Reverse Engineering a Loudspeaker

[JohnnieB]

As many here probably know, Ascend Acoustics has provided an upgrade path for their speakers, and I will be performing the upgrade on my Sierra Towers RAAL. I am considering the possibility of "reverse engineering" a set of cabinets so I can re-use the original drivers in a DIY type of build. I am considering this for a fun learning experience and if they turn out successful, a set of towers that I will enjoy in my billiard room. I am under no illusion that this is going to be as simple as it might seem to someone like me who is not experienced in speaker design. I will however be starting out with proven drivers and crossover, so I figure that's half the battle.

I would need to come in well under the price of new towers for this to make some sense otherwise I would just buy the ELX Sierras and skip upgrading my current towers. As I have it estimated, I come in at $1700 budget for cabinets to match new tower price. I would think I could build cabinets for under 1k so on the surface it seems to make sense. I'm still waiting on final numbers but can't imagine I'm too far off.

As I mentioned earlier, this probably won't be as simple as my imagination would have me believe. I am going on the assumption that I will not be able to acquire Thiele Small parameters for the drivers but if internal cabinet volume is kept the same, would they be necessary? I plan to use 3/4 MDF which will certainly have different characteristics from the original bamboo so considerations may have to be made regarding resonances, bracing and cabinet thickness. If the front baffle is made identical in width, height and thickness I would think any necessary adjustments in overall cabinet size could be accounted for in depth. Lastly, would I need design software given I have a proven enclosure to go off of? If so, what software is recommended? I have found a few mentioned throughout the forum but all seem to be geared toward subwoofer design. Can these also work for a 3-way cabinet design?

This is my starting point. Does this sound like a viable endeavor or am I fooling myself. I realize there are probably many more considerations to be made that I am not aware of, but I humbly submit to the experience of the forum for insight.

Thank You

 

[TLS Guy]

As many here probably know, Ascend Acoustics has provided an upgrade path for their speakers, and I will be performing the upgrade on my Sierra Towers RAAL. I am considering the possibility of "reverse engineering" a set of cabinets so I can re-use the original drivers in a DIY type of build. I am considering this for a fun learning experience and if they turn out successful, a set of towers that I will enjoy in my billiard room. I am under no illusion that this is going to be as simple as it might seem to someone like me who is not experienced in speaker design. I will however be starting out with proven drivers and crossover, so I figure that's half the battle.

I would need to come in well under the price of new towers for this to make some sense otherwise I would just buy the ELX Sierras and skip upgrading my current towers. As I have it estimated, I come in at $1700 budget for cabinets to match new tower price. I would think I could build cabinets for under 1k so on the surface it seems to make sense. I'm still waiting on final numbers but can't imagine I'm too far off.

As I mentioned earlier, this probably won't be as simple as my imagination would have me believe. I am going on the assumption that I will not be able to acquire Thiele Small parameters for the drivers but if internal cabinet volume is kept the same, would they be necessary? I plan to use 3/4 MDF which will certainly have different characteristics from the original bamboo so considerations may have to be made regarding resonances, bracing and cabinet thickness. If the front baffle is made identical in width, height and thickness I would think any necessary adjustments in overall cabinet size could be accounted for in depth. Lastly, would I need design software given I have a proven enclosure to go off of? If so, what software is recommended? I have found a few mentioned throughout the forum but all seem to be geared toward subwoofer design. Can these also work for a 3-way cabinet design?

This is my starting point. Does this sound like a viable endeavor or am I fooling myself. I realize there are probably many more considerations to be made that I am not aware of, but I humbly submit to the experience of the forum for insight.

Thank You

If you keep the internal volume, porting, driver spacing identical, and the width of the front baffle the same as the original cabinets, then your project should work. So you need to copy the original cabinets as closely as you can.

I'm sure Ascend used the correct enclosure volume and porting, so if you copy that exactly the drivers should be properly tuned. You also need to use the same quantity of internal damping material as the originals.

 

[JohnnieB]

Thanks for the reply TLS

I’m glad to hear it’s a viable project. I sure hated the idea of having these great components go to waste.

 

[Swerd]

I agree with everything TLS Guy said. I would add, just for emphasis, that when he said "keep driver spacing identical" to that of the original cabinet, that means not only the distance between drivers, but also the distance between drivers and the edges of the front baffle.

If you reproduce the interior cabinet volume and keep driver spacing & placement the same as in the original, then you can use the original crossover.

