Anatomy of a Polk RT3

[Guiria]

The HM130C0 5.25 Audax driver as the following:

Frame 136mm square
Frame 152mm diameter at rounded corners
Cut out 116mm
Depth 65mm

116mm is 4.57". If my cutout is 5" how does that work...

I need to bite the bullet and build new boxes. I could probably sell the Polks and use that money to stretch my budget a little.

It's time to have a heart to heart with my SO...wish me luck.

 

[TLS Guy]

Thanks for researching out those drivers. $90.00 per woofer is a little steep for my budget. I could spend up to about 200.00 (kind of a don't ask don't tell with significant other ) total for either direct replacement speakers or a brand new set of DIY front speakers. At the price point of $200 I'd rather not consider brand name pre-built speakers.

I currently have a pair of DIY TB 3" bamboo drivers

http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=264-844

I really like the midrange they produce. I've never heard a carbon fiber cone driver. How do they sound in comparison to bamboo drivers I wonder???

I'll take your word on suggested drivers TLS_Guy I just wonder if we can find another match that is a little less $$.

You are really lucky if you can find a driver to fit an existing cabinet. I have been through all likely suspects. That driver is excellent, and used widely in commercial designs. I will put the pdfs together when I get time and you can see that it makes an outstanding small speaker. F3 is in the sixties, and it matches well to a very inexpensive tweeter. The whole package will make a very high quality units, a lot better than you will get buying new at that price. There are 8 elements in each crossover by the way.

 

[dorokusai]

I sent an email two days ago. Did you not receive it?

No, I did not.

Mark
Polk Audio CS

 

[TLS Guy]

The HM130C0 5.25 Audax driver as the following:

Frame 136mm square
Frame 152mm diameter at rounded corners
Cut out 116mm
Depth 65mm

116mm is 4.57". If my cutout is 5" how does that work...

I need to bite the bullet and build new boxes. I could probably sell the Polks and use that money to stretch my budget a little.

It's time to have a heart to heart with my SO...wish me luck.

I think you can make a filler ring. You might not have to though. The chassis will be over the cut out all the way round. The chassis is square and you will likely get a good seal if you use a little silicone.

You will have to enlarge the recess for the tweeter. I think you will fit these drivers OK, and it will be a lot less work than building new enclosures.

You might build new speakers for less money though, as you won't have the constraint of having to find a driver compatible with your box.

I will get your pdfs ready for your mod, and another highly cost effective speaker I have worked out. I will do this after everyone leaves from the holiday weekend at the lake.

 

[Guiria]

Thank you for all your help. After sleeping on it I'm inclined to use the existing enclosures with slight modification. Obviously it would definitely be easier and have me up and running again much sooner.

I'll wait for your other recommendation before making a final decision.

Thanks again for your help. Have a great Labor Day.

 

[TLS Guy]

Thank you for all your help. After sleeping on it I'm inclined to use the existing enclosures with slight modification. Obviously it would definitely be easier and have me up and running again much sooner.

I'll wait for your other recommendation before making a final decision.

Thanks again for your help. Have a great Labor Day.

Here are the simulations of your Polk RT3 Modification.

See what you think. I think actually the cost of this mod will be significantly less than building a speaker from scratch. It is lucky there is a woofer that is about the right size, with T/S parameters that can be made to work.

 

[TLS Guy]

The Bemidji mini

Here is a small low cost speaker, the Bemidji Mini, that I have been designing lately. It is highly cost effective, and all the parts can be obtained from Parts Express..

The F3 is 50 Hz. The crossovers will be simple to build and have a component count of 5 in each crossover.

 

[Guiria]

I reviewed both designs. The modified RT3 seems to be more linear but with a sharper rolloff than the Bemidji mini.

How would you describe the dynamics of both speakers?

Would you claim one as a superior performer over the other?

 

[Guiria]

I need some crossover training. Forgive my ignorance

Poly caps are good - got it - I'll use Solen since TLS_Guy recommended that brand

What awg wire should I use for my air core inductors?

