posted
Yes, Normally you will get a peak above tuning. In the case that you do, The port and speaker have a different resonance frequency. The sub will be higher than the port. You basically end up with a peak in the output between the two resonances, which as you said is above tuning. For a given speaker, you can design the system to have the speaker’s resonance, ports resonance, and spl peak all at the same frequency. And like I said, you will not have a choice in box size and tuning frequency. They will be chosen for you. So you can see how this could become a challenge. The parameter of the sub that would change this is its mass. Some people fiberglass their cones to make them heavier.
When a sub is further loaded acoustically, like placing it in a car, The entire impedance curve will rise within a specific bandwidth. If you look at horn loaded designs, you will see that they have significant impedance rise at the lower frequencies that have better acoustic coupling (more loaded) due to the horn.
This is what I designed for my car and it worked out as I calculated. I can run my 2000x's on a 1-ohm load all day. I didn’t tune for spl though. I just wanted a loud daily driver that has a flat response.
I know its often described this way but, saying a port is 90 degrees ahead of the front of the sub implies there is time travel taking place. It’s actually -270 degrees. But again it really doesn’t matter mathematically.
Posts: 1247 | From: Fullerton. CA ,USA | Registered: Oct 1999
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posted
well in my experience, I've never been able to get an amp to work reliably for daily driving when loaded down
Posts: 358 | From: .. | Registered: Apr 2002
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quote:Originally posted by Bassaholic_au: well in my experience, I've never been able to get an amp to work reliably for daily driving when loaded down
To get mine to work, I measured the parameters myself, took measurements in my car on an old box at multiple tuning frequencies, simulated the old box with leap, compared the results, determined the biggest box I could fit, ran simulations and corrected them based on what I learned from the old box comparisons, determined the tuning frequency to give me the minimum bandwidth I was willing to accept, determined the minimum dcR the sub could have, bought a new sub with the impedance I determined, Then built the box. The amps don’t over heat., but I can shut them off if I crank up 125hz on the eq, play the right song, and well its usually turned up loud enough anyway.
So I guess it’s not that easy. But if you’re like me, you'll never be satisfied and always have new systems in your car, So you might as well give it a try. As far as what I did was to get the impedance curve so that in between both peaks where it hits a minimum was about 3.5 ohms in car. If I moved the peaks any further apart which would have given me more bandwidth, that 3.5ohm impedance would drop to low for my amps. The subsonic filter on the 2000X's just happened (got lucky) to hit a -3db at close to where the dc side of the ports resonance frequency was down to 4ohms. The active low pass filter had a -3db at the bottom of the inductive side of the subs impedance where it was down to 4 ohms. For the small signal spl response, I think I ended up with, -3db at 40hz, and -3db 80hz for bandwidth, with +/-3dB ripple.
Posts: 1247 | From: Fullerton. CA ,USA | Registered: Oct 1999
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posted
I'm not going to get into the finer points of this post though i will say it is a good one, and many people are obviously right on track.
this is about the leading/trailing issue.
long ago electrical engineers decided that when describing phase angle relationships (the angle between to sine waves on a scope) that they would always refer to X angle LEADING Y angle.
ex. in our case Y actually trails X by 270 deg.... but it is refered to as Y leads X by 90 deg.
IF... Y trailed X by only 90 deg. it would be refered to as X leads Y by 90 deg.
the reason for this is actually simple this makes all phase angles positive. when adding Phasers Quadrant I and Quadrant III are the same. (ex. sine of 30 deg is = to sine of 210 deg.) by eliminating negative angles it simplifies the addition & subtraction problems.
Posts: 12 | From: New Madison, Ohio | Registered: Oct 2002
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posted
Sorry to ask this but can someone run the whole concept by me one more time. I'm trying to build the box that would be best for my subs and after this I think I will try a ported enclosure. My e-mail is caraudiophile@yahoo.com.
posted
I could use clarification on the whole port tuning concept. I will be building a ported enclosure for two 12" woofers. The Fs is 21.7 The Qts is .35 and the Vas is 5.5(cubic feet). I want high s.p.l. but don't want the bass to sound like ass. My highest spl so far is only 147.1 with a 1.4 cubic foot per woofer vented enclosure with a 3" diameter 13" long peice of pvc pipe for the port per woofer. This time around I want to go square or slotted port.I am so confused at this point . I need some help. I got a lot of space to work with as I drive a suburban. only thing is I don't want to put up a wall.Any help would be greatly greatly appreciated. Thank you caraudiophile@yahoo.com
posted
Area for area, a port won't change much of your box (that is, if you keep the port area the same, a square port vs. a round port vs. a flared port won't change too much) however Dickason mentions some issues with diffraction on flared ports at high SPL.
