Passive radiator vs. ports

[Bonzo]

I was curious how many speaker companies, besides Klipsch and JBL, have used or still use passive radiators? The concept, to me anyway, seems rather complicated and unnecessary. I have had three speakers with radiators, Forte' I's and II's along with JBL L-222 Disco's. I had the same issues with all three, PLACEMENT. No matter how or what I tried, I just couldn't put any of them in the right place for decent bass response. The JBL's were a bit easier maybe due to the passive being in the front of the cabinet and the 14" woofer thumps pretty good. I could never get either Forte right. The K-stacks I put in the II's helped a lot and seem to work great for the new owner but not for me. Must be a combination of the room, passives??? I just got a pair of Chorus I's, ported in the front, BOOM, BASS! May be an unfair due to the 15" woofers but it semms they sound good no matter where I put them. Anyone done an A/B with Chorus I's and II's. I would be curious on the results.

Mark

[boom3]

A 'passive radiator" (drone cone) allows a lower tuning for the same cabinet volume versus an equivalant port, whose length might be too long for an acceptable box. A PR also provides a closure for the air in the box, so there is always a definite volume of air to control the woofer below system and driver resonance. A port does not offer this below Fb. A PR also prevents out-of-band (i.e.) midrange signals from the back of the woofer from getting out of the box.

With all these advantages, I don't like PRs for most applications. If you optimize the overall system, you have adequate output down to the Fb and a rapid rolloff below that, with control of the driver down a point where the active electronics should be rolling off the infrasonic signals anyway. PR systems, by comparison to a correctly implemented port with a adequate box volume, (think Cornwalls) sound 'mushy'. If the PR is mounted on the back, you can have exaggerated bass from loading one part of the bass spectrum differently than another (i.e the PR is closer to the wall/floor boundary or a corner than the front facing woofer).

[DrWho]

As far as there being a "closure for the air in the box" - this is not the case. In fact, cone excursion for the active driver is more uncontrolled below the tuning point than that of a port. It has to do with the fact that the PR is a more linear system...which is my next point. Ports suffer from port noise, port resonance and port compression. Sure, port noise can be accounted for in most situations, but the compression is still a rather large concern - even at low SPL's. It has to do with the fact that the air in the port isn't all moving in unison - effectively changing the mass of the system and lowering the tuning point. It also gets a bit more complicated when you take into account that music has a large dynamic range, so the effective tuning point isn't a linear function of SPL either (due to "mass hanging around" after a louder note). 1dB swings here and there aren't huge, but they are audible...you don't have any of these issues with PR's - not to mention a better impulse response (higher damping).

There are only three downsides to passive radiators - they cost more, they are excursion limited, and they aren't as versatile (you can't have a down-firing PR system and tuning them is a bit trickier). There are also some low SPL issues with a higher resistance to motion at zero excursion, but these are far smaller in magnitude compared to ports.

As far as rear-firing versus front-firing - the wavelengths in question are so long that a 1 foot difference in propogation path isn't going to be audible. Also, the wavelengths in question are much larger than the diameter of the driver so you effectively obtain a 360 degree dispersion pattern...There will of course be some minor differences, but it is far more a function of the room than the speaker itself. Just to demonstrate the point, put a rear firing PR/port system outdoors where there are no boundaries and then play some frequency sweeps...you will find that the frequency range handled by the PR/port isn't quieter than the active driver.

So why do you hear such a difference between front and rear firing in your setup? I have to assume that you have merely shifted your room modes to a different set of frequencies that don't affect your source material as much. If the new speaker sounds better, then great - all the more power to ya. It will only be a matter of time until you start noticing the other frequencies that the problem has been shifted to [ The better tool for improving your situation would be to invest in some acoustical treatment - though I can understand why most people don't go this route.

Anyways, I don't profess to be an expert on passive radiator systems, but I do know that Klipsch has spent an insane amount of time researching them. The research is intellectual property of Klipsch so I don't get to learn anything more than all the limitations of ports that the PR's overcome. Those much smarter than me will claim that PR's are without a doubt superior in every way. But the law of diminishing returns came into play with the Forte/Chorus lineups and that's why Klipsch has gone back to ports on their floorstanders. The market would rather have cheaper and convenience than better performance. Though you will find that PR's start to become much more convenient at tuning points an octave lower. Thus why you have the very low distortion RSW-12 and RT-12d - that should both outperm the ported KW's.

