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Re: Okay....

"Actually, an impedance peak is irrelevant. Sure, you can't get as much power into the driver, but it's an effect of resonance - you NEED less power at resonance to move the driver. Bass impedance peaks aren't a problem at all, regardless of what some experts in the audio community claim. Quick reality check - does the frequency response of your sub have a big dip at the impedance peak? If not, then the less power delivered at the peak isn't a problem."

Just the opposite is true of a vented design. That is, you have a peak(s) at resonance, not a dip. This is because the driver and / or vent is self-oscillating and is NOT responding to the input of the drive signal in a linear fashion. If a speaker produces a peak ( or a dip for that matter ) ANYWHERE in its' passband, it is responding in a non-linear manner. That is a distortion and it is directly related to a lack of linearity i.e. increased output with decreased drive.

"Note that with higher peaks, the damping factor of the system goes UP; damping factor is load impedance divided by amplifier output impedance; increase the load impedance, damping factor goes up. Meaning the amp can actually control the driver better. Less power (but that's irrelevant as described above), but the amp can control the driver even better."

Time for a reality check here. While damping factor goes up, all that this does is reduce the ability of the load to modulate the output of the amp. This is due to a greater deviation in impedance matching between the source and the load.

Since the driver should only respond to input fed into it, and the amp can't load into the driver efficiently at resonance due to what is effectively a high vswr, performance is reduced and signal reflections are increased. At this point, the driver / vent itself is contributing its' own "sonic signature" / non-linear distortions to the signal path. After all, you're not putting in as much power at resonance due to an increased impedance mismatch, so why should you get equal or greater signal out? This is not to mention the phase shifts that take place at or near resonance(s), so why would you want to introduce yet another resonance via a vent of some type to the system when you've already got one major resonance to deal with on a sealed system?

In effect, the amp is NOT controlling the driver at resonance, it can't load up and the driver is running away in self-oscillation. By maintaining a more linear impedance across the passband, the amplifier loads in a more linear manner, the driver responds in a more linear manner and power transfer, which is a direct measure of the amount of "control" or "influence" that the source has over the load, is improved.

For those that are not technically inclined, let's put it this way. If i push you two inches and you move two inches, that is linear input and output. As such, i have "control" over how much motion takes place, regardless of how far or hard i push. Now if i pushed you one inch and you moved four inches, i am no longer in control of the motion involved and the reaction to input is no longer linear. Such is the same with the amplifier / speaker interface at the point of uncontrolled resonance.

"As far as a sealed box versus ported box, it's easy to optimize both. I had trouble at first, but it's not a problem any more. Like one of my old college math profs used to say, it's the first few thousand integrals that are the hardest... For a seasoned professional, working out a bass alignment should be child's play. If it's not, then I'd take the other more difficult aspects (crossover design, or even more difficult driver design) with a block of salt."

From my point of view based on the examples that you've provided and the lack of common understanding displayed here, i would not want you designing a crossover for me.

"Passives are often quite superior to ports, especially in terms of linearity with SPL. Compression in a port is a serious issue; compresison in a PR is easy to control, as it's just linearity of suspension Cms and Rms over excursion. And both of those tend to be a lot more linear - and much wider in terms of effective pressure output - than a vent. Vents are considerably lower cost, though, and that is why they are so often used."

While i agree that a PR ( passive radiator ) can be more linear than a port in terms of dynamic compression and linearity, much of this has to do with the lack of understanding of ports and the geometries / configuration. Major advances have been made in terms of both the max flow of a port while also improving / maintaining ( or at least trying to ) velocity of said flow through the port at various spl's.

Having said that, both designs introduce their own drawbacks into the equation. As previously mentioned, a port is nothing more than a window to the world for the out of phase back-wave of the driver above the point of its' own resonance. At low spl's where port velocity is minimized, this becomes more of a problem. Since there is less flow through the port introduced by active motion of the woofer, there is less restriction for air to flow both in and out of the box. While, and as mentioned above port technology has improved in these regards, this can be reduced by using smaller ports to maintain velocity or "pressure" in the ports, the problem with limited port capacity comes into the picture. The end result in most cases is dynamic compression and port noise or "chuffing". In order to get around this, you have to use a bigger port capable of more flow, but then you are back to square one with reduced velocity. As such, one typically "picks and chooses" their poison in terms of max flow capacity and flow velocity and designs for a specific spl range / amount of air flow through the port. The end result of such a design is that bass linearity is optimized over a narrow range and sonics suffer both above and below that range. Hence the common occurance of "these speakers have no bass until you drive them" or "the bass sounds congested at higher volumes". Since a sealed box maintains constant pressure within the box and the amount of damping applied to woofer excursion remains constant, you don't have these problems with a sealed design.

