Passive Radiators - out of phase question

[hoosier21]

It is pretty well known that you do not want your speakers wired out of phase, and that a general rule of thumb is that if you have a subwoofer in the front and one in the rear, you will have to set the rear subwoofer 180deg out of phase with the front, generally speaking.

This is to eliminate cancellation of the sound waves. If the right front speaker is moving out, and the left speaker is moving in, there is canceling of sound, positive plus negative equals nothing. Similarly, you WANT the rear subwoofer out of phase with the front, when the front subwoofer driver is moving out, you want the rear subwoofer driver moving in, that way they work together and not against each other.

Finally my question/thought/rambling:

I am looking at building a subwoofer with a driver and a passive radiator, and want to but them both on the front face of the cabinet (ok I am looking at stealing the North Creek design for the Peerless 12” XLS driver and passive radiator) . The Passive radiator is a slave to the driver, it only gets it’s movement from the drivers movement, so when the driver moves out, the PR moves in.

How does this not cause cancellation?

Many here own early Polks that use a PR design, same question.

And if you own SDA’s, man what a mess, they use a PR, I have the same question as above but add to it, the SDA drivers are out of phase with the stereo drivers, so you have stereo drivers in phase, PR out of phase with the stereo drivers, and a PR that is out of phase with the stereo drivers, but in phase with the SDA drivers, my head hurts!

[RuSsMaN]

Russ,

I think it all boils down to resonant frequencies of the active drivers, and the passive radiator.

PR's are tuned lower than the active drivers (sometimes substantially), so they aren't producing the same frequencies - so there is nothing to cancel out.

Good question though, I'd like to know more of the tech side of it.

Cheers,
Russ

[PolkThug]

Originally posted by hoosier21

How does this not cause cancellation?

A PR doesn't generate a sound wave (nothing to cause cancelling), it just allows the cone on the active subwoofer to move more.

[HBombToo]

Originally posted by PolkThug
A PR doesn't generate a sound wave (nothing to cause cancelling), it just allows the cone on the active subwoofer to move more.

I was thinking they were in phase and with the PR in conjunction with active woofer the effective surface area has increased providing a higher spl.

??? HBomb ???

[hoosier21]

Originally posted by RuSsMaN
PR's are tuned lower than the active drivers (sometimes substantially), so they aren't producing the same frequencies - so there is nothing to cancel out.

That may be it.

Originally posted by PolkThug
A PR doesn't generate a sound wave (nothing to cause cancelling), it just allows the cone on the active subwoofer to move more.

I am pretty sure it generates sound, why not?
The driver makes sound by moving in and out, same with the PR.

Originally posted by HBombToo
I was thinking they were in phase and with the PR in conjunction with active woofer the effective surface area has increased providing a higher spl.
??? HBomb ???

I am thinking out of phase, driver goes out, PR goes in. But you are right about more surface area = more SPL.

[Dennis Gardner]

They are "out of phase" using your analogy just as a port or vent would act. Air pressure from the woofer moving in pushes the PR (air)outwards.

This isn't a phase problem though since there is no electrical signal causing them to push against each other in a cancellation mode, only the weight of the PR giving back pressure for the woofer to load against helping to keep the driver from bottoming.

The tuning of the PR will normally be lower than the woofers resonance frequency to give more bass extension just as a port tunes the box lower than the fs of the driver.

No more sound comes out of a PR than comes out of a port unless it resonating a frequency. There is no electrical energy to release. When you physically push on a woofer with your hand, no note is heard. Same with the PR.

[gatemplin]

PR's are tuned lower than the active drivers (sometimes substantially), so they aren't producing the same frequencies - so there is nothing to cancel out.

Yep that's basically it. A PR is basically the same as a port. Instead of the mass of the air in the port you have the mass of the driver. It works basically the same way.
Above the resonant frequency of the PR they are in acoustic phase and the PR is used to damp the motion of the active driver.
The reason PRs are used is to tune the system lower without having to bend the port. For example, in the speakers I am building right now, the port required is about 15" long which requires a very deep cabinet.
The PR's aren't exactly the same as a port, they have a steeper rolloff. They are also considered to have a higher group delay and worse transient response but whether these things are a problem at low frequencies is debateable.

A PR doesn't generate a sound wave (nothing to cause cancelling), it just allows the cone on the active subwoofer to move more.

It definitely produces acoustic output and the PR dampens the motion of the active driver near system resonace reducing excursion.