[D-MAN]

Boom3, I cannot agree with all of yer post...

However, it seems that you are talking about specific speaker (sizes at least) where there is some arbitrary limit on the length of the port or duct that can be employed for some reason. Both ports and ducts can be turned using radius turns inside the cabinet, sort of like folding a horn. However, in alot of manufactured speakers, this is deemed to be too much time and trouble.

In a real-world situation, the cabinet can be sized for virtually any low resonant frequency that the driver is capable of supporting, and the port therein can be of the size required...

The main difference between a passive radiator (or drone cone) is that it is a WIDE-BANDPASS frequency device, where a port of any type (except perhaps the exponential slot) is a very narrow bandpass device by comparison.

Each has its uses. One is not necessarily better than the other, it depends on what issue is being addressed by the speaker designer.

DM

[Bonzo]

Thanks for the insightful information gentleman. It sounds like more science than sound. I have heard the term "port noise" before and don't know if I would know it if I heard it. The Chorus I's have large ports so I "***-u-me" there should be more noise from them? During casual listening however, I can't detect any anomalous sounds that draw attention to themselves. Maybe it's the 35 years of playing music or maybe it's not as prevalent in my speakers, or maybe, it's not there? In any case, I guess my point is the low end seems to be tighter and more coherent with the Chorus' than with the other speakers I mentioned.

Oh, hey Doc, do the cones of the woofer and passive move in unison or is it a push/pull action? I have always been curious about that.

Mark

[DrWho]

For the record....PR's are an ever so slightly narrower bandpass than a

port (18dB/octave slopes versus 12dB/octave). They are virtually

identical as far as audibility goes though.

Port noise will sound like air blowing through the port and is more a

function of high SPL listening only. It's a chuffing wind sound - the

same sound you make when blowing out a candle. When you can hear it,

you will also be able to feel the air rushing by your hand if you put

it up to the port mouth.

As far as bent ports go....they even further sacrifice performance.

Port noise and compressions go up exponentially. And it is for this

reason that manufacturers do not go with them. Yes, I know the KW-120's

are using a curved port, but it was a design sacrifice to make them

capable of flush-mounting - but the 12dB of baffle gain at the tuning

point of the system reduces the air velocity by a factor of 16 so it's

not a huge concern.

The motion of the PR's and the active driver aren't really either

"push/pull" or "in unison." Think about a slinky that you are dangling

out in front of you with a mass on the bottom. As you move your hand up

and down, the mass at the bottom has a delayed response. Now connect

another slinky with a mass on the bottom so that you have

slinky-mass-slinky-mass in a row. As you move your hand up and down,

the entire system behaves in a very complicated matter. This is the

same thing that is occuring inside the speaker cabinet. The driver is

the hand moving back and forth. The air behind the driver that is

trapped by the enclosure is the first spring. This spring is then

interacting with the spring of the PR/port. In the ideal world, the

active driver is not moving at all at the tuning frequency and the

sound coming from the PR/port is 180 degrees out of phase. If you look

at a phase plot for a vented system you will notice that the phase is

gradually increasing. This is both a product of the vent and the

natural behavior of the driver (due to the size of the wavelength

versus the size of the driver). So at the tuning point the PR/port is

really only shifting the phase of the system around 70 degrees.

I hope that made sense...just know that everything is behaving like a

spring and you've got lag in the system from it. Thig lag can be

measured and is referred to as the "group delay"

[D-MAN]

"PR's are an ever so slightly narrower bandpass than a port (18dB/octave slopes versus 12dB/octave). "

Argue with Harry F. Olson, not me. "Acoustical Engineering". http://www.audioxpress.com/bksprods/books/bkpa1.htm

DM

[boom3]

As far as there being a "closure for the air in the box" - this is not the case. In fact, cone excursion for the active driver is more uncontrolled below the tuning point than that of a port. It has to do with the fact that the PR is a more linear system...which is my next point.

