box parameters

Okay cool thanks buddy! I appreciate the help! Yea I was planning on making a double baffle so it would be flush with the woofer and I have always read to use the tee nuts. We can't use resin at work so I probably won't even bother with that then! I wish I worked at a place with more freedom, but I guess it pays the bills for now! Lol Oh and I see that you used PVC on that box.... Does it really matter if the port is PVC or just a slot made out of mdf?

zoom... I know this is not what you want to hear, but in every and ALL realities, the pressures inside a vented enclosure can be significantly higher than the pressures inside a sealed enclosure. The "louder" aspect (at tuning, (Fb) to be sure) of a vented enclosure requires higher air pressure gradients outside the box, right? That's what SPL is, right? What do you think happens INSIDE the enclosure? Yep... Higher air pressure gradients (again, at Fb). Never, EVER skip bracing. ANY amount of vibration you can feel in ANY enclosure wall is loss. It is also mistuning of the overall system. agraves, DO NOT SKIP THE BRACING. I realize that this is overkill, but that's what I do. That's a 10" woofer, and less than 1 cubic foot, net... I am only linking it to show you how important *I* consider bracing. It is not an option, it is *necessary* in a properly constructed enclosure, whether sealed or vented! In answer to your "PVC pipe vs. MDF slot" query, it doesn't matter the shape or the material, as long as the cross-sectional area is the same and correctly sized.

Yea I have heard when you thought you had enough bracing then add some more bracing, but in a lot of the enclosures I see there is little to no bracing. Thanks for the example. It looks great!

I guess I never looked at it from that logic before. I always thought the increased SPL was directly related to the increased air flow from a port and that the pressure inside the box was less because of the ability of the air to move in and out of the box. I believe you but I would appreciate it if you could give an example so I can thoroughly understand why. I am an engineering student so I welcome any math if that's what would help clarify. Thanks ham

Below resonance, you are correct. The enclosure does breathe, just as you think it does. That's why the need for a subsonic filter. Subsonic is a bit of a misnomer, as the frequency of such a filter is really designed to be set AT the tuning of the enclosure, to prevent frequencies below resonance from getting to the woofer at all, thus preventing unloading. The name for that filter came about in the vinyl days, where it really WAS applied to prevent running out of amplifier power, while trying to amplify turntable rumble. Above resonance, and really only AT resonance, but above the frequency there is a decrease in damping with an increase in frequency, the port's air motion is in phase with the woofer, meaning as the woofer moves out, the air in the port is doing the same. (guess where the increase in efficiency comes from? Fifteen bonus points if you said the port's tuning!) Now, seeing as there are two masses, (the woofer cone and the port's air volume) both moving out of the enclosure, the air pressurization and rarefaction effect has to be greater than if there is only one mass... Right?

So let me make sure I got it right. When the woofer plays below the tuning frequency the volume of the air in the port is being pulled into the box as the cone moves out (out of phase) because the waves are longer than the port? And starting at and above the tuning frequency the mass of that volume of air is unable to move fast enough to keep up with the increase in frequency? Or is it that the sound waves at and above the tuning frequency are shorter than the ports length? And that's why it becomes in phase? I understand that if the sub and the port are firing in phase that would really increase the pressure inside the box though. I haven't taken a fluid dynamics class yet which would probly help me?

Well... It really isn't the wavelength. The waves will ALWAYS be longer than the port. 100Hz is a wavelength of 11.3 feet at STP, and they only get longer as your frequency goes down. It's resonance. It all has to do with the "springiness" of air. It's also why the port length changes with box volume - to remain tuned at the same frequency. Unfortunately, I might be at my personal limit of understanding/explanation capabilities... I've never taken any engineering courses, and I am not an acoustician. I wish I had more understanding of it myself, I Just don't! Here are a few more possible sources for Helmholtz Resonantor formulae, the first address is the google search: http://www.google.com/search?client=ubuntu&channel=fs&q=helmholtz+resonator&ie=utf-8&oe=utf-8 http://www.phys.unsw.edu.au/jw/Helmholtz.html http://physics.kenyon.edu/EarlyApparatus/Rudolf_Koenig_Apparatus/Helmholtz_Resonator/Helmholtz_Resonator.html http://en.wikipedia.org/wiki/Helmholtz_resonance And here is the link to "Helmholtz Resonator Formula": http://www.google.com/search?client=ubuntu&channel=fs&q=helmholtz+resonator&ie=utf-8&oe=utf-8#sclient=psy&hl=en&q=helmholtz+resonator+formula&aq=0&aqi=g2g-o1&aql=&oq=&gs_rfai=&pbx=1&fp=7b9141da4f416ce8 I don't know how much a fluid dynamics course might help; it may or may not. I know quite a bit of DYohn's pedigree might come in handy here; if he's reading, maybe he can chime in.

Here's a simple explanation; a port works like blowing across the top of a bottle.  The sound is created by activation of the air inside the volume, not by air moving in or out through the port.  At the most basic level, the woofer moves air across the end of the port and it makes a sound like the bottle does.  Thus why it's size is critical: just like if you add water to an empty bottle and blow across it the tone changes.Support the12volt.com

That helped a lot. Now instead of thinking of a ported box as a box that air can flow freely in and out of, its more like a box with a passive radiator because the air can move but it doesn't leave the box it just springs back and forth in the port like...a spring, and like the links you posted. I found this diagram the most helpful only instead of compressing the air at the port your compressing it with the speaker.

That's the right stuff... :) Now. How do we feel about bracing, even in a vented enclosure? That's rhetorical. You don't have to answer, but I bet the next pictures I see from one of your installs has bracing! (You do pretty sharp work, BTW...)