an audio confusion conclusion *Lots of PICS* :D

[shoteen95]

this is a continuation ofThis Thread ...but i decided to throw this thread together to give you guys a little write up as to what i've done.

and like the title promises, i took pics.

Originally i was looking for a cheap upgrade to the jbl system for a little more bass.. I ended up with a 236 watt (RMS, 4ohms) JBL amp and an 8 inch polk/momo subwoofer.

As i write this i am not complete upgrading..so expect some more pics and ramblings tomorrow after i finish..

First off...

a little more wattage anyone?


drools


:***:


8 inches never looked so good


..polk sub > stock sub


now this box has some balls (mods can i say balls?)

I already have the power cable and and rca's run under the carpet.

My speakers have been crapping out lately..so far one of my rear 5X7's is completely dead, and my passenger side door speaker cuts out occasionally. I hooked an old speaker i had up to the connecter for the rear 5X7 and it didnt work either (and i know this speaker was working when i took it out). So i'm thinking it's the array of metra connectors hooked up to my head unit. Tomorrow i'll fiddle with it to see if theres a short somewhere.

while i was reaching behind my dash..i found the infamous second connector! for some reason it doesnt fit into the metra connector though.

 

[shoteen95]

hmm...well..i got everything hooked up finally.

and it actually works!

only problem now is sealing the box to the rear deck..

The bass isnt very loud in the cabin, but in the trunk..wow..it just booms..

theres about an inch of space or so between the rear deck and the top of the sub box...and right now i have no idea what to do to seal it up against the rear deck..i'll have to fab something up..but i dunno what

anyone ever conjured up a solution? if so what did you use? I'm guessing i'll have to use something pretty dense.

Sorry i promissed more pics..but in my haste today i didnt snap any..and i have to leave for school tomorrow.

 

[Sho_Freak]

we put a little bit of fat mat (like dynamat, only cheaper)on the bottom of my jbl box with the r/f p1 in there and it boosted cabin bass a little...but not much, i, too am looking for a way to get most of that bass in the cabin.

 

[shoteen95]

it seems that little chunk of plastic that sits on top of the stock sub actually does something...all i know is..my new sub sounds awesome from my trunk

 

[93medusa]

maybe you should leave it there...

try switching the phase of the sub and see if that helps at all. My guess is that maybe with the larger output subwoofer, you are getting cancellation from the rear window.

Dave

 

[shoteen95]

maybe you should leave it there...

try switching the phase of the sub and see if that helps at all. My guess is that maybe with the larger output subwoofer, you are getting cancellation from the rear window.

Dave

haha..smart@ss, it'd be nice if that little piece of plastic went back on..

and here's a stupid question..but what does switching the phase mean?

 

[93medusa]

Swap the positive and negative wires to the sub.

 

[stevemainian]

Swap the positive and negative wires to the sub.

What would this do?

 

[EXP1787]

What would this do?

Essentially, it would make the sub hit backwards, causing it to push inward. Because the stock box is a sealed enclosure, it seems to me as if there would be a greatly increased risk of blowing the subwoofer.

 

[93medusa]

switching the phase will in no way cause you to blow the subwoofer. A speaker is a mechanical unit that converts electricity to sound, all that reversing the phase will do is cause the woofer to go in instead of out with the same electrical signal. It will in no way cause damage to the speaker. I doubt it will do much if you are getting cancellation from the rear window, but it's worth a try before you explore otehr avenues. The fact that the box is sealed has no effect on wheter it will cause damage to your sub...because it won't cause damage to your sub...ever.

Dave

 

[EXP1787]

So the increased pressure caused by the subwoofer hitting inward would be negligible?

 

[93medusa]

It's not increased pressure. A subwoofer doesn't only move out. It moves in both directions creating sound waves. For example, see the below graphics. The horizontal line represents the woofer at it's stationary state, for a given frequency, there will be a wave created by the speaker moving both in and out.

In phase

Out of phase

Switching phase is a very common tweak performed in car audio (and home audio for that matter, but it is on a deeper level there)

I repeat, no harm will come to your speaker, amplifiers or any other equipment by simply swapping the speaker leads.

Dave

 

[shoteen95]

i'll most likely give it a try next time i'm home and have free time to fiddle..

in the mean time..i've been contemplating a homemade solution involving home depot and glue...

 

[Sho_Freak]

if its not hot glue...its not cool...cuz hot glue is the COOLEST!!!!!

 

[shoteen95]

if its not hot glue...its not cool...cuz hot glue is the COOLEST!!!!!

haha fo ****

i was thinking my parts list might look something like...
Very large PVC piping
Hot glue or some other very strong glue/sealant
Some mocha colored carpet
Rubber or foam
metal L-brackets
and that awesome polk/momo speaker grille that came with my sub

EDIT

cute little drawing i made in paint...haha

I thought the kinda-curved "bass tube" would solve the rear window noise cancellation...that and the enlarged hole in the rear deck??? Plus the pvc piping would be thick enough to isolate the bass from the trunk. The whole setup would be as unobtrusive as a the third brake light on SLO's. Would it work? i dont know anything about the physics of this kind of stuff

PS it's snowing here in kalamazoo..supposed to get about a foot in the next day or so...gosh i sure do love walking to class here at western..where everythings always uphill and the winds always blowing

 

[NoSlo]

I can honestly say that there is no such thing as 'rear window noise cancellation'. Having put 4 kicker 10's in an isobaric box in the back window the only problem I had was having to pull out and reseal the back window twice because the bass blew out the back window seal.

Bass waves are 4 to 8 feet long, any reflection coming off the window is in phase with the direct output of the sub. If anything, this couples the speaker to the air mass more because you almost have a 'horn' there, where the sub radiates into 1/4 space. At low frequencies like the sub puts out, in a space as small as the inside of a car, you can almost think of the output of the sub as a pressure wave rather than as sound that bounces around.

