Speaker Basket/Magnet Weight and Dampening

[l_pad]

Hi There,

I am a hobbyist and have an interesting question regarding how to dampen the mechanical energy being transmitted by a speaker's cone/voice coil motor assembly to the mounting surface of a bookshelf speaker I am designing using old parts. Dampening/thickening the mounting surface has gone as far as possible, but what I would like to do now is add more mass to the raw speaker basket/magnet housing itself. You see, the speaker uses a very small neodymium magnet and motor assembly but which nonetheless is powerful enough make the raw speaker hop around on the bench. It is a 5-1/4" driver (Boston Acoustics Pro Series 50se), with a relatively tiny shielded magnet assembly and heavy polypropylene cone, and what I would like to do is add about 1-2 pounds of weight to the back of the magnet housing. Attaching the weight is not a problem, and the basket is extremely rigid cast aluminum, thus making deformation unlikely. It is my belief (!) that the additional weight, when coupled to the basket/magnet housing and thus coupled to the mounting surface by proxy, will increase the inertia and resistance both front and aft of the cone and motor assembly, thereby reducing resonances in and through the mounting surface.

Thoughts, comments, criticisms?

 

[TheWarrior]

You realize the magnet does not move, right?

 

[l_pad]

You realize the magnet does not move, right?

Hi TheWarrior,
Ah, I see my language is too loose. Sorry about that. Yes, I understand the magnet (and pole piece) assembly does not move as it is essentially one with the basket and (typically) a back-plate, though with the Boston driver a back-plate is questionable. As well, I understand the voice coil/cone assembly do move.

 

[Seth=L]

What is the application?

 

[TheWarrior]

Hi TheWarrior,
Ah, I see my language is too loose. Sorry about that. Yes, I understand the magnet (and pole piece) assembly does not move as it is essentially one with the basket and (typically) a back-plate, though with the Boston driver a back-plate is questionable. As well, I understand the voice coil/cone assembly do move.

Your language is fine. But you seem to be confusing passive radiators with drivers. Adding weight to a stationary magnet is pointless, and should that weight be metal, you could interfere with the magnetic field.

 

[Seth=L]

Your language is fine. But you seem to be confusing passive radiators with drivers. Adding weight to a stationary magnet is pointless, and should that weight be metal, you could interfere with the magnetic field.

http://www.woofersetc.com/catalog/product/gallery/id/41930/image/58314/

The basket is aluminum and the magnet is neodium (has very short but strong fields). That said he could affix something non-ferris.

 

[l_pad]

What is the application?

Just a small bookshelf speaker with 12db high pass at 63 Hz. I don't want the speaker cabinet to weigh a ton so wanted to attach weight to the back of the speaker. I'll try to upload a photo of the speaker I am talking about.

 

[l_pad]

Your language is fine. But you seem to be confusing passive radiators with drivers. Adding weight to a stationary magnet is pointless, and should that weight be metal, you could interfere with the magnetic field.

Oh, sorry again. Let me clarify: I know what a passive radiator is, such as on Polk's SDA SRS speakers from the 90's. Active radiators or transducers are attached to a voice coil, etc. The weight isn't being added to the magnet, per se, just to the, um, let's call it the backplate of the Boston speaker - even though it isn't necessarily a backplate as much as it would be called a housing (see my pic I just uploaded).

 

[l_pad]

The basket is aluminum and the magnet is neodium (has very short but strong fields). That said he could affix something non-ferris.

Yeah... that's the one! So, what I want to attach are 6 4" round steel plates, Gooped together into a non-resonant puck (thanks to the Goop) which I can affix to the back of the Boston (see new photo I just uploaded). Oh, and the magnet on the speaker is shielded... nothing gets attracted to it externally.

 

[KEW]

... thereby reducing resonances in and through the mounting surface.

Thoughts, comments, criticisms?

Looking at your strategy, Force = Mass X Acceleration.

The driver/magnet assembly will generate the same force as before, but because the mass has increased, the acceleration has lessened. So, it seems to me it would work,
I just don't think it is especially effective! The variations of air pressure within the cabinet will still excite resonances.

Why not do what speaker engineers and designers do and brace the mounting surface? Many companies will double the cabinet thickness for the baffle (mounting surface). There are also adhesive products designed to add mass and stiffness to the cabinet (dynamat). I think it would be more productive to address the actual area of the resonance.

As long as you have resonant walls for the cabinet, the air pressure waves inside the cabinet will excite their resonant frequencies!

