Passive Radiators

Polkaudioforumname

I did a search and went 5 pages into the result with no answers. Can someone explain exactly what they are and how they work?

F1nut

Saving myself a lot of typing......

https://en.wikipedia.org/wiki/Passive_radiator_(speaker)

Polkaudioforumname

F1nut, thanks but I saw that and I don't see the connection with antennas and parasitic elements. BTW, if we share the same meaning of F1, I'm with you.

F1nut

That's weird. Let's try this one.

http://www.humanspeakers.com/whatis/PR.htm

mhardy6647

To (heh-heh-heh) amplify @F1nut's resources

Passive radiator is no more and no less than another way of building a "Helmholtz resonator" (bass reflex or ported enclosure, in loudspeaker terms) that doesn't have an open port. The mass of the PR takes the place of the mass of air contained in the port.

Either kind of "Helmholtz resonator" speaker box (BR or PR loaded) has a resonant frequency which is tuned to complement the behavior of the bass radiator speaker used by the speaker to give the desired "Q" and frequency rolloff for the speaker system (which is called "Qtc"). These parameters are chosen by the designer.

bass response (extension) and "flatness" as a function of Qtc. The "frequency" (x-axis) is normalized to the Fc (resonant frequency) of the system (thus "1" = whatever the Fc of the system is):




All else being equal PR loaded enclosures have a different impedance curve and different bass rolloff curve compared to a "normal" bass reflex cabinet. They are also immune to noise from an open port ("chuffing") -- port-noise complaints being rampant in hifi.


bass rolloff: comparison of loudspeaker "alignments": BR, PR and sealed



This looks pretty good (quick glance):
http://www.theabsolutesound.com/articles/a-guide-to-better-bass-tas-197-1/

Polkaudioforumname

mhardy6647,

Thanks for the additional info. I can't say I really understand how a passive radiator works from the definitions though. Much of the information is oddly similar to my own experiences of designing automotive intakes. I understand tuning an intake because of the consistency of the physics. Moving the mass (air) through an engine and all that changes is the rate (RPM) and the resonant waves (valves). Very basically the goal is to match the waves to runner length to produce a laminar flow needed for the intended application. This is in contrast to a turbulent flow which creates resistance to the air flow which results in chaotic cylinder pressures. I'm not even going to get in to pulse waves or boundary layers.

Where I lose understanding with the passive radiator definitions is with the inconsistent nature of moving air with music. Each frequency moves it's own amount of air based on the wave length. I don't understand how a sealed box can be tuned to the chaos of the frequencies involved with music. As with an intake, turbulence from wave collisions has to be a factor. A sealed box by nature seems self defeating to tuning. Tuning to a single frequency and its harmonics makes sense but that's not music.

I'm thinking out loud here and I'm looking forward to researching this more. Thank again for the additional information.

mlistens03

I'll take a shot at this, although I have but a rudimentary knowledge on the subject...
IIRC, with ported cabinets, they are tuned to a specific frequency, and that is as low as they can play. they then make other modifications (whatever those may be) to the cabinets, drivers, or crossovers, in order to make a flat response... I think it'd be the same for a PR.
I am probably wrong in some way or another, so don't take my word for it!!

tonyb

Polkaudioforumname wrote: »

mhardy6647,


Where I lose understanding with the passive radiator definitions is with the inconsistent nature of moving air with music. Each frequency moves it's own amount of air based on the wave length. I don't understand how a sealed box can be tuned to the chaos of the frequencies involved with music. As with an intake, turbulence from wave collisions has to be a factor. A sealed box by nature seems self defeating to tuning. Tuning to a single frequency and its harmonics makes sense but that's not music.
.

Compare different speaker designs and you may find the answers you seek. Passives don't cover the whole freq. response and are mainly used for lower bass notes.

What you may find is, there is more to building speakers than shoving drivers in a box.

mhardy6647

It is not really about moving air as much as it is about exciting the particles (molecules) in the air. That may be where the confusion lies.

The wave moves, but the propagation medium does not (at least, it doesn't move in a net fashion)! That is fundamental to wave mechanics. The usual illustration of this is a float bobbing on the water. In practice, there is motion of the floating object, but there's no net motion.


source: https://ci.coastal.edu/~sgilman/770Oceansinmotion.htm

This being said, actually acoustic waves are longitudinal and water waves are transverse (more or less!). https://www.physicsclassroom.com/class/waves/Lesson-1/Categories-of-Waves

Folks will point out that they do feel air move from a woofer or from a port -- and this is true. I think that's one or both of two phenomena -- either that no net motion thing (i.e., you feel it "blow" but you don't notice feeling it s -u- ck back!)... or it's the fact that music waves aren't purely sinusoidal, but in fact are very asymmetric.

tonyb

Dang Doc, just once can ya make a post in English.

mhardy6647

Note the motion of a particle in a medium being excited by a transverse wave in this GIF. This is illustrative of air molecules excited by a sound wave (longitudinal pressure wave).



Here's the motion of particles in a medium excited by a transverse wave.



Waves in water are actually (derp) hybrid transverse and longitudinal waves



source: https://www.acs.psu.edu/drussell/demos/waves/wavemotion.html

SCompRacer

Beware the rogue wave....

pitdogg2

Doc is chewing on the paper tabs ..... again.

tonyb

pitdogg2 wrote: »

Doc is chewing on the paper tabs ..... again.

Before or after he's sniffing the glue ?

verb

Pay attention students! No shooting spitballs! Good stuff here Professor, in my biz we design hemholtz resonators to cancel out the noise from all of our fancy air inductions systems.

