this is for an AWT12X woofer which peaks over 6dB at tuned frequency in the 30 Hz+ range (!).

first part of the problem (the "is this a good idea" part):

i have a box with a removable face, with a little over 7" x 7" usable port area to build wierd ports. i have a simple 4" round (35Hz), 6" round (40Hz), and soon to come are 5" round and a 7" square EXTERNAL SPL port (once i find the transfer function). i wonder if i can do some strange stuff to try flattening out the response for normal daily driving.. one idea is to have multiple different ports, each tuned to different frequency, and so they will have similar flows i need to use different diameters, and try to match the airspeed in all of them (using diameter and length as variables). is this a crack pipe idea or is this feasable? is there a program which will let you use different length/size ports? how else would i begin with this?
another idea to flatten out the response is to tune it low as all hell, like 20Hz.. and the normal listening music probably will never get that low. it won't be loud as sh1t, but it will probably have a relatively flat response eh?

2nd part of the question (sound quality vs. flat response):

i understand that linearity and accuracy is SQ.. but also a "flat response?" exactly how flat? also, do you want it flat all the way from 20Hz to 100+Hz? same exact SPL all accross the board? or a curve up on both ends? a hump in the middle?
what if it was a bit "roller-coastery" with like 4 or 5 humps in the bass range? is that terrible or as long as it's w/in, say, 1.5dB per hump it's okay? is there a rule of thumb on this?

third part of the question (calculation):

3.42 cu. ft. gross box size.. woofer displacement: .42 cu. ft.
invert woofer for SPL comp's (and external SPL port for max internal vol)

2.8 something cu ft net... after port displacement depending on port dimentions..

i was thinking, if you can bare envisioning this, build a square port with 2 cross members. the 2 middle crossmembers will be common port walls to all the 4 "smaller quare ports" that this makes up. the outer walls will have different lengths to make up different tuning frequencies. the positioning of the crossmembers will dictate the port area of each of the four ports.

to find port dimentions, can i isolate each case and treat each port as the only port in the system (?) then estimate the final response to be a graph roughly made up from the 4 graphs given by each isolated case (?)

could having 4 peaks instead of one make it sound worse? will it flatten out at all or what?
i'm thinking.. if i want, say, 15 m/s airspeed overall, i can calculate each port seperately with a 60 m/s airspeed

i'd like to get an equation for airspeed so i can just do this the "old fashioned way" with a pen and paper...

port 1, 2, 3, 4 have, say, 60m/s airspeed ea.

area port 1 + 2 + 3 + 4 = 39 sq in.

each port can be, say, 5 Hz different in tuning

all i need is a couple of equations and i can plug and play.. but will this turn out okay?

if you haven't realized yet, i am matching airspeeds so the flow doesn't favor one port over another.. am i on the right track with this?

another thing to take into account is the transfer function of the vehicle. i have yet to measure the transfer function, but i can use it's function and add it to the four other functions i come up with and see what happens... change the tuning of the ports to account for this in order to flatten the response even more (total of 5 humps, 10 troughs).

any help?

quote:

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Originally posted by stabmaster:

i have a box with a removable face, with a little over 7" x 7" usable port area to build wierd ports. i have a simple 4" round (35Hz), 6" round (40Hz), and soon to come are 5" round and a 7" square EXTERNAL SPL port (once i find the transfer function). i wonder if i can do some strange stuff to try flattening out the response for normal daily driving.. one idea is to have multiple different ports, each tuned to different frequency, and so they will have similar flows i need to use different diameters, and try to match the airspeed in all of them (using diameter and length as variables). is this a crack pipe idea or is this feasable? is there a program which will let you use different length/size ports? how else would i begin with this?
another idea to flatten out the response is to tune it low as all hell, like 20Hz.. and the normal listening music probably will never get that low. it won't be loud as sh1t, but it will probably have a relatively flat response eh?

2nd part of the question (sound quality vs. flat response):

i understand that linearity and accuracy is SQ.. but also a "flat response?" exactly how flat? also, do you want it flat all the way from 20Hz to 100+Hz? same exact SPL all accross the board? or a curve up on both ends? a hump in the middle?
what if it was a bit "roller-coastery" with like 4 or 5 humps in the bass range? is that terrible or as long as it's w/in, say, 1.5dB per hump it's okay? is there a rule of thumb on this?

