more output. Twice the internal area, over 2.3 times the input/output area, and the flaring, and he gained roughly 78% more flow. In other words, doubling the vent area (and more than doubling termination area) did NOT double linear flow.
    Jack,
    Typically, a flared port will chuff at about the same SPL level as a longer straight vent with the same effective outside diameter. For example, assume your 6" vent with a 1" radius. The effective outside diameter, for chuffing and excessive levels of nonlinearity, is around 8" (6" diameter plus two times the radius). In terms of linear flow, the two vents will be close. My guess would be the actual SPL maximums between the two would be within 0.5 dB or so - basically a non-issue in terms of sonic differences.
    HOWEVER, where there's a difference is in the pipe resonances, vent Q, box pump up, etc. The shorter length of the 6" vent, the less issue from these effects for subwoofers (note that in a vented two way speaker, you still have to consider pipe resonances well into the 2-3 kHz range!).
    Again, it's all a matter of tradeoffs. Vents that are too small do hinder performance; however, vents that are too big also hinder performance. It depends upon which set of compromises you want to make.
    Dan Wiggins
    Adire Audio
     
  13. Tom Vodhanel

    Tom Vodhanel Cinematographer

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    JJ writes...
    >>>This article does talk about some important issues with long vents. In addition to what it says, for quite some time now I've mentioned losses in long vents compared to short vents. Will a pair of 4" diameter, 20" long vents allow for more max SPL than a single 4" diameter 10" long vent? Most likely yes, as long as the driver is not at it's limits. However the pair of longer vents will have a higher QL due to 4 times the internal surface area and 4 times the friction, so at any given input power the single shorter port should have slightly more output, up until it begins to compress.
     
  14. Tom Vodhanel

    Tom Vodhanel Cinematographer

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    DW sez...
    >>>John,
    Exactly! I believe Gander's paper even shows that doubling port area does NOT gain you double the output. One test he did was with a high efficiency 18" driver (roughly 96 dB @ 1W, 1m) tuned around 40 Hz. He did this so that he could test at medium SPL levels (96 dB with 1W delivered) and high SPL levels (116 dB SPL with 100W delivered) without the driver compressing or distorting. The results are completely transferrable to other tuning frequencies, since the driver was completely linear for the two tests.
    He tested with a single 110mm diameter port, and with twin heavily flared 110mm diameter ports (I think it was a 19mm flare radius on each end, but I'll have to go back and check the original paper). The single port did show less output than the twin ports; however, the twin ports had roughly 2.5 dB more output. Twice the internal area, over 2.3 times the input/output area, and the flaring, and he gained roughly 78% more flow. In other words, doubling the vent area (and more than doubling termination area) did NOT double linear flow.
     
  15. Jack Gilvey

    Jack Gilvey Producer

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    I should add that my Hsu 1220HO had a 4" flared port 28" inches long. It didn't seem to suffer any ill-effects (that I was in a position to identify). Nothing amusical,etc.
     