3" isn't enough for a good sized 12" driver, especially not with the type of air movement we're getting out of them nowadays. Look rather to something along the lines of 6" round ports or so, if you do a slot port, its good. Generally as much port as driver area isn't a bad idea. Plus, in your suburban, you'll be able to hear port noise quite easily.
The "sounding like butt" that you're referring to is usually a misaligned box. That said, you might think of it as putting springs on childrens shoes. The big heavy kid might jump just as high, but he needs different springs than the little skinny kid to do so. Making the box and tuning it is just picking the springs for the size woofer you have.
With a ported box, you generally have to sacrifice ultimate sound quality and response for a reasonably peaky response that will net you a higher SPL score.. For SQ, you want the peak of the box to fall a little bit under the start of your car's transfer function so that the two sum overall to a flat response, IN car.
Now, port tuning works kinda like this, in essence: You pick a tuning and you vary it until you get the sound you want.
If its big bad SPL, start with a good box volume for your driver and then move the box tuning around to match with your car peak. Mathematically the situation is complex just to describe how a port interacts with the box.. When you change the environment around to something complex like a car, and power it with a voltage source like an amplifier that can't supply infinite current, you make the situation much too difficult to model before hand.
So look at your manufacturers specs for the ported box.. If its not an SPL intended driver, then add a couple cubes to the volume to get into the region where your driver can unload a bit.. Then make yourself some 4" pieces of PVC of a certain length (say 15 inches to start with) (or bigger than 4" if you can) and then test test test.
ShadowStar
-------------------- You can't build a reputation for what you're GOING to do.. But you can build one for TALKING about it!
It's all about knowledge, love and respect. Posts: 2578 | From: Somewhere In the Northeast | Registered: May 1999
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posted
caraudiophile, Your doing 147 with 12's in 1.4ft^3 in a suburban? That sounds really high to me...... If you’re building a new box and have the room put the subs on one side of the box and the port on the other. Have both the port and subs firing into the back corners, and use baffles to create a path between the port and subs so that the air has to travel 5-6ft between the two.
And like Shadowstar said, spl and sq are a trade off. The more you tune for SPL (which is considered a misaligned box by sq people) the more it sounds boomy......
Posts: 1247 | From: Fullerton. CA ,USA | Registered: Oct 1999
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posted
ok I'm gonna have to read this a couple times before i makes much sense.. what are some good topics I should research on to learn more? cuz I really want to learn as much about this stuff so when I get money to start building my system I'll have something to work on..
-------------------- 1990 300zx NA Posts: 744 | From: Vancouver BC Canada | Registered: Apr 2002
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posted
ok I think I just figured it out if anything or everything is off tell me and I'll ask some questions then.. ahah
ok here is what I have figured out.. at least for one part.. ok you have a ported box.. the sub has a front wave and a back wave.. the front wave goes off and the back wave goes into the box and out the port.. obviously it doesnt travel faster out the port then from the front so the back wave is travelling behind the front wave.. that is why ported boxes tend so sound muddy cuz the sound wave is getting to you at 2 diffrent times.. so what you want to do is to try to delay the back wave so that when the front plays that same wave again the back wave is at that point too.. so then you have 2 times the wave travelling forward at the same time which will make the meter go higher..
I know it is hard to read.. I wrote it in newbie terms cuz I'm a newbie and this is how I understand it.. so am I completely off or do I got it? haha
-------------------- 1990 300zx NA Posts: 744 | From: Vancouver BC Canada | Registered: Apr 2002
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posted
I know that when I start competing in DB Drag, I'll do a series of experiments to determine how the ratio of the volume of the air in front of the woofer to the volume behind the woofer affects SPL, with a given tuning frequency. The experiment will also be done many times, varying several other variables, to note which increase or decrease the max SPL. With lots of experiments, one could make some general rules on what variables really affect SPL, and what their relationship is to SPL.
This has probably been done on one scale or another, but I'm sure not many would reveal the results of their tests.
I'll see you guys in the lanes when I get out of school. I'll do my best to get to World Finals, and perhaps one day win my class. Whether win or loss, I'll have loads of fun.
