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Passive radiator to woofer question


Dave A

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Looking at all the various things done with modifications and I have not found the following. Is the idea of a sealed plenum going from a woofer to a passive radiator of worth? I wonder if this has been done and what the results were. Is there some value to cubic inches of space (also I would imagine interference to some degree from other components radiating sound waves) over a sealed volume of direct connection to a woofer/radiator allowing perhaps for more dynamic energy transfer and better isolation from the squawker and tweets? Pondering building some speakers and just wondering.

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If you are still talking about a woofer radiating into a room with the passive doing the same while just keeping a "good" connection between the air movement of the woofer to the passive then that what John's pointing out is different.  If you are just trying to keep the back wave of the woofer from the other components in the system because you feel some gains can be made, there could be a plus to what you suggest.  The key with a passive radiator isn't so much the connection of the air but to have the box completely (good sealant on edges and joints along with gaskets around drivers) as the passive works as a port does but is can not work correctly in a leaky cabinet.  Any movement of the woofer needs to do a transfer to the passive via the air in the cavity.

 

A good passive radiator calculator if you don't already have one:

http://www.mh-audio.nl/PassiveRadiator.asp

 

Very helpful in design.

 

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  • 2 weeks later...

OK a picture is worth a thousand words and here is what I mean. A sealed chamber from front to back and side to side. Besides adding stiffness does this accomplish anything? I ponder improvements and recognize the genius of Klipsch. Someone will come up with the exact perfect acoustic variables some day I suppose but are we there now? I go back to everything is a compromise in manufacturing and what we can do today as individuals does not depend on some CPA penny squeezer cutting corners. I am interested in utilitarian solutions and wonder if this plenum idea would work.

speaker internal lenum.PNG

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Dave what you have posted is a plan if you want to do is to throw away about half the total volume of your cabinet. I am not really sure what it is you expect will happen if you do this? Just about every speaker box speaker out there is already designed smaller than optimum simply because consumers don't want a big box. Your idea just makes matters worse and may possible reduce the available volume to the point where the bass response of the woofer is lost (due to lack of volume) and it couldbe so reduces as to inhibit the woofers ability to drive the passive. For a woofer to drive a passive into resonance (read to make the passive work) you must have an open volume of air which is totally free to resonate, it is the resonating air volume which couples the vibration of the woofer to set in motion or "drive" the passive into its resonant mode which is what damps the woofer and also which extends the bass response. At the lowest frequency it is the passive which is making the bass. You can run bass tones from a test disk and see that once the passive gets going the woofer slows down at full passive motion the woofer does not move much if any. You could experiment with damping the cabinet both above and below the woofer and passive and keep the area directly between the woofer and the passive open to allow free resonance and that ought to be more than enough volume to couple the two units as one. The damping material would buy you additional apparent volume which would not be a bad thing. The best materials to damp with would be either bluejeans damping material or good old fiberglass or rockwool. Polyester batting is all bu useless at low frequencies and is a waste of money space and time. Hope this helps some.

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So, where's the plenum?  What you've drawn is a sealed chamber that could have a woofer in the front and a passive in the rear, just like a Chorus, kg2 or a Genesis III, except that the volume between the woofer and passive will likely have to be larger.  Size the box and the weight and stiffness of the passive and it works.  Use Thiel-Small parameter and run a few equations.  All good hard engineering, nowdays.  You could build it with a PVC pipe, it you cut it to the right length. 

 

Here is the equivalent:

5937089_f520.jpg

 

Just substitute a passive radiator for the port, and maybe resize the box/tube so it all works.  Remember, the 2 are physically equal (resonating mass just below the natural limit of the woofer to extend the low bass output. 

 

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OK maybe sealed chamber would be better as a description. I can still take and recreate the volume of the, lets say a Forte, and make the cubic feet inside the chamber the same as the open area inside the Forte. What my question really is does a a contained "stiff" column of air offer any improvement over a box with air which would I assume act to some degree as a shock absorber and be subject to potential interference from mid range sound waves freely roaming in the same area?

 

  That is an interesting site John. Do you suppose that plastic tubes would be a violation of sonic purity construction methods :D

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On 8/15/2017 at 7:15 PM, Dave A said:

OK maybe sealed chamber would be better as a description. I can still take and recreate the volume of the, lets say a Forte, and make the cubic feet inside the chamber the same as the open area inside the Forte. What my question really is does a a contained "stiff" column of air offer any improvement over a box with air which would I assume act to some degree as a shock absorber and be subject to potential interference from mid range sound waves freely roaming in the same area?

