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View Full Version : NEW BUILD IDEA, need help with port dimensions :)



XORstatus
08-23-2011, 07:08 PM
Hello all I am designing a box for a SINGLE memphis MOJO 15.

I am putting 1.2k watts to it daily.

The OVERALL dimensions that I have worked out are 4cubic feet total.

And the port tuned to 31 Hz is 14.5 x 2.15 x 25, I have to "U" turn the port which is giving me problems, I've never done that before and I want to double check my dimensions.

PLEASE give my your two cents!!!!

THANK YOU!!!

This is the FRONT VIEW, sorry about the quality I made these on paint.
http://i.imgur.com/yHFUj.png


This view is ON TOP of the BOX looking down.
http://i.imgur.com/6SOm3.png

---------- Post added at 07:08 PM ---------- Previous post was at 07:07 PM ----------

Will there be a problem in the back corner behind the port wall?

XORstatus
08-23-2011, 07:53 PM
ALSO where does one buy auto audio carpet?

akheathen
08-24-2011, 12:36 AM
there will be a problem, you have to ballance your net volume [internal, not counting the port(and 1/2 width beyond) or the speaker displacement] with a good port area ratio of 12-16"^2 per cu. net. right now, you are somewhere between 7.5, and 9" per, not knowing the net. that makes for allot lower output. it does help to put 45's on the radius of the bend to make it more round, just as long as it does not constrict the port width. keep it square, like you did, do not slope with the back of the box, and you should be good. i buy my carpet/vinyl at a local autosound shop, but i've bought it at walmart in a pinch.

Moble Enclosurs
08-24-2011, 06:20 AM
If the mounting depth does not interfere with the port being ran along the rear wall, you could benefit more acoustically from that, accounting for the obvious characteristics of the internal volume requirements. This will be easier to design as well as more acoustically efficient.

akheathen
08-24-2011, 10:28 AM
are you saying that the 1:5 angle on the back wall is not going to have a negative effect on the port as it makes the bend?

XORstatus
08-24-2011, 10:49 AM
there will be a problem, you have to ballance your net volume [internal, not counting the port(and 1/2 width beyond) or the speaker displacement] with a good port area ratio of 12-16"^2 per cu. net. right now, you are somewhere between 7.5, and 9" per, not knowing the net. that makes for allot lower output. it does help to put 45's on the radius of the bend to make it more round, just as long as it does not constrict the port width. keep it square, like you did, do not slope with the back of the box, and you should be good. i buy my carpet/vinyl at a local autosound shop, but i've bought it at walmart in a pinch.

I am not exactly sure what you're saying?
Would you please clarify as I am a noob.

I talked to memphis tech support and thats where I got the 4 cubes net internal (including port and sub displacement)
They told me to keep the port 31 square by 25 long, which should tune it to 31 Hz they said.

Do you see problems with this design?

Moble Enclosurs
08-24-2011, 12:25 PM
are you saying that the 1:5 angle on the back wall is not going to have a negative effect on the port as it makes the bend?

Regardless of the shape of the port, the path the sound travels is constant as long as the port is constant. So, yes there will be effects at the bend, but nothing that cannot be controlled. Here is an example of where the pressure will affect the output.
http://img220.imageshack.us/img220/7141/exampleofpressureonangl.jpg (http://imageshack.us/photo/my-images/220/exampleofpressureonangl.jpg/)


The darker red is the most pressure in the port. This area needs to be controlled for optimum output, but since the output is based on pressure in this type of design, there will be minimal effects anyhow, but no one can say they will not be audible yet. I bet they will be with a trained ear, but for general music, not so much different. it will however affect the tuning when calculated, unless this area is accounted for as well.
If you want to get technical, then controlling these red areas is the best option.

XORstatus
08-24-2011, 01:13 PM
Should I just block them off?

THe port is designed to be 16 inches long on one side, then wrap wround and an additional 9 inches so the entrance of the port wont be near the "red zone"

But is causes too much problem I can just remove it.

Keep the prot the same and just use extra mdf to completely encase that portion of the box.

Is that a good idea?

OR IS THIS WHOLE IDEA STUPID?
SHOUD I START OVER?


AGAIN I KNOW HOW TO ASSEMBLE THEM, don't know too much about designing them.

And as always thanks guys :)

Linkz
08-24-2011, 01:17 PM
Perhaps you could use some aero ports instead of a slot port.
Otherwise, I would just block off the angle, or get rid of it all together.

Moble Enclosurs
08-24-2011, 01:28 PM
Should I just block them off?

THe port is designed to be 16 inches long on one side, then wrap wround and an additional 9 inches so the entrance of the port wont be near the "red zone"

But is causes too much problem I can just remove it.

Keep the prot the same and just use extra mdf to completely encase that portion of the box.

