Problem with passive radiators,.. possible solution?

[Nixter]

Passive Radiators (PR from here on), do not have as much "punch" compared to a properly designed ported system. Most, but not all of us, more or less agree with this assessment. What got me thinking about this topic was the feeling that the cheaper flat disk type PR with a conventional surround only, sounds "dead" compared to a regular cone with a stiff spider assembly type PR. If this is true, then I think that is a clue as to what is causing the difference in sound. My guess is that ALL current PR designs have too much damping. Adding a spider suspension around where the VC normally would be on a PR adds a "spring" effect to the system, maybe that is causing the PR to resonate more than one without a spider. So I propose someone, or some of us, collaborate on experimenting with a PR purposely designed to resonate, (or ring), at a specific frequency, not a normal fs, but a highly "springy" very "under damped" configuration. Audio designers usually design speakers to be well behaved and well damped to not color the sound too much, but a vented port may have a less damped nature than an equivalent PR system. To replicate a vented systems sound quality and efficiency, decreasing the PRs damping should change the sound quality audibly, whether or not it will actually sound "better" is to be determined through experimentation, trial and error. Replacing the spider with either a leaf spring or coil spring of the proper weight and resonant frequency would make the PR actually "ring" at specific frequencies. It could turn out to sound terrible, but it also could be applicable for adding a certain type of punch to a bass system using a such a highly resonant PR. The mass of the spring system may cause a damping effect of its own that will neutralize the wanted effect, but using a lighter spring should counter that. It hasn't been tried yet, but it could be worth looking into, I'd like to have it kept open sourced and available to speaker builders and companies alike for free if it has any benefits or value.

I need help in determining the best way to measure a "spring loaded" PRs actual fs. I have modified a PR with a single fiberglass rod as the spring, but I am stuck trying to measure the fs of a PR. I can upload an image of what I have done so far if there is any interest in seeing it.

[bobgpsr]

Re: Problem with passive radiators,.. possible solution?

I have never heard a respected authority say anything about PR's lacking "punch". Do you mean a midbass peak? Not normally desired IMHO - I'd rather have it flat when combined with room response. I will agree that PR's have more group delay.

The PR's that I have used from Acoustic Elegance have a heavy cardboard tube going from the radiating disk to a normal looking spider.

[jsr]

Re: Problem with passive radiators,.. possible solution?

Maybe Nixter meant poor impulse response. Low tuned PRs with heavy weight respond slower and thus fast bass notes may sound blurred.

[bobgpsr]

Re: Problem with passive radiators,.. possible solution?

Maybe Nixter meant poor impulse response. Low tuned PRs with heavy weight respond slower and thus fast bass notes may sound blurred.

Possibly for midbass woofer applications but subwoofers with Fb below 20 Hz do not have anything to do with "fast bass". Not in their passband. That is why the LLT (Large Low Tuned) movement got going to get the Fb of ported systems below audibility -- but you definitely can still feel the very low freq bass. Pipe organ music and modern day action movies. My favorite is the opening battle scene from Master & Commander.

[fastbike1]

Re: Problem with passive radiators,.. possible solution?

Maybe Nixster should listen to some of the better PRs when correctly tuned and applied.

It's also rash to come on with the "most but not all of us" broad assessment, especially when new on a board. Just asking to be thrashed.

Maybe Nixter meant poor impulse response. Low tuned PRs with heavy weight respond slower and thus fast bass notes may sound blurred.

[Pete Schumacher]

Re: Problem with passive radiators,.. possible solution?

bobgpsr posted the best example I can think of for rebutting the naive OP.

Those AE PRs are very loosely suspended, have a very low natural resonance (single digits) and long throw. They're as close to an ideal for a PR as is currently available. Their parameters make them behave more like a port than most other PRs featuring higher Fs and stiffer suspensions.

There is no such thing as "fast bass." There's coloration due to excessive harmonic distortion, and non linear response. But by definition, bass is slow. How the harmonics of a bass instrument are handled by the woofer and mid are what will determine if the sound is "fast" or "slow".

[charlielaub]

Re: Problem with passive radiators,.. possible solution?

Those AE PRs are very loosely suspended, have a very low natural resonance (single digits) and long throw. They're as close to an ideal for a PR as is currently available. Their parameters make them behave more like a port than most other PRs featuring higher Fs and stiffer suspensions.

I will enthusiastically +1 your comment.

The AE PRs are a combination of all the right stuff for great performing PR systems. Just like a vented system, if the designer doesn't know what they are doing, doesn't measure anything, and expects high-fidelity response the results may be much less than what is desired.

-Charlie

[Nixter]

Re: Problem with passive radiators,.. possible solution?

