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Old 21st September 2009, 09:13 AM   #11
Did it Himself
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That port is way too small area, when you turn it up it will make wind noise.

It's more complicated than "what would be the upper and lower range" really, but for automotive use, booming, 35Hz would be pretty good. If you want SQ then maybe think about tuning lower.
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Old 21st September 2009, 12:15 PM   #12
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i dont think i am after sound quality at all. i dont have much of a trained ear. i mostly use the stereo for watching movies and also for bass heavy music. ill i am really after is the sledge hammer in the chest effect when i listen to music or watch a movie and a car explodes

well now i have already cut a hole in the side of the box. is it possible to keep the 3.5x2 dimensions and make a longer port? if so how long should i make it?

is there any advantage of using sqaure port or round port?
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Old 21st September 2009, 06:46 PM   #13
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You gave the impression that this was for a car. Where are you using it?

As I said the port area is too small. You need to be looking at an absolute minimum of a 4 inch round port, or the equivalent area of rectangular port. Can you make either of the 3.5 or 2.0 dimensions bigger?
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Old 21st September 2009, 07:19 PM   #14
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it is for a car, well actually a work truck. I just watch a lot of movies in it. Is there another calculator that I can use that would give me something closer to the port volume that you are talking about. I'm using the calculator on bcae1.com.

Is my port volume off because my speaker box is larger than recomended, because I can make it a little smaller if that is the cause of my problems. When I turn it up too loud I get a violent crack noise from the speaker which I never got before from a sealed box.

Can I stick with my square port, it just that much easier to work with and change if needed, or do you suggest i switch and start using a round port? I was thinking of doing a square port at 3.5x3.5 id. how can I calculate the dimension of the port. I don't want to just guess at the dimensions.

Is it possible to tune the port to be able to listen to bass heavy music and still listen to movies and have them sound decent?
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Old 21st September 2009, 10:27 PM   #15
GM is online now GM  United States
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Quote:
Originally Posted by scampo77 View Post
ok i installed the port into my box today and i must say im a little dissapointed.

i can tell where the port stops braking the speaker and i wonder if i have done something wrong.

-why not just tune the box to 15-20 hertz?

- is a ported box signifigantly louder/deeper then a sealed box?

does a sealed box ever run into the problems of not being able to act as brake to the speaker at low extremes?
If the sim I did in WinISD is remotely accurate using the S12L74 specs, I can understand why if not in a sealed up small car and then only if a low XO point is used due to its high inductance. It's useless in a HIFI/HT app IMO.

No, you didn't other than not sizing the vent for higher power. As it is, it's only good for ~25 W before vent compression starts becoming a factor. The box is just way too small for a vented alignment except some car audio ones. Factor in that a sealed alignment will protect the driver somewhat from overdrive down low whereas vented unloads below Fb, usually requiring some sort of high pass filter.

It's never a good plan to tune a vented alignment < ~0.707*Fs unless it's either a prosound one or a compression horn since the vent provides no protection plus you risk bottoming it out above Fb since the box efficiency can't protect the driver good enough either at higher power.

Yes, vented typically plays lower/louder, but the price you pay is a bigger box. With the specs I have, it will need to be at least 6.2 ft^3 tuned around 27-28 Hz with an 8" diameter vent 23-24" long for the driver to reach Xmax at low compression with 400 W (yellow plot), but I see this is for a truck, so probably too big and tuned all wrong to blend well with its cabin gain unless you want the extra gain, so you can EQ it to suit.

For your existing vented box (green plot), it takes a 7" diameter vent 32-33" long for a ~34 Hz tuning. Yes, you can use a square or rectangular vent of the same area.

As long as the box doesn't leak, the sealed's air mass spring will provide an increasing resistance as the driver tries to compress it at higher excursion, so what happens is the VC heats up, raising Qts/Qtc, weakening the motor, so while it protects it from bottoming out, it can burn it up and/or tear it apart and one reason why sealed isn't normally used in a prosound app.

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Last edited by GM; 21st September 2009 at 10:33 PM.
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Old 22nd September 2009, 01:11 AM   #16
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geez man 6.3 cubic feet??? i have room to build it i just dont know if i want to. is there some sort of middle ground?

right now the longest dimension i have on the box is 24 inches. so unless i am aloud to have the port sticking way outside of the box then im left either with building new boxes or shorting the port length. am i aloud to shorten the port if i make it fatter?

i should be clear as to what kind of a "truck" this is. it is a 6 wheeled city delivery truck. it is a GMC T7500 i have always sort of been interested in car audio as a hobby but i kept running out of power from the engine to drive alternators. so now i have this truck and i have no shortage of space or power. i have 4 forklift batteries on the frame and enough power and space to run 6 alternators if need be. i mostly watch movies while i am stuck in traffic or on a long road trip but i do use it for music aswell.

