Sasha

iTrader: (10)

The rms rating on the sub is not really an indicator how it will perform with certain amp power, it is more up to the enclosure. RMS rating on the sub is more of a thermal rating of the voice coil. For example, a 600WRMS sub on 300 watts may sound better and louder that a 300WRMS sub, so this would throw your theory ou the window.

Here is a little info tip. You have 500 watts. The subwoofer of your choice has a 1000wrms rating and is designed to be in a 1 cube sealed box. Usually, that 1000wrms rating is for the limit of the voice coil, and also for that size box. Thermal rating always stays the same. However, if you choose a bigger box, the air suspension inside the box is softer, than designed, therefore the sub is easier to move now, and because of that full xcursion is easier reached, so you dont need that much power anymore, as the sub becomes more efficient. On the flip side, the sub can be overexerted easier, so the power rating will have to go down to lets say 800wrms depending on how much bigger the box is. If you place the same sub in a smaller box, you can send it more power, cause the suspension will now be tighter, and the sub is rendered less efficient. But I repeat, during all this voice coil rating is the same, as the current will still be travelling. So, in lamest terms, the bigger the box, the more efficient the sub gets, and the less power you need, and vice versa for the smaller box. So you can figure out how big your box needs to be based on these ratings.

Some subs are designed to play well in smaller enclosures, and some are designed to be in bigger ones and will not play well in smaller size boxes. But the sub in a smaller box will usually sacrifice it's SPL, and the sub in a bigger box will almost always sound louder based on box size alone. There are of course hundreds od subs out there, and all have different performance. You just have to find the happy medium which you will enjoy with the gear and power you have. 500wrms is not much, but it can be made to work in your advantage, if you make a proper size box for your sub. Usually ported box, and on a bigger side than recommended will yeald more output on smaller wattage. So, no matter what sub you choose between the two, box size will ultimately make or break the performance you are going to have.

SQ subs with softer cone are usually meant for smaller enclosures, as the cone is easier to move, so it gets efficiency out of the cone, and the rms rating on the sub for example is 1000wrms in 0.6 cubes sealed after displacement. DIYMA is a perfect example, but once you put it in a bigger box, and still send it the rated power, it will flap like a fish out of water, and you will have to feed it like 500 watts in 1 cube to have it work properly.

TragicMagic

Well, I'm looking at the Eclipse SW7200 from MR2NR. What I cannot find is this particular woofers recommended vented enclosure volume. Appears to be last years model, and they don't have a PDF file of the owner's manual on the Eclipse site (which sucks by the way).

I get what you're saying. The air in the box helps to "control" the cone. If there's little resistance from air, and you feed it lots of power then there's less stopping the cone from reaching its excursion limit, or bottoming out.
So can we assume that while the smaller box sacrifices SPL, it helps to optimize SQ? I've been reading up on the port tuning. A lower tuning frequency (30-35) is better for SQ, while a higher frequency (40-45) is better for SPL competition.

I'd like to have a smaller enclosure, and port tuned to 32Hz. That's what I'm thinking anyway. She'd much prefer the hard hitting bass, as opposed to deep droning bass.

Sasha

iTrader: (10)

Smaller boxes are better for SQ by default here, cause they dont overexert the sub, so the sub has lower distortion due to less output. It is just common sense physics. If you do not try to overdrive the sub, you will get better SQ, so it is not really about box size, but knowing your sub's limitations. Higher port freq is for SPL - yes. You can tune your sub to the spec, and then still use the subsonic filter anywhere between 25-30hz depending on the port tuning. And do use the subsonic.

If that Eclipse was made by TC Sounds, it is an awesome sub, and you can try to google for equavalent version of TC sub for specs, or contact Eclipse customer service on their website.

TragicMagic

So I wrote Eclipse customer support an email regarding their recommended vented enclosure specs...

This is their un-edited reply, lol.

Now I need to figure out what frequency a 3" diameter port, 6" long will result in.

http://www.carstereo.com/help/Articles.cfm?id=31

I simply kept typing in a different value for the frequency until the length of the port equaled 6".

I used 1.95 cubic feet for the volume. If I have a box made of 3/4" MDF, at 16.5 for the outside dimensions of the length, width, and height; the internal volume equals 1.95.
Used Rockford box wizard for that.

veeman

iTrader: (3)

I'd use 15" x 1.5" x 22" rectangular port personally, this will prevent port noise. The box is still tuned to 34-35hz as before. Their specs (and most monufacturer's for that matter), seem to ignore port noise as an issue when they give their recommendations. According to WINISD, their 3" x6" round port would create a port air velocity of 55 m/s @ 500 watts rms, as opposed to 17 m/s with the rectangular port I mentioned. 17m/s is the limit that is recommended by the program, and most speaker designers.

quote from WINISD:
"Very important here is to make your ports large enough, so that peak air velocity won't exceed 5% of sound velocity in air. That is, about 17 m/s assuming normal environmental conditions."