Thanks for you input, I was referring to PRs output vs vent output in the same scenario, if you could mount both in the same cabinet and switch back and forth, I think that the port will have more spl in the range being enhanced by the port or PR. My theory is that the difference might be due to the difference in resonant behavior. I am saying you "may" get a higher spl in the PRs fs range than you could normally get with a well damped PR. By damping in a PR, I mean, how many cycles will a naked unmounted PR get with a singe impulse before settling down to zero? I do not mean fast or impulse, I am after duplicating the spl boost found in a vented application, but with a PR. You know how the bass response can be extended with a properly tuned port? That is what I am talking about using the PR for, say a woofer has fs of 40, and you want the bass to be extended a bit at that point, by tuning the fs of the PR as you would a regular port, you should be able to get a sizable peek at the PR's resonant frequency, especially with a ringing type resonance that a spring would "maybe" provide.

You can tune a PR low and not worry about it, but that's not what I am after, I want to use it exactly like a port, tuned at a point that will extend a woofers response to a specific point. by making a PR that will resonate in a ringing mode, the frequency at that spot should be noticeably more active, and probably much louder at that specific frequency.
No one is understanding what I am saying it seems. I have friends who can help me on this project, I thought those here would be more open to accept new ideas. I'll let you know at a later date if I get any positive results on this idea.This site has gotten a lot less friendly since I joined in 2008. New Ideas? Don't want then here! Go away, we aren't interested! OK bye!

Pete Schumacher, what did I do to you that I deserve to be called names? You consider me a "naive" Original Poster? (wow!)
You like to make enemies without knowing anything about them? Weird, OK you don't like me, then please forgive me if I go the other direction and say, I think we should respect our opinions and if we disagree we should do so nicely, without strange and unfriendly exchanges that try to belittle others serious ideas and communications. I am far from being naive. If you don't have anything nice to say, don't say anything,.. I always say.

[Jay1]

Re: Problem with passive radiators,.. possible solution?

You simply need a PR with adjustable mass tuning. Another thing you can do is add an additional PR to the same volume which will raise the tune.

http://www.creativesound.ca/pdf/APRG.pdf

[Pete Schumacher]

Re: Problem with passive radiators,.. possible solution?

Passive Radiators (PR from here on), do not have as much "punch" compared to a properly designed ported system. Most, but not all of us, more or less agree with this assessment.

Pete Schumacher, what did I do to you that I deserve to be called names? You consider me a "naive" Original Poster? (wow!)
You like to make enemies without knowing anything about them? Weird, OK you don't like me, then please forgive me if I go the other direction and say, I think we should respect our opinions and if we disagree we should do so nicely, without strange and unfriendly exchanges that try to belittle others serious ideas and communications. I am far from being naive. If you don't have anything nice to say, don't say anything,.. I always say.

Lighten up Nixter . . .

Your opening line, quoted above, was a grand guffaw moment for me, and came across as rather condescending and haughty, hence, my snarky response to your rather bogus assertion. We don't all mostly agree, and some have even explained why.

PRs, when properly implemented, don't do worse at bass than vented systems. Not all PRs are made equal, and a lesser PR, with a stiffer suspension and higher Fs, can cause what you assert, a drooping response compared to the same enclosure with a vent tuned to the same frequency.

If the PR has sufficiently low Fs, and large Vas, it will act more like an ideal port when tuning an enclosure to frequencies well above its natural resonance. You can see this effect for yourself using a program like WinISD. Heavy, loosely suspended PRs, like the one's pictured above, perform marvelously, and in many applications, will provide better bass than you could get with a ported box, especially when the boxes are small, and need to be tuned low.

Truce?

[Nixter]

Re: Problem with passive radiators,.. possible solution?

You simply need a PR with adjustable mass tuning. Another thing you can do is add an additional PR to the same volume which will raise the tune.

http://www.creativesound.ca/pdf/APRG.pdf

Hey finally a friendly reply, thanks Jay!

The PRs I have are from Parts Express, they do have a threaded hole in the part where the Voice Coil would go for adding mass, I know all about that, thanks for your help.