I don't see the connection here. More linear, yet less controlled? Are you trying to say the active/box/PR system has higher mass? It should since the point is that tuning can be lower

Ports suffer from port noise, port resonance and port compression. Sure, port noise can be accounted for in most situations, but the compression is still a rather large concern - even at low SPL's. It has to do with the fact that the air in the port isn't all moving in unison - effectively changing the mass of the system and lowering the tuning point. It also gets a bit more complicated when you take into account that music has a large dynamic range, so the effective tuning point isn't a linear function of SPL either (due to "mass hanging around" after a louder note). 1dB swings here and there aren't huge, but they are audible...you don't have any of these issues with PR's - not to mention a better impulse response (higher damping).

You just agreed with my first point about driver control from "closure for air in the box"...maybe my phrasing was awkward, but still valid. Paul agreed when he said that the Rebel and other "horn loaded ported speakers" lost control sooner than horns with back chambers to provide a load when the horn and driver suspension loads fell to zero.

There are only three downsides to passive radiators - they cost more, they are excursion limited, and they aren't as versatile (you can't have a down-firing PR system and tuning them is a bit trickier). There are also some low SPL issues with a higher resistance to motion at zero excursion, but these are far smaller in magnitude compared to ports.

As far as rear-firing versus front-firing - the wavelengths in question are so long that a 1 foot difference in propogation path isn't going to be audible. Also, the wavelengths in question are much larger than the diameter of the driver so you effectively obtain a 360 degree dispersion pattern...There will of course be some minor differences, but it is far more a function of the room than the speaker itself. Just to demonstrate the point, put a rear firing PR/port system outdoors where there are no boundaries and then play some frequency sweeps...you will find that the frequency range handled by the PR/port isn't quieter than the active driver.

Have you actually done this? It's well known that the closer a driver (or PR in this case) is to two room boundaries, the more the room's eigentones will be excited. Corner placement excites all possible eigentones, which is why some folks sniff at corner placement.

So why do you hear such a difference between front and rear firing in your setup? I have to assume that you have merely shifted your room modes to a different set of frequencies that don't affect your source material as much.

Also confirms my statement about rear vs. front PRs

If the new speaker sounds better, then great - all the more power to ya. It will only be a matter of time until you start noticing the other frequencies that the problem has been shifted to [ The better tool for improving your situation would be to invest in some acoustical treatment - though I can understand why most people don't go this route.

Anyways, I don't profess to be an expert on passive radiator systems, but I do know that Klipsch has spent an insane amount of time researching them. The research is intellectual property of Klipsch so I don't get to learn anything more than all the limitations of ports that the PR's overcome. Those much smarter than me will claim that PR's are without a doubt superior in every way. But the law of diminishing returns came into play with the Forte/Chorus lineups and that's why Klipsch has gone back to ports on their floorstanders. The market would rather have cheaper and convenience than better performance. Though you will find that PR's start to become much more convenient at tuning points an octave lower. Thus why you have the very low distortion RSW-12 and RT-12d - that should both outperm the ported KW's.

It's possible to "fold" a tubular port to get the right length, but a folded tube in any commercially viable box is going to be small in cross-section, which promotes port noise

Small box, wide bandwidth, lower efficency=consider PRs. Big box, wide bandwidth with carefully chosen Fb, high efficency=ports best choice. Personally, I would not trade my ported CW IIs for any PR speaker.

[D-MAN]

A radial turn on a ducted column does NOTHING to the so-called port noise, IF it is of a REASONABLE cross-section.

If you are assuming a too-small overall duct diameter, then yes, velocity goes up and you get noise, but again - that has NOTHING to do with radial turns in the column. It is NOT the same as a transmission conduit like a horn.

DM

[boom3]

A radial turn on a ducted column does NOTHING to the so-called port noise, IF it is of a REASONABLE cross-section.

If you are assuming a too-small overall duct diameter, then yes, velocity goes up and you get noise, but again - that has NOTHING to do with radial turns in the column. It is NOT the same as a transmission conduit like a horn.

DM

All I said was:

It's possible to "fold" a tubular port to get the right length, but a folded tube in any commercially viable box is going to be small in cross-section, which promotes port noise

and I think you're agreeing with that...

Your answer to a statement I didn't pose (about the effect of turns) I am not sure I agree with. Any medium flowing through a turn will exhibit different particle velocities at different parts of the turn. If you listen to water flowing through household plumbing, you will hear most noise at the elbows. Is this significant for a loudspeaker duct? I'm not sure. The folded ducts I've seen have all been one turn of 90 degrees, and I would imagine the added noise-if any-is swamped in the total system response, not to mention room effects.