-------------------- Mike Miguez
All motor CRX! Who wants some? In memory of 9/11/2001 THE FLAG STILL FLIES! Posts: 2209 | From: Beaumont TX, USA | Registered: Jan 2000
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posted
I should clarify how I acheived that (very unofficial)spl. I first folded all the rear seats down to create a flat surface. I then laid down a plywood platform to reduce the amount of air space inside the interior. I made the right side of the platform raised almost to the ceiling. To effectiviely seal the patform I made a rubber gasket around it. The whole thing fits perfectly but is a pain in the ass to put in and take out. this settup reduced the air space in the interior by about 85 cubic feet or so. allowing higher sound pressure levels. As for the box I just follwed the manufactorers specs for the recomended vented enclosure. When I really look at it the reeding does seem kind of high so I am questioning the acuracy of the meter used. Realisticly though, I have heard enough stereos, and seen enough settups to know I am doing at the very least 140 with the platform in. I'm just not to crazy about paying 50 bucks to have it metered. Any ways I have decided to dump this rig and go with an x-audio 182 18" sub instead. I already have the woofer but just need to build a box.
quote: With a ported box, you generally have to sacrifice ultimate sound quality and response for a reasonably peaky response that will net you a higher SPL score.. For SQ, you want the peak of the box to fall a little bit under the start of your car's transfer function so that the two sum overall to a flat response, IN car.
ShadowStar
In your SQ scenario, would it still be louder than a sealed box? And what is transfer function? How do you find it? Thanks.
quote: - in an ideal system, these will cancel each other out resulting in a flat response... -------------------------------------------------- ------------------------------
Dont you mean flat Impedence response the impedence response can be wobbly-but the output doesnt resemble it.
quote:But back to your other point, since there will be a significant amount of impedance, once you have got the port/box tuned correctly, loading down the amplifiers is an easy way to get more *actual* power..
the impedence peak just shows its wasting its energy-but actual output stays because its a resonant system
a ported box cant hve a flat impedence curve
the graph Does show impedence Minimum compared to sealed box impedence maximum in the same zone.. so back-EMFs are there, just at different areas.
Not many people know much about this subject..
well a shiva in 4cubes modeled up- Fcb=33hz impedence phase=0 @ 32hz
posted
First off this post is very interesting and confusing at the same time but I have a question. I am doing a super street car for next year now when building a wall i heard the 1/4 theroy works the best because it aligns both waves but I know this will result in a high peak note but the car's peak frequency is 55hz so how do you think I should tune my box.
posted
toxicbass_1 - I was indeed talking about a flat frequency response, not a flat impedance curve. By "loading down" I did mean running the amplifier below the rated "nominal" impedance, taking advantage of the fact that the impedance will rise and not actually be as low as the "nominal" impedance..
Also, when driven into full clipping, a class D amplifier will work much the same as a class A/B amplifier - Class D can only have an efficiency advantage over the class A/B where the A/B output stage would be operating in a linear manner.
Yes, not many people know much about this subject.. I sure as hell didn't (and to be honest, still don't) know anything when I posted those replys last year. But I'll leave them there because I find them quite amusing.. Posts: 358 | From: .. | Registered: Apr 2002
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The volume of the box acts like a spring. The air in the port acts like a weight.
Now, go find a spring (long one) and some weights to add to it, then pluck it to make it vibrate. Try a light weight, or none, then try a heavy weight. Which one vibrates faster? The light one.
If you ever make a big ported box, before you cut put the sub hole, kick the biggest panel of the box, and you should hear the tuning frequency. The air just moves back and forth. Look at the air in the port as a solid, not a fluid.
The more air in the port you can get to move, the higher the SPL will be. Usually a big port and a big box.
In an SPL box, the air in the port will be too heavy to move until the tuning frequency is very close, then the momentum of the air will make it move alot, and make tons of SPL.
In an SQ box, the port is small and the air in it isn't heavy, so it will move at frequencies farther away from the tuning, and you get better bandwidth. But at tuning, the weight is less, so there is less momentum to make it move more, so less SPL.
posted
dont a larger box/port also show a higher spl peak aroudn a certain freq...like lets say a 4cu box show a 2db peak at 50hz..and havea slower roll off where a big bos may havea 4db peak and a very sharp roll off...this is one of the reason why i always thought we used biogger boxes for spl...just wanted to drop that to see if anyone could comment or add to it ...
-------------------- Alan Hall Broken Silence CC 2007 usaci mod 1001-2000 champion 163.6 2003 usaci mod151-300 legal record holder 4th 2002 usaci finals mod 0-150 6th 2003 usaci finals mod 151-300 4th 2004 usaci finals prostock 1001-5000 4th 2005 usac finals mod 301-900 Posts: 667 | From: Troy IL | Registered: Apr 2000
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In an SPL box, the air in the port will be too heavy to move until the tuning frequency is very close, then the momentum of the air will make it move alot, and make tons of SPL.
Think about that. Get a mental picture of the response, and how it behaves.
quote:Originally posted by Mouser: Fairly easy question. A larger enclosure simply acts as a higher Q resonator, same as having a larger port does. If you have less damping on the system the
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