posted
ok, at the sound levels we reach, it's all about the pressure in the cab, right? so why would we port are boxes?
follow me here, in a ported box, and the sub goes in, the port pushes air out, but when the sub comes out, the port sucks in air. so don't ported boxes just move the air and not add "pressure"? the volume of the area of where the cab air is never changes.
but in a sealed enclosure/cab, when the subs hit, the have the same air, but less air space. cause the sub push it in. the air has no where to go, just pressurize. isn't that the goal?
posted
BUT with spl it is not constant pressure, the spl is measured for a brief second, so having ports increases the air that can be moved. I am still new to this but thats what I think is correct.
------------------ Get LOUD or Go HOME
Posts: 107 | From: Sequim, Wa., USA | Registered: Mar 2000
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posted
the way I understand it is... at the port tuning freq the front wave and back wave of the speaker are "in phase" add completely (lots of preasure) which is why you get more gain from a ported box. when you go too far below the tuning freq of the box the front and back waves cancel (no preasure), basically unloading the box and possibly causing damage. (rereading my post I don't know if I expressed what I was trying to say exactly.. but you get the idea.. I hope)
------------------ 9 12's in the hatch of a golf
Posts: 735 | From: Calgary AB | Registered: Oct 1999
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posted
I was thinking the same as you a week ago But there are many varibals in spl a speaker perform different in different boxes wich make the woofer louder also the air movement can make a difference In thereoy the woofer moves at the same time then pushes air out of the port so its still building pressure it just is suckd back in but it goes back out and builds back up Then you have all the other varibles resonaceFeq heck some cars a sealed works better than ported and the same with different woofers. The truth is spl is not a exact science there is alot of trial and error.
Ah, yes.. The question that drives us... ("What is the matrix?" NO! BAD keanu!)
A sound wave IS pressure.. It is the pressurization and rarefaction of air due to motion at specific frequencies, correct?
So, why do some cars get louder when you open the windows?
Answer- The ratio of speaker displacement to volume to be displaced (IE, cabin volume) dictates SPL... A smaller number of speakers have trouble pressurizing all of the air in a cab, therefore, when you open the windows, it become easier for them to "push" the air... Think of it this way- When you have subs in a box of say, 1 cubic foot, and the subs produce, say, 96 db, then you boost the box up to say, 4 cubic feet, the subs will be louder at the same power level, because the enclosure does not act like as powerful a "spring" as the smaller box did. When you open the windows of a car, the compliance of air in the car goes up, making it easier for the speakers to move... Making it louder.
(The previous paragraph is the result of a few discussions i have had... Any one and everyone who disagrees or can better illuminate this point, i WANT to hear from you!)
When the ratio of volume of speaker displacement to volume to be displaced passes a certain point passes a certain point, this above scenario will no longer happen.. You will LOSE sound, or at least gain none, because the speaker does not displace more air... the same principle applies to the port, because it creates more volume to be displaced, which gains you nothing...
Now- Tear this apart, people.
ShadowStar
------------------ Computers are a lot like air conditioners.. Open Windows and they become useless..
The day Microsoft makes something that doesn't suck is the day they make vacuums.
Posts: 2578 | From: Somewhere In the Northeast | Registered: May 1999
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posted
After reading another forum, the folks at a2000 firmly believe that several smaller drivers in a sealed enclosure with a ton of power and huge motors will be what breaks the 180dB barrier. Reason being, a smaller driver can better pressurize the volume of air directly in front of it, due to the smaller diameter cone having a higher stiffnes due to they're being less cone area to flex. Shadow, I've noticed the same phenomenon with the windows as you noted, however, I always found that was mainly due to the woofers being in the wrong size box for that particular car. Sometimes I found that aiming the woofies toward the rear of the car corrected some of the problem, possibly because the woofer was now loading into the added volume of the trunk. With large driver's there is a point of diminishing returns, and if you really think about it, it makes sense. Hey, you forgot to mention that woofers in larger boxes lose they're ability to dissipate larger amounts of power due to that lack of springiness in the box, but I wont mention it!
------------------ STILL waiting to hear a GOOD pair of HLCD's!!
If it dont fit, force it... if it breaks, it needed to be replaced anyway...
posted
I just wanted to comment. When you suck the air out of a car you still would create pressure.
For example...Take a SPL mic. And cup your hand over it. If you blow you get a reading...BUT..also if you suck...you also will get a reading. You still are moving the diaphram...
That make any sense?
------------------ Robert (Rob) - 1996 Chevy Cavlier 2.2 Member Of"Team Gates" and "Team Shockwave" USAC Sactioned Judge
I've known cold air to be more dense than warm air. So on cooler days do you get better spl scores due to the barometric pressure increase? Would it help to run the A/C in the car an get it as cold as possible before hitting the lanes...?