[ranjith]

Assume you are talking to a beginner, why is it that bass in PR speakers are generally said to be "smoother" than say a ported speaker? What gives it that "smoothness"?

It took me a little while to find the right place to put my KG2s, the first few placements were sort of boomy. But right now the bass is punchy and tight, sweet sounding. In fact I like the sound of the right speaker thats closer to a corner, don't know how that relates to any scientific theory though. I heard a lot of different bookshelfs in different stores, most had smaller woofer than the 8" on the KG2s, but even the ones with 8" woofers didn't nearly have the bass and clarity of the KG2s. Why is that? Just to give you an idea, I heard a pair of PSB Alpha B, Polk RTi6, JBL E20, some Tannoy and a few more. Like my KG2s way better.

[D-MAN]

"Your answer to a statement I didn't pose (about the effect of turns) I am not sure I agree with. Any medium flowing through a turn will exhibit different particle velocities at different parts of the turn."

Ah! the proverbial answer to the question that nobody asked! Yes, indeed.

But nevertheless, the difference and the point of confusion is that the acoustical port is an virtual air plug EXCEPT at its resonant frequency when it becomes subject to movement of the air mass contained within it, back and forth, essentially like a piston.

With an appropriate duct diameter, there would be no audible port-noise resulting from a full radius turn inside the duct, providing the velocity does not exceed more than 5% of the speed of sound (per Small of T/S fame). Smaller velocities can be produced for a given displacement by using a larger diameter duct, etc., and that is what I mean by the terms "appropriate duct diameter". The full radius turn is inconsequential as far as causing an impedance to the air mass.

DM

[DrWho]

"PR's are an ever so slightly narrower bandpass than a port (18dB/octave slopes versus 12dB/octave). "

Argue with Harry F. Olson, not me. "Acoustical Engineering". http://www.audioxpress.com/bksprods/books/bkpa1.htm

Well what kind of research has he done on the subject? Even I could write a book... Everyone I talk to that has been researching PR's and actually uses them supports the figures I posted. In fact, every website says it too...here, how bout a pic:

Btw, a curved port DOES increase turbulence and therefore also increases port noise. It also increases the amount of port compression because the change in direction is yet another acceleration. The difference is huge btw...I've got a bunch of promedia subwoofer drivers and a few stock cabinets which have a curved port in them. To test the difference I built a temporary enclosure keeping the port and cabinet volume all the same, but converting to a straight instead of curved port. I would say a good 10dB reduction in port noise was accomplished - I didn't even bother measuring because my only goal was to see if it reconstructing the cabinet was worthwhile and I got the results I needed. This is something I talked to Deon about too....I suppose I could always post the transcripts of my conversations, but he doesn't like his name to be dropped on the forums.

Can you make the curved port even bigger to accomodate? Probably, but you can't undo the extra port compression introduced. It's the non-linear port behavior that you want to avoid. What does it sound like? Like somebody wiggling the tone control back and forth. The louder it gets, the more wiggle that occurs.

[D-MAN]

Perhaps you need to get out more...

Harry F. Olsen is one of the giants of acoustical research.

His book is one of the virtual "bibles" of acoustics along with Berenek's "Acoustics". I suppose you have never heard of him, either.

The frequency repsonse curve and discussion is on page 162-163 of Olson's book. Those who ignore the past are doomed to repeat it.

As for the physics involved, please explain why a baffle mounted passive radiator would have less available bandwidth than a port, please.

The only reason that I can feature is that the passive in question has mass and/or suspension characteristics associated with it, i.e., IT WAS SPECIFICALLY CHOSEN for its narrow bandpass characteristics!

A "correctly" chosen drone cone could be employed to produce a wider frequency bandpass than the curve posted above, even though it would exhibit an certain amount of loss, it still could outperform the inherent narrow bandpass of a port, and that is an undeniable fact of physics.

DM

[jacksonbart]

I prefer when people are passive as opposed to ports. When it comes to speakers it really depends how each one sounds in your particular room.

