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View Full Version : Woofer loading and differing distance to port, probably a quick and easy question :)



jolly_26
01-01-2012, 08:49 PM
26529595

Could someone please explain what would be the best option and more importantly why it would be? Thanks in advance, I'm really enjoying designing boxes for myself and friends but wish I knew more.

On another note, if someone could point me towards some reading on the more advanced aspects of box building that'd be great :)

hispls
01-01-2012, 09:04 PM
Personally I don't like to keep a woofer too close to a port. Conventional wisdom is that you don't want to.

Your best bet would be to build a box with a removeable baffle and build baffle with both arrangements and see which works best in your application. IMO the vehicle accoustics will play a lot more of a hand in what it sounds like than placement of the woofers on the baffle.

jolly_26
01-01-2012, 09:15 PM
Thanks, I thought about that but with the size of the box I'm designing it isn't actually possible, each different option requires a box with entirely different proportions. The option on the left is much easier to design, and I have done 7 or 8 drafts trying to sort the one on the right to work. I might just go with the option on the left and run the subs at about 100rms less than I have been (run them at what they're actually rated for). The option on the left will result in both woofers being further from the port opening than the other option, is this what I'm after?

hispls
01-01-2012, 09:23 PM
From what I've been told the woofer right next to the port can result in some turbulence with the front wave of the woofer and the port. I've always avoided that sort of thing when possible.

AFAIK the subs location in relation to the inside opening of the port isn't terribly important (provided they're not right on top of it...though I've actually had that work pretty well...not by preference, just because that's how I could fit things). Like I said I've found that just the aiming of subs/port in the car makes a way bigger difference than that will. AND there's really no way to figure it out without making some sawdust.

jolly_26
01-01-2012, 09:32 PM
Oh I was only concerned about the woofer distance from the internal port opening, now I'll have to look at the external opening too haha. Thanks for your help though, and congrats on cracking a 150! When I finish uni and have some money I'll go chasing one for sure, at the moment i'm in the process of embarrassing my friends with much cheaper gear, all thanks to you guys on this site

Kangaroux
01-01-2012, 09:37 PM
To add on to what hispls said, if the sub is too close to the port you won't be able to utilize the whole chamber the sub is in. The sub will load off into the port before it can make sure of the whole chamber

jolly_26
01-01-2012, 09:47 PM
To add on to what hispls said, if the sub is too close to the port you won't be able to utilize the whole chamber the sub is in. The sub will load off into the port before it can make sure of the whole chamber

The internal port opening you mean? This would increase the tuning frequency and decrease mechanical power handling also I assume?

jolly_26
01-01-2012, 09:57 PM
Also, while you guys are here, what are the advantages/disadvantages of running, say, 16 square inches of port per cubic foot as opposed to 12 square inches of port per cubic foot?

hispls
01-01-2012, 10:30 PM
Oh I was only concerned about the woofer distance from the internal port opening, now I'll have to look at the external opening too haha. Thanks for your help though, and congrats on cracking a 150!

Thanks. A good 2db of that was had just by proper aiming of the port.

Distance from the external opening is going to be more important with either of those designs. Neither should have any impact on the internal opening. Really as far is internal you would just worry about the woofer motor too close to where the air is trying to flow. (keep in mind the port extends beyond the actual internal opening air flows along that wall). Either way you have those set up the rear wave is just creating preasure inside the box, interference with the air from the port and the air rolling off the cone is what you should be worried about.

More port area generally gains output (particularly with windows up and doors closed), but port freq can start to become dominant in your response. Also if you wind up with more port area than cone area you'll find the port unloads at pretty much anything more than a few cycles outside of your tuning..... I doubt you'll get into that though. Unless you're chasing 10ths I wouldn't worry much about it. Some people even find a port can be too big, but there's too many other variables to really tell you how to optimize. It's going to take building, testing, and rebuilding.

jolly_26
01-01-2012, 10:37 PM
Going to run 48sq in. of port with 3 cubes and two (low end) 12s at 30Hz, I assume there will be no problems as I should have about 185 sq. in. of woofer area and an 11.5 Xmax. Was thinking 40sq. in. but 48 gives me the port length i'm after.

Thanks heaps man!