 

[l_pad]

Looking at your strategy, Force = Mass X Acceleration.

The driver/magnet assembly will generate the same force as before, but because the mass has increased, the acceleration has lessened. So, it seems to me it would work,
I just don't think it is especially effective! The variations of air pressure within the cabinet will still excite resonances.

Why not do what speaker engineers and designers do and brace the mounting surface? Many companies will double the cabinet thickness for the baffle (mounting surface). There are also adhesive products designed to add mass and stiffness to the cabinet (dynamat). I think it would be more productive to address the actual area of the resonance.

As long as you have resonant walls for the cabinet, the air pressure waves inside the cabinet will excite their resonant frequencies!

Okay, I think I am getting it now: increasing the mass of the speaker by adding weight to its basket/magnet assembly directly would work, in a narrow sense. However, adding mass to the coupled surface does the same thing (i.e. adds mass to the speaker via its attachment to the more massive mounting surface), and has the added benefit of favorably treating the resonance-propagating surface, the front panel, in my case. Second, the big limitation in my idea is that adding weight to the speaker basket does nothing to inhibit/convert the energy in the air which excites the mounting surface, again, even if the mechanical energy is reduced by adding weight to the speaker basket. So, there are actually two potential problems, mechanical and acoustic, both of which can be more effectively addressed by adding mass and resonance trapping/converting material the mounting surface, or front panel. Right?

 

[KEW]

Here is a pretty good article on bracing speaker cabinets:
http://www.audioholics.com/loudspeaker-design/detailed-look-proper-loudspeaker-cabinet-bracing

 

[highfigh]

Hi There,

I am a hobbyist and have an interesting question regarding how to dampen the mechanical energy being transmitted by a speaker's cone/voice coil motor assembly to the mounting surface of a bookshelf speaker I am designing using old parts. Dampening/thickening the mounting surface has gone as far as possible, but what I would like to do now is add more mass to the raw speaker basket/magnet housing itself. You see, the speaker uses a very small neodymium magnet and motor assembly but which nonetheless is powerful enough make the raw speaker hop around on the bench. It is a 5-1/4" driver (Boston Acoustics Pro Series 50se), with a relatively tiny shielded magnet assembly and heavy polypropylene cone, and what I would like to do is add about 1-2 pounds of weight to the back of the magnet housing. Attaching the weight is not a problem, and the basket is extremely rigid cast aluminum, thus making deformation unlikely. It is my belief (!) that the additional weight, when coupled to the basket/magnet housing and thus coupled to the mounting surface by proxy, will increase the inertia and resistance both front and aft of the cone and motor assembly, thereby reducing resonances in and through the mounting surface.

Thoughts, comments, criticisms?

You could do that, but it's easier to add bracing by installing 1/2" dowels that connect opposing panels of the box and gluing them in position. You can press one finger into the panels that vibrate most and move it to the position that deadens the box best and use that as the location of the dowel. One dowel in each axis is all you need, although the front-rear dampening would probably be best with two dowels, placed adjacent to the woofer's frame. The length of the dowels should be just greater than the distance between the panels and to make installation easier, you would round the ends so the length wouldn't be affected by corners creating a hypotenuse.

 

[l_pad]

You could do that, but it's easier to add bracing by installing 1/2" dowels that connect opposing panels of the box and gluing them in position. You can press one finger into the panels that vibrate most and move it to the position that deadens the box best and use that as the location of the dowel. One dowel in each axis is all you need, although the front-rear dampening would probably be best with two dowels, placed adjacent to the woofer's frame. The length of the dowels should be just greater than the distance between the panels and to make installation easier, you would round the ends so the length wouldn't be affected by corners creating a hypotenuse.

Thanks! That makes a lot of sense. I have also been flirting with the idea of a 3/16" steel plate cut to the same dimensions of the front baffle (internally), Gooping it to the baffle, then putting a layer of Dynamat or just smearing a handful of DAP on the inside. So, it would be 3/4" MDF-->Gooped-on 3/16" steel baffle-->Dynamat or DAP on interior. I think that should handle all the resonances directly to/through the front. Then I could use the dowels in the manner you describe. The driver would be mounted externally to radiate from the plane of the front baffle. Finally, I could contour the front baffle and put leather on it for a final layer of deadening, but I doubt I'd hear anything unwanted by this point.