[Ray Garrison]

There is at least one advantage to a passive radiator vs port that I don't think anyone mentioned... you can "tune" the bass response of a speaker with a PR by adding / removing mass from the cone, typically by using small sections of rope caulk. VMPS did this (think they still do) with all of their larger systems. If the speaker and room interacted poorly in the bass, you could experiment with adding and removing bits of the caulk from the drome cone and, by altering the bass response of the system, possibly improve the sound in the deep bass. I used to have a pair of (gargantuan) VMPS SuperTower II A/R se speakers, and when I moved from one house to another, the ability to tune the response was quite helpful.

This is one of their current larger speakers, with 4 (four) 30 CM (roughly 12") active woofers and a 30 CM mass loaded passive resonator.

[DrWho]

I know of Olsen, but what research is he doing with PR's? And then why does he contradict other research (like from dB and TN)? In this case I will defer to the people that I know have researched the design specifically...I think the biggest reason PR's are a narrower bandwidth is simpy because they have more mass than that of air making them harder to accelerate. Another issue with PR's is that you have to keep their fundamental resonate frequency below the pass band that they will be operating in (or you get the little peak like you see in that one graph).

re: Ray....a lot of the current PR designs have screws coming off the back where the VC normally is so you can attach weights to change the tuning as well.

[D-MAN]

Well, since he's been dead since '82, I doubt that he's been doing any research lately.

What you have shown in your posted response curve is in direct conflict with published research, so I have to doubt the circumstances of the frequency test you chose to form your opinion, and the particular configuration being tested.

Drawing conclusions from particular configurations means that you need a wider variety of tests to draw the conclusion from! Like I said, please explain the physics involved.

As a general rule, passives are capable of wider bandpass than ANY port, and they inccur less losses due to frictional effects. They are not subject to port-noise, of course. The elements that require consideration is the amount of moving mass, its suspension characteristics and the amount of air being moved by the drone cone.

DM

[DrWho]

In direct conflict? To what? That one lone book written by a dude who probably didn't even specifically research the topic himself? I shouldn't make presuppositions like this without first reading the book, but from my perspective that book (if you even read it correctly) is in direct opposition to all sorts of measurements and more current research. No need for an ego war here...and really it is a very stupid discussion because there is no audible difference (as far as raw linear low signal level frequency response is concerned).

Anyways, I do have a grad level acoustics text book right here with all the equations for determining resonance of systems...the mass of a PR is many orders of magnitude greater than that of the "trapped air in the port" and is therefore harder to accelerate and decelerate...the end result requiring a suspension that slightly narrows the bandwidth in order to maintain the same tuning. All the variables are interrelated, you don't have control over just one part of the "response." Sure, there is a case where you can move the resonance of the PR system (not to be confused with the 'box+port/pr resonance') to be just lower than the final tuning of the final system so as to widen the response with a dual resonance, but doing so entirely destroys the time-domain response (you'll get a very long and very audible resonance - probably on the order of half a second). You get similar issues when your box volume and even the active driver all have resonances at similar frequencies. But such a discussion is not an apples to apples comparison. In this case there is an audible difference, but for the worse. And we haven't even touched the concept of the delayed transfer of energy between the two medium transitions - so you've got two complex impedances and need to take the cms of the PR into account as well....

But don't take it from me - textbooks are available to anyone and I've even asked my prof about it....I've probably ruined some of the concepts by trying to bring everything down to "normal terminology" so this is my disclaimer not to take everything I say literally []

But ya, I would love to see some real life measurements that contradict the theory....and then I can go to my acoustics prof and have him provide some explanations...

[D-MAN]

Oh, I get it - it's too tedious to explain, huh?

Your posts are beginning to smell of self-important hyperbole.

Here's some physics:

1) there is no limiting of frequency for the passive mounted on a baffle - it is NOT a frequency sensitive device except when limited by its own MASS and suspension qualities. That goes without saying.

2) The resonance of the back chamber has no effect on the passive drone cone device. The passive cone on a baffle mount is not "tuned" for a specific frequency as a port would be. It is simply a phase inverting mechanism and is physically coupled by and to the volume of air in the back chamber which undergoes compression and rarification as the active cone moves back and forth.

Please explain why you think the passive drone should be limited in frequency response beyond its suspension qualities, diameter, and moving mass? I'm very suspicious that you don't know what you are talking about, and are unwilling to go to the effort to look something up to find out for yourself.

DM