Passive Radiators - out of phase question

PolkThug

Sound simplified:

http://science.howstuffworks.com/hearing1.htm

Tour2ma

Originally posted by hoosier21
And if you own SDA’s, man what a mess ... the SDA drivers are out of phase with the stereo drivers ...

Couple thoughts here:

Is this true for the low frequency range that the PR's reinforce? I don't think so. I think all the MW's are moving together in the Low Frequency range.. at least this is what I see during a pronounced bass drum kick.

Have to remember that the left SDA's are out of phase with the right stereo signal, and vice versa, so for strong right or left side images, there is no potential for cancelation within a given cabinet.

hotwheelman

Originally posted by dorokusai
To late, I'm on it!

Hand at 100 hertz = one BIG blister.:D

amulford

If I remember right, the PR does generate some low FR. But it's more of a case of moving air mass in conjunction with the active driver, kind of pneumatic reinforcement. Since sound waves are actually "ripples" made in the air the larger surface area of the PR would create a longer wave, thus effectively lowering the FR and slightly increasing the SPL.

I don't see it being a "phase" problem, since it is a reaction of the active driver's air mass excitation.

That's my take on it, anyway.

Oh yeah, Mike, that's a slow stroke. I get the tempo of hovering hummingbird sometimes... The ole' lady is still on me to fix the cracked bathroom window...;)

HBombToo

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.

and they are in phase by definition(just air movement) and making noise. Front speaker either pushes or pulls the passive so they are in phase.

a simple active circuit can change the phase inexpensively. I can't understand why I would want to do that but hey... muck it up;)

1/2twin

HBombToo

BTW, I am suffering from the worst hang over in long while.

Hair of the dog sounds good right now.

HBomb

Mjr7531

Unless I'm mistaken, it reacts with the tuning of the box, like a port, if the woofer is playing above tuning freq, the air/PR moves in and out with the port, below tuning frequency and the air moves opposite to the woofer, thus causing the possiblility to unload your woofer etc.
Phase problems are unlikely, and yes Passive radiators do make sounds, It's comparable to a port.

PolkFreak

I agree with Mjr7531 about the PR being "tuned" to the cabinet.
The PR is like a spring and the "stiffness" is tuned to the cabinet size and the FR of the drivers. If you had the wrong PR that PR might allow too much movement or not enough. Lets say for instance the PR were too loose and it caused the drivers to overextend. If the PR were too tight the drivers would not get the proper bass extension. Compare PR design to acoustic suspension.With acoustic suspension there is no way to relieve cabinet pressure like PR or PORTED. Correct me if I'm wrong, but you also have to dump more AMP power into an acoustic suspension design. It makes the amp work harder. Acoustic suspension makes the speaker less efficient. The PR absorbs the "tones"and energy of the cabinet and extends the overall low frequency response. When the drivers go low and move inward for bass the PR is forced outward. It extends the low tones bouncing off of it and relieves cabinet pressure. This is how passive radiator design works to me anyway.
I too have read about it. I thought I heard someone mention that a PR can make sound buy itself which made me laugh and made me think of a pair of monitor 10's with all of the active drivers replaced with passive radiators.
Why isn't it making any sound? LOL Could have been my overactive imagination though:D

nadams

Just my 2 cents... I've read through most of the thread here and would just like to say this-

Speakers produce sound by moving air. Doesn't this mean that a PR can produce sound, as long as it's coupled with active drivers? As long as it's moving, it's creating air waves, and therefore producing sound. PRs are only good for lower frequencies, because tweeters wouldn't move enough air to make the PR move at all.

Just my thoughts.

gatemplin

Unless I'm mistaken, it reacts with the tuning of the box, like a port, if the woofer is playing above tuning freq, the air/PR moves in and out with the port

Right, but only near Fc (system resonance).

Correct me if I'm wrong, but you also have to dump more AMP power into an acoustic suspension design.

Yes, acoustic suspension is very inefficient at low frequencies.

The PR absorbs the "tones"and energy of the cabinet and extends the overall low frequency response.

When the drivers go low and move inward for bass the PR is forced outward. It extends the low tones bouncing off of it and relieves cabinet pressure. This is how passive radiator design works to me anyway.

No and no. The PR and drivers are coherent above Fc. They are always in phase above the system resonance but the PR is not always excited, only near Fc.

Speakers produce sound by moving air. Doesn't this mean that a PR can produce sound, as long as it's coupled with active drivers?

Yes. Sound is made up of compressions and rarefactions of a medium (in this case air). If the frequency of these vibrations is in the audible range (20Hz to 20kHz), you will hear it. Here is a animation of a longitudinal wave like a sound wave.
http://www.silcom.com/~aludwig/images/pwave.gif

There is nothing mysterious about passive radiators. It is covered in any good speaker book. Doro and I mentioned Vance Dickason's book which has a section or PRs. They are Helmholtz resonators. This is covered in many physics textbooks, or google it.