What type of resistors? Madisound's are way cheaper than partsexpress...does the resistor type matter? Also there is no 6.2 ohm resistor. Can you combine a resistor say 1.2 and 5 in parallel to make 6.2?

I priced everything out with drivers and inductors from Madisound, tweeters and other crossover parts from PE.

Total was 303.22...roughly $80.00 in crossover parts. Seems a lot to me but I've never built a custom crossover before...maybe that's normal. I can make things a little cheaper by finding more crossover parts from Madisound but it won't be much.

So I'm spending $300.00 to retrofit a current enclosure...these things better sound amazing.

While I'm at it should I be adding any kind of rockwool or anything to the inside of this enclosure?

Talk about a leap of faith...

 

[TLS Guy]

I need some crossover training. Forgive my ignorance

Poly caps are good - got it - I'll use Solen since TLS_Guy recommended that brand

What awg wire should I use for my air core inductors?

What type of resistors? Madisound's are way cheaper than partsexpress...does the resistor type matter? Also there is no 6.2 ohm resistor. Can you combine a resistor say 1.2 and 5 in parallel to make 6.2?

I priced everything out with drivers and inductors from Madisound, tweeters and other crossover parts from PE.

Total was 303.22...roughly $80.00 in crossover parts. Seems a lot to me but I've never built a custom crossover before...maybe that's normal. I can make things a little cheaper by finding more crossover parts from Madisound but it won't be much.

So I'm spending $300.00 to retrofit a current enclosure...these things better sound amazing.

While I'm at it should I be adding any kind of rockwool or anything to the inside of this enclosure?

Talk about a leap of faith...

Use this inductor for L2, this for L1

Use this for C1, and this for C2 and this for Ce.

Use this for Rp1, for Rp2 and for Req. Forget the 0.2 ohm. Six ohm will be just fine.

I would build them on this board.

Total cost per crossover is $33.05 per crossover $66.10 for the two. So you should come out at $290 for the pair. Education is a free bonus. I guess you have to ask yourself what could you buy for that money with quality European drivers with quality crossover components. The other thing is that you can tweak it without voiding the warranty.

The savings of DIY come into play when you put together the more esoteric systems. Although I certainly hope this speaker mod will perform better than one with cheap Far Eastern drivers and electrolytic caps.

I know you have been itchy to get your feet wet in speaker building and this will be a good starter project.

I would put some Rockwell behind the driver, and cover half the internal surface.

As to linearity, both are pretty good. Both designs are xmax limited, this will always be the case when using cheaper woofers. The Bemidji Mini has a lower Fs and 3db higher spl below 100Hz.

I have never used that Audax driver, but it has a very good reputation.

This small Vifa woofers are excellent, and I'm familiar with them. They produce a nice tight bass in a good cabinet and have a good midrange and excellent off axis response.

The Bimidji Mini is a slightly bigger speaker with a bigger driver, so that gives it an advantage. The mod however looks a good speaker for the size.

I would cross the mods over to the sub at 80 or 100 Hz. If you are not going to use a sub, you should probably increase Rp1 to 4 ohms. I would spend an extra ninety cents and see which you prefer in any case.

 

[Guiria]

Trigger is pulled.

All parts should be here by Monday the 9th.

I just realized however that the tweeter mount diameter is 104 mm which is 4.09". The recess tweeter mount on my RT3 is 3.75 with little room to spare. If the driver recess doesn't have to be enlarged on the top side then I should be able to make it work.

Hopefully it fits.

Also after careful examination of the boxes I'm thinking the front baffle is the only mdf piece. The others are a larger chip size particle board (not huge but larger than mdf). I don't think it's 3/4" either, probably 1/2".

I don't want a mediocre box with 290 worth of components. I might be building new boxes after all

 

[TLS Guy]

Trigger is pulled.

All parts should be here by Monday the 9th.

