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?

    OK, so first, a couple things about polyfill.

    - It's sold by weight so if you buy a 10 pound box/bag of it, it's 10 pounds of spun polyester.
    - There is another weight rating for it and it has to do with how thick the strands are, like speaker wire has a gauge. Typically, you want a lighter weight because you can get a higher density which is important.
    - You only really see the higher weights in batting because it's used as insulation, stuffing for stuff like padded quilts and seat cushions. The loose stuff in bags is typically for stuffing toys, pillows, etc.
    - You can buy it in loose bundles or rolled batting. Either way, it will do the same thing in a speaker enclosure.

    Stuffing a speaker box, should you?

    Quick answer: you can but you probably shouldn't. It's honestly a band-aid in most cases.

    Long answer:

    Look, I don't care what your "ears tell you", stuffing a speaker that didn't have polyfill in it prior to your bright idea is going to hurt performance. Now, with that out of the way, here's why.

    First, how polyfill works.

    Polyester is a very smooth, reflective surface. So is the alternate, spun nylon. Nylon is also more rigid and you don't see it as much in speakers because it's more apt to damage motor structures. But because of that smoothness, it's very reflective. Reflective is also deflective. It does two things, a physical difference and an audible difference.

    The physical is the most important. A speaker enclosure is essentially an air spring. It creates a vacuum when the driver moves out of the basket and it creates a high pressure pocket when the driver moves into the basket. The vacuum keeps the driver from over-excursion damage, the pressure pocket keeps the driver from being able to bottom out. When you stuff a speaker with polyfill, it essentially slows down the air movement in the enclosure because it is diffuse. The multiple fibers in random arrangement will make the air bounce off them and in all directions. This slows the air movement down and creates turbulence. So every time the driver moves in or out, the air movement is slowed by the polyfill. The effect this creates is that it makes the enclosure behave like a larger enclosure. We'll get to what that does in a minute. It makes it behave like a large enclosure by slowing the air movement. Slow the air, you slow the backwave as well. It takes longer to reach the walls of the enclosure. When it does that, it can actually reduce how fast the pressure or vacuum is built up in the enclosure. That can affect how far your driver extends and whether it bottoms out or not. Not a big deal, right? Well, no, it is.

    If your VC is designed to stay in a certain spot in the motor structure, it can extend past that sweet spot from the over excursion in either direction. At high volume levels, there is a possibility that you can send the VC out, past the ends of the pole piece in the motor structure. That can cause the driver to actually overheat because if the VC goes past the magnetic field then you effectively reduce the VC's size for the magnet to act on so you can momentarily decrease the resistance while increasing the load on the part of the VC that is still in the magnetic field. You're doing it at the driver's peak as well. Because of that, the driver is not operating at it's highest control potential and you are relying more on the surround and spider to control the movement. That taxes those parts more than normal and can wear them out sooner. If they wear out and lose their control capability, you can actually blow the driver because the VC being extended past the pole piece can either impact the pole piece and seize or it can tear the cone/spider/tinsel leads.

    Also, when you stuff a speaker, you move and vibrate the fibers of the polyfill with the driver movement. When that happens, they can eventually fatigue and if the fill is too close to the driver, the motor structure venting can clog up and cause overheating. It can also cause physical failure by making small holes in the control structures that can be stretched out and expanded under even just normal use. Yes, I am not making this up. But, over-stuffing can also slow air movement enough that even a venting system on the driver that isn't clogged isn't moving enough heat out of the motor structure to be efficient. It's more pronounced in, say, a subwoofer where the forces are much larger than what you would find in a full-range speaker. However, overdrive your full range speakers and stuffing a speaker can become an issue.

    Now, the sound stuff. We already know what polyfill does physically to the enclosure. So that artificial expansion of the enclosure space will do a few things. We'll go over the positive later. The negative is that it can dampen response and cause the resonance of the enclosure to change. The problem with that is it can cause things to rattle that previously did not rattle. It can also cause the driver to hit it's resonance points and kill efficiency which can cause audible distortion. That's typically found in a sealed enclosure.

    In ported enclosures, it can reduce port flow velocity which can hamper low-end frequency response by changing the port tuning. Typically, you want the port to be tuned to compliment the driver at it's peak efficiency point. That requires length and width calculations. It can also cause port noise or even cause the driver to suck the air column in the port back into the port because the air spring behind the driver is weaker. That makes noises like blowing across a bottle neck. I've actually heard these noises.
  • Re: Hi folks, not that yall care

    lightman1 wrote: »
    Jstas wrote: »
    I can't believe I'm holding up lightman as sage wisdom but the dude is right. Get back to you.

    Hell hath seemed to have frozen over.

    Well, considering I was standing on my porch this morning drinking coffee, watching the fam leave for work/school and it was about 17 degrees with 20-30 mph wind gusts, at least NJ has frozen over if not Hell proper.
  • 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: Need help finding a stylus

    I think I figured it out.

    At first I turned the anti-skip off. Not much change.

    The stylus apparently came with one of those little dustbuster brushes. It wasn't on it. I found it in a recessed screw hole for the tone arm mount. So I stuck it on and bent it back straight, looks like it was pinched at some point.

    Anyhoo, I stuck it back on and tried playing the album that skipped the worst again. I didn't adjust drag or tone arm balance or anything. It barely skipped. I turned the anti-skip back up to 1.5 where it was and only the biggest scratches caused issue.

    I'm guessing the drag is set to compensate for the dustbuster brush.

    Works pretty good now. Kinda quiet but I'm guessing that's the output stage. My Micro-Seiki has a higher gain output stage so my phono pre is set pretty low for that.

    So I might go dig up a stylus for it just in case but right now, this is going as is. Hope her dad likes it.

    Needs a good scrubbing though. All kinds of groady funk on it. The platter, tone arm and dust cover are pretty clean, though.

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