 

  That is an interesting site John. Do you suppose that plastic tubes would be a violation of sonic purity construction methods :D

I think what you are missing and is at issue is that the passive radiator is not there to pass on the sound of the woofer so you don't necessarily need a particular coupling.  It is a resonating point, just as a port, based on a volume of air.  Look at the calculators, information on bass reflex and helmholtz resonators.  The key is pressure built up in the sealed box.  Will the box have pressure built up in different areas differently?  If you can model that perhaps you can find a better location in a box for a port or passive.

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On 8/15/2017 at 5:15 PM, Dave A said:

OK maybe sealed chamber would be better as a description. I can still take and recreate the volume of the, lets say a Forte, and make the cubic feet inside the chamber the same as the open area inside the Forte. What my question really is does a a contained "stiff" column of air offer any improvement over a box with air which would I assume act to some degree as a shock absorber and be subject to potential interference from mid range sound waves freely roaming in the same area?

 

  That is an interesting site John. Do you suppose that plastic tubes would be a violation of sonic purity construction methods :D

   what is it that you are thinking about when you say "a  stiff column of air"? You are not making sense, a chamber with the same volume of air as a Forte cabinet is going to behave like a Forte cabinet because they are the same thing, the same volume. Unless you stretch things out to the point where the volume turns into a pipe and that pipe length coincides with the woofer resonance then you are now in the realm of a tuned pipe and not a box volume. Inside of a sealed box the pressure is the same everywhere how can it not be? Further what mid range sound are you referring to? The mid horn is a sealed unit it has zero interaction with the air inside of the cabinet. The woofer continues to radiate past its crossover point rolling off at a rate equal to the crossover slope which most often will be 12 or 18 Db per octave. That's why some damping is useful to further attenuate this unwanted radiation from bouncing its way back out through the woofer cone passive cone or reflex vent(s). Mid frequencies are shorter in wavelength than bass ones and so they are more easily damped and attenuated than are bass frequencies.

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Moray let me try to explain it like this. A direct smaller connection will have a smaller volume of air to deal with. Air  is compressible and therefore my thought is larger volume with the whole speaker interior to cushion the effect of sound waves, reducing compression so to speak, would not yield the same compressive transfer of energy to the passive as a controlled direct square pipe between the two would. The answer is something not addressed yet and it is  will this work and does it make sense or no it will not work and here is why. Yes I get the idea of volume for bass but is there a way around that is the question I am trying to ask. I am figuring to some degree sound is like air. Blow it into a box and yes it will hit the other side and every other part to. I blow that air into a sealed chamber between two points it goes from point A to point B and does not fool around with dissipating its energy. I don't know how to explain what I mean better than that.

 

  Listening to how directional the sound output from my La Scalas are tells me that sound can be directed and channeled. Horn designs have an angle of sound coverage. So to my mind without technical training in this field shape can improve sound wave transmission.

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3 hours ago, Dave A said:

Moray let me try to explain it like this. A direct smaller connection will have a smaller volume of air to deal with. Air  is compressible and therefore my thought is larger volume with the whole speaker interior to cushion the effect of sound waves, reducing compression so to speak, would not yield the same compressive transfer of energy to the passive as a controlled direct square pipe between the two would. The answer is something not addressed yet and it is  will this work and does it make sense or no it will not work and here is why. Yes I get the idea of volume for bass but is there a way around that is the question I am trying to ask. I am figuring to some degree sound is like air. Blow it into a box and yes it will hit the other side and every other part to. I blow that air into a sealed chamber between two points it goes from point A to point B and does not fool around with dissipating its energy. I don't know how to explain what I mean better than that.

 

  Listening to how directional the sound output from my La Scalas are tells me that sound can be directed and channeled. Horn designs have an angle of sound coverage. So to my mind without technical training in this field shape can improve sound wave transmission.

"A direct smaller connection will have a smaller volume of air to deal with." it's not clear what you mean by this statement however I will assume for the moment that you are trying to imply that given a plenum or tube of approximately the same cross sectional area as the woofer cone and a total contained (plenum) volume which is the same as the Forte cabinet volume would behave differently from each other and that is incorrect they would behave in more or less the same manner. As I mentioned if the plenum were ling enough to have fractional wave resonant modes which interacted with the woofers own resonant modes there would be erratic dips and or peaks in the woofers response as a result of those resonances.

    You are asking if there is a way around this and the answer is no. However I am not sure that I understand what your goal is? I am also not sure if you understand that a passive radiator is essentially the same as a reflex port. Both require a resonating body or volume of air to function. The volume of air is driven into resonance by the woofer the passive of reflex vent are both tuned to resonate near to (could be lower could be higher) the woofer/cabinet resonant frequency. The air vilume is driven into resonance by the woofer and the resonating air drive the air in the vent or the weighted cone in the passive in to a anti resonance which can if designed properly extend below the frequency of the woofer. When either the air in a reflex vent or the cone of a passive are in full resonance you will see little to no motion from the woofer. So long as you have a reasonable volume of air which is free to resonate (no damping mater within this volume) the woofer and the passive will effectively couple and work as desired.