Is that a good idea?

OR IS THIS WHOLE IDEA STUPID?
SHOUD I START OVER?


AGAIN I KNOW HOW TO ASSEMBLE THEM, don't know too much about designing them.

And as always thanks guys :)

Good idea. As the angle is really only needed for flushing the exterior of the design to the seats or wherever you have it. It really does not do much for acoustics, so sealing it off on the inside will be easier for you.

on1wheel01
08-24-2011, 01:56 PM
Go with aero ports. I did on my newest box 2 6" and man I love them. U could always just have someone design u a box, pro rabbit designed mine for my mojos

http://i79.photobucket.com/albums/j160/grnreaper01/IMAG0263-3.jpg

XORstatus
08-25-2011, 12:35 AM
How would one go about using or designing aero ports?

How does one tune those?

akheathen
08-25-2011, 03:47 AM
to me, comes out to 26.5hz that way.....i would go with 14.5x3.5 and it would need 29.5" length to get you to 31hz....... ok, i just run some #'s..... going off what you said, i calculated an appropriate port for a 4^3 net, and it's displacement came to just over 1^3..... then i looked at your box, and that 21.5" back wall stood out (should have been 16.5") and, anyways, your gross internal, subtracting the wall thickness, is only ~4.2.... so, that would knock you town to the range of 3'^3 ported or less..... as your net, guessing on sub displacement, and what the new port would need to be...36-43"^2, imho at that point, and that may be under-estimating sub displacement...... the t/s parameters of the sub would be nice to have....
mobile- i'm all ears, it just has never seemed right to run a port like that in my book, but you are the pro builder, so i will take what you have to say. i just see the high pressure area and un-even port pressure causing a bit of turbulance and break-up as the un-even volume tries to blend with the even slot. what if the bottom half of the port allong the back wall mirrored the top, making it even from top to bottom?think that would work well?

Moble Enclosurs
08-25-2011, 11:05 AM
How would one go about using or designing aero ports?

How does one tune those?

Far too much to explain in a thread without ample time and energy to explain it. I recommend getting the loudspeaker cookbooks for that information. For me, I do it a little different than most, so my explanation may not be recognized, and too many questions would evolve from it, which I do not have time right now to keep up on answering. But you should be fine if you read the recommended literature I mentioned.....if you can get your hands on it.

XORstatus
08-25-2011, 11:09 AM
I think Im gonna scrap the angled back, and run the port along the out side wall.

After talking to memphis they say 4 cubes @ 31 Hx, which includes port displacement and diver displacement.

Are their calculations wrong?

XORstatus
08-25-2011, 03:12 PM
WHAT ABOUT THIS???

http://i.imgur.com/sAKtt.png

Moble Enclosurs
08-25-2011, 05:26 PM
WHAT ABOUT THIS???

http://i.imgur.com/sAKtt.png

That would definitely be much easier for you to calculate. :) Just make sure you account for the bend area in those dimensions.

XORstatus
08-25-2011, 06:26 PM
HMMM

Ive always calculated the length of the separating port wall as the summation of the port length.

For example:

http://i.imgur.com/a2uBU.png

XORstatus
08-25-2011, 06:29 PM
IS THE TUNING OF THIS BOX CORRECT?

I told the memphis tech guy that I wanted it tuned to ~ 32 Hz, he came up with this design.

Total 4 cubes including port and driver displacement.
Port 31 sq. inch (14.5 x 2.15) by 25 in long which he claims to be 31 Hz tuning.

Also is the port width small? I have built a few boxes and the port width seems small to me.


I WANT TO BUILD THIS TONIGHT / TOMORROW, as soon as you guys OK it ill start and POST PICS.

:)

XORstatus
08-25-2011, 06:41 PM
ALSO when is a dual baffle necessary?
SOULD I USE ONE ON THIS BUILD?

Moble Enclosurs
08-25-2011, 07:28 PM
HMMM

Ive always calculated the length of the separating port wall as the summation of the port length.

For example:

http://i.imgur.com/a2uBU.png

it should be calculated as (19-0.75)+(8.25+0.75), which will give you 27.25", and can be even tuned more accurately with the bend accounted for if a 45 is used. But not a big deal for a simple design.

Now regarding tuning, that......I cannot tell you unless I figure for the driver specs along with the box information together, as the driver will affect the tuning depending on the mechanical parameters of the driver and displacement volume as well. Now, with double baffle, considering dampening factors of 0.75" wood against 1.5" wood, then doubling is ALWAYS going to be a recommended idea, but for simplicity, again, usually is more of a concern with excursion and power levels beyond average. The average can be said to be the mm distance equal to the drivers sd diameter. So, if a driver has an sd of say 500 square cm, then the sd diameter is 8.8" respectively(obviously as an average, as this is just an example). So, the xmax average for that driver would be 8.8mm(just replace inches with mm).
This is a good way to figure if the driver has better than average excursion, and why some companies consider 10mm "high excursion capability" even though we may not. So, if the driver exceeds this average, it may be recommended to double baffle.