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First, let me say its good that you're considering the box so in depth.

However- If you two different ports in the same chamber, you'll probably run into problems. In fact, if you use a common ported area for two woofers and they don't match well, you risk a lot of loss... Using two different ports is an unecessary complication.

As for how flat you should make your box.. Well, the ideal SQ response IN CAR is perfectly flat. However this might not sound ideal, it still represents the best an installer can do to stay truthful to the source material.

As you mentioned, there will be a transfer function. Large, SQ ported boxes that are tuned LOW just don't work well with most cars, because if the design is flat before its in the car, then once its in the car, the response looks just like the transfer function.

The ideal way to build a box is to test a box IN the car, IN the position that the final box will be in, with AHB (all high bits) source material. Then, test this box at 1m sitting on a stool in a large open area, and observe the difference between the two. (You'll have to normalize one of the graphs to the other so they can be compared) Then, find a general increasing trend, and design your box so it rolls off inverse to this trend.. This will sum to as flat as possible once in the car.

..however i do beleive that this won't change the outcome enough to offset the +6dB peak of that woofer (this is not your average woofer). for the record it's one single woofer. the box is alreaady built and there is really no other design that would work in terms of box volume. i spoke with some audiobahn techs and i gathered that this woofer will be sloppy (not stay linear) in a box not too far off the manufacturer reccomendations. so anyways to decrease that peak appreciably the box size wants to be like .05 cu. ft. which is obviously not going to happen (not to mention the port would be about 400 feet long).

so basically the best i can do with 1 port probably won't be an SQ trophy winner... and although adding extra ports adds "unnecessary complications," it would be a feat of modern engineering to pull it off. it would surely impress some people and as you saw, all i need is a few equations and a little time. if the effort is all on paper i think i can hack it (as this box has already taken a month to build). so my questions are simply if i'm on the right track.

if i am than all i really need is (1) the transfer function (2) a program or equation for airspeed (3) the equations for surface area and length and (4) desired tuning frequencies (optimal tuning seperation, etc).

quote:

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Originally posted by stabmaster:

it's kindof like an ABC box without a, b, or c..

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The sub initially creates a pressure in the box.
The pressure on one side of the port is different to the pressure on the other side.
It has a contact area, and the air in the port has a mass.
The air in the box has a springiness.

When the pressure is oscillated the air in the port will move with it. I would like to post some .swf animations of mine here to show how, but Wayne has unfortunately rendered me unable to do so.

Below tuning the air in the port moves the same way as the pressure inside the box is pushing it. At this stage the port lowers the pressure inside the box relative to a sealed box of the same volume by allowing air to 'escape'. The ports output to the atmosphere is 180 degrees out of phase with that of the sub. i.e. they cancel.

At the resonant frequency the air in the port moves a long way for very little input. This is a high Q design. The ports output to the atmosphere is about 90 degrees phase wise ahead of that of the sub. Due to the amount the port moves it creates quite a lot of pressure inside the box, so the sub doesn't move as much.

Above tuning the air in the port will actually move the opposite way to the force (pressure inside box created byt sub) driving it. At this point its output to the atmosphere is the same phase as that of the sub.

I made a page just for you so you can see this: http://www.procedos.co.uk/~colin/flash/substuff.html

The last one is a trick I normally do, which requires a little effort and sometimes some testing to get right. Basically it sounds a little better, and is a little more efficient. No set in stone rules on building though.