I just realized however that the tweeter mount diameter is 104 mm which is 4.09". The recess tweeter mount on my RT3 is 3.75 with little room to spare. If the driver recess doesn't have to be enlarged on the top side then I should be able to make it work.

Hopefully it fits.

Also after careful examination of the boxes I'm thinking the front baffle is the only mdf piece. The others are a larger chip size particle board (not huge but larger than mdf). I don't think it's 3/4" either, probably 1/2".

I don't want a mediocre box with 290 worth of components. I might be building new boxes after all

If you do build a new box you might want to increase Vb to 0.35 cu.ft, with a two inch diameter port of 6"

If you flare the port both ends, the length should increase to 7.5" This box will drop F3 to 59 Hz and put the 12 db point to 40 Hz.

I would modify the Polks first and see how you like the tonal balance.

If you want a little more bass the bigger box will get F3 just below 60 Hz which is pretty good for a woofer that size.

Good luck with your project, and have a lot of fun along the way.

 

[Guiria]

I will attempt to mod the RT3 enclosure first and see how that works.

Are there any baffle size constraints I should take heed of given the crossover design if I do build new boxes?

 

[TLS Guy]

I will attempt to mod the RT3 enclosure first and see how that works.

Are there any baffle size constraints I should take heed of given the crossover design if I do build new boxes?

I would try and make the front as narrow as you reasonably can, and round the corners if you can by placing half rounds.

I think you should be able to mount the drivers in the Polks, by my estimation anyway.

 

[Guiria]

I think you should be able to mount the drivers in the Polks, by my estimation anyway.

I just can't stomach the thought of putting these new speaker components into the RT3 box. I can also offset most of my box costs if I sell the RT3s, even with one broken. If I hack up the RT3 and things don't end up good then the boxes will be as good as trash.

I am taking the box design you created a couple of posts back.

I plan to keep the front baffle as narrow as the polks (7.25") and get the extra volume in depth and height.

I will be mounting the tweeter, woofer, and port aligned down the center of the front baffle. Unless their is some audible reason to offset one against the other.

 

[WmAx]

If you decide to scratch build cabinets, then please consider putting the effort into making them low in resonance. This means more than some bracing. One effective method is to use a standard material for the outer layer/exterior, such as 3/4" thick MDF or hardwood plywood. Then adhere 1/8" to 1/4" thick visco-elastic material to the inside, such as Dynamat or similar(Peel N' Seal - a roofing product available from Lowe's). Clean the aluminum foil facing with break parts spray and then score it with some sandpaper. Use a poly glue(such as PLH Premium Construction Adhesive, which comes in a caulking tube) to adhere a minimum of 1/2" thick Hardibacker brand fiber re-enforced concrete board to the visco-elastic layer. 1" layer is superior, obviously, but will increase weight even further. Use liquid poly glue such as Gorilla Glue or similar to glue the 2nd 1/2" layer of concrete to the first, if you want 1". Now use 1/8" thick 1" x 1" steel angle and run this vertically in at least 2 rows on each wall and continue it along the bottom and top walls. Apply to front panel, also. For the back panel, it may be easiest to only use the visco-elastic layer and concrete layer, then use spacers from the center line of the back panel to glue/adhere to the steel tubing that you will install as horizontal cross bracing. Adhere steel to the concrete using the PLH construction adhesive. Use steel square tubing and use metal epoxy to run steel cross braces every few inches. Note: use sand paper or other means to clean the steel at points that you use adhesive. The steel has a thin oxidized layer that must be removed if you desire maximum strength joints. Here is an example showing the layering of MDF/visco-elastic material/concrete and steel bracing: http://www.linaeum.com/productinfo/other/ascend_cbm170se_wmaxmod/9.jpg

-Chris

 

[Guiria]

I get the concept here...its the execution of it that worries me

 

[Guiria]