 

[TLS Guy]

Thanks! That makes a lot of sense. I have also been flirting with the idea of a 3/16" steel plate cut to the same dimensions of the front baffle (internally), Gooping it to the baffle, then putting a layer of Dynamat or just smearing a handful of DAP on the inside. So, it would be 3/4" MDF-->Gooped-on 3/16" steel baffle-->Dynamat or DAP on interior. I think that should handle all the resonances directly to/through the front. Then I could use the dowels in the manner you describe. The driver would be mounted externally to radiate from the plane of the front baffle. Finally, I could contour the front baffle and put leather on it for a final layer of deadening, but I doubt I'd hear anything unwanted by this point.

I would not do that if I were you. Steel has a high Young's modulus and you will drive up resonance where it will be more audible.

 

[lsiberian]

I would not do that if I were you. Steel has a high Young's modulus and you will drive up resonance where it will be more audible.

I agree this is not a good idea. Rubber is the best mass loader for speaker in my experience.

If you want to use CLD. I suggest you build the exterior box which will contain the baffle. Use Peel-n-seal or dynamat on the internal walls. Then place a 2nd layer of wood internally. From there you brace the internal panels together. Then stuff the inside with mineral wool. I see no need to decouple the driver mountings from the baffle, but I suppose you could if you want to overly complicate things.

I built my speakers using that basic layout. You do need to factor the displacement of the bracing and internal panels in your box dimension calculations.

 

[highfigh]

Thanks! That makes a lot of sense. I have also been flirting with the idea of a 3/16" steel plate cut to the same dimensions of the front baffle (internally), Gooping it to the baffle, then putting a layer of Dynamat or just smearing a handful of DAP on the inside. So, it would be 3/4" MDF-->Gooped-on 3/16" steel baffle-->Dynamat or DAP on interior. I think that should handle all the resonances directly to/through the front. Then I could use the dowels in the manner you describe. The driver would be mounted externally to radiate from the plane of the front baffle. Finally, I could contour the front baffle and put leather on it for a final layer of deadening, but I doubt I'd hear anything unwanted by this point.

If you double-layer the baffle, any resonances will be outside of the woofer's range, so they won't affect much of anything. If you want to add bracing to the back of the baffle, think in terms of how a joist works- narrow, deep structural members resist deflection better than square or wide & thin members. Deflection just means it won't bend in the direction of the force when the added braces are in place.

I would advise against gooping anything.

 

[l_pad]

I would not do that if I were you. Steel has a high Young's modulus and you will drive up resonance where it will be more audible.

Hey, that

I would not do that if I were you. Steel has a high Young's modulus and you will drive up resonance where it will be more audible.

Oh... okay. I just looked that up, and am grateful to know that now. It would seem if I use a hard substance, it must be able to prohibit transfer of vibration altogether rather than accentuate it or shift it up or down - sand and concrete probably work well for this. Otherwise, it will have to be heavy and flexible, such as rubber or other like materials which convert the vibrations into heat. Thanks again!

 

[l_pad]

I agree this is not a good idea. Rubber is the best mass loader for speaker in my experience.

If you want to use CLD. I suggest you build the exterior box which will contain the baffle. Use Peel-n-seal or dynamat on the internal walls. Then place a 2nd layer of wood internally. From there you brace the internal panels together. Then stuff the inside with mineral wool. I see no need to decouple the driver mountings from the baffle, but I suppose you could if you want to overly complicate things.

I built my speakers using that basic layout. You do need to factor the displacement of the bracing and internal panels in your box dimension calculations.

Thank you! I am glad you mention rubber being a very effective mass loader. One of these days I am going to just coat the inside of a box with a 1/4" of plasti-dip - you know, the stuff they use to put rubber grips on tools - and see what happens. In the meantime, I think I will do something similar to what you mention, and go with Dynamat or something similar. Thanks again!

 

[l_pad]

If you double-layer the baffle, any resonances will be outside of the woofer's range, so they won't affect much of anything. If you want to add bracing to the back of the baffle, think in terms of how a joist works- narrow, deep structural members resist deflection better than square or wide & thin members. Deflection just means it won't bend in the direction of the force when the added braces are in place.

I would advise against gooping anything.

Okay, this helps! Sort of how a rod op dowel, when upright, can support a very heavy object, but if the same rod or dowel were horizontal (supported at both ends only) and the heavy object were tied to its middle, it would bend or break? Air inside a sealed box presses outward and sucks inward nearly equally in all directions, and it would seem the mechanical energy in the material from the driver would seek the same paths of dissipation; that is, perpendicular to the long plane of the material. So, rods or dowels for bracing would be a low material volume, high rigidity bracing solution that would work quite well. Am I on the right track there?