Please note that on all animations the displacement of port and sub are shown only for clarity. They won't really move the same distance at all frequencies.

but what i would like to know is if there are some huge holes in my reasoning.

is there something big that i'm missing? so far nobody has said NO THIS WON'T WORK. i don't even need anyone to say "yes it will," because probably nobody has ever done such a thing.. but if there's no reason NOT to try this than i think i'm going to.

as far as the mass on a spring model- i think this also makes the same kind of sense if the port velocities are similar at tuned frequency. the "spring" would favor the movement of the "mass" at it's resonant frequency and hopefully the "mass" will move like it is supposed to. the other vents would be harder to push air through so i would think there to be a dB increase (peak) at each frequency.

quote:

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the "spring" would favor the movement of the "mass" at it's resonant frequency and hopefully the "mass" will move like it is supposed to

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that concept isn't totally correct.....a mass by itself will not have a resonant frequency.....the spring attached to the mass is what gives the spring-mass-SYSTEM its resonant frequency......now i know that ports by themselves will have pipe resonances, but that is not of any concern in this matter.........that aside, i think i see what you're getting at.......isn't that what the abc box does? the cone movement favors the resonance in the smaller chamber/port if frequencies are in the vicinity (~octave higher) and because it is a seperate helmholtz resonator? and since it is tuned higher, that is why the box works as a whole at lower frequencies: the smaller chamber/port system unload below its tuned frequency and therefore acts as part of a whole based on how the woofer "sees" it.......any of this making any sense to anyone?

the conclusions that i drew were that for each resonance you have to have a seperate chamber......that is why a vented box has one ported chamber, the abc box has two ported chambers, a 4th order bandpass has a single ported chamber, 6th order bandpass has two ported chambers, an 8th order triple reflex has three ported chambers.....each with their own respective ports

sorry for mixing up thoughts and questions in there.......i'd like to hear some feedback on this too

[ 01-24-2002: Message edited by: guitar_maestro ]

quote:

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Originally posted by stabmaster:
NO THIS WON'T WORK

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NO THIS WON'T WORK

they work on the principle of porting to every freq within a certain range (depending on a few variables)

so the answer so far is:

no it won't work, but it worked for Bose... and try it out for yourself

quote:

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Originally posted by guitar_maestro:
hate to be a pessimist, but a question came up while reading this: say you have a box with just two ports instead of four......one tuned at (just as an example) 40 Hz and the other at 30 Hz......tunings based on the same net volume available inside the box .....neglect vent air speed for now......would the sub not just simply unload below ~40 Hz no matter how closely or separated the lower tuning frequency is? isn't this the reason the abc box has a divider? to have more than one resonance in a vented box (driver exposed to atmosphere) must you have separate chambers with correct volumes and ratios and also be connected properly to each other (via 3rd port in abc box)?

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I am not a box guru, and I am not nearly as advanced as anoyone who has posted here, but before I got to this post I had the same idea.

It seems that it would work if you used an ABC-type box with say 3 or 4 different chambers. Of course you would have to factor in the fact that for a regular ABC to work, the ratio must be correct. I'm sure this would hold true in a box with more than 2 chambers also.

Like I said before, I have a very good basic knowledge but am not anywhere near as advanced as you guys are. Please let me know what you guiys think?!?!

quote:

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Originally posted by Raymondnoodle:

It seems that it would work if you used an ABC-type box with say 3 or 4 different chambers. Of course you would have to factor in the fact that for a regular ABC to work, the ratio must be correct. I'm sure this would hold true in a box with more than 2 chambers also.

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if you add another chamber, the response would be hard to predict and the new tuning freqs would be too spread apart, and the upper tuning freq would probably be too high to be of any use

quote:

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Originally posted by Audio Stress Test:
but the ports on an ABC are all tuned to the same freq, to give 2 peaks.. 1 close to the tuning freq, the other just above ~twice the tuning freq

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the ports are tuned to the same frequency to give two peaks?

that was a mistake in my wording

what I meant was, with you only choose 1 tuning frequency, and the other is automatically 1.9x the freq(almost an octave above)

quote:

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Originally posted by Audio Stress Test:
if you add another chamber, the response would be hard to predict and the new tuning freqs would be too spread apart, and the upper tuning freq would probably be too high to be of any use

also the box will need to be huge

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Couldn't you change the ratio of the chambers in order to make the seperate tunning frequencies closer in range and not so spread apart?

Or like say you are using 3 chamber instead of 2, you make the largest chamber VERY large and tune it to say 20hz and then the other two would be what? around 30hz and 40hz? Assuming the use of the 2/1 ratio of course?????