I took a few minutes to model the driver you have in a 70l enclosure with 2 - 3"x12" ports - the modeling shows that everything should work properly and the air vleocity through the ports should not cause "chuffing"
However, in the modeling with 100W applied it also shows that driver will exceed xmax on all frequencies below 26Hz
I think it is time to ask you some more questions
1. Since this is a DVC driver - how have you wired the two coils - in parallel? + to + and - to - ??
2. What sub amp are you using ?
3. Does that amp have a "bass boost" knob/switch and is it turned up/on ??
4. Does the amp have a High Pass filter - if so is it adjustable
CharlieLaub is thinking along the same lines as myself - with 2- 3"x12" ports that 70l box is tuned to about 33Hz. By tuning it lower to 26Hz (by changing to 2 - 3"x20") ports this will move your cone unloading much lower in frequency and also result in moving where the driver will exceed xmax down to about 20Hz
However, in the modeling with 100W applied it also shows that driver will exceed xmax on all frequencies below 26Hz
I think it is time to ask you some more questions
1. Since this is a DVC driver - how have you wired the two coils - in parallel? + to + and - to - ??
2. What sub amp are you using ?
3. Does that amp have a "bass boost" knob/switch and is it turned up/on ??
4. Does the amp have a High Pass filter - if so is it adjustable
CharlieLaub is thinking along the same lines as myself - with 2- 3"x12" ports that 70l box is tuned to about 33Hz. By tuning it lower to 26Hz (by changing to 2 - 3"x20") ports this will move your cone unloading much lower in frequency and also result in moving where the driver will exceed xmax down to about 20Hz
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I did wire the two voice coils in parallel and I am using MCM's 100 watt amp. I don't think that there is a bass boost button, if there is I have no idea where to find it. There is a filter which is adjustable with a knob. Even if I play with this filter there is always the same bad sound.... Your idea of putting longer ports is really interesting.. I'm at school now so when I get back home I'll look into it. Thanks!! 
I had a look at the specs for the MCM 100 watt plate amp and confirmed that it does not have bass boost - the only controls appear to be for level, XO frequency and phase.
The one thing that does concern me is that there is no indication that this amp has any High Pass filtering to prevent cone over excursion at lower frequencies ( ie this could be part of your problem as well)
To summarize I would suggest two things
1. Try the longer ports I suggested
2. With the amp not having a High Pass filter(probably one of the reasons the amp is so cheap) you should really consider adding on an external HPF such as this one which is the least expensive external way to add one. The HPF will prevent cone over excursion at subsonic frequencies - not having a HPF in the amp could be one of the factors contributing to the problems you are having.
The one thing that does concern me is that there is no indication that this amp has any High Pass filtering to prevent cone over excursion at lower frequencies ( ie this could be part of your problem as well)
To summarize I would suggest two things
1. Try the longer ports I suggested
2. With the amp not having a High Pass filter(probably one of the reasons the amp is so cheap) you should really consider adding on an external HPF such as this one which is the least expensive external way to add one. The HPF will prevent cone over excursion at subsonic frequencies - not having a HPF in the amp could be one of the factors contributing to the problems you are having.
Seriously try this for bass notes with ports!
Just try to ignore the chugging sound and listen. Play some media at a reasonable level (not rocking out) and listen to the bass. Find a solid ball (like softball for baseball) and block the port hole while listening.
In over 90% of the cases there will be more rumbling bass with the port open. With the port well blocked bass guitar and other bass sounds will suddenly have notes and the low rumbling will cease.
Which would you rather have, low rumbling or notes? I choose notes myself. Anyone can try this simple exercise and discover the facts for themselves.
Just try to ignore the chugging sound and listen. Play some media at a reasonable level (not rocking out) and listen to the bass. Find a solid ball (like softball for baseball) and block the port hole while listening.
In over 90% of the cases there will be more rumbling bass with the port open. With the port well blocked bass guitar and other bass sounds will suddenly have notes and the low rumbling will cease.
Which would you rather have, low rumbling or notes? I choose notes myself. Anyone can try this simple exercise and discover the facts for themselves.
sumaudioguy: I will try your thing.
