OK, ported boxes are a pain in the butt to design!
I can do a sealed box easily! This tuning stuff is a drag when you are going for big pressure!
The box I'm putting together from this thread:
http://www.polkaudio.com/forums/showthread.php?t=67143
for a pair of Memphis M3124D's has been a thorn in my side.
Currently the box they are in is a 6+ cubic foot beast with a gigantic rectangular vent. The box is tuned for about 47 Hz or so they say. I honestly think it's too high. After plotting out response on a graph, the "hump" in response that the guys in that other thread say is there doesn't seem to be there or at least not nearly as pronounced as they say it is.
I've already established that the resonance measurement of the car is off. How much, I don't know. Maybe we'll find out after I get this box put together.
The box is 35 inches by 17.5 inches by 16.125 inches. I think it might actually be a tad wide for the space between the shock towers but we'll see. The owner of the car can't seem to get off his dead butt and get me measurements.
Anyhow, I was going to build a dual chamber box but, I could not keep port velocity below 30 f/s without a common chamber box. Consequently, I have three 4 inch ports to keep the velocity down and the port tubes in as straight a line as possible in the depth of the box. Both ends will be flared so my port velocity of 23.2 f/s should not be that big of an issue because the flared ends manage air flow better.
The box is tuned at the resonant frequency of the sub because I can't trust the car measurements. It's about 30-35% smaller than the old box so moving it around should be easier too. I'm going to mount the 3 ports in a vertical line between the subs. That way air flow isn't obstructed by the sides of the trunk compartment. That should help port velocity and cabin loading too.
A couple things I learned about "tuning". You aren't tuning a resonance like so many people think. You are managing air flow. When you tune to a resonance, you are making it so that the air pocket behind the woofer has enough space so that the timing of the backwave moves the air in the port in sync with the woofer cone so it compliments the woofer instead of fighting it. Granted, this is in reality what everyone is doing, I just don't think that as many people really understand it as much as they say they do.
Air flow is crucial. It's how the cabin gets pressurized. If your box/ports are too big, they will not have enough velocity to effectively load the cabin to it's maximum. If the cabin is not loaded to its maximum, you can tune to the resonance of the car all you want, it will just fight you until you hit the maximum SPL you subs are capable of producing. For that, you need to build the enclosure properly.
Enclosure construction is not a "wham, bam, thank you ma'am, slap it together any old way and call it a day" kind of proposition. Especially if you are tuning the enclosure to increase output. It is pretty much an exact science and requires accurate measurements from the company that designed and built the driver. Numbers matter.
SPL is volume. There is no difference. Someone can try and tell you until they are blue in the face that SPL and Volume are not the same. There are several definitions of volume, one of which is esoteric but for the most part volume can be described as amplitude, dynamics, loudness and SPL. If someone tells you that Volume is not the same as SPL ask them how they measure SPL. If they say anything other than dB (decibels) they are wrong. Then ask them how they measure Volume and if they say dB...I rest my case.
Audio is physics. Electronics and the electricity that runs the electronics is also governed by physics. Numbers describe EVERYTHING in physics. If audio, electronics and electricity are based in or described by physics then physics and the numbers used to describe physics can describe anything in audio. Any audible manifestation or even inaudible manifestation, unless it's just in one's own head, can be described with numbers. Specs don't describe everything nor tell you how something will sound because how the something interacts with the environment it is installed in is where those manifestations come from. But that environmental interaction can be described with numbers. Most people who tell you that it can't just don't really fully understand what is going on with those manifestations.
So I dunno if this box is going to produce more favorable results. We'll see I suppose. I do know that the box I built is about 4.009 cu. ft. which is in line with what the subs should be performing at their best in. The response hump is pretty wide too and goes above a -3 dB slope around 36 Hz and stays high all the way to about 52 Hz where it falls off below -3 dB again. The old box seems to have a drastic spike that doesn't rise as high as this box. I tuned the port for this box to be around 40-42 Hz, right smack in the middle of that peak. I think the car's resonance is closer to that too than the 47 Hz they measured it at. I got a bunch of towels and stuff that I can rubber band up and stuff in places to see how the dampening will change the car too. Maybe I'll get some more out of it, maybe I won't. All I really want is 2-3 dB more because that will put him over the top in his class and no one will be able to touch him.