Now, for those who want to rip apart what I just mentioned, this is ONLY used as an averaged example of what I came up with for the question of double baffling. This does not mean that a driver WILL be 8.8" diameter if the area is 500 square cm, as some drivers are unique, like square drivers, or the TB 8X12. And this does not relate to the drivers sd. The sd of a driver can be calculated by measuring the ACTUAL diameter of the cone and using a formula to figure for sd from that measurement.

So, in general, if the xmax exceeds this simple little example I put together as an average, then a double baffle will be recommended...and should always be used regardless, for added support and resonance control.

XORstatus
08-25-2011, 07:44 PM
Here are the specs of my woofer: I have the 15-M3MOJO15

http://i.imgur.com/VaRh1.jpg

Yeah its X max definately surpasses the 8.8mm mark.

I also used the RE box calculator With the same demensions of the box that MEMPHIS designed. RE calculator stated that the memphis design was ~ 27 Hz.

THE RE calculator however did NOT account for my driver. So how ACCURATE can this information be?

Further it is saying that my design is NOT 4 CUBES, its saying that my 30 x 19 x 16 design is 3.73 cubes?
Im assuming its discounting a portion of my cubic volume for the port?

I dont knwo whats CORRECT?

I dont want to build a MISTUNED box, that would make me REALLY REALLY SAD!!!! :(

Moble Enclosurs
08-25-2011, 07:55 PM
Yea, for the 12 on that list, for instance, the maximum excursion for single baffle use is 9.06mm. It has just over 15mm. SO, double baffle would be recommended, yes. It's not 8.8mm for every driver, lol, it depends on the drivers sd and power levels. But the example I gave was for 500 square cm-as an example, not a constant.

Now, if you are worried a lot about tuning and such, I have not recommended this yet, because I usually do not throw myself out there as a sign of need, but if you want one of us designers on this forum to do the enclosure for you, by all means, it will be worth the cost of a design if your concern it great for accuracy. i would NEVER consider using any port calculator, or program that does the math for you, other than checking your work. As, quite a few of them do not consider the most important parts of the acoustical aspects of the design.

So, myself, pro-rabbit, surreal, Ram, and a few others (forgive me if I left you out), can definitely make sure this happens the RIGHT way for you. Just ask. :)

XORstatus
08-25-2011, 08:01 PM
How much does this usually cost?

Moble Enclosurs
08-25-2011, 08:10 PM
How much does this usually cost?

From myself, a design is $45. From others, they might have to chime in for that. I cannot answer for that. Others may be less expensive than myself, so it's worth asking around. And by all means, this is no advertising thing for me right now.....it is clear that you need someone experienced to help you with this more than pure advice, so again, recommended.
Now, if you would like those of us who build them as well, to do a build design, this will vary upon different design approaches, so that will need to be discussed through PM, or during a request form for a more accurate quote. I will say this, that I am reverting to a new way of purchasing for customers. The initial price will only include the base price, and the shipping can be paid after the item is built and ready to ship. This allows for accurate shipping/handling costs rather than overcharging you for what the company calculates for. I have had a LOT of people request this, as it is more accurate for costs to you, and can be split up in case you do not have the costs of the total amount up front.

Just an idea, if you choose to have a build done as well.

XORstatus
08-25-2011, 08:25 PM
Thank you for you input, I appriciate you going above and beyond.

I like the feeling of doing it my self tho. it makes the outcome better to me. BUT I WILL KEEP IT IN MIND.

I have redesigned the box and calculated the tuning.

New box is:
30 x 16 x 21
Gross vol: 4.66 cubes

Port:
14.5 x 2 x 14 tuned to 32.99 Hz
NET vol: 3.9 cubes

Im building this baby TONIGHT!!!!

Moble Enclosurs
08-25-2011, 08:27 PM
Thank you for you input, I appriciate you going above and beyond.

I like the feeling of doing it my self tho. it makes the outcome better to me. BUT I WILL KEEP IT IN MIND.

I have redesigned the box and calculated the tuning.

New box is:
30 x 16 x 21
Gross vol: 4.66 cubes

Port:
14.5 x 2 x 14 tuned to 32.99 Hz
NET vol: 3.9 cubes

Im building this baby TONIGHT!!!!