so if i am stuck with having to build such a massive box (and ill have to do it twice) will i be better off going back to sealed? because i use this radio aprox 40-60 hours a week i am really afraid of burning out the voice coils and its why i thought i would try at a sealed box how much gain will i see ported vs sealed? because if i run sealed boxes then i can have room for 4 speakers.

why did kicker reccomend the specs that he did (see first post).

i dont know if this matters or not but i am running JBL bp1200.1 amps for each speaker
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Old 22nd September 2009, 02:03 AM   #17
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i am still having a hard time understanding what the job of the port is. does it matter where i put it? can i have the port aimed at the ceiling of the truck?

i sort of picture it like the speaker box taking a breth in then compressing the air because there is now now more air inside the box then atmosphere, then it forcing down on the air trapped inside faster than it can escape out the port. is this accurate?
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Old 22nd September 2009, 02:27 AM   #18
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L7s take big boxes. That's one thing I've noticed around here, people use L5s and L7s with tiny boxes and barely any port area, and wonder why their SPL scores aren't what they thought they'd be.

Personally, I'd go with another driver that'd be suitable for a smaller sealed enclosure, but that's just me. For watching movies and listening to most kinds of music, that'd be all you need.
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Old 22nd September 2009, 03:45 PM   #19
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Quote:
Originally Posted by scampo77 View Post
geez man 6.3 cubic feet???
Qts dominates net box Vb to the point that at a ~0.403 Qts, Vb = Vas, ergo for a Vb < Vas requires a < ~0.403 Qts and vice versa. Note that in this scenario Fb = Fs when Vb = Vas, ergo Fb = > Fs with a < ~0.403 Qts and vice versa assuming a maximally flat alignment.

There's always a 'middle ground' between two points (or alignments in speaker design), but the trade-off is always the loss of some box efficiency (useful gain BW), so it's always up to the end user to to decide where this point is.

Yes, the vent can be completely out of the box, though normally if it's a long one, best overall to fold it up to get its terminus (mouth) back to someplace near the the driver, but in such a limited LF BW app such as yours it's moot due to the long WLs combined with a low XO point and presumably a high slope order.

A vent's cross sectional area (CSA) sets its basic tuning, so making it larger raises its tuning and vice versa. Lengthening it therefore tunes it lower, so from this we see that making it larger and shorter will raise box tuning considerably.

Hmm, assuming the cab's floor-pan is relatively flat, then the interior is large enough and rectangular enough to treat it as a tiny, well sealed room with the windows up, so a low Fs, Vas, ~0.31 Qts driver in a ~max flat (~0.7 Qtc) sealed cab with high linear excursion (Xmax), high peak power handling (Pe) is what you need for HIFI/HT. IOW, a different driver with specs appropriate for the truck's cabin gain curve.

Then again, heavily stuffing your existing box/vent may work well enough, so get some scrap R-30 fiberglass insulation from a local Htg/AC contractor and pack it at 1.5 lbs/ft^3 and if the driver is rear vented, stretch a scrap piece of pantyhose over it to prevent any fiberglass from getting into the motor. http://web.archive.org/web/200210070...ces/fiberfill/

This will lower the box Q considerably, making it sound like a larger, lower tuned one, though with the trade-off of less peak SPL, but it should sound more tonally balanced, i.e. deep LF more in line with its harmonics higher up instead of its current ~ 'one note' tone.

Can't comment about Kicker's recommendation. I mean depending on what you asked for it may have been the appropriate alignment, so like with anything else always be careful of what you ask for.

From just reading a single review, the amp appears to be serious overkill for your app, though there's no such thing as too much power in a LF BW app, just a point of diminishing returns. .

GM
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Last edited by GM; 22nd September 2009 at 04:05 PM.
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Old 22nd September 2009, 04:04 PM   #20
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Quote:
Originally Posted by scampo77 View Post
i am still having a hard time understanding what the job of the port is.
See my previous response, though I'll add that for strictly 'sub'/LFE channel duty, having the driver closely coupled to either the floor or ceiling (whichever sounds smoothest overall) with the vent opening at other end creates an acoustic 'infinite' line array.

Note that in theory air doesn't compress in a vented alignment and AFAIK it's true in reality except for high compression ratio (CR) horns, but vented alignments do 'take a breath' (time delay) before they begin 'pumping up' since the air mass plug in the box has to ~ bump start the air mass plug in the vent. Maybe this nifty series of animations will help: http://web.archive.org/web/200711280...ayporting.html

GM
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