 

   "I blow that air into a sealed chamber between two points it goes from point A to point B and does not fool around with dissipating its energy. "

The suggestion above is not correct. The air, be it in a plenum or in a box of some other dimension will behave in the same way. I believe that you may be under the impression that there is a better way to couple the woofer and the passive/reflex vent and that that is your reason for suggesting the plenum? The existing system of using a reasonable volume of air works perfectly. As I mentioned in the example of a Forte cabinet not all of the cabinet volume need be un damped only a volume coupling the woofer and passive need be open and un damped (free to resonate). You could as I explained damp the lower portion and the upper portion (above the woofer and the passive) of the cabinet in order to  1) capture upper out of band radiation from the woofer (in this case frequencies above 750Hz.) and  2) increase the apparent cabinet volume to obtain a lower overall system response.

   If I am missing the point that you are attempting to make I apologize. So far as I understand things it would seem that you are making a case for finding a fix for a problem when there is no problem. 

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8 hours ago, Dave A said:

Moray let me try to explain it like this. A direct smaller connection will have a smaller volume of air to deal with. Air  is compressible and therefore my thought is larger volume with the whole speaker interior to cushion the effect of sound waves, reducing compression so to speak, would not yield the same compressive transfer of energy to the passive as a controlled direct square pipe between the two would. The answer is something not addressed yet and it is  will this work and does it make sense or no it will not work and here is why. Yes I get the idea of volume for bass but is there a way around that is the question I am trying to ask. I am figuring to some degree sound is like air. Blow it into a box and yes it will hit the other side and every other part to. I blow that air into a sealed chamber between two points it goes from point A to point B and does not fool around with dissipating its energy. I don't know how to explain what I mean better than that.

 

  Listening to how directional the sound output from my La Scalas are tells me that sound can be directed and channeled. Horn designs have an angle of sound coverage. So to my mind without technical training in this field shape can improve sound wave transmission.

You are misunderstanding how a passive radiator works.  Passives don't channel sound.

 

The only way you could do better is to use a tube that connects the woofer to the passive directly containing enough air to provide the appropriate low frequency tuning and load on the passive and woofer.  Unlikely to provide a noticeable difference.  As Moray said, you are trying to fix a problem that doesn't need fixing.

The passive radiator merely vibrates due to and in unison with the "air load" in the box.  Increase the air load or mass of the passive radiator, you lower the frequency of the vibration.  Same way a port works.  You don't harvest the sound from the rear of a cone (you don't want to harvest the sound as a matter of fact for bass, that is why passives are used over ports in some speakers - no upper/mid leakage) unless you are doing a horn.  That is why there are speaker boxes such as bandpass.

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16 hours ago, Dave A said:

Moray let me try to explain it like this. A direct smaller connection will have a smaller volume of air to deal with. Air  is compressible and therefore my thought is larger volume with the whole speaker interior to cushion the effect of sound waves, reducing compression so to speak, would not yield the same compressive transfer of energy to the passive as a controlled direct square pipe between the two would. The answer is something not addressed yet and it is  will this work and does it make sense or no it will not work and here is why. Yes I get the idea of volume for bass but is there a way around that is the question I am trying to ask. I am figuring to some degree sound is like air. Blow it into a box and yes it will hit the other side and every other part to. I blow that air into a sealed chamber between two points it goes from point A to point B and does not fool around with dissipating its energy. I don't know how to explain what I mean better than that.

 

  Listening to how directional the sound output from my La Scalas are tells me that sound can be directed and channeled. Horn designs have an angle of sound coverage. So to my mind without technical training in this field shape can improve sound wave transmission.

 

 

I think I'm beginning to understand.  Simplified, a passive radiator MUST be "decoupled" from the woofer so that is resonates at a lower frequency than the woofer's natural roll-off in the size of box chosen and more strongly (more movement).  The resonance of the box volume and passive radiator requires that "looser connection".  If the woofer and passive were in a small chamber or the space were filled with a non-compressible fluid, the woofer and passive would always in phase.  The effect would be like that of a woofer in free air, the back wave cancelling out the front wave, no bass. 