Do you have any woodworking experience? And, more importantly, do you have a way to countersink the circular cutout holes for the drivers? Most DIY speaker builders use a plunge router with a circle cutting jig. There are other ways as well, but I've only done it with a router. If you haven't done it before, I suggest you practice on some scrap MDF. Ask, if you want more details on that technique.

I'm glad to see you choose to use MDF and not bamboo. Bamboo will make sharp new tools and router bits quickly get dull. Ascend Acoustics is prepared for that, but most DIY builders are not.

Ascend Acoustics houses the midrange driver in its own damped & sealed enclosure within the cabinet. Make sure you understand how this was done in the original cabinet. Also, be sure to look at how Ascend did any internal braces in their cabinet.

 

[MrBoat]

As someone who has done a lot of woodworking, sometimes, just knowing I can do something is enough. In other words, building boxes is not difficult. I could essentially build the exact copy and nobody else would care to know the difference. Now if the cabinets are not built to the highest necessary standards, then sure.

I have thought about doing this before, but it all ends up coming down to the actual finish, which is the most difficult and observable part. Since I know I could make the exact same box, I'd be more inclined just to refinish the old ones, whether that be new (perhaps more exotic) veneer or paint.

 

[JohnnieB]

I agree with everything TLS Guy said. I would add, just for emphasis, that when he said "keep driver spacing identical" to that of the original cabinet, that means not only the distance between drivers, but also the distance between drivers and the edges of the front baffle.

If you reproduce the interior cabinet volume and keep driver spacing & placement the same as in the original, then you can use the original crossover.

Do you have any woodworking experience? And, more importantly, do you have a way to countersink the circular cutout holes for the drivers? Most DIY speaker builders use a plunge router with a circle cutting jig. There are other ways as well, but I've only done it with a router. If you haven't done it before, I suggest you practice on some scrap MDF. Ask, if you want more details on that technique.

I'm glad to see you choose to use MDF and not bamboo. Bamboo will make sharp new tools and router bits quickly get dull. Ascend Acoustics is prepared for that, but most DIY builders are not.

Ascend Acoustics houses the midrange driver in its own damped & sealed enclosure within the cabinet. Make sure you understand how this was done in the original cabinet. Also, be sure to look at how Ascend did any internal braces in their cabinet.

I do have the necessary tools for this project, table saw, router, jig saw, bar clamps etc. Between woodworking and construction experience I feel I should be able to handle the build.
Front baffle being the critical component here is understood. Since I’m not sure of the wall thickness with the bamboo until I can measure them, I figured any loss/gain of volume due to thicker/ thinner walls could be adjusted with cabinet depth. I may end up getting some larger snap gauges to be precise about measuring internal volume, not sure I trust a Stanley tape measure in this regard.

As someone who has done a lot of woodworking, sometimes, just knowing I can do something is enough.
I have thought about doing this before, but it all ends up coming down to the actual finish, which is the most difficult and observable part. Since I know I could make the exact same box, I'd be more inclined just to refinish the old ones, whether that be new (perhaps more exotic) veneer or paint.

I was planning to veneer the cabinet. Since 4x8 sheets of wood veneer range from $130-infinity, I’m looking at other possible options. I have found a resin impregnated paper with a clear melamine top coat that looks quite good. I can get it in many different wood grains. It is 27mil thick and looks like it should work, about $60.
I also considered paint but a nice automotive with clears would be as much as the veneer.

Thanks for the replies. I feel less apprehensive than I was initially. I think this will be a fun project and should make a nice addition to my billiard room.

 

[Swerd]

I do have the necessary tools for this project, table saw, router, jig saw, bar clamps etc. Between woodworking and construction experience I feel I should be able to handle the build.

Good. There's probably a lot you know better than I do.

I'm relieved that I don't have to explain in full detail how to cut countersunk circles. Basically, I use a plunge router with two different bits. First, I cut a mortise to fit the outer dimension of the driver, using a 1¼" mortise bit. I cut it deep enough (roughly 3 to 5 mm) to allow the driver's rim to sit flush while there is 3/8" or 1/2" compressible gasket tape beneath it. It helps to use a plunge router and it's depth gauge. The photo shows the mortise cut for a tweeter with a 4⅛" diameter mounting plate.

Next, I use a ¼" spiral upcut bit to make a smaller diameter hole through the MDF for the driver's basket (see below). If you cut the through hole first, and then use a rabbit bit to cut the flange, the resulting flange is barely wide enough to use #8 pan head wood screws to mount the driver. (Those "ears" on either side of the hole are for the tweeter's terminals.)

