Bad Speaker Box Design?

Hey Guys

This is a box that I designed and built for my 1999 Nissan Frontier King Cab. As you can see, it sits behind the driver seat. It currently holds a 12" Rockford Fosgate XLC 8 ohm driver. I am using a Fosgate Punch 250a2 for power.

I pretty much designed this box with space in mind. The sub points downward and the amp is on the other side of the box in a recessed space. Box design is not really my area of expertise, but I had this same setup in my car with a more traditional shaped box and it sounded great.

MY COMPLAINTS: when I first got this thing all hooked up the box seemed like it was little bit mushy sounding or ringy. I was also having a problem with hearing a lot of vocals and higher frequencies through the sub.  So I got some poly fill and stuffed the box. I didn't fill it up totally, but pretty close. I think I might have put in too much poly though.

These are what I think the problems are with this setup, please let me know your thoughts.

1. I am using the crossover on the amp, would a deck crossover or external active crossover be a better solution? Would that help eliminate higher frequencies better than the amp crossover, or is the amp crossover sufficient?

2. Is the sub facing the wrong direction? I am not getting the kick out of this that I want. Should the sub be facing another direction?

3. I have the amp bridged, but its a 4 ohm amp and the speaker is an 8ohm driver. Although I seem to have plenty of power. The box size is approximately 1.2 cubic feet. It is a sealed box design.

Is my design all wrong, or is it workable?

Let me know.

Thanks for any advice you can provide!

1.  If you hear high frequencies through your sub, your crossover is not set correctly.  Either the deck or amp xover should work - check your settings - but an external is usually higher quality.

2.  Not all subs can be mounted down-firing and function properly.  The RF Punch series has very low linear excursion specs (0.3") and is not a good candidate for down-firing mounting.  That being said, it will finction in this configuration, although not at peak performance.

3.  Running an 8-ohm speaker to a bridged stereo amp is fine.  The amp will operate at its 4-ohm stereo rating level.

1.2 cu ft sealed enclosure with 50% polyfill will result in a 1.32 effective enclosure volume.  That should work for this driver.  Be sure your enlosure is sealed and has no air leaks (including around the driver after mounting.) 

I suspect the ringy sound is mainly due to high frequencies in the system.  If the bass is too mushy, try removing half of your stuffing to tighten the box.  Also try elevating the driver more than you have it to allow more air flow in front of the driver. 

You may find this speaker will never sound right in a down-firing configuration.  Try rotating the enclosure so it is firing close to vertically and see if the sound quality improves and you will know.

DYohn gave you some very solid advice

Great feedback. Everyone's got some great ideas!!!!

My $.02

Either xover will work. Just use one or the other,not both.

Downfiring depends on the driver/enclosure. Also, its best that space in front of driver be 10-12% driver diameter. Also best if it fires into a firm surface. use MDF for enclosure (I would use at least 3/4" for a 12). Add internal bracing if sides are unusually wide or long (eliminates ringing). Direction of driver is really personal preference of what you want it to sound like.

Keep plugging a way at it. Most of the best systems went through many changes a little at at time.

8 ohm load on a 4 ohm amp won't "hurt" it. The amp just won't give full rated output (some higher end units will give same rated output over a range of loads).

Thanks for all the input. I am pretty sure that the crossover is setup correctly. i haven't a chance to look it at it all really since i got it in there. I will try playing around with the firing direction.  I will first try putting a scrap piece of MDF underneath it to see if that improves the wave dispersion. The box is really built quite well from the construction standpoint, but probably not so great as far as size and shape goes. It is made with High density MDF and is totally sealed with silicone. although it is an 8ohm driver it seems to get plenty loud, just doesn't have the tight response that I am looking for. As for airspace, someone told me that it is safer to have the box be a little bit smaller than recommended for the driver as opposed to be too big, a problem that can't be fixed :)

Thanks for all your great input. I will try several things and let ya know how it turns out.

THanks

Brad

Be sure to let us know what you decide and if you can solve the problem, Brad. 

