sympathetic frequency?

after recently installing a 15" memphis pr154d in a 36X16X10 sealed box being pushed by an alpine v-350, running at 2 ohms and pushing around 350 watts with the gain set around 2/3 of the way up my sub sounds muddy at higher frequencies, i'm wondering if the car is having resonant frequency issues or if the higher notes= shorter waves and i'm gettin cancellation issues?   On really deep songs, like bass test tracks, or 36 mafias late nite tip its pretty clear except for my trunk vibrating really bad, but on ciaras song Oh the deeper bass line plays pretty good but the one slightly above it sounds out of tune with itself, there is a deffinate oscilation in the bass, kinda sounds like cancelation.  the crossover on the amp is set to 75hz the head units lp is 78hz and high pass is 78hz when i had my 10" rockford i never noticed any problems any ideas what it might be?  also should i put some bigger supports in the box, its not flexing now but i'm wondering if i take up some air space will it tighten up my low end alittle more? 

Also what is a good sound deadening material that i could add to my trunk lid to minimize the vibration alittle? the only one i know of is dynamat and that craps expensive, also would deadening material help the box at all? or if i wrap it in carpet will that help anything?

thanks in advance

J~

Agreed with DYohn and tcss, and please respond if you are using a voltmeter to read voltage.  If not, set the amp gain using the method described on this forum so many times.  Here is what is happening:  the higher notes of the sub are naturally louder than the lowest notes because of the rise in the SPL on the response curve.  With that additional SPL, you are also experiencing the clipped signal from the amplifier. 

You need to do two things:  use Dynamat or something equivalent like Raamat or Fatmat and line the trunk lid and surrounding areas of sheet metal in the trunk.  Until you do that you cannot rely on your ears to make any kind of judgement about the quality of sound.  Then, search this forum for some topics on setting gain and use the prescribed method.  The gain obviously has to be turned down, but to what point you have to determine.  When all is said and done, the 80 Hz tones will sound as good as the 40 Hz tones.

As for the box, if it is well built, sealed completely, built to specs for the sub, mounted securely to the car's chassis......altogether structurally sound........then no amount of carpet or deadener will make an improvement to its sound.  If you were going to use egg crate foam or something of the sort inside the box you would have built it to spec with that additional feature in mind.

the preouts on my head unit are 4v according to sony, my gain on this amp is set to .5v according to the gain dial, the eq boost is at zero, and my crossover is at 75hz,  to set my gain i turned all eqs off set everything flat and cranked the stereo till it distorted then backed it down until all distortion was gone and went a couple more clicks down, then i turned my gain up until the sub hit decent without distorting or working itself to death, should i be setting my eq on and then doing this first or am i correct with this procedure? sound deadening is my next step so hopefully i can get alittle cleaner response.

J~

well the eq i was talking about is the built in eq in my sony head unit, after rereading what i posted that sounded alittle confusing to me as well lol, all i'm really doing is boosting the low and high frequencies and pulling some of the mids out with the eq settings, my sub is rated at 200 watts my amp is rated at 350watts @ 2 ohms, the sub is running at 2 ohms so i started with my gains set  hair past half way up, and turned it up alittle more, i guess i'll just keep playing with it. One thing i'm wondering is if my rear deck speakers and the sub sharing the same air space could be causing some of the out of phase sounds coming from my bottom end? the bass also cleans up by a noticable amount when the trunk lid is open.

J~

1.  The rear speakers will become distorted at times because of the pressure from the sub.  Make a baffle and seperate the rears from the trunk.

2.  The amp is overpowering the sub with its 350 watts into a load that can handle 200.  You should be getting another sub, but in the meantime you should set your amp according to voltage output by using a meter.  If you want 200 watts and the load is 2 ohms, the voltage output of the amp should be 20 volts using a test tone at 50 Hz and with the deck at loudest clean level (Ohm's Law E=sq rt[P * R]).

3.  When you use the deck's tone controls (EQ) it might be changing slightly the highest clean level of output.   Set the volume of the deck after the EQ is engaged and proceed with the gain setting / testing at that point.

4.  Create a pass-thru for air pressure to escape into the cabin area of the car.  You'll get less bottled-up pressure pushing thru the rear speaker cones and louder, cleaner bass.  It will be easier to turn down the sub bass when you do that.

after playing with some settings in the driveway for awhile, i've gotten it to clean up alittle bit, the song that i refered to earlier ciara's Oh has a spot in it that the bass is out of phase with itself no matter what i do, even with the gains turned all the way down and the eq set flat the sub still produces and out of phase section thru the chorus.  I'll have to check with my brother to see if he has a fluke around so i can check actual voltage on the amp, and as far as test tones wheres a good place to pick up cds with test tones and bass tracks for system setups?

J~