What I am talking about is reducing a PRs damping, all PRs I've seen so far are purposely designed to not resonate very much on their own, they have very lossy, soft surrounds, same as most woofers. What I'm trying to get across is what would happen if a PR acted more like a coil spring, you stretch it out a little and let one end go, and the spring vibrates on its own for several seconds. Most woofers or Passive Radiators on the other hand behave more like it they have a shock absorber connected to them, tap on the unmounted cone to get it vibrating and its motions will damp out, or die down within a second or two at most. That works fine for imitating an infinite baffle, or a transmission line box design where the goal is to have a neutral bass sound, you are trying to get the sound waves from the woofers front surface while damping out or attenuating the back wave. The same woofer in a transmission line compared to a properly designed ported box will sound VERY different. The back wave will excite the air in the port at a specific frequency, depending on its diameter and length, extending the sound levels out to a lower frequency than would happen with a sealed infinite baffle type design. I think I am correct when I say that most PRs have a lower bass boost at the tuning frequency frequency than an equivalent properly designed port. This is the crux of my idea, it is the highly damped nature of a PR that causes the lower boost or spl at system fb,(Enclosure resonance) even when tuned to the same equivalent frequency of a port. I think that there is something worth exploring here that has never been tried before in the speaker building world, something that would or could result in a very different sounding bass cabinet design, maybe an audiophile would not want this, but perhaps it would be good for something like an automotive design where people just want more bass output.

By adding or reducing the mass we are adjusting the fs, (Driver free air resonance), of the PR, but we are still not changing the damping of the cone and suspension, there will be nothing different sounding compared to similar setups. I want to have a PR that vibrates wildly at the Fb (Enclosure resonance), contributing a massive boost (maybe), of bass just below the woofers Fs (Driver free air resonance).

If I can get someone here to understand what I am trying to say, I think I'll get some agreement from a technically inclined speaker builder. If properly understood, I think this will be an exciting new idea worth exploring. I can do this on my own, I was hoping that this would generate some interest within the speaker building community.

[Nixter]

Re: Problem with passive radiators,.. possible solution?

Lighten up Nixter . . .
Truce?

Of course, I just want to explore the possibility that there is some aspect of PR design that can be changed to get a different resonant behavior and hence a different sound from it, my point is that no one is exploring the idea of using a highly undamped Passive Radiator. One that will vibrate in a ringing mode at Fc (System resonance) Maybe it is a bad idea, that is fine, but I want to build a prototype PR and do a comparison between a traditional PR and a "tweaked" version. I have a S&L Woofer Tester Pro to run the tests, I just want some fellow speaker builder types to talk to as I go forward, you know how a second opinion is always, (well usually), helpful, that's all I seek.

[Jeff B.]

Re: Problem with passive radiators,.. possible solution?

Hey finally a friendly reply, thanks Jay!

The PRs I have are from Parts Express, they do have a threaded hole in the part where the Voice Coil would go for adding mass, I know all about that, thanks for your help.

What I am talking about is reducing a PRs damping, all PRs I've seen so far are purposely designed to not resonate very much on their own, they have very lossy, soft surrounds, same as most woofers. What I'm trying to get across is what would happen if a PR acted more like a coil spring, you stretch it out a little and let one end go, and the spring vibrates on its own for several seconds. Most woofers or Passive Radiators on the other hand behave more like it they have a shock absorber connected to them, tap on the unmounted cone to get it vibrating and its motions will damp out, or die down within a second or two at most. That works fine for imitating an infinite baffle, or a transmission line box design where the goal is to have a neutral bass sound, you are trying to get the sound waves from the woofers front surface while damping out or attenuating the back wave. The same woofer in a transmission line compared to a properly designed ported box will sound VERY different. The back wave will excite the air in the port at a specific frequency, depending on its diameter and length, extending the sound levels out to a lower frequency than would happen with a sealed infinite baffle type design. I think I am correct when I say that most PRs have a lower bass boost at the tuning frequency frequency than an equivalent properly designed port. This is the crux of my idea, it is the highly damped nature of a PR that causes the lower boost or spl at system fb,(Enclosure resonance) even when tuned to the same equivalent frequency of a port. I think that there is something worth exploring here that has never been tried before in the speaker building world, something that would or could result in a very different sounding bass cabinet design, maybe an audiophile would not want this, but perhaps it would be good for something like an automotive design where people just want more bass output.

By adding or reducing the mass we are adjusting the fs, (Driver free air resonance), of the PR, but we are still not changing the damping of the cone and suspension, there will be nothing different sounding compared to similar setups. I want to have a PR that vibrates wildly at the Fb (Enclosure resonance), contributing a massive boost (maybe), of bass just below the woofers Fs (Driver free air resonance).

If I can get someone here to understand what I am trying to say, I think I'll get some agreement from a technically inclined speaker builder. If properly understood, I think this will be an exciting new idea worth exploring. I can do this on my own, I was hoping that this would generate some interest within the speaker building community.

I never set out or intended to become an authority on passive radiators, but after working with three different driver manufacturers on passive radiator design and implementing them in several commercial systems I feel I have a very good understanding of how they work.

