[ADVANCED]Awesome Technical simulation of an electronic subwoofer eqivelent! and TECH - Car Audio Classifieds!
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Old 06-02-2011, 08:37 PM
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Default [ADVANCED]Awesome Technical simulation of an electronic subwoofer eqivelent! and TECH

[Warning, there is alot of info in here which may be too much for you, but if you care about subwoofer design then this is good info] This is for people interested in some core fundamentals of subwoofers.

I wanted to make a few vids of a few simulations i did.

It is an equivalent "Tank" Circuit of a Subwoofer with a given set of TS Paramaters. This is for those who enjoy subwoofer physics, i probably wont go very far with it, i just mainly wanted to touch on damping and TS Paramaters.

First thing to know is that a subwoofer is an Oscillator, the moving mass oscillates with the suspension system(at FS). The two components, spring and moving mass are each storage components. They are analogous to an capacitor and an inductor, respectively. For those who know math, they are 180Degress out of phase(at fs), being so, they complement each other to keep moving together. The degree thing is not really important to get into for this basic TUT, but basically, you should know that once a signal is applied to a subwoofer, after it is removed, the subwoofer keeps moving. How long it moves depends on it's damping which is the HEART of the subwoofer, and system design. I don't want to create a HUGE read here since people don't like to read much so i wont write a book in a thread.

First following VID below is a TANK circuit, it shows the yellow dots moving which represent the current, and obviously current wouldn't move as slow as it shows, but i slowed it down so you can see it. All i want to show is what happens to a subwoofer when it reaches FS, mind you this is a free-air representation of a sub. Later i may show a 10" i just built. I can get the circuit eqiv by my woofer tester which is AWESOME!

[1stVID]
First up is a Sweep from 20hz-80hz, with an FS of 35hz, remember that!
In free-air at FS, the subwoofer's Impedance PEAKS. In the vid i have a scope showing the current at FS and like you should guess, it is least at FS, which is 35hz like i said. The response is a mountain TOP peak of impedance. 35hz is the center, the peak, and the current will raise as it goes above and below it.

The current is YELLOW in the SCOPE at the bottom and you should look to the right side of the scope when the video pauses at FS. The Freq in time is the ac source on the left side...

http://www.youtube.com/watch?v=Ha-gChP26cw


[VID2]
Now, my second vid is a vid of it playing a constant 35hz sine. So it shows it playing at FS. It then shows how the current KEEPS going in the TANK section on the right after the signal is removed. The TANK section represents the diaphragm with it's mass, and the suspension, which are resonating together. It goes on for a certain amount of time, the time is displayed to the right of the scope in milliseconds and you can see that the current dies down as time goes on.


http://www.youtube.com/watch?v=5Cde5WzbC1w



This also shows that the subwoofer moves fine on it's own during impedance rise because the energy in the resonance is highest at FS. So impedance peak happens BECAUSE the resonance energy is high and so efficient. It is like so because the 90degree PHASE at FS makes it resonate easier then any other phase angle. So less current is even required in the first place! And co-incidentally, less is there at the time from imp rise.


HOW LONG IT GOES ON IS THE DAMPING. This is why systems with HIGH TOTAL Q don't sound accurate, because the subwoofer doesn't stop as fast as it should. A lOW Q will stop faster. The lowest possible Q is not the best though because then there is no low end and a bit of inefficiency.

As you can guess, a HIGH Q means that the subwoofers MECHANICAL system is EFFICIENT, where a low Q is inefficient. This is true because the mechanical components store energy longer then a low Q's does. Both can be said about either subwoofer type though since Low Q subwoofers are more efficient then highQ subs at upper frequencies and visa versa. This Efficiency is also a free-air efficiency though.

So Q really depends on the RESISTANCE in the circuit, yea thats true, it does. This is why you cannot wire a resistor to a system to match the amps impedance wants for top power. What i mean is you cannot practically take a 2 ohm resistor and add it to a single 2 ohm coil in parallel to get 1 ohm. YOU CAN, but it will change the damping!! which means you need to change the box! Wiring a DVC sub together is different though.
Reactance is another thing you can change to change the damping. The box you put the sub in does this, it adds to the suspension stiffness.

This info is NOT designer information that is going to help design a box and system it is the base of what goes on, but now all you need to know to design good boxes is terminology and things. For instance, what Q am i talking about? There is Qms, Qts, and Qes etc...If i am designing a sealed box i dont just pick a sub with the FINAL preferred Q, i have to build the box to GET to that total Q, Infinite baffle is an exception to this. So, adding the box actually ALWAYS increases TOTAL Q. The subwoofers Qts is the one that you combine with the box to get to your desired Q. Then, there is ported boxes which don't go by Q factor. They have a Q, but they ALSO have another resonance which is the box-port. A pretty low Qts sub in a sealed box does not really do as well in the low frequency domain as a higher qts one does. Thus, when you apply a port to a sealed box you are allowing the port to work with the sub and air in the box to extend lower. So the port is important for LOW Qts subwoofers on music setups that have good low-end response. But then you also have your cabin gain, but CG has it's disadvantages.

SO, AFTER READING ALL THIS STUFF, HOW THE HECK DOES THE CONE AND SUSPENSION EVEN INDUCE THESE EFFECTS ON THE COIL SUCH AS IMPEDANCE RISE? Well i answered it, they INDUCE it on the coil. A subwoofer is like a transformer, what happens one one side manifests itself on the other side.

There is so much i want to write about but cant since not many people like to hear this much stuff.

Ask whatever question you need...

Last edited by Novicaine; 06-27-2011 at 02:58 PM.
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Old 06-02-2011, 11:06 PM
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I did the Videos backwords so 1 is 2 and 2 is 1...But i corrected it. Now i get double videos! How do i change that?

I want to show anoter vid(#3 below) with an "amplifier" attatched. Basically i am using a .01ohm resistor to short it because thats what amps do, they provide very low resistance to shunt the sub, there is a BIG difference, it dissipates ALOT faster. Damping factor in amps is important, but comparing different amps damping is useless since going from a DF of 50-2000 will not make a real difference in Q, but a DF below 20 will. Tube amps have a pretty low damping factor though and some like it.

How does the shunting, or short circuiting of the VOICECOIL stop the subwoofer from moving Like i said earlier, the subwoofer is a transfromer, what happens on one side manifests on the other. Whenever the subwoofers coil moves in the feild it generates a reverse voltage which is an induction thing, it is known as the impedance rise we see with subwoofers, it opposes the direction it is going. This is why the subwoofer experiences impedance rise, because the coil is moving in the feild.

Above Fs, the subwoofer dosn't move far. It needs to accelerate faster to be at it's higher frequency. high freqs faster, low freqs slower.
So since it dosnt move far, and needs to go faster, it has no time to move far like low freqs need to do. It moves less distance and generates less impedance via backward voltage. It is however damped because the coil ITSELF has less impedance on IT and less impedance/resistance(amp DF) will allow it to be dampened, or stopped sooner since a short circuit is "faster" with less impedance.

http://www.youtube.com/watch?v=q7_0NwgfU0Q

Last edited by Novicaine; 06-24-2011 at 12:42 AM.
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