Question about passive radiators

[superbassman2000]

So in my quest to misunderstand everything amplification-wise, I looked up some speaker designs and i came across the passive radiator design. The only thing I could really find in the TB search is that the mesa walkabout cab is one, but that doesn't really answer my question.

Anyways, I have the understanding that the passive radiator basically takes the place of a port. But what happens if the I were to do something like take a cheap 210, wire it for a 110, and replace the other speaker with a passive radiator (or remove the magnet from the speaker and leave it unhooked, if i have that right)?

would it work any different? if so, how?

[Sundogue]

Originally Posted by superbassman2000

So in my quest to misunderstand everything amplification-wise, I looked up some speaker designs and i came across the passive radiator design. The only thing I could really find in the TB search is that the mesa walkabout cab is one, but that doesn't really answer my question.

Anyways, I have the understanding that the passive radiator basically takes the place of a port. But what happens if the I were to do something like take a cheap 210, wire it for a 110, and replace the other speaker with a passive radiator (or remove the magnet from the speaker and leave it unhooked, if i have that right)?

would it work any different? if so, how?

I went through the whole scenario of possibly using the passive radiator in a cab design. They make actual passive radiators for that purpose. They have stronger surrounds than regular speakers and they also have a way to "weight" the cone for tuning purposes. IMO, not worth the effort especially when the advancements in speaker technology have made more simple designs that work better, possible.

[jnewmark]

Originally Posted by Sundogue

I went through the whole scenario of possibly using the passive radiator in a cab design. They make actual passive radiators for that purpose. They have stronger surrounds than regular speakers and they also have a way to "weight" the cone for tuning purposes. IMO, not worth the effort especially when the advancements in speaker technology have made more simple designs that work better, possible.

You might have some Walkabout/Scout users, not quite agreeing with you.

[SPQR]

Well I'm not quite sure what you're looking for, but I can highlight some of the pros/cons for you. There are two very good things a PR can do: a) reduce vent noise to nothing b) work well in small enclosures.

a) So long as you can keep the PR operating in its normal range, it is very happy and wont give you any fuss. But if you mismatch the PR size or just plain overdrive it - you will get some nasty response back from it as it locks up or "rocks" side-to-side. This is why you see a lot of home theater designs use two 12" PRs to one 12" high-excursion subwoofer - they will want to displace a lot of air at the tuning frequency and you want as much linear excursion as possible.

The good thing is you dont have any whooshing like a small port gets when driven hard.


b) When you try to tune a small enclosure with a port you end up having to build either a long square port that wraps around the box or attempt to squeeze in rather long tube ports - PRs can be quite slim compared to fitting 4" ports into a <2 cu ft box.


So why would we not want to use PRs? Well, if our goal is to reproduce/create large amounts of bass we need a large box, so we should have plenty of room to fit the port area we need with a reasonable length. And cardboard or a little more lumber is often cheaper than a PR.

PR's make sense for subwoofers smaller than 12" in small enclosures- which really means they'd be best suited for small HT or car systems. Or possibly small portable cabs like the Mesa.

[Sundogue]

Originally Posted by jnewmark

You might have some Walkabout/Scout users, not quite agreeing with you.

IMO, not worth the effort especially when the advancements in speaker technology have made more simple designs that work better, possible...

Walkabouts/Scouts notwithstanding.

[pfschim]

Originally Posted by Sundogue

IMO, not worth the effort especially when the advancements in speaker technology have made more simple designs that work better, possible...

Walkabouts/Scouts notwithstanding.

Walkabout users everywhere applaud your timely correction

not sure I would want to tackle building one as a DIY, but I'm sure glad Mesa tackled the PR issues with the Walkabout.

[Sundogue]

Here's my "active" radiator cab. I just built a box around the downfiring speaker, and the speaker is loaded in the same cab/compartment as the front firing one...and it's hooked up just like the front firing one.

Not really a passive radiator, but it takes the idea of a downfiring speaker that is mounted in the same cab but with both speakers connected to the amp it puts out an enormous amount of upper bass and lo-mids (LOUD). I can also dial in quite a bit of low end for such a small box.

Pretty small for a 215 (with dedicated mids) at 25 X 22.5 X 17 and 38 lbs.

A passive radiator style cab will only work as intended if it is on the floor. Tilt it back or put it up on a stand and it loses most of it's ability to contribute to the sound. With mine, the downfiring speaker is in an open chamber and sounds the same regardless of orientation.