TragicMagic

Veeman, that's great info. Definitely going to implement that into the box design. Thanks. I'd imagine that some of the companies give you the easiest way to achieve a certain tuned frequency that doesn't necessarily take port noise into consideration.

I'm trying to figure out how two ports that are so different in design produce similar tuning... By my math the volume of air in the slot port would equal 15 x 1.5 x 22 = 495 cubic inches. While the 3x6 round port would equal 3.1416 x (1.5x 1.5) x 6 = 42.5 cubic inches. Not sure if the volume of air in the port has anything to do with tuning or not. lol I was figuring this out simply to determine if there was a direct relationship between the volume of air in a port and its air velocity/tuning frequency.

I'm so lost.

I thought that a narrower slot, like one that is 1.5" wide, would generate more port noise

veeman

iTrader: (3)

well in this case it won't, since the vent area is 3x that of the 3" port. With this sub, and power level, it'll be more than ok. however, you are correct in assuming that such a narrow vent could be cause for concern. Ideally, vents should be as close to square as possible, the problem is in the restrictions with the box dimensions. You could use 2x 3" by 14" round ports as well, but in the given size box, the port opening would be too close to the wall. You could alter one of the dimensions to accomodate this, and still keep the volume at 2.0 cubes, but i was basing my recommendations on what you gave.

As far as the explanation for the vent differences, read this:
DUKK explained it rather well.

"Where your theory failsthough is trying to relate tuning to air speed. If you place a 4" woofer with 1mm Xmax in that 1cuft box will it be tuned to 35hz? yes. If you place a 10" woofer with 25mm Xmax in that 1cuft box will it be tuned to 35hz? yes. Will the relative air speed in the vent be the same for the 4" as the 10"? nope. Why? Because the 10 is moving in excess of 150 times the air per stroke than the 4" and displacement dictates air speed. This is key in determining the area of the vent, but not the length, ie tuning.

Consider this 10" woofer playing a single note - the cone moves more or less depending on amplitude. So when at low volume and cone displacement the air speed in the vent will be much less than at high volume and cone displacement, yet the tuning stays the same (if the vent is sized properly). The frequency is not changing but the air speed is because air speed is related to displacement. The tuning of the vent is unchanged though.

A higher tuned vent does need to have more area though as the relative airflow through it IS faster from a design standpoint. How come? It is due to the total displacement of air over time. At resonance, a 30hz tuned vent has to move XX air 30times per second. At resonance, a 45hz tuned vent has to move XX air 45times per second. The theoretical total displacement over time for the 45hz vent will be 50% more than the 30hz vent and therefore it needs to have more area as a vent is always tuned based on its resonance.
Remember though that the air speed in either vent is dictated by the frequency being played by the driver and the excursion of the cone - it has nothing to do with the length of the vent. Whether that vent is 13" long or 7" long, it will have to move 45 excursions of air per second if 45hz is being played. All the length of the vent does is tailor how that sounds to our ear and how well the driver reacts to it. "

veeman

iTrader: (3)

Just so you can feel confident that i didn't just shoot off a theoretical recommendation to you. i actually still have a box with 2 10's and ran it with 600 watts rms, and it had zero vent noise with a 1.25" wide vent by 13.5" tall, and 27" long. So your single 12" with 500 watts, will not give you any vent noise with those dimensions. I just don't want you to feel the anxiety of going that route.

TragicMagic

I don't necessarily feel anxiety about the slotted vent. I just want to understand the concept of how it actually works. The dimensions I posted are not set in stone. What I need to do is measure her backseat, and figure a box that fit 'comfortably', while maintaining the 2 cubes of volume.
What if I were to use a port that went through the box on an angle? The corner to corner measurement of a rectangle, through the center point, is longer than any of the sides. Maybe I could achieve a square port, and still maintain enough length, without having to change the outside dimensions of the box.

veeman

iTrader: (3)

yeah, but it would be hard to build. Easiest and most efficient port type is usually the round tube, so if you can re-work your dimensions to use 2-3"x14" pvc tubes, then that's the route i'd go. You just have to make sure you have atleast the width of the pipe's distance from the wall facing the port's opening (inside the box). Example:
instead of 16.5 x 16.5 x 16.5, you could do 16.5 tall x 18 deep (this would allow the ports to be on the same face as the sub if you like) x 16.5 wide. This will give you 2.0 cubes including the ports displacement (not sure what the speaker's is or i'd have calculated it for you).