Jstas

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Jstas
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  • Re: Is there a way to measure polyfill accurately?

    The polyfill in SDAs is not there for driver dampening. It's there for port tuning.

    SDAs have passive radiators which is essentially a speaker port.

    Ports work because they have a column of air in them that gets moved when the driver vibrates. That moving air in the port creates complimentary sound waves that enhance the performance of your speaker. Just like how you can feel and hear the THWHUMP of air when you shut a car door or when you shake a trashbag to open it up. The same thing happens with a port.

    Port tuning is essential because you need to be able to get the maximum flow rates without hitting the cavitation point in that port to move the air and take advantage of the air spring it creates. If you move it too fast you will either compress the air in the port and enclosure too much and possibly cause physical distortion in some or all of the drivers. Alternatively, you will blow all the air out of the port and create a vacuum in the enclosure which can also cause physical distortion, attenuated performance of the driver and even cause overheating issues in the motor structure.

    Port tuning means that you have an appropriate amount of air in the port and enclosure, it's weighted properly to compliment the drivers because of the port size and it's moving at the highest rate possible before cavitation or other performance killing phenomenon happen.

    A passive radiator does the same thing as a port but instead of a column of air, you have a rigid, polymer disc (which is actually a cylinder) that takes the place of the air. It does the same thing which is why they have to be weighted properly so they don't move too fast or too far and make farty distortion noises.

    So like a ported enclosure uses polyfill to make a box and port perform well by changing the velocity of the air being moved by the backwave off the rear of the driver, polyfill in an SDA evens out the velocity of the backwave pressure inside the enclosure. That makes the air pressure work evenly on the rear surface of the passive radiator so you get nice, tight bass response without having to have a giant driver or getting the flappy, **** noise from distortion rippling across the surface of the passive radiator.

    The benefit there is that you can have a crazy, complicated load to drive in a fancy crossover network like what is needed to drive the SDA tech and you can reduce or at least make the load more reasonable by having small, efficient, easy to drive drivers. That way you get your SDA wizardry but you have a huge tower speaker that performs on the high end like a 2-way bookshelf but digs deep like 15 inch sub and can do it on 200 watts per channel for the entire array, not just the sub.
  • Re: Is there a way to measure polyfill accurately?

    machone wrote: »
    How can you type that much!!!!

    This is completely uncalled for and why I don't post much anymore. I go through all this effort to help some people out because I can. Then I get mocked and dumped on because I took some time out of a lunch break to provide an answer with complete thoughts formed with proper grammar, spelling and punctuation. All because it's beyond your comprehension that someone can form a complete answer and explain complex concepts with a keyboard.

  • Re: Is there a way to measure polyfill accurately?

    So why SHOULD you stuff a speaker?

    Well, compromise.

    As we said the fill makes an enclosure behave like it's larger than it is. When you have a small package/footprint restriction, like in a car trunk, stuffing a sub box can vastly improve response. It's not nearly as good as if you built your enclosure correctly. Sometimes that's not an option, though so you band-aid with a wad of fill.

    One of the reasons you will see it in tower speakers is because it can be used to slow air around stuff like bracing. Air will move like water and around a sharp edge/corner it will create eddys that create high pressure pockets. That can make the enclosure seem smaller than it is. So if you stick some fill or batting around the bracing spots, it will tame the effect by not allowing the high pressure pockets to develop. But speaker engineers know this and will compensate for it in design of not just the enclosures but also with crossovers.

    Another reason it is used is to tame resonances. We've all see the oddballs speakers from like Bang and Olufsen or the cheapy plastic enclosures for computer speakers or HTiBs or Best Buy specials with the plastic or other kind of composite or metallic enclosures. They tend to ring like bells at lower frequencies. So stuffing the enclosure with some polyfill tames that. But, again, it's a big compromise. Then again, those teeny speakers or the crazy designs are compromises as well.

    Yet another reason is poor cabinet materials. Using something like particle board can be troublesome because it can start to separate under constant abuse of a driver. So lining the enclosure with batting can knock that down a bunch.