There is a good section in The Loudspeaker Design Cookbook by Vance Dickason on passive radiators and there is some good info here:
http://www.diysubwoofers.org/prd/

[PolkThug]

Originally posted by hoosier21
I am pretty sure it generates sound, why not?
The driver makes sound by moving in and out, same with the PR.

I don't think its the same, a driver has a voice coil. Depending on how the PR is weighted, it will allow the driver to move more at certain frequencies. I've had PR boxes before with 8" drivers and 10" PR's, while its playing you can place your hand on the PR to stop it from moving, this in turn, limits the excursion of the driver, acting as if it were in a sealed box, then take your hand off the PR and allow it to move and in turn the driver is able to move more.

[hoosier21]

Originally posted by PolkThug
I don't think its the same, a driver has a voice coil.

movement of the cone in and out makes sound, either a speaker driver with a coil driven by amp, or a PR without a coil driven by the air pressure in the cabinet, both "cones" are making sound waves.

[tryrrthg]

A port doesn't have a voice coil either but it makes plenty of noise. The PR works just like a port.

http://www.diysubwoofers.org/prd/

[jdhdiggs]

Russman's pretty much 100% right on this one. The PR reacts to thos frequencies close to it's tuning and starts taking the load off of the active drivers by producing those sounds themselves by resonating. So while your mids are driving a 30, 100, 700 Hz signal, the PR is taking the load of the 30Hz signal. There is also some funky phase correlations that have to do with box size and sound wave length that I really need to learn a bit more about.

[PolkThug]

Originally posted by hoosier21
movement of the cone in and out makes sound, either a speaker driver with a coil driven by amp, or a PR without a coil driven by the air pressure in the cabinet, both "cones" are making sound waves.

So the PR generates bass off its surface when it moves??

Not trying to argue, I'm just having a really hard time grasping this concept. I would think that only a poorly designed PR/port/vent would make its own noise.

Regards,
PolkThug

[PolkThug]

Originally posted by tryrrthg
A port doesn't have a voice coil either but it makes plenty of noise. The PR works just like a port.

http://www.diysubwoofers.org/prd/

I don't think anywhere on that site is says the PR makes noise. Looks like the resonating point "notch"of the PR itself causes a null down low which relates to Hoosier's original post about cancellation.


Linked from that page:
"The PR's allow for lower tuning in a smaller enclosure where a typical port will not fit. PR's also do not have the issues with compression or noise that ports do, allowing for higher maximum output, and lower distortion."

[jdhdiggs]

PR's move back and forth and in doing so they move air so they do make noise. No ifs, ands, or buts about it.

[PolkThug]

Originally posted by jdhdiggs
PR's move back and forth and in doing so they move air so they do make noise. No ifs, ands, or buts about it.

Audible noise at say 2 meters?

[gatemplin]

Yes ports and PRs do produce sound. They are Helmholtz resonators. Think of the air in the enclosure like a spring with a resonant frequency. Dont think of the system as hydraulically coupled pistons (like a slave and master).

At high frequencies the PR hardly moves and ignores the driver. When the driver produces frequencies near the PR resonant frequency it will dampen the motion of the active cone and will be producing most of the output.

Here is a FR graph of a PR system. You can see where the PR and driver are in phase and the output sums greater, and down low where they are out of phase the total output is less than the sum of the driver and PR.

[tryrrthg]

Originally posted by PolkThug
I don't think anywhere on that site is says the PR makes noise. Looks like the resonating point "notch"of the PR itself causes a null down low which relates to Hoosier's original post about cancellation.


Linked from that page:
"The PR's allow for lower tuning in a smaller enclosure where a typical port will not fit. PR's also do not have the issues with compression or noise that ports do, allowing for higher maximum output, and lower distortion."

I don't know much about PR subs but a ported sub is is tuned to a certain frequency when a the playback material approaches that certain tuning frequency the driver produces less of the sound, the port is actually making the bass around the tuning frequency, then again after a certain frequency the port stops and the driver takes over again.

I imagine PR's work the same way. they're weighted (or tuned) at a frequency and when you reach that frequency the PR is doing most of the bass reproduction.

gatemplin, beat me to it. He summed it up nicely.

[PolkThug]

Cool! I wonder how they measured the SPL's coming off the passive radiator itself.

[madmax]

The PR is tuned to a given frequency dependent upon the weight you load it with and the box size. It has a peak output at this tuned frequency and drops off above and below. On frequencies above and below the resonance it will be very much out of phase (although at a much lower sound level) and as you approach the tuned frequency it will be more in phase which will give you a higher output at those frequencies.

