okay so what mi getting at is... if you tune a box to lets say some random number... 35hz... and you send it a higher frequency to it... what exactly is happening? and same question for if you send it a lower frequncy?... sending a lower frequency than the boxes tune it bad for it right??? thats what subsonic filters are for?
MAN you dont know jacksh!t you wannabe go back to work at best buy were you belong with all the other wannabes
First you have to understand resonance. Resonance is a point in terms of frequency where the resistance provided by the inertia of the moving mass and that of the suspension are equal. That frequency is referred to as the resonant frequency, in the case of a ported box, this is called the tuned frequency, just for those that don't know that. Since resonance is the point where the two suspensions are equal, it's easy to say that it will mean at other frequencies they are not equal. Simply put, when you are above resonance, the port's suspension is dominant, and when you're below resonance the suspension of the sub is dominant, and the resonant frequency is where they become friendly.
If you're above tuning, the resistance of the air mass (port) is high due to the fact that the port isn't quick enough to react to the quick changes in direction. This results in an inactive port and basically a large sealed box. This is gradual, not instantaneous, for a range of frequencies you still get a minor boost in output.
At tuning, the port becomes active since it matches the resistance of the port matches the resistance of the frequency played (inertia). It uses the speaker cone as a fulcrum for it's own output, and sucks energy through the internal air pressure and loads the speaker in the process. This means higher output, lower excursion for the sub since it's controlled heavily by the port's activity, and low distortion.
Below tuning, the port is no longer adding any significant amount of resistance and the suspension of the sub is higher at this point. While the inertia remains the same, as the excursion requirements become greater and greater at any given output level, the influence of the suspension increases, becoming more of a factor than the port. The port is now a hole in a baffle and doesn't load the driver anymore, causing cancellation and sending the subwoofer to very high excursions due to the lack of control.
Why not tune to the lowsest frequency the sub can handle?
In some cases 20hz or even down to 5hz.. wouldnt you get the little SPL burst over the sealed box that you mentioned if you above tuning.. and have alot less lack of control and high excursion mentioned when below tuning..
would tuning to 20hz or even lower have better SQ (less spl than higher tune.. but more than sealed) from the ported box vs a box tuned to 35hz or higher?
Utilize a low tuned enclosure, when the subs t/s parameters call for it, usually a low fs, say around mid teens to low 20's is considered "low", would call for a low tuned box. Yes a low tuned enclosure would have "less lack of control" than a higher tuned enclosure, but it is entirely frequency dependant and that is based on the type of music and the frequencies in that music that you listen to. In dolby digital home theater there is a lot of content in the low mid teens,(star wars pod race scene, finding nemo glass tapping scene to name a few) but in "popular music" low-mid teen content is relatively rare. Classical music on the other hand can have organ tones down to 12-16hz! Then you have the "bass" cd's that may have content that low. Otherwise most music does not go that low, so no real need to tune at 10hz, for music.
"Why not tune to the lowsest frequency the sub can handle?
In some cases 20hz or even down to 5hz.. wouldnt you get the little SPL burst over the sealed box that you mentioned if you above tuning.. and have alot less lack of control and high excursion mentioned when below tuning.. "
You've just stated the basis of SQ tuning . Like E said, music rarely drops that low, really it's rare for music to drop even in the 20s. Movie soundtracks are a different story, though. Anyway, in order to maintain control you need to keep the tuning frequency above the resonant frequency of the sub itself so that you don't lose driver damping within it's range, that is a factor that weighs in with the frequency you choose as most subs don't have an fs below 20hz, many don't even get that low. Your basis is correct, tuning as low as you need to will add SPL at lower frequencies without adding a lot of stress to the driver mechanically. The main reason people don't do this in car audio is that they're looking for added SPL with music, and that means they typically tune in the 30s and 40s. Another limiting factor is that a 20hz tuning frequency requires a very long port, and more space, which is to another thing to be considered in a car.