    One more thing that you can have happen is standing waves. Sound travels down a surface which is why you see odd designs of mounting baffles. That's designed to control stuff like off-axis response, reflections and even port noise. It can happen on the inside of an enclosure as well. That's fine, usually, because it doesn't really impact stuff since lots of companies make enclosures with very few hard corners inside anymore. That's because gradual corners let the sound waves disperse as they cancel each other out. But, when you have a square enclosure, like in a subwoofer box, you can get those sound waves traveling down the interior and then they hit an abrupt wall which also has a wave on it. The stronger those waves, the bigger the air pocket they can create. That creates an eddy in that corner which, again, can cause the enclosure to seem smaller. But it can also reflect air and sound back towards the driver which under high-power/extreme situations can cause distortion. Lining the inside with a bit of polyfill can eliminate that.

    One last thing is that in subwoofers with high power and an enclosure that has a reasonable WAF rating, the driver can perform too well and overpower the enclosure. Especially in ported enclosures. If the driver is moving enough air, it can create port flow problems. Not just the low-frequency hum of port noise but also with port velocity. Air can only move so fast in a confined space, like a port. The port also slows the air down. So if your port is long enough, the air entering the port from the inside of the enclosure can be moving significantly faster than the air at the other end of the port. What happens then is that the faster moving air runs into the slower air and it's like Black Friday Walmart crowd. The air cavitates and then the flow doesn't work at all and you get this port farting noise. Doing things like Polk's Power Port to smooth all that flow out can help a bunch because it helps maintain velocity and flow at the port mouth by smoothing the transition from high pressure port flow to atmosphere. But in lieu of something like that, polyfill can be used to slow the air in the enclosure enough that you can maintain the proper volume of air to be moved in the port but drop the velocity enough so that you aren't exceeding the dynamic limitations of the column of air in the port.

    So there are benefits and drawbacks. Stuffing can seem to improve bass response but more often than not, all it did was change the tuning frequency and drop efficiency of the speaker system (i.e.: drivers, enclosures and crossovers). It can be damaging if not done correctly too. So you want to pay attention to what you are doing.

    Polyfil can degrade over time too. It gets dirty or filled with dust and even mold/mildew. That can weigh it down and it can change how it behaves in the enclosure. So sometimes, when you over stuff, you compress the old junk with the new stuff and it can seem like there was an improvement. Well, there was but that's because the original performance was so degraded that it restored most of what was missing. What should be done is the old stuff removed completely and replaced with new stuff.

    As far as stuffing with a passive radiator, I honestly wouldn't unless you are willing to fiddle with the weighting of the PR. The weighting is like port tuning and if you change the airflow in the enclosure with stuffing, you can affect how the PR moves because the stuffing changes the air flow and pressure levels acting on the PR. Some are stuffed from the factory and that's designed into it. So if you feel that over-stuffing improved performance, it might mean that you need to replace the stuffing. Adding to it might mostly solve the problem but can cause others as well.

    So feel free to experiment. They are your speakers. Just be aware that there's a bunch going on here to be considerate of.

    Also, stuffing has been going on for decades. Before polyfill was available, manufacturers used fiberglass insulation.
  • Re: What Are You Listening To?

    ZZ Top -- Sleeping Bag
  • Re: Model RR or real photo?

    Nah, that's probably India.

    They are still running electric overhead lines and since most of the infrastructure there was first built by the British, lots of it dates back to the 1800's with early 1900's modernization. It's all way out of date now.

    But the stuff that you think is inefficient was built well before modern construction techniques were available. So for the time of it's construction where steel girders that would allow for a larger span were not available, a multi-arch stone trestle was made with what was readily available and that's copious amounts of rocks.

    Also, unlike today where we have massive machines that bore tunnels, it wasn't uncommon for multiple lines to converge in a convenient, natural mountain pass to not only save time and money on construction efforts but reduce costs in distributing resources for the train lines.

    But it honestly does look real to me. I could be mistaken and it's just a very well done diorama but despite all the protestations about leaves and scale and such, it's the lighting that makes me believe it's real. It's far too natural, like an overcast day and the way the light gets diffused by the clouds.

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