Enclosure design is just as much trial and error as it is how to use formulas and how to cut and glue.  I must have parts from twenty different failed enclosure designs in my garage!  Sometimes bad sound is caused by enclosure geometry, sometimes it is poor construction, sometimes too many baffles, other times too few, and many times (indeed, more often that not) it is the wrong driver for the enclosure I want to build.  Sometimes playing with the stuffing will help.  Sometimes swapping the woofer solves all the problems. 

I once built a nice octogonal enclosure for a customer's $1200 MTX sub that looked really cool but sounded like crap, with poor response and flabby bass with no punch at all.  Since I had spent 40 hours on the enclosure I tried to fix it.  During trouble shooting (looking for air leaks) I didn't want to risk the expensive sub so I swaped in a $20 Pioneer POS sub I had laying around.  Suddenly it sounded GREAT.  Awesome, in fact.  That cheap-ass Pioneer was pumping out 100 db @ 50 hz with a 100 watt amp and sounded very tight for a 15".  I had simply built the wrong enclosure for the MTX.  I built another, more conventional shaped one and voila, the MTX rocked.  I sold the Pioneer system to someone else!

Sorry about the long story, the moral is keep trying and see if you can make your plans work.  If you are serious about staying with a down-firing design, read THIS web page from my friends at Parts Express to help you decide which driver would work best.

Cheers

phase and cancellation. if 2 speakers are 180degrees out of phase, then 100% cancellation will occur.

cant play with one without affecting the other.

nope. a speaker is just as loud from the front as it is from the back. Ever seen subs mounted with the magnet  mounted outside the box with the cone facing in? makes no difference to the sound volume which way the cone moves.

as for which speakers put out sub bass? none, but your other speakers do put out bass (50Hz - 100Hz) which is what is cancelling.

This thread has turned into something completely different from where it started, but as pointed out in an earlier thread, and as illistrated on THIS WEB PAGE, it is possible for one speaker to cancel itself in a car installation due to standing waves and reflections.  In the case of this original question, however, it is unlikely there is a polarity, phase or cancellation problem going on.

Cheers!

well guys, this discussion has been nothing but...completely irrelavant. So let me bring it back to where I began it at.

I had some time to play around with settings and positioning.

1. I verified the settings on the Fosgate crossover. It WAS set to Low Pass, but the frequency sweeper knob is a little bit confusing as this picture shows.

To me it looks like when the knob is cranked totally to the left and the crossover is set to Low Pass, you have a 50hz bottom end and an 80hz top end. So I set it at about 55hz which seems to cut most of the vocals. I've got the punch bass set pretty much where this picture shows it. This didn't totally clean up the muddiness.

2. I took off the grill and then the driver out. I took out over half of the stuff I had in there as someone suggested.

3. I placed a piece of particle board underneath the box to help provide better deflection, as seen in the picture below.

So now I gotta clean up the extra pieces of wood and cover them with carpet.

I think it was the combo of all three of these adjustments that made the difference. I think if I got a 4ohm sub that would definately increase the punch, but it definately sounds better. I am not positive this box is the best design, but not the worst either. It could possibly use some bracing on the bigger sides.

Thanks for all the info guys. Feel free to keep it coming. I realize this setup it isn't the pretiest or the best, but its going to serve my purposes fairly well.

Un-stuffing the enclosure probably helped a lot.

Vocals happen above 600Hz, so your crossover should have nothing to do with them unless it is not in Low Pass.

The "Crossover" 3-position switch determines what kind of function the electronic crossover will perform.  In the center, as shown in your drawing, it is in "Full" mode and the crossover does nothing.  To the left, it is in "High" and will cut out bass below the setting.  To the right (where you want it for a sub) is "Low" and it will cut out frequencies above the setting.  The control labeled "Crossover Frequency" sets the point where the electronic crossover begins cutting the frequencies.  It can range from 50Hz to 200Hz.

For your setup, I suggest the "Crossover" switch be pushed to the right (in "Low") and the frequency sweep should be set to around 80Hz (in the middle.)  The Punch control can be fiddled with an set to wherever sounds best.