Passive Radiator Line Source

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Joe_Limon

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Passive Radiator Line Source
« on: 16 May 2020, 11:57 pm »
I was watching the video below. And had an idea.  It looks like one of the constraints of the line source is designing a baffle strong enough to be rigid and damp oscillations yet thin enough that it doesn't mess with the open baffle in upper frequencies.

https://youtu.be/7bwU5HagoQo

What if. To thicken the speaker. You added a row of passive radiators. Simply a thin membrane offset from the column of planar drivers.  This would technically create a cavity. But it would also provide a smaller distance to the rear driver allowing you to thicken/stiffen the speaker.

The only potential issue I see is I do not know how much sound could/would reflect off a passive radiator and mess with the planar driver.
« Last Edit: 17 May 2020, 01:40 am by Joe_Limon »

Danny Richie

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Re: Passive Radiator Line Source
« Reply #1 on: 18 May 2020, 02:36 pm »
I was watching the video below. And had an idea.  It looks like one of the constraints of the line source is designing a baffle strong enough to be rigid and damp oscillations yet thin enough that it doesn't mess with the open baffle in upper frequencies.

https://youtu.be/7bwU5HagoQo

What if. To thicken the speaker. You added a row of passive radiators. Simply a thin membrane offset from the column of planar drivers.  This would technically create a cavity. But it would also provide a smaller distance to the rear driver allowing you to thicken/stiffen the speaker.

The only potential issue I see is I do not know how much sound could/would reflect off a passive radiator and mess with the planar driver.

You can't add PR's to an open baffle speaker.

Joe_Limon

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Re: Passive Radiator Line Source
« Reply #2 on: 18 May 2020, 06:28 pm »
You're right, you couldn't call it open baffle.  It would have an enclosure.  But it would still create a similar acoustic pattern in the room as an open baffle speaker.

Danny Richie

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Re: Passive Radiator Line Source
« Reply #3 on: 18 May 2020, 06:52 pm »
You're right, you couldn't call it open baffle.  It would have an enclosure.  But it would still create a similar acoustic pattern in the room as an open baffle speaker.

No, the pattern would be different. The PR would only increase output down low, and the air space needed would be that of an equivalent ported box. 

Joe_Limon

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Re: Passive Radiator Line Source
« Reply #4 on: 18 May 2020, 07:14 pm »
When I say passive radiator, I am not talking about a row of drivers that have no coil.  I am talking about a thin membrane that can pass higher frequencies as it has almost no mass behind it and a very large surface area to pass subtle pressure differences.

Hobbsmeerkat

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Re: Passive Radiator Line Source
« Reply #5 on: 18 May 2020, 10:43 pm »
Its less effective than you'd probably expect. I've used thin pieces of tape to cover the tiny speakers that beep every time they scan a product. it actually makes them much quieter and reduces the frequencies that are able to pass through.
To be fair, the openings on these speakers are quite small, (1-2mm) and a larger suspended membrane, might work somewhat better, but would still be largely restrictive, not to mention more easily damaged by things like vacuums, children's fingers, sharp objects, etc. Esp as the membrane deteriorates over time You're much better off just having them play directly into open air. Anything else would make them sound muffled/veiled/distorted.

Joe_Limon

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Re: Passive Radiator Line Source
« Reply #6 on: 18 May 2020, 10:57 pm »
I mean of course it will have some negative effect.  I don't think it will be a muffled effect though as you aren't actively interfering/degrading in the music signal (although potentially for enclosure reflections although I am unsure how much a thin membrane passive radiator will actually reflect).  More so you would be muffling ambient noises that otherwise already get degraded by the room prep.  And if that loss in quality meant that you could construct out of mdf, vs a custom cnc'd block of aluminum...  That would be a phenomenal product value wise.

Woodsage

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Re: Passive Radiator Line Source
« Reply #7 on: 18 May 2020, 11:44 pm »
I was watching the video below. And had an idea.  It looks like one of the constraints of the line source is designing a baffle strong enough to be rigid and damp oscillations yet thin enough that it doesn't mess with the open baffle in upper frequencies.

https://youtu.be/7bwU5HagoQo

What if. To thicken the speaker. You added a row of passive radiators. Simply a thin membrane offset from the column of planar drivers.  This would technically create a cavity. But it would also provide a smaller distance to the rear driver allowing you to thicken/stiffen the speaker.

The only potential issue I see is I do not know how much sound could/would reflect off a passive radiator and mess with the planar driver.