I post this just to show that there are alternatives to a passive radiator if one is going the DIY route. But hey, if you want to try and build a cab with a passive radiator, please do and post it for us to check out! I'm all for DIY and trying different things (or alterations to the same old).

[Crabby]

I f you have never actually tried a Scout cabinet in a big room, you would be impressed!

[superbassman2000]

is there a way to do a passive radiator design in WinISD? it seems to me that you could just set the port to the size of the passive driver, however, that doesn't sound right because while they do a similar action, don't they act in a different way?

[pfschim]

Originally Posted by Sundogue

A passive radiator style cab will only work as intended if it is on the floor. Tilt it back or put it up on a stand and it loses most of it's ability to contribute to the sound.

hmmm, I don't believe that this is a true statement, either in a acoustics/physics sense or in a hands on experience sense.

I use my Walkabout tilted up all the time and do not experience any change, or loss of performance. Of course, I do not push the WA's limits of performance at all, so that could be part of my experience.

I thought that with a passive radiator, which acts like another form of porting/tuning .... the passive element is weighted for a specific freq and does not require a surface to "fire" against. It also does not produce any sound per se, just provides air resistance enough to equal an equivalent target tuning freq. -and allows this to happen in a smaller box dimension than would be possible with std porting.

could be wrong, but I have read up on PR's before, and followed a few well informed posts on the topic here on TB.

[30.87hertz]

Originally Posted by superbassman2000

But what happens if the I were to do something like take a cheap 210, wire it for a 110, and replace the other speaker with a passive radiator (or remove the magnet from the speaker and leave it unhooked, if i have that right)?

would it work any different? if so, how?

You are taking about a sealed cabinet, right? What is your goal? Less sound output? Less power handling? Big, heavy one 10 cab the size of a 210? I don't think you can gain anything good in doing this.

Passive radiator systems are very sensitive to misaligned parameters; not something I would just try without T/S parameters and a real tunable passive radiator. There will be a big dip in the frequency response at the tuning frequency, because the PR will move out of phase with the active driver, so you would want the tuning frequency to be quite low.

[SPQR]

Originally Posted by Sundogue

IMO, not worth the effort especially when the advancements in speaker technology have made more simple designs that work better, possible...

Since a PR is a replacement for a port - what does "advanced speaker technology" have to do with the resonators' performance?

Originally Posted by superbassman2000

is there a way to do a passive radiator design in WinISD? it seems to me that you could just set the port to the size of the passive driver, however, that doesn't sound right because while they do a similar action, don't they act in a different way?

Yes, I believe WinISD Pro Alpha will model PR's. You choose it as a box type and then enter the PR parameters. PR's act in a different way in that you have to tune them below what you normally would for a port because they have a stiffer suspension than a port (which is nothing) and require more mass to achieve the equivalent tuning. But if you model them side-by-side you'll see the active driver excursion, impedance, and spl response can be made quite similar.

Originally Posted by pfschim

I thought that with a passive radiator, which acts like another form of porting/tuning .... the passive element is weighted for a specific freq and does not require a surface to "fire" against. It also does not produce any sound per se, just provides air resistance enough to equal an equivalent target tuning freq. -and allows this to happen in a smaller box dimension than would be possible with std porting.

Correct, a PR does not require a surface to fire against. But it does produce sound as it generates a lot of volume velocity at the tuning frequency, just as ports do.

The only reason you could get away with a smaller box is if the port volume was greater than the PR's volume you're replaceing it with.

Originally Posted by 30.87hertz

Passive radiator systems are very sensitive to misaligned parameters; not something I would just try without T/S parameters and a real tunable passive radiator. There will be a big dip in the frequency response at the tuning frequency, because the PR will move out of phase with the active driver, so you would want the tuning frequency to be quite low.

Vented cabinets are just as finicky about tuning - its kind of a draw.

And there will be a dip in the SPL reponse - but only of the active driver! The PR will be working so efficiently it will cause the active driver to move very little- but the PR will do all the work and generate more output (in phase as well). It works just as ports do and its the reason we use vented/PR boxes at all - it's essentially "free" energy we can get from the box.

[pfschim]

Originally Posted by SPQR

Correct, a PR does not require a surface to fire against. But it does produce sound as it generates a lot of volume velocity at the tuning frequency, just as ports do.

fair enough, far be it from me to argue with real knowledge about speaker design. I only know what I read here and elsewhere on the interwebs.