PolkFreak

Nadams, the point of my little joke about the Monitor 10's was that the passive radiator would not be able to produce sound
without active drivers. I thought I read a post that implied that a PR can produce sound by itself which is wrong. I also was in no way implying that the tweeter would move the PR because we all know that is physically impossible. Good points gatemplin:)

Tour2ma

... but here is the crux of the matter...

---

COMMENT: When the drivers go low and move inward for bass the PR is forced outward. It extends the low tones bouncing off of it and relieves cabinet pressure.

---

REPLY: No and no. The PR and drivers are coherent above Fc. They are always in phase above the system resonance but the PR is not always excited, only near Fc.

---

Those of us that have pushed on our PR's know that the MW's do move iin the opposite direction, i.e., out of phase. It's this observation that leads to the questions about phasing...

Granted, there is a time delay between the MW's motion and the PR's reaction based on distance the pressure wave must travel and the speed of sound (~1/4 of a cycle for the SRS's for 100 Hz), but there is not sufficient baffling to produce a full cycle delay... and even if there was at 100 Hz, they's be out of phase at 150 Hz...

I dunno... just thinking out loud.

Found plenty of info on Helmholtz and the like, but not a definitive read on the phasing issue. Close as I found was from: http://www.hometheaterhifi.com/volume_1_1/v1n1spk.html
"Above the tuned frequency, the port contributes relatively little to the total output, moving more or less in phase with the active driver.
As the frequency reproduced moves down to the tuned frequency, the reflex system starts to dominate the output, as it's sucking energy so efficiently from the active driver that while the active driver barely moves, the output from the port or passive radiator may be tremendous.
Below the tuned frequency, the output of the port or passive radiator becomes more and more out of phase with the active driver, and more and more equal, so that at lower frequencies, while the driver and the port may be huffing away, it results in very little real SPL, resulting in a much sharper decline in response (24 dB/octave) below the low-frequency cut off than a sealed box (12 dB/octave.) In other words, a bass reflex system can improve bass extension and output to a point, after which it makes it worse.
The difference between a port and a passive radiator in terms of performance, generally speaking, are that a port doesn't have any output limitations relating to limited air volume, compared to a passive radiator with a fixed displacement limit.
On the other hand, a passive radiator cannot suffer from turbulence noise which afflicts many ported designs."

So if I am "getting it", it seems that:
- at higher frequencies the PR's stiffness/ mass renders it "mute" to significant sound production, and what is produced cannot be significantly out-of-phase due to the short wave lengths;
- at very low frequencies the phasing issue does arise and leads to wave cancelation (validation for we, PR pushers); and
- in between, i.e., around the system resonance/ tune point, the PR overwhelms the MW('s) in terms of output, rendering whatever phase issues there are insignificant...

Still can't wrap my bean entirely around this last point (I'd think there'd have to be peaks and nulls in this FR zone), but sobeit...

Maybe we can get a lecture/ Q&A during the April(?) Polk HQ tour...

PolkFreak

REPLY: No and no. The PR and drivers are coherent above Fc. They are always in phase above the system resonance but the PR is not always excited, only near Fc.



I probably have an even harder time wrapping my even smaller brain around the above comment, but thought it would be nice of me to say "good point" even if I didn't understand the statement.
Saying "good point" said Ya, I HEAR ya, but I don't think I could be LISTENING because of the fact that I don't understand the comment:o Tour2ma, that looks like the same info that I got my passive radiator info from, but I read it so long ago that I should have read it a few more times before posting my worthless crap:D Thank you Tour2ma for getting the proper information out in the open!

gatemplin

So if I am "getting it", it seems that: at higher frequencies the PR's stiffness/ mass renders it "mute" to significant sound production, and what is produced cannot be significantly out-of-phase due to the short wave lengths;

I think you are close, the driver and PR are in phase above the system resonance. The PR has a resonant frequency, it needs energy at or near that frequency to resonate. The energy is taken from the active driver (specifically the air in the enclosure). So at 1000 Hz it basically doesn't move, try it with a test tone. Also, think of the air inside the enclosure as a spring, not a non-compressible fluid.

at very low frequencies the phasing issue does arise and leads to wave cancelation

Yes, below the system resonance they are out of acoustic phase. see my PR response graph on page 1.

around the system resonance/ tune point, the PR overwhelms the MW('s) in terms of output, rendering whatever phase issues there are insignificant...

Yes, most of the output is coming from the PR at the tuning frequency, BUT, they are in acoustic phase, they constructively interfere. Also see the graph.