Or is it just not enough of a change to make a difference.
Well, by that ideal, cold air would be harder to move.
Because pressure is caused by the collision of air molecules, heat, which causes molecules to move faster, would cause more collisions. This would cause your pressure to go up. I would expect that, if you heat up your interior (let it sit in the sun, for example) and keep your amps cool, you would be a little above your normal pressure.
any thoughts?
ShadowStar
------------------ Computers are a lot like air conditioners.. Open Windows and they become useless..
The day Microsoft makes something that doesn't suck is the day they make vacuums.
Posts: 2578 | From: Somewhere In the Northeast | Registered: May 1999
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posted
Just my two cents on the port thing: In a sealed box half the energy is wasted, hence the 3db gain (perfect world) when you port it. I don't like the complexity of ported and bandpass boxes myself. although Freeair being the most appealing to people lazy like me, I hate the way they preform. The cone has no control and flops around, similar to humongo boxes. And a oddball question: Howcome nobody makes dome woofers? If you can make a square sub you can do anything.
------------------ "look at me, i'm big woofer headed boy. now give me some candy"
Posts: 323 | From: Great Falls, Montana | Registered: Feb 2000
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Making a true "dome" subwoofer would be moderately hard... It would come out looking a lot like what most of the newer aluminum subs do.. Cabasse, a moderately expensive home audio company, makes a 23 or something odd inch sub, with a true inverted dome diaphragm.. Very neat, very expensive.
ShadowStar
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Posts: 2578 | From: Somewhere In the Northeast | Registered: May 1999
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posted
Um our cars are louder than the Space Shuttle? I don't think so. Well maybe since the closest we can get to the shuttle at launch is a few miles. But at the pad, the shuttle does 194 dB- I think.
About cars being louder with the windows down, are you not in effect creating a bandpass box when you do this? The open window is the port and the cabin is the ported chamber?
About the A2000 guys, funny that they built a near 6 foot sub...
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Posts: 7866 | From: Charlotte,NC USA | Registered: May 1999
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posted
I think that heating up the cabin would be a good idea if you could keep the amps cool and not to forget the voice coils cool also. I have seen I lot of new compeitors park and before the show run there subs for 30 min straight and then wonder why there score went down.
------------------ -Life moves pretty fast, If you don't take a look around once and a while, you could miss it. http://lowspage.cjb.net My E-mail: Low_Rider@mailcity.com
posted
Back to the top, to answer original question from Solosier.
Air in the box/port can be compressed, and decompressed, so it can act as a spring.
Just imagine that you have some weight hanging on long rubber strap from your hand. Now imagine what will happen, if you will move your hand for inch up and down. If you will move rubber strap slowly, weight will have the same speed as your hand. If you will move your hand too quickly, weight might stand steel. But if you move it with right speed, weight can move much more than one inch originally moved by your hand.
So you must have ideal amount of air in the box, and in the port, to gain some advantage.
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Posts: 1820 | From: Ljubljana, Slovenia | Registered: Jan 2000
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posted
Winslow, sorta. My theory is: if your subbox is incorrectly sized/loading incorrectly, yes, opening the window sorta resembles a bandpass box, but really all your doing is giving the sub the airspace it wants to load up right. Change box orientation, and if that doesnt work, correct the box size. It is possible that the box size recommended for your car is incorrect...
------------------ STILL waiting to hear a GOOD pair of HLCD's!!
If it dont fit, force it... if it breaks, it needed to be replaced anyway...
posted
Audiophyle, now that is true. The first that I heard about the relationship between box size and interior size was last year at a JL meeting. Which, now that I think about it makes sense. A larger car could use a samller box box because the transfer function of the vehicle itself will take care of filling the low end that is normally missed when using small boxes. Is that waht you were refering to? Did I butcher that badly? It is late and my brain is going into shut down.
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Posts: 7866 | From: Charlotte,NC USA | Registered: May 1999
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"the subs will be louder at the same power level, because the enclosure does not act like as powerful a "spring" as the smaller box did. When you open the windows of a car, the compliance of air in the car goes up, making it easier for the speakers to move... Making it louder"
Thats sorta what I was talking about, the compliance of air (the function given by enclosing air behind the speaker) changing when the window is open.
I like it.
Lets go farther.
So... Is that why a sealed box is less efficient, because of air compliance? And a ported box more efficient because of the same thing (air compliance), then what is the effect of the back wave released from the box?
ShadowStar
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Posts: 2578 | From: Somewhere In the Northeast | Registered: May 1999
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posted
Damn, Shadow, why do you have to ask such questions? Guess I need to read the Loudspeaker Design Cookbook more. That is a really good question, probably has a simple answer too.
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Posts: 7866 | From: Charlotte,NC USA | Registered: May 1999
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