Polkaudioforumname

Wow, a lot to digest here that's going to take some time.

@Verb, This is something I understand. I don't see the relationship here with an intake resonator. Typically a intake resonator is controlled by a MAP or MAF sensor which is continuously "retuning" wave and pulse propagation to match the demands of the engine. Hemholtz Resonators on an intake are not tuned to a specific operating frequency. They are constantly monitored and electronically matched or "retuned" to changing engine and intake temperature, pressure and engine RPM. The only condition where they might be considered similar would be during a cold start condition. An induced rich fuel condition is used to overcome the effects of the resonator during cold starts, among other requirements. This is why there are no EPA smog standards during cold starts. Furthermore, engines built to run at WOT don't use a intake resonator. They're nothing but a restriction to airflow at WOT. WOT=Wide Open Throttle

An intake resonator is also considered an air mass staging area.

Have to say thanks everyone, this is fun. Physics is physics. Pi r round, cornbread r square.

verb

Yep they are designed to improve volumetric efficiency, but also to attenuate engine noise. Depending on how sophisticated (higher cost), they are tuned to a typical operating frequency.

Polkaudioforumname

Verb, I agree but when tuned I'm only aware of 2 conditions that strategy is used. One being when a pre-ignition chamber is used like with the Honda CVCC engine. The second is when they are used to attenuate or cancel undesirable engine noise which is usually identified as vibration. This is where I think you may have hit on my initial questions.

Polkaudioforumname

SCompRacer, I get the humor but this is a good example of the chaos I referred to despite a wave not being in a closed system.

verb

Polkaudioforumname wrote: »

Verb, I agree but when tuned I'm only aware of 2 conditions that strategy is used. One being when a pre-ignition chamber is used like with the Honda CVCC engine. The second is when they are used to attenuate or cancel undesirable engine noise which is usually identified as vibration. This is where I think you may have hit on my initial questions.

Most naturally aspirated engines nowadays use em. The pressure waves coming out of the intake ports from unused air "bouncing back out" is what causes the noise. Simple ones (constant volume) could only be tuned to a specific operating frequency.

Some of em today (depending on how much money you want to spend) have active controls and adjustable chamber volume to cover a wide range of operating conditions.

xschop

Toyota's factory plenum-intake on their 1GZ V12 engine was an engineering enigma. I think I understand what they were out to accomplish with the variable plenum volumes over the rpm range... now that someone else is explaining it other than Dinsdale.

mhardy6647

Dinsdale?

https://youtu.be/mBL3XB-PhE8

Viking64

When the Piranha Brothers napalmed Cheltenham, even the police stood up and took notice.

mhardy6647

Whew! Now I finally understand this thead!
Sheesh, guys start talkin' about plenums and stuff...

Polkaudioforumname

verb wrote: »

Most naturally aspirated engines nowadays use em. The pressure waves coming out of the intake ports from unused air "bouncing back out" is what causes the noise. Simple ones (constant volume) could only be tuned to a specific operating frequency.

Some of em today (depending on how much money you want to spend) have active controls and adjustable chamber volume to cover a wide range of operating conditions.

This being a speaker forum leaves me doubting the appropriateness of pursuing this with you. I would suggest you look into the research done by Chrysler on Ram Charging. Wave vs. pulse and their cyclic nature due to positive and negative pressures along with where and how pressure equilibrium in a naturally aspirated engine is achieved is explained.

Polkaudioforumname

Dinsdale? Monty Python?

Viking64

Polkaudioforumname wrote: »

This being a speaker forum leaves me doubting the appropriateness of pursuing this with you. I would suggest you look into the research done by Chrysler on Ram Charging. Wave vs. pulse and there cyclic nature due to positive and negative pressures along with where and how pressure equilibrium in a naturally aspirated engine is achieved is explained.

Polkaudioforumname

xschop wrote: »

Toyota's factory plenum-intake on their 1GZ V12 engine was an engineering enigma. I think I understand what they were out to accomplish with the variable plenum volumes over the rpm range... now that someone else is explaining it other than Dinsdale.

Want to scare a mechanic? Just say I have a Toyota BEAM Engine.

Polkaudioforumname

Viking64 wrote: »

Polkaudioforumname wrote: »

This being a speaker forum leaves me doubting the appropriateness of pursuing this with you. I would suggest you look into the research done by Chrysler on Ram Charging. Wave vs. pulse and there cyclic nature due to positive and negative pressures along with where and how pressure equilibrium in a naturally aspirated engine is achieved is explained.

Does this suggest it would be appropriate? Being new here I'm careful. Some forums are full of "police" just waiting to shoot someone.

Viking64

Polkaudioforumname wrote: »

Does this suggest it would be appropriate? Being new here I'm careful. Some forums are full of "police" just waiting to shoot someone.

As far as I am concerned, this is your thread so you can talk about whatever you like in it. The pug was just my way of having fun.

The member "verb" is one of the nicest people here, plus he has a great sense of humor. Definitely someone worth getting to know.

"mhardy6647", AKA "Mark", "Doc", and "Professor", is also very knowledgeable on a number of subjects. He is known for providing PLENTY of information when a thread interests him. And his sense of humor ranges from the obscure to the brilliant.

I don't get the initial reference to "Dinsdale", but I am quite familiar with the Piranha Brothers (Doug & Dinsdale), although I have never had my head nailed to a table.

lightman1

Yall see! First and foremost...
Jesse is an as$hole....