Cokewithlime: Thank you for being so helpful, Isn't there a cheap way to put in a HPF after the amp (putting resistors and caps and stuff like that)? or is this really the only solution? I will try the lower tuning, but maybe not now since I have so many other things to do. When I play the music louder (turn the knob 3/4 to max) there seems to be so much air coming out the ports (like if it was a cannon) and the speaker reaches it's Xmax. The higher notes (around 80 to 100 Hz) play really loud and clear, but when it goes down, the sub just moves alot of air but doesn't make much noise or that low clean bass that we all look for. Will putting a HPF help alot?
Cokewithlime: Thank you for being so helpful
, Isn't there a cheap way to put in a HPF after the amp (putting resistors and caps and stuff like that)? or is this really the only solution? I will try the lower tuning, but maybe not now since I have so many other things to do. When I play the music louder (turn the knob 3/4 to max) there seems to be so much air coming out the ports (like if it was a cannon) and the speaker reaches it's Xmax. The higher notes (around 80 to 100 Hz) play really loud and clear, but when it goes down, the sub just moves alot of air but doesn't make much noise or that low clean bass that we all look for. Will putting a HPF help alot?Blocking the port will just raise your F3 to about 60Hz and you will get very little bass below that with your current sub amp.sumaudioguy: I will try your thing.
Cokewithlime: Thank you for being so helpful
Try tuning the sub lower first by putting in the longer ports and also adding the eggcrate foam on the back, sides, top and bottom - this should help a lot and doesn't cost much. The HPF can always be added if the other things don't completely resolve the problem. There is no cost effective way to do a speaker level HPF after the amp.
For fun easy testing and dual voice coil EQ
For dual voice coil 2 low pass filter section can be used, one for each coil. Need to model this to get it perfect. The filters either have different frequencies for first order or if 2nd order different Q and/or same/different frequency. This allows the driver to be equalized such that total drive increases as frequency decreases. This results in lower efficiency at higher frequencies because less energy is going to one voice coil. The advantage is boost down low. Have done this in the past and it worked really well using 2 different Q second order filters. The Watkins woofer system does this either with band pass or first order filters- i forget.
To the point- this could allow a sealed box and quality bass with a higher box resonance.
In the sealed box add a little mass to the driver using modeling clay. Say stick about 50 grams on there in sealed box and listen. Lower efficiency but lots of other things change also.
Just for reference- if you have some form of oscillator you may simply sweep down in frequency and watch the cone motion. Put a little dot, 2-3mm, on cone with tape to make the motion easy to see. With port, cone motion will increase with decrease in frequency and then decrease to a motion minimum. The minimum is the port tuned frequency. Same test in sealed box will provide a cone motion that increases with decreasing frequency to a maximum motion and then decreases in magnitude or at the very least no longer increases in motion. This frequency is the effective cutoff for the seal box.
Free computer based oscillators are available from several sites. I use Marchand one. He is a nice guy and recommend his stuff for fun.
For dual voice coil 2 low pass filter section can be used, one for each coil. Need to model this to get it perfect. The filters either have different frequencies for first order or if 2nd order different Q and/or same/different frequency. This allows the driver to be equalized such that total drive increases as frequency decreases. This results in lower efficiency at higher frequencies because less energy is going to one voice coil. The advantage is boost down low. Have done this in the past and it worked really well using 2 different Q second order filters. The Watkins woofer system does this either with band pass or first order filters- i forget.
To the point- this could allow a sealed box and quality bass with a higher box resonance.
In the sealed box add a little mass to the driver using modeling clay. Say stick about 50 grams on there in sealed box and listen. Lower efficiency but lots of other things change also.
Just for reference- if you have some form of oscillator you may simply sweep down in frequency and watch the cone motion. Put a little dot, 2-3mm, on cone with tape to make the motion easy to see. With port, cone motion will increase with decrease in frequency and then decrease to a motion minimum. The minimum is the port tuned frequency. Same test in sealed box will provide a cone motion that increases with decreasing frequency to a maximum motion and then decreases in magnitude or at the very least no longer increases in motion. This frequency is the effective cutoff for the seal box.
Free computer based oscillators are available from several sites. I use Marchand one. He is a nice guy and recommend his stuff for fun.