 

Here are Windowz signal generators:

http://www.dxzone.com/catalog/Software/Signal_Generator/

 

You can use your PC and receiver to feed tones to your Chorus and feel the motion of the woofer and passive.  Somewhere above 100 Hz, the passive will be effectively motionless.  Near 50 Hz, it will start going nuts compared to the woofer and will appear to be "in phase" (moving out when the woofer moves out), but really that is out of phase since the passive is on the back.  This situation pressurizes the room with bass and requires that "loose connection" to the woofer to happen, otherwise, little bass.  It becomes just like having 2 woofers playing at that point, getting most of its energy from the resonance of the trapped air (spring) and weight of the cone (mass). 

 

A horn works in a different manner.  It traps and contains a column of air and "holds" it against the cone or diaphragm.  The trapped air helps the cone build pressure and move the air more forcefully.  It is just like a bicycle pump.  It you disassemble a bike pump and push the piston up and down in the air, you get a little breeze.  It you put it back in the tube, you can get 130 psi with a few strokes.  A horn is then shaped to change the output from a laser (tube) to a spotlight (narrow angle, long-throw horns) to flood lights (90deg x 40 deg short-throw horns used in homes and small spaces).  Bass notes are of such long wavelengths, that they cannot be directed as easily as highs, though. 

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13 hours ago, John Albright said:

 

 

I think I'm beginning to understand.  Simplified, a passive radiator MUST be "decoupled" from the woofer so that is resonates at a lower frequency than the woofer's natural roll-off in the size of box chosen and more strongly (more movement).  The resonance of the box volume and passive radiator requires that "looser connection".  If the woofer and passive were in a small chamber or the space were filled with a non-compressible fluid, the woofer and passive would always in phase.  The effect would be like that of a woofer in free air, the back wave cancelling out the front wave, no bass. 

 

Here are Windowz signal generators:

http://www.dxzone.com/catalog/Software/Signal_Generator/

 

You can use your PC and receiver to feed tones to your Chorus and feel the motion of the woofer and passive.  Somewhere above 100 Hz, the passive will be effectively motionless.  Near 50 Hz, it will start going nuts compared to the woofer and will appear to be "in phase" (moving out when the woofer moves out), but really that is out of phase since the passive is on the back.  This situation pressurizes the room with bass and requires that "loose connection" to the woofer to happen, otherwise, little bass.  It becomes just like having 2 woofers playing at that point, getting most of its energy from the resonance of the trapped air (spring) and weight of the cone (mass). 

 

A horn works in a different manner.  It traps and contains a column of air and "holds" it against the cone or diaphragm.  The trapped air helps the cone build pressure and move the air more forcefully.  It is just like a bicycle pump.  It you disassemble a bike pump and push the piston up and down in the air, you get a little breeze.  It you put it back in the tube, you can get 130 psi with a few strokes.  A horn is then shaped to change the output from a laser (tube) to a spotlight (narrow angle, long-throw horns) to flood lights (90deg x 40 deg short-throw horns used in homes and small spaces).  Bass notes are of such long wavelengths, that they cannot be directed as easily as highs, though. 

 

9 hours ago, Dave A said:

THANKS John, it was the physics of the situation I was wondering about and you have explained it.

All dictated by the air load presented by the box (or weight of passive) of which you would think the pressure in all the parts of the box are the same to drive the passive.  This air load can be affected and adjusted (why passives are used in many instances) by the weight of the passive radiator.  Just as with ports and the weight of the air volume.

As John said, a woofer and a passive without the appropriate amount of air in the box or weight of the passive radiator to make up for the air load, will resonate in unison.  Not good but you do bring up something interesting.  What about a more tightly coupled woofer and heavy passive?  I think that is how some manufacturers are able to get very extended output without having to do it with crazy amounts of eq and power in a small cabinet.  You should play with this calculator and plug in some numbers http://www.mh-audio.nl/PassiveRadiator.asp using a passive such as https://www.parts-express.com/dayton-audio-sd315-pr-12-passive-radiator--295-496 or two or three, adjusting the weight.

 

 

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The passives in my VMPS Larger Subwoofers have additional weight added to the dust cap for tuning.  The OM suggests you can change the "speed" of the sub by changing the amount of rope caulk added.  I fooled with them, but did not detect a noticeable difference.  There is just so little energy below 40 Hz. 

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1 hour ago, John Albright said:

The passives in my VMPS Larger Subwoofers have additional weight added to the dust cap for tuning.  The OM suggests you can change the "speed" of the sub by changing the amount of rope caulk added.  I fooled with them, but did not detect a noticeable difference.  There is just so little energy below 40 Hz. 

Yes for music.  Depending if you are getting below the woofer fs and tuning frequency, I think that's pretty much where you are changing the roll off to add extension.  That extension needs power and you have to decide if it is worth it or not.  No real free lunch from a power perspective but you will have those lower frequencies for HT available by adding mass or volume. 

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