I am not sure exactly where to begin here. Some of what you say is correct and indicates some understanding of the subject, but some of it is pretty far off-base and even completely incorrect, which makes me wonder if you are not understanding how a resonant box actually works. It is interesting that you use the spring analogy, because that is exactly the mechanical equivalent for a vent box and a passive radiator box. But your description goes off course.

The air inside a box has compliance, which is a spring effect, so it is modeled as spring. The mass of air inside a port behaves as a weight that has been attached to the spring. The weight of this mass lowers the resonance of the spring. So, in effect, by changing port sizes you are changing the tuning of the spring or the box itself. It is not the port that is resonating, it is the air inside the box. An ideal passive radiator is designed to work in exactly the same way - by attaching a mass to the spring inside the box. In fact, much of the math is the same too. If you had a 12" diameter port it would need to be extremely long, the mass of the air inside that huge port would end up becoming the same amount of mass you need to arrive at for your 12" passive radiator.

Now, it is true that PR's have losses, and they are not all created equal, as Pete has pointed out. Some do, in fact, behave as nearly ideal radiators. These losses are measureable and are defined by a lumped parameter called Qmp which represents the mechanical Q of the passive radiator. A radiator will also have an inherent Fs, referred as Fsp. The higher the value for Qmp the lower the losses in the mechanical system of the radiator. A passive radiator made out of a standard woofer cone and suspension will have a Qmp of around 7, which can be quite lossey. The Acoustic Elegance radiators have a Qmp of 52 or so, so losses are very low and its behavior is nearly ideal. However, what you said about adding mass to a radiator is incorrect. When you add mass to the radiator several things happen - you lower the tuning of the air compliance of the box itself, you lower the inherent Fsp of the radiator, and you raise the Qmp value as well - reducing the lossess. Both Fsp and Qmp are related due to their relationships to Mmp - moving mass.

Take a TC Sounds VMP-12 for example. It starts off with an Fsp of 28Hz and a Qmp of 7, but has very low Mmp (moving mass). Once you add the appropriate amount of weight to the PR to tune the box you will lower Fsp down close to 10Hz and raise Qmp into the high 20's or 30's. Adding mass does indeed change the damping as shown in the increased Qmp.

But most of all it is your original supposition that is incorrect - that passive radiators have less output at box resonance than a port. Although this is true when a port is ideal for the design, like when operating with higher than subwoofer Fb's, and in that case a port should be used and not a passive radiator. It is not true in instances that call for a passive radiator, and that is when the dimensions of the ideal port are too large for the box. Try tuning a 2cuft box to 18Hz with a port. You will end up with a port whose size creates port compression at higher SPL's, and the onset of this can be quite low, at only a few Watts of input power. This compression results in significantly reduced port SPL, higher air noise, and audible distortion. The passive radiator is then the best option. And when used its output (SPL) will far exceed that of the smaller port, as well as having very low distortion. Use a port when a port will work best, use a PR when it will.

This article I wrote gives an overview on the topic that may be of benefit:
http://www.salksound.com/wp/?p=144

You may want to investigate passive radiator design a little. You will find that some of what you are targeting - a very loose suspension for example - is already a part of the design of the good ones. You are on the right track with that part. But you need to know that the people who design drivers and passive radiators are pretty knowledgeable of how they work and what constitutes an ideal radiator. Trust me, these people know their business very well. I have worked with them, I know.

These PR's pictured below are made by Eminence for an OEM application. They are not available for use to purchase for DIY. However, they also represent nearly ideal radiators with an Fsp that is down around 4 -5 Hz, a very high Qmp, and a suspension that is so loose that they must be clamped down for shipping or storage so as to not destroy the extremely flexible suspension. Once mounted in a sealed box the air compliance of the box becomes part of their suspension, controlling their motion, and they become part of the mass of that air, changing its resonance. It is now a system that is tied together, and tuned to a specific Fb.





By the way, my box modeling program, available at the link below does passive radiator modeling. It may help to show the relationships I referred to above and mode easily see how it all works.

Jeff B.

[rone]

Re: Problem with passive radiators,.. possible solution?

One thing of note for the OP - there is no output in a PR system at the PR's resonant frequency, not sure if that has been covered as I only read the first couple replies.

[Jeff B.]

Re: Problem with passive radiators,.. possible solution?

One thing of note for the OP - there is no output in a PR system at the PR's resonant frequency, not sure if that has been covered as I only read the first couple replies.

I don't think that was covered, but yes, that's correct. At the PR's resonance the cone and PR are out of phase and the system's response sees a deep null at this frequency. This can cause some real problems in the deep bass if the Fsp is very high. Ideally you want it as low as possible.

Below Fsp the system will roll-off second order like a sealed system, which is an interesting characteristic too.