SWEET! Post a pic or two! And obviously, you have to let us know how it sounds, lol.

akheathen
08-25-2011, 09:12 PM
ah, they just give it to you in the wrong format. it is actually about 3cu, maybe less ant then their port dimensions would be 31hz, but i'm telling you that you do loose some effectiveness of the port once you get below 12"^2 per cu. generally, we go off net displacement, not displacement before the port and driver. i do not know about how people are calculating aero-ports, but i remember reading on their site, and i don't 100% know if it's correct, that you tune them like normal round ports, just calculate them as bigger ports. only used them a couple times, but generally, i prefer to stick to slots, but that is my own preference.

XORstatus
08-26-2011, 01:06 PM
Here are some pictures of the build!

http://www.caraudio.com/forums/general-discussion/521889-new-build-memphis-mojo-15-a.html#post7723870

Tell me what you guys think.

akheathen
08-26-2011, 10:54 PM
oh, tripple wowee..... first, i need to start making sure there is not another page before posting, lol, 2nd, i can't belive i read that right. mobile measured the port from the outside, not the centerline?!?!?!?, and 3rd, you went way backwards on that port area. that is only 7.3"^2 per cu, which is absolutely tiny.... like those truck boxes that had the tiny 1.5" round ports in them or even bigger prefabs back inthe day, the ports really did nothing.......... and yours will be highly inefficient and barely effective. at lower power levels, say 300watts, the port will have an effect, but with some ral power, not so much. if you hadn't already glassed it, i would suggest putting it in raw, with just sealant and seeing how you like it. then, when it doesn't reall have the output you expected, then add 1.5-2" width to the port and it will be more than you hoped for.... your tuning and volume #'s were just about spot on, so you are doing good there, just remember lenght of the port is measured down the centerrline of the port, and port displacement also includes 1/2 width of the port beyond.....

Moble Enclosurs
08-27-2011, 10:54 AM
oh, tripple wowee..... first, i need to start making sure there is not another page before posting, lol, 2nd, i can't belive i read that right. mobile measured the port from the outside, not the centerline?!?!?!?, and 3rd, you went way backwards on that port area. that is only 7.3"^2 per cu, which is absolutely tiny.... like those truck boxes that had the tiny 1.5" round ports in them or even bigger prefabs back inthe day, the ports really did nothing.......... and yours will be highly inefficient and barely effective. at lower power levels, say 300watts, the port will have an effect, but with some ral power, not so much. if you hadn't already glassed it, i would suggest putting it in raw, with just sealant and seeing how you like it. then, when it doesn't reall have the output you expected, then add 1.5-2" width to the port and it will be more than you hoped for.... your tuning and volume #'s were just about spot on, so you are doing good there, just remember lenght of the port is measured down the centerrline of the port, and port displacement also includes 1/2 width of the port beyond.....


Another remark without understanding acoustics........I love it. Yes, most of the audio community will measure centerline, but this completely disregards the high pressure at the bend, therefore without using 45s to control the propagation of the wave, the high pressure area HAS TO BE COUNTED!! The measurement was not exterior, it subtracted the thickness of the wood. :). If he were to put 45s in the line, then yes, a centerline measurement would be recommended. You cannot disregard the portion that causes the most effect in the output in terms of nulls and peaks. A lot of people do not know HOW to account for this, so they go simple.

Do you understand now?

Moble Enclosurs
08-27-2011, 10:59 AM
oh, tripple wowee..... first, i need to start making sure there is not another page before posting, lol, 2nd, i can't belive i read that right. mobile measured the port from the outside, not the centerline?!?!?!?, and 3rd, you went way backwards on that port area. that is only 7.3"^2 per cu, which is absolutely tiny.... like those truck boxes that had the tiny 1.5" round ports in them or even bigger prefabs back inthe day, the ports really did nothing.......... and yours will be highly inefficient and barely effective. at lower power levels, say 300watts, the port will have an effect, but with some ral power, not so much. if you hadn't already glassed it, i would suggest putting it in raw, with just sealant and seeing how you like it. then, when it doesn't reall have the output you expected, then add 1.5-2" width to the port and it will be more than you hoped for.... your tuning and volume #'s were just about spot on, so you are doing good there, just remember lenght of the port is measured down the centerrline of the port, and port displacement also includes 1/2 width of the port beyond.....


Also, it has to be added that measuring port area by "per cubic ft" is very inaccurate. It does not account for all parameters in calculating for minimal port noise or coupling. People need to get away from this common misconception of port area. Just because a port is smaller than you like, does not mean it is ineffective. In this design, you may be right or wrong, I do not know as I have not personally designed it myself, but again, to assume it's too small based on basic area needs is like saying the small ice cream cone is too small to hold more than 3 scoops....which has been proven incorrect as well, lol. But in this case, port area is less physical, and more acoustical. You can't say, "it's too small" without knowing how it was calculated or what it was calculated for.

akheathen
08-27-2011, 03:59 PM
lol, um... ok