The description of having front subs out of phase with rear subs is not correct. If you do that it is only to smooth out some irregularity in a given room and is not the norm. For most rooms the multiple subs should be of the same polarity no matter how they are oriented. Keep in mind that you are trying to pressurize a room, not move air back and forth between speakers.

madmax

[PolkThug]

Ya know, there's plenty of room on the side of the SVS PB10 for a 15" PR!

[gatemplin]

Originally posted by PolkThug
Cool! I wonder how they measured the SPL's coming off the passive radiator itself.

Actually that is just a theoretical frequency response I did in Unibox. It is just predicting the response based on the T/S parameters.

[PolkThug]

Originally posted by gatemplin
Actually that is just a theoretical frequency response I did in Unibox. It is just predicting the response based on the T/S parameters.

Oh no I thought we had real world objective data! Looks like we're still in the theory stage of this thread....

Ok, I will play devil's advocate to myself and prove that a PR can make its own sound.

Let's start out with a box with a driver and a PR in it, with your stereo off. You can push on the PR all day long with your hand, as fast as you want, and no audible sound will come out of it (unless you have a squeaky PR).

Now, turn on the stereo on low volume, the driver immediately starts making sound and bouncing sound waves around in the box. You may now feel a slight vibration on the PR itself, although it is not visibly moving back and forth yet. With your ear up to it you can hear some sound passing through it, just like you can hear sound passing through a thin wall (my wife reminds me of this).

Now turn up the volume. The driver moves back and forth and the PR is now visibly moving. The thing that has changed is now the sound waves on the inside of the box are hitting a moving PR and not a still one. This moving PR alters the sound that is passing through it, thus creating its own sound.

Regards,
Polkthug

[gatemplin]

Originally posted by PolkThug
Oh no I thought we had real world objective data! Looks like we're still in the theory stage of this thread....

No, not theory. That is a theoretical response, meaning anechoically correct. It is as accurate as the measured T/S parameters I used.

[PolkThug]

The theory is "PR's make their own sound."

So far we have our thoughts, and as you point out, theoretical responses, but no real-world objective data.

Either way, I'm having fun thinking about how this works.

[dorokusai]

PolkThug - No offense, but you seem to immediately question the validity of comments regarding speakers and the effects of cables, due to recently purchasing test measurement devices and software.

Building a speaker enclosure requires theoretical AND hard data, but not everything needs plots and graphs to show an improvement to do with audio. An individuals opinion on the effects of something added or subtracted from their system, with or without measurement data, is still an opinion regardless of content. I think we all get this, don't we

Am I to expect the "toilet flush" plot or the "angry wife" graph?

Again, seriously, no offense. I'm just trying to understand what your intentions are. I will certainly agree that hard data is something good to have and to perform.

Speaker design has been around for more than a couple months. There are some excellent resources in hardcopy format that would help you see these theories and the applications explained.

[madmax]

Originally posted by PolkThug
The theory is "PR's make their own sound."

So far we have our thoughts, and as you point out, theoretical responses, but no real-world objective data.

Either way, I'm having fun thinking about how this works.

Take hold of something, anything, and move it to and fro 100 times a second. You will hear a frequency of 100 hz. Can you do this with your hand?

Not you doro...

[dorokusai]

Originally posted by madmax
Take hold of something, anything, and move it to and fro 100 times a second. You will hear a frequency of 100 hz. Can you do this with your hand?

Not you doro...

To late, I'm on it!

[PolkThug]

Don't blame me, I don't trust anybody since the Bose people tricked me a few years ago.

Nothin' to do with test equipment, I'm just trying to figure out how a "weighted panel on springs" creates its own sound wave. Which I think I *may* have figured out in my last paragraph a few posts up.

Intention is learning (and I'll admit it, I'm bored at work).

Regards,
PT

[dorokusai]

Originally posted by PolkThug
Don't blame me, I don't trust anybody since the Bose people tricked me a few years ago.

Nothin' to do with test equipment, I'm just trying to figure out how a "weighted panel on springs" creates its own sound wave. Which I think I *may* have figured out in my last paragraph a few posts up.

Intention is learning (and I'll admit it, I'm bored at work).

Regards,
PT

I completely understand, rock on Polk daddy!

[PolkThug]

Originally posted by madmax
Take hold of something, anything, and move it to and fro 100 times a second. You will hear a frequency of 100 hz. Can you do this with your hand?

I see your point. This is the kind of info/explanation I'm looking for.

Thanks,
PolkThug