"so thats why avalanches hit sooo low?... they have a Fs of 15.7 i think..." That has an effect, but in the case of the Avalanche the bigger factor is that it uses a loose suspension to achieve a high efficiency.
The car cabin resonates at a certain frequency, so you're tuning the enclosure to that so that you'll get even more SPL, since the car helps it out. Like said above, cabin gain. To determine that, you take a speaker with a known frequency response in a large room (preferably flat freq. response) and you place that in the car. The frequency that you get the highest SPL at is where your res. frequency is, usually it's 60-70hz or so, sometimes in the 50s depending on the vehicle.
you can safely tune down as low as the fs of the sub.. is that what i am getting? (without causing problems) Even though a sub manufacture my suggest higher tuning. So like the eclipse subs that have fs of the low 20's can u actually tune to 22hz and it sound good? or will it be a waste of space to build that long of a port
To find the resonant frequency of your vehicle, measure the frequency response of your subwoofer system, in open air(outside of the car with no reflecting walls within 10 feet of the enclosure), use test tones starting with 35hz as a basis, use a good spl meter and take readings at 35,36,37hz and so on all the way up to 60hz in 1 hz increments. Record your findings(example 90db at 60, 89db at 59hz and so on). Now install(place) the subwoofer system in the vehicle that you want to find the resonant frequency, do the same test, only this time it will be in the vehicle. Take note of the spl readings at 35,36,37hz and so on. Where there is the largest spl gain is where the resonant frequency lies, for example an 8db gain at 48hz over the "outside measurement" but only a 6db gain at 47hz, would mean that the responant frequency of the vehicle COUPLED with that enclosure lies somewhere near 48hz. Now for base line tuning, you'll need to start with an enclosure tune roughly 8-9hz below the resonant frequency that you found earlier., this is a good place to start with. Every vehicle is different, as well as placement, will determine a long way in how you do in competition(placement within 1-2 inches farther back in a suv for example can make a 1-2 db difference). Port area also matters, the more port area the larger the gain, there are ports in competition vehicles that you can literally crawl in! It is said that a ported enclosure peaks at 7-9hz above tuning ie. a 30hz tuned enclosure will peak at 38-39hz, but experimentation is key here. Keep building those boxes until you can achieve the highest spl reading you can. Now this does not work for a daily driver though, as your subwoofer system will be a somewhat "one note wonder" and will not sound good for your "streetbeats", some competitors "plug" the port holes for everyday music in effect making a sealed enclosure, but it is at best a poor solution, if you use the same woofers for comps as your daily driver, then use 2 different enclosures, one for comps and one for your daily driver. There are exceptions to the rule but keep measuring and experimenting and you'll get it(thats part of the fun of competition).
or you could do my plan with the adjustable airspace box and adjustable sealed/ported design with different port lengths and save on building box after box and use his method above for testing the sub in and out of the car...i also have a daily user street box with adjustable ports and a sealed option built into my enclosure for the 12's i used to have...its all flush mounted and carpeted no air leaks and looks nice
slide the box forward or backward simple as that.. on a track system of course so it could slide back and forth...the box volumes would be calculated and then written on the outside and marked so you knew what volume you were at
E explained it very nicely up there the only thing ill add is you could buy either a epic 160 or 150 with a built in spl meter and volt meter or you could do what i did before i knew about them and buy a radioshack digital spl meter for 50 bucks but they only go up to 126 db and the epic 160 and 150 do 150 db and 160 db....for right now im fine with my digital handheld meter i dont need to go all out i just want to see if im gaining at low volumes to keep voltage consistant and get the most spl for as little power as possible eventually i will buy an epic 150 or 160 but i did realize one thing just now if you dont have a hand held meter you cant take readings outside the car using E's method described
ps...write down your spl readings outside the car and inside the car and youll have a good chart to look at ....you may learn even more charting the in car results as you vary the box sizes and port tunes in car and chart them and compare you could also write down notes to what the sound charactersitics were like playing music if you were using the box for that reason and you could get into more but ill stop there (hint clamp meter)