If you decide to scratch build cabinets, then please consider putting the effort into making them low in resonance. This means more than some bracing. One effective method is to use a standard material for the outer layer/exterior, such as 3/4" thick MDF or hardwood plywood. Then adhere 1/8" to 1/4" thick visco-elastic material to the inside, such as Dynamat or similar(Peel N' Seal - a roofing product available from Lowe's). Clean the aluminum foil facing with break parts spray and then score it with some sandpaper. Use a poly glue(such as PLH Premium Construction Adhesive, which comes in a caulking tube) to adhere a minimum of 1/2" thick Hardibacker brand fiber re-enforced concrete board to the visco-elastic layer. 1" layer is superior, obviously, but will increase weight even further. Use liquid poly glue such as Gorilla Glue or similar to glue the 2nd 1/2" layer of concrete to the first, if you want 1". Now use 1/8" thick 1" x 1" steel angle and run this vertically in at least 2 rows on each wall and continue it along the bottom and top walls. Apply to front panel, also. For the back panel, it may be easiest to only use the visco-elastic layer and concrete layer, then use spacers from the center line of the back panel to glue/adhere to the steel tubing that you will install as horizontal cross bracing. Adhere steel to the concrete using the PLH construction adhesive. Use steel square tubing and use metal epoxy to run steel cross braces every few inches. Note: use sand paper or other means to clean the steel at points that you use adhesive. The steel has a thin oxidized layer that must be removed if you desire maximum strength joints. Here is an example showing the layering of MDF/visco-elastic material/concrete and steel bracing: http://www.linaeum.com/productinfo/other/ascend_cbm170se_wmaxmod/9.jpg

-Chris

A note of curiosity. How much does that box in the picture weigh after your modifications?

 

[Guiria]

What quantifiable measurements are used when testing a cabinet with internal dampening vs. without? I'm learning here.

Is it off axis response? linearity? etc, etc...

Are their any polymers out there that can absorb the frequencies of a full range speaker or even a bookshelf speaker?

I must admit for my first build with a 2 way the above bracing seems a little tricky to execute and I will most likely go with some internal wood bracing for now.

 

[avaserfi]

A note of curiosity. How much does that box in the picture weigh after your modifications?

They likely weight 25-30lbs a piece.

What quantifiable measurements are used when testing a cabinet with internal dampening vs. without? I'm learning here.

Cabinet resonance is the measurable aspect of added bracing/internal dampening. The coloration from cabinet resonance does not show up in typical measured frequency response graphs. Rather taking multiple impulse response measurements at various angles in an anechoic chamber and averaging them to derive a waterfall plot will show resonance. Also, use of an accelerometer can give one a less accurate, but general idea of cabinet resonance.

Are their any polymers out there that can absorb the frequencies of a full range speaker or even a bookshelf speaker?

The entire audible frequency range does not require equal construction methods to fully attenuate resonances. For example a subwoofer only requires simple bracing techniques that are common, but as the pass band rises the required dampening increases exponentially.

There are certain materials that are far more capable than others at converting energy into heat and thus minimizing resonances. Chris recommended peel n seal which is an asphalt based roof flashing material. Asphalt happens to be an excellent material for use in a viscoelastic constrain layer because it so efficiently makes the conversion previously mentioned. When such a material is placed between two materials such as plywood and cement board as pictures the viscoelastic layer's effectiveness increases greatly due to the added mass and created sheer forces created.

The use of a high grade acoustic damping material within the cabinet such as OC705 or 8lb mineral wool removes all internal reflections which also can cause such resonances.

I must admit for my first build with a 2 way the above bracing seems a little tricky to execute and I will most likely go with some internal wood bracing for now.

If you take your time measuring and cutting as well as gluing it is a doable project. The reason for recommending use of steel is because it is far more dense than wood allowing for a less dense bracing matrix and a audibly inert cabinet. Also, this allows for a smaller cabinet to be build since less volume is dedicated to bracing.

If you decide to go with a wood bracing method use of oak stock would be ideal. Also, to achieve the same results you would need to construct a bracing matrix that leaves no gap larger than 3" (ideally 2") in any direction on the XYZ axes.

Another example (do note there are two constrain layers in use in my build):