  16. DanWiggins

    DanWiggins Second Unit

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    Tom,
    I think Gander's measurements are completely valid as done, and were probably the BEST way to do them, rather than trying for a lower tune. I'll explain...
    Gander was looking at effects from port compression. Ports compress from too much airflow. The frequency of the airflow does NOT affect the compression; it's strictly an issue of trying to push too much air through the port.
    Now, to make sure the nonlinearities and compression effects you're seeing are from the port alone and not from the driver, you want to make sure the driver is extremely linear. Running at a higher frequency, the driver isn't running nearly as hard. In fact, doubling the frequency quarters the required volume displacement of the driver. Combined with an 18" unit, that results in VERY little motion compression from the driver. Additionally, by going higher, the driver efficiency is higher, which means less issues of power compression.
    As a result, Gander's results are VERY scalable for lower - and higher - tunes. His paper is cited by many others as an excellent reference on the effects of port compression (Collom's works, being one referrer).
    As far as the overall vent area goes, I don't think anyone here is now, nor ever has argued against moving from twin 4" flared vents to a single straight 4" vent. I don't think you'll even be able to find that kind of recommendation in Dick Pierce's article. He addresses points that are a fundamental issue of vented boxes: Helmholtz resonators. I don't think going to twin 4" vents aroudn 20-24" long is a problem at all for most subs, given Pierce's guidelines. What he's addressing are huge vents.
    A few weeks ago, with another thread about vent sizing was introduced, it mentioned trying to use vents similar to a 12" diameter vent on a 16" sonotube. That is what I'm talking about, and what I believe Pierce is commenting on. Vents can be too big, where they actually no longer gain you anything in terms of output (can actually start LOSING output narrow-band) and introduce/exacerbate many other problems.
    Once a vent's area becomes large in relation to a panel of the box, or the vent becomes excessively long in relation to wavelength, it moves from simple Helmholtz resonator operation, and starts moving to open pipe operation. You start losing vent Q. This is from several effects: air no longer moving as a single mass in the vent, but as a distributed system, drag on the walls, termination impedance issues, etc. But the end result is that, once you hit the "limit" you start to lose output, and the output broadens over bandwidth. Just like taking a Q=5 bandpass filter and lowering it to Q=1. Less gain at the peak, and it operates over a wider frequency range.
    Additionally, some effects, like pipe resonances, will ALWAYS be an issue with vents. Long vents will resonate. A vent of 3 feet long will have a quarter-wave resonance of a 12 foot wave; around 90 Hz. Now, your sub may be crossed over below that; however, consider that, if you're tuned around 18-20 Hz, driver excursion at 30 Hz isn't affected much. The 3rd harmonic of 30 Hz is 90 Hz. Suddenly driving your sub hard at 30 Hz results in a good excitation frequency for that 3 foot long vent.
    The pipe resonance issue is a real concern for long vents. Both Dickason and Collom's warn about it, and Dick Pierce warns about it too in the article I posted. This is an issue that you CAN'T overcome, without lowering the Q of the vent (via stuffing, or placing acoustic resistances across the ends of the vent).
    Anyway, just to sum up, here's what I'd say:
    - you can definitely have too small of a vent; Small's recommendations, while not the last word in minimum vent size, are decently well set
    - you can definitely have too large of a vent; too much vent moves you from the realm of vented boxes to TL boxes
    - pipe resonances are a real issue. Long vents will resonate with your harmonics, and in extreme vent cases, with the fundamentals
    - vent linearity is NOT 100% scalable. Doubling vent area does not double linear SPL output capability. It reaches a point of diminishing returns, where you can double the vent area again, and actually start losing output
    Anyway, that's my take on this issue about vent lengths. It's about understanding there can be a thing as too much vent. For most Shiva systems, a single flared 4" vent is sufficient; Tempest or MASS based systems, dual 4" flares should be sufficient for most applications. You can make additional gains by going to more vent, but they start returning less and less, and you need to keep the downsides into perspective (loss of vent Q, pipe resonances, box pump up (sometimes called acoustic rectification), operational mode changes, etc.). Lastly, consider that the downsides manifest themselves at higher frequencies, where your ear is more sensitive, and the impact of these becomes that more critical.
    Dan Wiggins
    Adire Audio
     
  17. Tom Vodhanel

    Tom Vodhanel Cinematographer

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    >>>Tom,
    I think Gander's measurements are completely valid as done, and were probably the BEST way to do them, rather than trying for a lower tune. I'll explain...
    Gander was looking at effects from port compression. Ports compress from too much airflow. The frequency of the airflow does NOT affect the compression; it's strictly an issue of trying to push too much air through the port.>The pipe resonance issue is a real concern for long vents. Both Dickason and Collom's warn about it, and Dick Pierce warns about it too in the article I posted. This is an issue that you CAN'T overcome, without lowering the Q of the vent (via stuffing, or placing acoustic resistances across the ends of the vent).
     
  18. Hank Frankenberg

    Hank Frankenberg Cinematographer

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    Real world, gentlemen, not bass-freak highest possible maximum level SPL that violate the WAF. Dan, remember I built one of your recommendations: Tempest in 24" dia sonotube with one 4" port kit (large flares on each end), 6.2 ft^3, tuned to 17.5 Hz. I still hear no port noise and no distortion up to levels that make people drop jaws and get very concerned looks. Recall that I plugged the AVA250's wattage into WinISD, rather than the Tempest's max handling wattage, and the port velocity wasn't bad. One 4" dual-flare port is fine for real world listening, IMHO.
    [​IMG]
     
  19. DanWiggins

    DanWiggins Second Unit

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    Touche, Hank... [​IMG]
    In reality, for a large majority of the public, all these designs are way overkill. For most people, you don't need a huge box, massive power, and lots of port to get satisfying levels of bass!
    Dan Wiggins
    Adire Audio
     
  20. Patrick Sun

    Patrick Sun Studio Mogul

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    And my Sunosub III was a close approximation to Hank's Tempest sub, based on Dan Wiggin's suggestion when I had some leftover sonotube that yielded about 5.8 ft^3 of internal volume. Pretty impressive subs for the money.
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