Turning my trunk into a bandpass box

chucked

Member
2016-03-02 10:47 pm
So i have a 10" pioneer subwoofer in a sealed enclosure (about 1.4 cubic feet i think) in my trunk of my 1997 miata. its a big help, but im hoping to improve it.

im in the planning stages of trying to turn the whole trunk into a bandpass box that ports into the 'cabin' with a port that runs along the driver side tunnel.

im planning to cut a hole in the sheetmetal besides the driver and put a tube into the tunnel and remove the panel that separates the tunnel from the trunk. if i can fit some port flares great, but not crucial right now. i dont yet have a clear prediction of the diameter and length of the port until i work through the math.

im working with my friend whos a speaker designer for a large PA company.

has this been done before?

im using this calculator to determine the size of the port that i want.
Port Length Calculator

my goal is to add SPL on the cheap(esp below 40hz)! i think it will be quite easy, take carpet off, cut a big hole with a dremell or something, shove tube in and seal with epoxy.

other perks im hoping for, reducing the amount of force the trunk lid experiences, and therefore hopefully be a contributor in my rattle reduction strategy

reducing the impact that top down speed has on sound quality (i currently have to adjust my EQ and amp settings as i increase my speed, esp above 90)

im a big bass freak and love spending time in front of massive WALLS of 21" PA subwoofers at desert/underground music festivals/gatherings, i love the feeling of organ rattling bass and hair moving wind currents. im hoping to be able to feel a decent amount of wind with my hand/elbow when i put it in front of the port, to better connect with the low low bass that i love so much but will never really get out of a 10" (and yes i usually wear ear plugs, lets not get hung up on this)

no impact on trunk space


for reference, my system consists of a pioneer HU, pioneer gm-d1004 power pack amp giving 90w rms to front polk db6501 component speakers, 3.5" coaxial cheap headrest speakers (powered by HU), 10" subwoofer powered by 300 w rms soundstream amp (in passenger tunnel), 2 dayton bass shakers in each seat powered by cheap BOSS amp in passenger tunnel (in progress).

thanks for the help, i will do a lil writeup with some pics and results once its done
 
Okay, right off the bat you're going to need more speaker. A big wall of 21" drivers can be doing 140dB plus, easily. You're not going to do below 40Hz with a single 10" at 140dB, even with the not-inconsiderable cabin gain available.

To get close, I'd start looking at tapped horn designs. What you're proposing is somewhere between a 4th order bandpass and a slot-loaded sealed box. You're only using one side of the cone, so you're immediately losing out on some SPL there. If you add a bass reflex port (probably requiring a larger cabinet), you'll get more low end. Slot-loaded boxes tend to gain a lot at the top end, over 80Hz. It makes the crossover more difficult, but the headroom in the kick region is useful.

A tapped horn uses both sides of the cone, like a ported box, but thanks to some very clever physics, you can get quite a lot louder than a ported box. There's no free lunch, though - THs need relatively big cabinets.

Going from sealed to TH might gain you 10dB at the bottom end, whereas adding the slot you propose will add ~6dB at the top end. I know which I'd pick.

Going back to my original point, however, if you want serious low end, you're going to need more than 1x10".

Chris
 
There are obvious performance advantages to this this, however, it is not easier than making a box. To start, forget about intentional reactive tuning. You will have enough trouble getting less reactance out of natural resonances. All leaks between the front and rear of the cone have to be plugged with something rigid. Most likely body panels and structure behind the seats reinforced. In other words, you will have a ton of work to do starting with a sealed enclosure. The upshot is at the power levels which you think you're going to operate, you could see where you have work to do by running the system and visually inspecting around the trunk for physical motion.
 
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chucked

Member
2016-03-02 10:47 pm
im well aware i wont replicate a wall of subwoofers lol
we hit well over 150dB in the pressure zone, im not looking for that. (and my bass shakers will really make up for a lot of the <45hz lack of OOOMPH)
just trying to improve with a fun challenge of radical frugality.

the amp is a nanostream 450D, looks like its only 260w rms. it was chosen because its tiny and therefore fits in one of the nooks of the miata passenger side tunnel that hides behind the spare tire. and because i got it on craigslist for 35$.