Where one can drastically reduce these compromises with a port is to use a port that allows a greater amount of flow while maintaining port velocity at lower spl's / air flow. By radiusing the bends at the inlet and outlet of the port, air flow is neither restricted nor slowed down. Just like driving a car and going into a sharper turn, you must slow down quite noticeably in order to navigate such a sharp bend. Air does the same thing going into a "flat" port due to the lack of radius involved. By flaring the radius / entry way into the port, this is much the same as providing a wide turn with a bank to it. Just as you can "hammer down" into a turn of this sorts, so can the air entering or leaving the port. Flow velocity is maintained in a more linear manner due to less pressure drop / consistent speed of flow. While you still have the potential for reverse flow of the backwave from the out of phase woofer signal, this is drastically reduced. At the same time, port linearity is improved AND the sonics produced by such a system ( if properly designed and implimented ) remain much more consistent over a wider spl range. As such, if you are going to use a port, make sure that you use a "flared" or "radiused" port for best results. Now, onto the drawbacks of a PR ( passive radiator).

Rather than just using pressure through the port to push the air, PR's use a drone cone ( driver with no active motor structure ) to do much the same thing. There are some major differences here though even though both designs work by putting the out of phase signal from the back of the woofer to work.

One of the major differences between a port and PR, as Dan mentioned above, is that passive's do not suffer dynamic compression anywhere near as badly as most "conventional" port designs do. Since you can only flow so much air through a small hole / tube ( port ), it runs out of surface area and compresses / distorts the signal. By using a drone cone of equal or greater surface area* to the active woofer, the drone cone is capable of moving as much air ( or more ) as that of the woofer driving it. This in itself is a big improvement in terms of linearity and "functionality" when compared to a port, as PR's tend to produce more consistent sound regardless of spl's involved.

As one might guess, the PR also gets rid of a large portion ( but not all ) of the problems with a port in terms of out of band high frequency leakage. This is because a PR is sealed and no longer offers a direct "window to the world" like the open hole of a port. Having said that, a PR does contribute sound at frequencies other than that of its' main tuning due to what is called "sympathetic resonances". Just as other things in your room vibrate / resonate due to the moving of air, especially at higher spl's, so does the PR. This occurs to a much greater extent though as the air is not only acoustically coupled via the sound waves, it is coupled via the internal pressure of the speaker cabinet. As such, ANY movement of the woofer, regardless of frequency, is both acoustically and pneumatically coupled to the PR, causing it to respond in equivalent or near equivalent excursion. This is where we start getting into other problems i.e. due to "coupling" and the actual mass of the drone cone.

As mentioned, a PR uses a cone whereas a port uses the air. The port works as a Helmholtz resonator i.e. blowing across the top of a bottle produces a resonance. The only mass involved is that of the air. With a PR, you've got the mass of the cone to deal with. By altering the mass of the drone cone itself, you vary its' point of resonance or "maximum output due to sympathetic resonance". Since one can add a great deal of mass to such a cone i.e. FAR more than what air weighs, one can tune a PR for extreme bass extension and achieve better results than that of what you get with a port. But like anything else, added mass introduces another "variable" into the equation.

As mass is increased, it becomes harder to move. While this in itself can reduce the "excitability" of the drone cone at higher frequencies and help to minimize "out of band contributions", it also takes longer to respond and recover from direct and more forceful input i.e. larger low frequency excursions. It is this "lack of response time" or "lag" that gives most PR's "greater apparent bottom end weight", but also poorer transient response. Remember, the port only has the weight of the air and the PR has the mass of the drone cone.

What happens here is that, as the active woofer moves forward, the pressure inside the box is altered and that pressure change causes the PR to "suck in". When the active woofer recoils on the return excursion, it changes the pressure inside the box again, which causes the PR to push out. In effect, the PR is an "internal pressure regulator", BUT, it can only respond to a pressure change AFTER the active driver has already started in motion. Otherwise, their would be no internal pressure change in the box and since the PR has no active motor of its' own, it would sit still. As such, the drone cone is always trying to play "catch up" in terms of equalizing the internal "air spring" or "internal pressure" in the box. As a side note, sealed boxes maintain a consistent "air spring" in the box since the only thing that can "relieve internal pressure" is the cabinet itself flexing. Hence, the need for rigid cabinets and increased weight of a sealed design.