Since I’m not sure of the wall thickness with the bamboo until I can measure them, I figured any loss/gain of volume due to thicker/ thinner walls could be adjusted with cabinet depth. I may end up getting some larger snap gauges to be precise about measuring internal volume, not sure I trust a Stanley tape measure in this regard.

I don't believe real precision is required. If you're within 10% of the theoretical cabinet volume, you'll be good. Ascend Acoustics says their cabinets are made from 20 mm thick stock, just a bit more than ¾". Make the cabinet volume a bit larger than what you need. Then, if needed, add some extra cross bracing to subtract some volume.

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[JohnnieB]

Good. There's probably a lot you know better than I

Thanks Swerd but…let’s not get ahead of ourselves here.
As Boat commented, making a box isn’t hard, however if one isn’t paying attention it’s entirely possible to make a mess of that too.

 

[JohnnieB]

Update….
The components were somewhat cheaper than I budgeted for so that’s a plus.

OTOH, Dave recommended not using mdf as I would have to double up to 1.5” thick to match the rigidity of bamboo. He did provide a link for 20mm layered bamboo ply but as one can imagine it’s considerably more. Not to mention shipping and the added cost of damaged tooling due to working with the bamboo. Swerd alluded to this and I know Dave has commented that their cabinet maker goes through tooling bits like crazy but I’m only making one pair of cabinets not several hundred.

I’m also not sure if cabinet configuration would make a difference. IOW, if the cab were made deeper to compensate how does this affect porting? If internal volume is the same and port diam and length are the same, the tuning should be the same, am I correct on that?

 

[Swerd]

I’m also not sure if cabinet configuration would make a difference. IOW, if the cab were made deeper to compensate how does this affect porting? If internal volume is the same and port diam and length are the same, the tuning should be the same, am I correct on that?

Yes. As long as the internal volume, port length & diameter are all the same, the bass tuning will be the same as in the original.

If you use two layers of ¾" MDF for the front baffle, the outside depth of the cabinet will be ¾" longer, while the internal volume stays the same. Do you plan to mount the port on the front or rear of cabinet? How long and what diameter is the tube?

You can mount the port at the rear of the cabinet. If the rear panel is ¾" that would avoid the problem.

If you need a longer tube to deal with a thicker front baffle, you can get the same tuning with a longer and wider tube. There are online calculators to help with those port tube dimensions. @TLS Guy probably has software for that too.

From the Impedance vs. Frequency curve, the bass is tuned to about 38 Hz.

 

[Swerd]

I just looked over the long list of calculators available at Madisound that could help you find out what length and diameter port tube to use:
http://www.mh-audio.nl/Loudspeakers.html#top

On that list, this one looks the simplest and might be just what you need:
http://www.mh-audio.nl/Calculators/WVC.html

Others are a bit more complex, but they might help:
http://www.mh-audio.nl/Calculators/RBC.html
http://www.mh-audio.nl/Calculators/BRA.html

Bass Reflex Box Calculator (max. flat amplitude) with the possibility to customize Vb

www.mh-audio.nl

 

[JohnnieB]

Thanks for the links Swerd.
The plan is to port the rear of the cab like the original.
I am now considering plywood for its stiffer properties and I think finishing may actually be easier. Plywood is still considerably cheaper than the bamboo would be and can be sourced locally.

 

[Swerd]

Thanks for the links Swerd.
The plan is to port the rear of the cab like the original.
I am now considering plywood for its stiffer properties and I think finishing may actually be easier. Plywood is still considerably cheaper than the bamboo would be and can be sourced locally.

When you choose plywood or MDF, another point to think about is how you plan to finish the cabinets.

MDF has a flat smooth surface making it easier to apply veneer. Painting especially with glossy paint, will require a lot more work, filling, sanding, and many many coats of paint, with fine sanding between coats. In my hands, I hated painting MDF because it telegraphs every minor flaw in the wood's surface. I much prefer veneer over paint.

Expensive plywood can be almost as easy to finish as MDF, but the cheaper plywood varieties have a rougher surface than MDF.

If you want to paint cheaper plywood, I suggest bringing the cabinet (minus the drivers, crossover board, & wiring) to an auto body shop. They are equipped to apply filler to smooth out a rough surface, and have painting tools, spray booths, and dust control better than I've ever seen in any DIY set up. Once I saw the results from an auto body shop, I knew I would never again try to paint speaker cabinets.