I must be thick because I don’t see how a thin membrane is going to do anything to stiffen the baffle or reduce oscillations. It seems to me, and evidently others, it would cause more problems than it would solve.

There are lots of better mechanical ways to stiffen and damp a baffle.

Once you’ve tried it let us know how it works though, I’d be curious to know.


Joe_Limon

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Re: Passive Radiator Line Source
« Reply #8 on: 19 May 2020, 12:48 am »
The membrane wouldn't thicken it.  It would allow you to create a thicker frame/enclosure without having to worry about having the driver as close as possible to both the front and the rear baffle surfaces.

Danny Richie

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Re: Passive Radiator Line Source
« Reply #9 on: 19 May 2020, 01:12 am »
There is nothing like that to be done to them.

Adding such a film or damping material would not have any positive effect and it would be costly to add.

Woodsage

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Re: Passive Radiator Line Source
« Reply #10 on: 19 May 2020, 02:19 am »
There is nothing like that to be done to them.

Adding such a film or damping material would not have any positive effect and it would be costly to add.

That goes without saying Danny (or I would hope it would). I consider it very poor form to arm chair engineer someone’s finished product. Especially a commercial design by a talented designer on  his own forum (regardless of ones intentions).

I was just trying to understand Joe’s idea and I still can’t.

After all, these just have to play Dark Side of the Moon not go to the dark side of the moon.

Mike

Hobbsmeerkat

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Re: Passive Radiator Line Source
« Reply #11 on: 19 May 2020, 02:48 am »
Ive re-read the original post a few times now and im still not sure i quite understand the point being made?
How would a membrane add rigidity to an OB platform, esp a linesource? And i don't understand how vibrations are a big factor since each driver plays rather quietly on its own, but when added together in multiple parallel sets of drivers, sounds large. Plus you can use thin gaskets to dampen any vibrations to the frame.

Even creating a box with the entire rear as a single thin membrane wouldn't be ideal, it would cause standing waves within the enclosure. Making for a muddy sound from the rear at best..
You'd be better off just using a large grill cloth fitted around an open frame thats been dampened with a product similar Dynamat or "lizard-skin." But even then that's a lot more money and engineering than it'd be worth..

Maybe it would be helpful for us to understand if you drew some simple diagrams explain your idea?

Joe_Limon

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Re: Passive Radiator Line Source
« Reply #12 on: 19 May 2020, 05:14 am »
I'll try drawing something tomorrow. I don't mean to criticize the design. It looks wonderful and if cost isn't a factor, the current design is better than what I am suggesting.

The thing that bothered me is having to have a big ole cnc milled then bent chunk of aluminum rather than a cheaper wood based solution.

My understanding is that aluminum was chosen to allow the thinnest possible baffle profile while maintaining enough physical rigidity through the speaker.  Thin being beneficial since you want the driver to be as close as possible to flush on the front and back for higher frequencies.

The membrane idea was my attempt at an idea that would allow you to use a thicker/cheaper material. And potentially even a full enclosure if you really needed to cut down on the oscillations in a soft wooden 7 foot tall speaker filled full of drivers.  The membrane itself would not be a structual member, but rather allow a the oscillating membranes (driven and passive) to simultaneously be flush with both sides of a thicker panel/enclosure.

mlundy57

Re: Passive Radiator Line Source
« Reply #13 on: 19 May 2020, 10:46 pm »
I'll try drawing something tomorrow. I don't mean to criticize the design. It looks wonderful and if cost isn't a factor, the current design is better than what I am suggesting.

The thing that bothered me is having to have a big ole cnc milled then bent chunk of aluminum rather than a cheaper wood based solution.

My understanding is that aluminum was chosen to allow the thinnest possible baffle profile while maintaining enough physical rigidity through the speaker.  Thin being beneficial since you want the driver to be as close as possible to flush on the front and back for higher frequencies.

The membrane idea was my attempt at an idea that would allow you to use a thicker/cheaper material. And potentially even a full enclosure if you really needed to cut down on the oscillations in a soft wooden 7 foot tall speaker filled full of drivers.  The membrane itself would not be a structual member, but rather allow a the oscillating membranes (driven and passive) to simultaneously be flush with both sides of a thicker panel/enclosure.

The DIY cabinet is made from a composite material instead of aluminum.

Joe_Limon

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Re: Passive Radiator Line Source
« Reply #14 on: 19 May 2020, 11:55 pm »
Oh that's cool!