Now, not only have we got the "lag" associated with the "tag along" drone cone, we also have the mass of the drone cone to deal with. Not only does the passive mass of the drone cone have to be accelerated when standing still, it has to be slowed down once in motion. Since the drone cone doesn't have an active motor of it's own to do this, it is reliant on the motor of the active woofer to do this. The end result is that a drone cone that is tuned quite low with a lot of mass ends up suffering from increased lag ( heavier is harder to get moving ) and over-shoot ( greater mass is harder to stop moving ). Now think about this and how the passive works in terms of how it is coupled to the active woofer.

Just as the active woofer's motion changes the pressure inside the box, which results in the movement of the PR, what is to stop the greater mass of the PR, once set in motion, from "pressurizing" or "modulating" the active woofer? The answer is nothing. Nothing other than the active motor of the driven woofer. As such, you really need a "over-kill" motor structure on the active woofer if you want to keep it under control.

To put this in "easy to understand terms", look at it this way. You are driving a car that has a reasonable amount of power. You now add a "trailer" aka "U-Haul" to the back end, increasing the mass that the motor and suspension have to respond to. When starting from a dead stop, the "reasonable" motor now has more drag on it, has to work harder and can't accelerate as fast. After trying to do this a few times, you really wish you had more motor to pull the heavier "passive" load that you've got lugging behind you now. Due to the lack of motor, you just can't accelerate like you want or maintain the speed that you want. This is equivalent to using a "weak" woofer and trying to "band aid" it for extension and output with a PR. EVERYTHING suffers and nothing feels right.

In order to remedy this, we install a bigger motor with more power. Now you can more easily whip into action, which in turn pulls the passive load that is in turn responding in like fashion. Now we can literally "jerk" the passive load behind us on fast acceleration. Not only does the motor respond so much more effectively, but now we can really get going in terms of speed. BUT, and this is a whole new problem, we've now got the added mass of the passive load behind us in terms of momentum and it is time to stop. Not only is that added mass hard to stop, especially since we were able to pull it faster, it wants to keep going, causing our suspension to react in an uncontrolled fashion. If the passive load that is attached to us develops too much inertia, that energy will be transferred into our the driven vehicle, possibly even influencing how much control we have over the situation using the motor. Pretty scary to say the least, huh??? In effect, that is exactly what happens in a PR system every time the active woofer makes an excursion.

In plain terms, a PR is trying to achieve the extension of a port ( and even more ) with the tightness via "controlled internal pressure" of a sealed design. The difference here is that you only have the mass of the air to deal with using a port ( internal pressure & external pressure ) and in a sealed box ( internal pressure only ). With a PR design, you now have the added mass of the drone cone to factor into the equation as a variable and added mass ALWAYS equates to poorer transient response and less agility.

To sum things up, WHY would you want to make things more complicated, add mass, reduce transient response, increase the quantities of resonances & phase shifts / reduce power tranfer & output linearity, etc ???? ALL of these take place when trying to impliment a port or passive. There is NO real reason to do this other than if one is willing to sacrifice performance on the whole in order to gain performance in specific areas? As pointed out elsewhere in this thread and in others, vents offer increased quantity of bass at the expense of quality. Anybody that tells you something different has an agenda.

I don't make / market / work for anybody that offers any type of product associated with audio. My only "agenda" is to share & exchange ideas with those that share a common love of music / music reproduction. If another's ideas clash with those of known fact and common sense, i'm going to do my best to both refute those points AND try to learn from them at the same time. That is, IF they have something new to teach. As i've said before, with a little common sense and the help of technology, you can go a long way in audio without spending a lot of money. Most ideas that are contradictory to common sense / common knowledge are promoted in the attempt to get into your wallet or anybody else that will allow them to. Sean
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*I'm not going to go into specifics here, but for optimal performance, the drone cone selected should be of measurably larger diameter than the active driver being used to excite it. In effect, one would use a 10" drone with a 6.5" active woofer or a 15" drone with a 12" active woofer. If using two active 6.5's, you would use a 12" prassive or two active 12's would use a passive 18", etc... Nothing is set in stone here, but you get the idea.

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