The only issue you would have with cancellation is if the package-tray mounted subwoofer is not sealed between the passenger compartment and the trunk (like your stock sub box), then you may have a standing wave in the trunk between the sub and the back of the trunk that can cause reinforcement or cancellation depending on the frequency. The best way to solve this is to have a solid baffle of MDF/Medite on your sub box that covers the entire package tray and is sealed, so the sub doesn't radiate into the trunk. You can also mount and seal a MDF sheet behind the back seat so road noise and bass doesn't go through the back seat.

It is important to note that the air volume inside the stock JBL sub box doesn't match what the new sub requires for optimal output. That combined with the fact that the thin plastic box is going to vibrate and radiate bass into the trunk, which is not only wasted sound energy, but also the output from the back of the speaker radiated by the plastic is going to be out of phase and cancel with the output coming from the front of the subwoofer. Building and mounting a strong wood subwoofer enclosure made of 3/4 inch MDF (medium density fiberboard - a heavy duty dense particle board) with the right volume for your sub, packed with polyester fiberfill, and sealed against the package tray would make a world of difference.

I ran your speaker's thiele-small parameters through software, and the best box for you by far would be a 1.0 cubic foot PORTED box. As you can see in the graphs below, even the ideally designed sealed box volume for this speaker (purple) is far passed up by a ported box (teal) in low end response, efficiency, and maximum power handling. In a sealed box, the amp you have can drive the sub to the limits of excursion before using a fraction of its power; in fact, a sealed sub can only handle 40W of power at 40Hz before it reaches it's excursion limits. The power handling of the ported box is far above the 116dB/230W solid line across the graph all the way down to 33Hz (also the -3dB point in my design), it's above the 400W max thermal power rating for your speaker for all bass above 38Hz.

This box would have one 3" inside diameter port 9" long, or one 4" diameter port 17.5 inches long (the 4" port is better but may need an elbow to fit inside the box). Inside dimensions need to allow for the volume of the speaker and port; an example box INSIDE measurements would be 12" x 12" x 14" (subtracting ~288 cu in for the port volume and speaker volume). A box like this you can also just stick in the back corner of the trunk (by the taillights, not the back seat - remember that standing wave) and the bass will find it's way through the back seat, package tray, etc.

There also should not be any need to change the phase of the subwoofer, except to match it to the phase of the output of your other speakers. Even being 180 degrees out of phase would only affect sound around the crossover frequency between the sub and the other speakers.

Enjoy!

 

[93medusa]

NoSLo,
Good point about the length of bass waves and the rear window not causing cancellation. It was really just a shot in the dark as far as the issues he's having. I've actually never had a problem with cancellation in the installs I've done. Also, Based on your reply, I'm thinking that the best cure for his problem is to make sure that the sub is sealed between the cabin and the trunk, and to cut the hole in the rear package tray a larger than the diamater of the new sub. Kind of like he is proposing.

I have to disagree with your box design though, just because you can plug the numbers into a box design program doesn't mean it is going to make a great sounding sub. First of all, I don't see cabin gain figured into either of those box designs, and secondly, just because a ported is louder, does not mean it is going to sound better. It sounds like you have some experience with car audio, so I don't want this to seem like I am talking down to you, because that is not what I am doing. But the sealed box will have a much more natural and controlled sound in comparison to that ported, as is evident by the big peak on the ported enclosures graph, versus the nice smooth **** for the sealed. Also, an f3 of 33 hz is way lower than you need for an 8 inch sub in a car environment. 33hz might be a good tunign frequenciy for a larger sub, perhaps a 12 or a 15, but it is going to be pushing the limits of a small sub. Especially since the Fs of the polk unit is 43 hz, tuning below the free air resonance of the sub is a big no-no. Notice that Polk tuned the box to 60hz. This is why you are showing the sub reaching maximum excursion at 40w of power. If you increase the tuning freq, you will get a smaller box with better power handling, and the cabin gain will take care of boosting the lower freqs. As for using two different sized ports, that's just bad practice, how did you come to those two sizes? I don't see that information in the box program on that either. The following is the information on the polk site, notice the graphs for both in car response, and anechoically. Notice that in car in a sealed enclosure, the subs will play an average of about 83-84 db with 1 volt of power below 100 hz. Note that anichoically, it will drop off very quickly. These two response graphs show the effects of a speaker being in a car audio environment, aka very reflectivce, and an environment lacking any reflections. .35 cubic feet sealed will provide more than adequate bass. Yes, a ported will be louder, but once again, look at the product specifications on Polk's site, .6 cubic feet tuned to 43 hz. A lot different than what you are showing.

http://www.polkaudio.com/car/toolbox/subboxplans/subbox_plan.php?id=76&mesurements=standard

http://www.polkaudio.com/car/toolbox/subboxplans/subbox_plan.php?id=53&mesurements=standard

Like I said, I'm not trying to talk down or start an argument here, so lets keep it civil. I'd like to hear your response, and have an intelligent conversation about this.

Thanks

Dave

 

[NiNeTy Fo SHO]

while i was reaching behind my dash..i found the infamous second connector! for some reason it doesnt fit into the metra connector though.

Finally! someone has found it!

I try to tell people its there (I am using it) and it seems like no one believes me...

Good luck on the setup.

 

[Sho_Freak]

NiNeTy Fo SHO: ive been trying to tell that too.......nobody want to spend less for their installation....thats what im using in my car.....


FINALLY!!!!!!!
Good Job Shoteen!

 

[shoteen95]

Jay and Dave...you guys are too smart for me....

but i am very interested in the science of this all.

Indeed..we can all agree my speaker box needs a better seal to the cabin.