As far as the PR producing sound; it is my understanding that the PR tuning frequency is usually well below the 20hz range, so while it may technically produce sound, the sound it does produce is below the threshold of normal human hearing.

no?

[SPQR]

Originally Posted by pfschim

As far as the PR producing sound; it is my understanding that the PR tuning frequency is usually well below the 20hz range, so while it may technically produce sound, the sound it does produce is below the threshold of normal human hearing.

no?

It is deceptive when you look at the resonance frequency of a PR as you're only looking at part of the system. Just as when you place a speaker into a sealed box - also called an Acoustic Suspension (for good reason) - the cabinet air volume acts as a spring. This resorative force runs in parallel with the drivers' internal spring, the surround and spider. These components not only ensure the voice coil is kept concentric in the magnetic gap and stop the coil from permanently leaving the gap, they provide the inertia-dominated mass with its reactive counterpart in order to form a resonance. This is the same as how an inductor (an electrically inertia-dominated component) and a capacitor react to form a resonance.

So when we look at a PR in a box we have to consider the mass and stiffness of the PR and the stiffness of the cabinet volume. Just as placing two capacitors in parallel will add up, the total stiffness of the system now reacts with the mass of the PR. So while the PR's resonance is below 20 Hz, by including the stiffness of the box the systems resonance will be placed where we wish to use it.

[winston]

In a design with a PR (or powered driver) mounted on a different side of the cabinet, is the output of that driver 90 degrees out of phase with the rest, or am I getting it wrong?

[Rick Auricchio]

The phase difference will depend on the distance between the drivers, not the way they're aimed. It's a function of the time for the sound to travel from the farther driver to the nearer one, with respect to the wavelength of the sound.

If we were talking about tweeters, it would matter because highs have a short wavelength.

But for low mids and bass, the wavelengths are so long that you'd need the drivers several feet apart to be an issue.

[pfschim]

Originally Posted by SPQR

It is deceptive when you look at the resonance frequency of a PR as you're only looking at part of the system. Just as when you place a speaker into a sealed box - also called an Acoustic Suspension (for good reason) - the cabinet air volume acts as a spring. This resorative force runs in parallel with the drivers' internal spring, the surround and spider. These components not only ensure the voice coil is kept concentric in the magnetic gap and stop the coil from permanently leaving the gap, they provide the inertia-dominated mass with its reactive counterpart in order to form a resonance. This is the same as how an inductor (an electrically inertia-dominated component) and a capacitor react to form a resonance.

So when we look at a PR in a box we have to consider the mass and stiffness of the PR and the stiffness of the cabinet volume. Just as placing two capacitors in parallel will add up, the total stiffness of the system now reacts with the mass of the PR. So while the PR's resonance is below 20 Hz, by including the stiffness of the box the systems resonance will be placed where we wish to use it.

so, with all the very specific and detailed info in your explanation above (which I vaguely understand ), what do you suppose the "real" resonant frequency of the WA 12 combo box and PR is, and do you think it would be in the range of human hearing ?

Maybe that's one of the reasons the WA sounds sooo deep - the combined tuned freq of the PR and the box characteristics make it operate in the sub-woofer range ? Even after using this amazing rig for several years, I am still surprised by the deep lows it produces from such a small size box.

[SPQR]

I'd have to see one in person to get an idea of where its tuned, but my experience would tell me its somewhere above 30 Hz.

[walterw]

Originally Posted by pfschim

As far as the PR producing sound; it is my understanding that the PR tuning frequency is usually well below the 20hz range, so while it may technically produce sound, the sound it does produce is below the threshold of normal human hearing.

no?

i don't think so.

i'm not up on PRs generally, but "tuning frequency" usually means the zone where something resonates the most, below which it starts to drop off, becoming unloaded by any air resistance.

i don't think it means that somehow it only produces sound at or below that frequency.

[billfitzmaurice]

Originally Posted by walterw

i don't think so.

i'm not up on PRs generally, but "tuning frequency" usually means the zone where something resonates the most, below which it starts to drop off, becoming unloaded by any air resistance.

i don't think it means that somehow it only produces sound at or below that frequency.

PR cabs are tuned to the same Fb as ported cabs of the same net volume for the same response. However, the Fs of the PR will usually be considerably lower than Fb. The reason for using a PR is when the required duct would be too long to fit in the desired size cab. The net internal volume of the two for the same tuning and response is the same, but the external size of the PR is smaller. The argument against the PR is that it's more expensive, PRs cost more than ports do.