Moble Enclosurs
01-03-2012, 04:23 PM
Just to clarify.....The relation of the driver to the distance of the port, like all other aspects of design acoustics, will have an effect either positively, or negatively, depending on the design construction, not so much the acoustical noise factor. The reason is....the area before the port opening inside the enclosure, if utilizing a compression chamber, is less sensitive to reverberation and resonances at the desired response range because of the size of the enclosure. So, things such as changes in the response curve other than that of cancellation from phasing, is not as relevant as one would think in the driver placement within a certain distance INSIDE the enclosure. The main concern should be placement and effects outside of the enclosure to the listening position. This is where the environment has the potential to become more sensitive to response changes involving placement such as dips and peaks.
In regard to noise factors, this is not important in placement near as much as area of the port opening, such as that in an RLH design, where the throat area is much smaller in most cases than the drivers acoustical suspension compliance in volume for proper compression and LF output. So, as long as the port is a proper area, to reduce port noise as a factor in itself, the placement of the driver should not be a major concern if you have no other choice.
When noise other than turbulence becomes an issue, regarding driver placement within the enclosure, then you might want to check to see what kind of distortion is occurring from the driver itself other than worrying about how close it is to the port. That is why this has become a common misconception about placement regarding port opening. Its when the physical part of the driver interferes with the port opening that it can be thought of as improper placement of the driver. The rule of physics is that anything less than the distance of the shortest length of the port area to the port opening is improper as that will cause the well known compression noise faults within the enclosure and then gets heard from the port in the environment.
So, neither of those drawings are considered improper when it comes to driver placement within the enclosure. More concerns will arise when dealing with placement in relation to the exterior port opening with phase linearity.

jolly_26
01-05-2012, 03:53 AM
Box is built, it tears **** up compared to the last one, but for a box tuned to 30Hz, doesn't quite kill the lows like I wanted. I think it's the car actually not the box though, as it has a very linear response right down low when I'm listening to it from outside with the boot open, but not inside all sealed up. Need to do a layer of sound deadening in the boot I think, and pull out my rear tray speakers to allow for more airflow. Thanks for the help guys!

Moble Enclosurs
01-05-2012, 04:19 AM
Box is built, it tears **** up compared to the last one, but for a box tuned to 30Hz, doesn't quite kill the lows like I wanted. I think it's the car actually not the box though, as it has a very linear response right down low when I'm listening to it from outside with the boot open, but not inside all sealed up. Need to do a layer of sound deadening in the boot I think, and pull out my rear tray speakers to allow for more airflow. Thanks for the help guys!

i have an idea of why that is. Pm me if you can.

duanebro
01-05-2012, 09:00 AM
I have been playing with hornresp a little. it takes distance from port and subs to your location into account. And box resonances also. It is a pain to use, but neat to see the difference of different designs layouts. (Not that there seems to be much change.)

GammaRadiation
01-05-2012, 11:08 AM
You want to minimize the interaction of the "air plug" in the port with the cone of the driver. It's a resonating system and the plug hitting the cone instead of springing against the volume of the enclosure can cause interference at the back of the cone. If it's constructive you're likely to cause turbulence and lose energy to heat (though you'll be really loud...at the back of the cone) or the cone will be flexing at an angle. If it's destructive you will witness a phenomena often called "unloading." That is, the cone of the driver interacting with the port will easily overdrive mechanically near resonance of the system rather than what is expected, minimum excursion near resonance. You'll also be losing a lot of sound because your port will lose velocity. In general, you want to have your subs away from the back of the port and have the front of the port near the subs.

GammaRadiation
01-05-2012, 11:16 AM
Also to answer the question pertaining to the picture, my intuition would say greater distance but unequal. Why? Well the first assumption in building most smaller volume ported enclosures is that the wavelength of the resonance is much greater than the distances in the enclosure. That is, pressure is nearly equal throughout the volume of the enclosure. When you think about it, a 40Hz sound wave has an 8.5m wavelength. You need a box over 2m wide/long/high to generate any significant resonance. On top of that, Even a 15" woofer is comparatively small. So a 15" difference in distance can be assumed to have nearly the same phase. You mostly just dont want a port opening being right by either of the cones, especially on axis.