[eryn shannon]

I am building a subwoofer box for my vw jetta. I will be putting 3 Alpine Type R subwoofers in it powered by the 1000W Alpine mrdm1000 amp. I calculated the internal box volume to be about 3.5ft2. I would like to use slot ports. My questions are how many ports should I use, and what frequency should I tune the ports to? Thanks!

[David.G]

well if you have 3 subs you need 3 ports. Where did you get the plans for the box? IMO, you should never have subs sharing the same internal space. I'd seperate them to their own chambers with seperate ports for each one. If you do seperate them into 3chambers with the box you have you're looking at 1cu ft per sub. Thats enough for a sealed box which will do quite well for your subs. A ported box only allows for louder output but cuts into SQ. with 3 subs you really don't need more output since really its air movement that counts and you have 3 subs. 10" subs=30in of air movement. 12"=36in. Thats more than enough for any system. I'd seal them into seperate chambers but if you want to stay ported you'll need a new box. Considering that a ported box requires more internal volume than a sealed box does.

[eryn shannon]

The 3.52cu ft is the volume I calculated for a ported box. I got this figure by using the theile-small parameters for the sub (Qts=.41, Fs=30Hz, Vas=1.02cu ft). I haven't built the box yet but for a sealed box the sub works best in a 0.5-.08cu ft box according to alpine. If I do go with a sealed box should it be on the high or low side of this range. The volume I calculated for a sealed box are close to 0.5cu ft per driver, which seems kind of small for me. Why when you talk about air movement why do you only measure the size of the sub? A 12" sub is only 2" bigger than a 10" sub but it has much more surface area, which I think is the important factor in air movement. I don't think 5 10" subs, 50" of air movement have greater output than 4 12" subs, 48" of air movement. However the surface area of 4 12" subs is much greater than that of 5 10" subs.

[eryn shannon]

The 3.52cu ft is the volume I calculated for a ported box. I got this figure by using the theile-small parameters for the sub (Qts=.41, Fs=30Hz, Vas=1.02cu ft). I haven't built the box yet but for a sealed box the sub works best in a 0.5-.08cu ft box according to alpine. If I do go with a sealed box should it be on the high or low side of this range. The volume I calculated for a sealed box are close to 0.5cu ft per driver, which seems kind of small for me. Why when you talk about air movement why do you only measure the size of the sub? A 12" sub is only 2" bigger than a 10" sub but it has much more surface area, which I think is the important factor in air movement. I don't think 5 10" subs, 50" of air movement have greater output than 4 12" subs, 48" of air movement. However the surface area of 4 12" subs is much greater than that of 5 10" subs.

[brentl]

If you plan on drving this hard go for a sealed box, it's MUCH harder to overdrive the subs in a sealed box.

You should still get PLENTY of output.

Tune your port at the point where you want the most output. remember that the resonant frequency of the car will have a large effect on output in that range.

You could also have a problem with port length depending on youy tuning frequency

B

[brentl]

If you plan on drving this hard go for a sealed box, it's MUCH harder to overdrive the subs in a sealed box.

You should still get PLENTY of output.

Tune your port at the point where you want the most output. remember that the resonant frequency of the car will have a large effect on output in that range.

You could also have a problem with port length depending on youy tuning frequency

B

[Dave Milne]

Eryn,
You're on the right track. Use the T/S params and calculations (or preferably a computer simulation program --there are a number of freeware programs out there) to determine what response shape you will get in various configurations. Some of the better programs have the option of simulating cabin gain to give you an accurate picture of in-car response. YOU decide what response shape works best for you.

Yes, vented alignments are easier to overdrive and "bottom" --especially if driven with significant content below Fb (system resonance). But they will typically have more output at low frequencies, so you shouldn't have to push them as hard. Again, simulation software will give you a plot of maximum acoustic power output for a given alignment.

I don't have the Alpine Type R woofer in my database, so I plugged the parameters you gave into BassBox 6.0. For maximally flat vented alignment (B4), it shows 0.88 cubic feet with Fb of 27.79 and F3 of 31.47. This is not a bad alignment and should fit nicely in a car. You're right in that the sealed alignments for this woofer are tiny and sub-optimal. The maximally flat (B2 Q=0.707) alignment is 0.25 cubic feet with F3 over 63 Hz.

So I'd go vented with 0.88 cubic feet per woofer. This will give a hump in the 30-40Hz region due to cabin gain... but I like the solidity this gives in a car system, compared to the typical mid-bass "boom". And you don't need three enclosures or three ports for three drivers. You can, of course, but it's not really any better than a single enclosure and single port. Just calculate for one and triple the enclosure volume and port area. Port length remains the same.