This requires active circuitry - an op amp some R & Cs plus a power supply - the RCA plug thing is far cheaper overall - as I said before I think the lower tuning and foam will solve 99% of the problem
The OP problem is related to cone over excursion - your suggestion of low pass filters is exactly the wrong type of filter to help solve his problem. Also if you read his other posts he does not have a sub amp with bass boost capability so going to a sealed enclosure is not a good option for him.
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MCM 100 watt amp
http://www.mcmelectronics.com/content/ProductData/Spec%20Sheets/50-6272.pdf
This is the schematic for the amp input section. It has a high pass filter built in. 12dB octave at like 14Hz but the Q of the filter is pretty low so roll off is not so steep. If the values of the 2 caps and 2 resistors around that filter (U200B designator) could be changed maybe everyone would be satisfied. Say a Q of 1-1.5 set at Ff of 35 or 40Hz. That would kill the subsonic and excess cone motion from out of band and EQ the bass.
Win win? Maybe.
Oh yah- and small ports still do not work. Must be at least half the size of the cone.
http://www.mcmelectronics.com/content/ProductData/Spec%20Sheets/50-6272.pdf
This is the schematic for the amp input section. It has a high pass filter built in. 12dB octave at like 14Hz but the Q of the filter is pretty low so roll off is not so steep. If the values of the 2 caps and 2 resistors around that filter (U200B designator) could be changed maybe everyone would be satisfied. Say a Q of 1-1.5 set at Ff of 35 or 40Hz. That would kill the subsonic and excess cone motion from out of band and EQ the bass.
Win win? Maybe.
Oh yah- and small ports still do not work. Must be at least half the size of the cone.
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Btw - the SD of that DVC driver is about 81 in^2 - he already has 2 x 3" ports which totals 44 in^2 and still has cuffing problems - the problem is not port area but cone unloading because the enclosure is tuned too high and also not damped.
If there is a HPF in the amp, a 2nd order at 14Hz will do nothing for him to prevent exceeding that drivers xmax. He really needs a HPF 2nd order at 20 to 25 Hz to be sure of staying within the xmax of that driver
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I believe you have some math error. Area of port is pi*r*r. Two 3 inch ports is about 14 square inches total. A long way from 44 square inches. At least half would be 44 and bigger is much better. Small ports still do not work!
I agree with you on the filter which is why I recommended 35Hz.
Yes, 14in^2 - typo on my part - even with this area cross section he should not be getting the problem, which I believe is more related to cone unloading than port air velocity. The issue with oversized(area) large ports is making them work in a reasonable size enclosure particularly when it comes to sub designs.
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key point
Yes typo no problem.
My key point is small ports do not work. That is it. I have proved this in the lab to many people over and over. Yes I know it is very popular. Does doing something for many times which does not work make it work? I do not think so.
You have also hit on an obvious fact- to make the port large enough the port size becomes a substantial part of the box volume. The only port which works well falls in the narrow category of passive radiator where the passive is equal to or one size larger than the active driver.
The lesson I am trying to teach everyone is "Small ports do not work as expected." That is it.
Yes typo no problem.
My key point is small ports do not work. That is it. I have proved this in the lab to many people over and over. Yes I know it is very popular. Does doing something for many times which does not work make it work? I do not think so.
You have also hit on an obvious fact- to make the port large enough the port size becomes a substantial part of the box volume. The only port which works well falls in the narrow category of passive radiator where the passive is equal to or one size larger than the active driver.
The lesson I am trying to teach everyone is "Small ports do not work as expected." That is it.
hmm... so there is a problem with the ports being too small. I'm starting to being confused. If I were to put ports big enough.. they would be of a huge size and would need to be very very long? I think I'm too far into this to start thinking to change it for a passive radiator system so I guess I'll forget about that. Are you guys still saying that the amp I have needs to be modified? Or that the ports are too small? Or both? The thing is.. even when putting 50 or 60Hz the speaker seems to be reaching Xmax. A HPF at 30-35Hz would help but if the driver is at Xmax around 60Hz... will it even do something? If I were to put the ideal ports, What size and length would they be?
Since you already have hole cut for the two 3" ports just try making them 20" long each and see if the lower box tuning helps - this cost virtually nothing but a few minutes of your time.
With lower box tuning, by lengthening the ports - this pushes where you will exceed xmax below 20hz and thus where the cone begins to unload and flap around
If you need to go with larger diameter ports or a slot port these can easily be added after the first test above.
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