the 10" sub is the pioneer here (single voice coil) (got it for 45 CAD$ on craigslist several years ago in a ported box which i subsequently sold for 40$ (was too big and poorly matched to the driver)) TS-W256C|DVC - 10 | Pioneer Electronics USA
http://www.pioneerelectronics.com/pio/pe/images/portal/cit_11221/199784721TRD1247-A_US.pdf

i may consider changing the sub one day, but im not interested in spending much money (call it 20-75$) so lets move past it. ive let my speaker freaks friends know that if they come across a ridiculous 10" (preferably DVC) from another friend who just wants it out of their storage for DIRT cheap, ill consider it.

i think what im proposing is actually exactly a 4th order bandpass box, though a very unorthodox approach.

im not interested in losing anymore trunk space, so lets not consider anything that involves changing the enclosure.

my friend is of great help, but his experience is quite esoteric and constrained to LARGE proffesional systems, so theres a lot of room for questions when it comes to 'radical frugal' (GHETTO) car sound systems.


one question i do have is:

is there any real discernable difference between using a 6" port with a 3" long tube VS a 3.5" port with zero tube length? the math indicates identical results
i know that theres a much higher chance of port noise, but i have a feeling that my trunk will have some leaks (most of which ill fix, but odds are there will be a few impossible to find ones) , and that given my tiny setup there wont be all that much airflow to create any discernible port noise when im blasting down the hwy top down at 80-100mph

how about 2 holes each 2" with zero port length (one on each tunnel), will that sound differently? (the math says similar physics)
 

chucked

Member
2016-03-02 10:47 pm
They are having all the fun. At diyAudio... we only get to clean the dust.:D:D:D

having spent lots of time at burning man, believe me when i tell you that i have cleaned SO MUCH F#$%&* DUST!

most of the sound systems that go to burning man end up needing a full service afterwards, we take everything apart and often replace every driver/diaphragm. playa dust is a big deal. a lot of underground/burningman PA companies coordinate their service schedule so that they just plan to service/refurbish everything after the burn.

oops strayed too far from topic, sorry. but hearing chopin's nocturnes at 140+ dB sitting on a couch on the edge of deep playa watching the sunrise on a million dollar sound system with my friend sitting next to me controlling everything from an ipad is something that frequently make me go radically tangential lol
 
is there any real discernable difference between using a 6" port with a 3" long tube VS a 3.5" port with zero tube length? the math indicates identical results
how about 2 holes each 2" with zero port length (one on each tunnel), will that sound differently? (the math says similar physics)
Port length at minimum is the thickness of the cabinet wall, if your cabinet wall has zero thickness it does not exist...

Your speaker displaces some amount of air (cone area times linear travel, Sd x Xmax), if the displacement is greater than the slug of air volume of air contained in the duct, the port volume "blows out" and the Helmholtz resonator becomes a blower, rather than making bass. Blowers move air, but don't make anything but wind noise, which you have plenty of already.

Go big.
 

chucked

Member
2016-03-02 10:47 pm
Port length at minimum is the thickness of the cabinet wall, if your cabinet wall has zero thickness it does not exist...

Your speaker displaces some amount of air (cone area times linear travel, Sd x Xmax), if the displacement is greater than the slug of air volume of air contained in the duct, the port volume "blows out" and the Helmholtz resonator becomes a blower, rather than making bass. Blowers move air, but don't make anything but wind noise, which you have plenty of already.

Go big.

well the 'cabinet' thickness is just sheetmetal seperating the trunk and the cabin, likely about 1mm of steel.

does that change anything?
 
well the 'cabinet' thickness is just sheetmetal seperating the trunk and the cabin, likely about 1mm of steel.

does that change anything?
1mm contains a lot less port volume than 18mm, (the usual cabinet wall thickness) and would get "blown out" at low drive levels.

Go with the 6" or larger duct.
Cover with a grill to keep critters from taking up trunk residence ;).
 

chucked

Member
2016-03-02 10:47 pm
ok ill plan to go with 6" .
but this pipe goes for 43$ at home depot, not cool!
6 in. x 10 ft. PVC Sch. 40 DWV Plain End Pipe-30577 - The Home Depot

im thinking i can prob contact a plumber and ask him for a scrap footlong section and gift him a 6 pack.

hopefully they make sleeves that i can use to join two sections together, because my plan currently is to:

cut hole, put short pipe (say3") and epoxy it into place, then run some ear and SPL meter tests, then add a 2" section with said sleeve and repeat test, then repeat for a 3" 4" 5" section, and find the best two, then make an extra section out of the unused ones that splits the difference of the two best, and choose the winner from those three. then glue the pipes together and put everything back.

will cut a hole into carpet and velcro it on to make it as clean looking as i can.