You're also right about the "air movement". It's all about cone area (Sd) which determines swept volume (Sd x xmax = Vd). A 12" woofer has at least 50% more cone area than a 10" (I don't recall cone area formulas at the moment). I do recall that a 15" woofer has roughly twice the cone area of a 12".

Finally,

Quote:

A ported box only allows for louder output but cuts into SQ

This is rubbish. There are many very high-end, very accurate vented loudspeakers out there. Tell Focal that their $80,000 per pair vented Grande Utopia speaker has poor "SQ". Yes, vented systems have higher group delay, but this can be easily managed. Bottom line is a well designed vented system can sound every bit as good as a well designed sealed system.

[Dave Milne]

Eryn,
You're on the right track. Use the T/S params and calculations (or preferably a computer simulation program --there are a number of freeware programs out there) to determine what response shape you will get in various configurations. Some of the better programs have the option of simulating cabin gain to give you an accurate picture of in-car response. YOU decide what response shape works best for you.

Yes, vented alignments are easier to overdrive and "bottom" --especially if driven with significant content below Fb (system resonance). But they will typically have more output at low frequencies, so you shouldn't have to push them as hard. Again, simulation software will give you a plot of maximum acoustic power output for a given alignment.

I don't have the Alpine Type R woofer in my database, so I plugged the parameters you gave into BassBox 6.0. For maximally flat vented alignment (B4), it shows 0.88 cubic feet with Fb of 27.79 and F3 of 31.47. This is not a bad alignment and should fit nicely in a car. You're right in that the sealed alignments for this woofer are tiny and sub-optimal. The maximally flat (B2 Q=0.707) alignment is 0.25 cubic feet with F3 over 63 Hz.

So I'd go vented with 0.88 cubic feet per woofer. This will give a hump in the 30-40Hz region due to cabin gain... but I like the solidity this gives in a car system, compared to the typical mid-bass "boom". And you don't need three enclosures or three ports for three drivers. You can, of course, but it's not really any better than a single enclosure and single port. Just calculate for one and triple the enclosure volume and port area. Port length remains the same.

You're also right about the "air movement". It's all about cone area (Sd) which determines swept volume (Sd x xmax = Vd). A 12" woofer has at least 50% more cone area than a 10" (I don't recall cone area formulas at the moment). I do recall that a 15" woofer has roughly twice the cone area of a 12".

Finally,

Quote:

A ported box only allows for louder output but cuts into SQ

This is rubbish. There are many very high-end, very accurate vented loudspeakers out there. Tell Focal that their $80,000 per pair vented Grande Utopia speaker has poor "SQ". Yes, vented systems have higher group delay, but this can be easily managed. Bottom line is a well designed vented system can sound every bit as good as a well designed sealed system.

[David.G]

for 80K, thats pretty pathetic. vented only allows for more deeper output which IMO means its compensating for the quality of the sub. A vented box can make almost any sub sound good. But ED sub hits the best bass i've heard fomr any sub and hits low. Your Alpine will not hit around 30hz well i can tell you that. Most car audio does around 50hz. It'll lose output i think at or below 30hz. I'd put the subs in a sealed box. I don't remember you mentining what size it was but usually in a sealed box something with 1.4-1.5cf will do and if its too big add sand bags. With the Type R's you're using i'd say go 1.5 so that when the sub sits in you'll have roughly 1.35cf of volume.

[David.G]

for 80K, thats pretty pathetic. vented only allows for more deeper output which IMO means its compensating for the quality of the sub. A vented box can make almost any sub sound good. But ED sub hits the best bass i've heard fomr any sub and hits low. Your Alpine will not hit around 30hz well i can tell you that. Most car audio does around 50hz. It'll lose output i think at or below 30hz. I'd put the subs in a sealed box. I don't remember you mentining what size it was but usually in a sealed box something with 1.4-1.5cf will do and if its too big add sand bags. With the Type R's you're using i'd say go 1.5 so that when the sub sits in you'll have roughly 1.35cf of volume.

[HowardLi]

How did you calculate the internal volume without the dimensions of the ports? The port dimensions depend on the tuning frequency AND the internal volume of the box.

IMO, vented enclosures are better for car audio since you can't really hear the difference between a sealed box and a vented box in a car environment anyway. Besides, most of the stuff that gets played through a car sub isn't high fidelity anyway. With vented boxes, you save a lot of power for the same amount of output.

David, cabin gain happens anywhere under 60Hz-80Hz, depending on the size of the cabin, not just between 30Hz and 40Hz.