woofer replacement recommendation

[yelkenli]

I have old 75 watt Realistic cabinets with 12 inch, 8 ohm woofers with low frequency at 50Hz. This is the Optimus 50 model.

the cabinet measures roughly 23.5"H, 12.75"W, and 8.25"D on the interior. there is damping cloth, a mid-range, and a tweeter, and some driver hardware. there is a 4 inch diameter cylinder that isolates the mid-range. there is no other partitioning for the tweeter and mid-range (i.e. the woofer is open to all of the interior volume measured above).

the cabinet is vented.

In replacing a woofer, it is recommended to look at the interior volume. I am unsure if the woofer interior measurement should include all of the volume measured above.
Also, I am not sure if I need to adjust anything - I have seen comments about 25% reduction for volume due to venting. Since I am not an expert, I am unsure of applicability.

Ultimately I need a recommended replacement. This is not a high end speaker system, as you can tell, but I would like to restore it to its baseline (there is a pun in there).

Can anyone recommend how to proceed on finding a replacement, and if there are any replacement speaker sellers that might carry a suitable replacement?

Thanks,
Ted

[llung]

Hi Ted,

Here are some thoughts. If I did the math right, the cabinet interiors works out to about 1.4 cubic feet or about 40 liters. You should account for anything that protrudes into that space so you'd want to deduct the space taken by the midrange cylinder. Of course, other things also take up space such as the magnets on drivers and crossover components, but you usually don't need to be that super precise on the volume. You can always error on the side of assuming it's smaller than it actually is.

But before starting your driver selection, see if the existing cabinet is good enough. What's it made of? how thick are the walls? From the description you gave I'd bet it's little more than 1/2 inch particle board. If you think that's too thin, you may want to consider reinforcing the interior of the cabinet - laminate a layer inside for added stiffness? add ribbing?

A 12 inch woofer in that cabinet is way too large IMHO. To use a smaller woofer would mean shrinking the existing hole. While you're at it, you may want to consider a 2-way design (depending on the drivers used). This would mean filling one of the other 2 openings. And let's not forget the port. You didn't say what size the port is - length and diameter of the port. Since you don't have an alignment yet (vented, sealed), ask yourself how you would handle either case - how to change the size of the port, how to fill it and make it a sealed box, etc. Also, chances are the eventual driver choices will not be an exact match for the existing cutouts in the baffle. So the summary is that the baffle will likely need a makeover. Think about how you want to do this, then calculate the volume you'll end up with.

Once you've arrived at a rough cabinet interior volume, do some research on drivers. Since this is not a high end design, I'd recommend something simple like an MCM or Parts Express house brand driver. Look for one or more choices that will work in something roughly the volume you calculated. My gut says to look at 6.5, 7 or maybe 8 inch drivers in a 2-way design. For simplicty, I'd go sealed but if you want that added bass, you can go with a vented design.

Sorry, I'm short of specifics for drivers and such. I guess the crux of my reply is to have you look at the potential issues faced with using the existing box before diving into the actual driver selection and design. A retrofit is not always easy nor a good thing but it can work so long as you plan ahead.

hope this helps
-lou

[DVDdoug]

The interaction between the woofer and cabinet is complicated, but with the Theile/Small paramaters and a computer program the results can be predicted.

I suggest you download a copy of WinISD (FREE speaker design software). Plug-in your cabinet volume and port dimensions, and then plug-in the Thiele/Small parameters for some potential repalcement woofers to see if you can find a good match.

In replacing a woofer, it is recommended to look at the interior volume. I am unsure if the woofer interior measurement should include all of the volume measured above.
Also, I am not sure if I need to adjust anything - I have seen comments about 25% reduction for volume due to venting. Since I am not an expert, I am unsure of applicability.

There are really no "simple rules". If you blindly change the woofer or blindly add a port, you can end-up with "weak" bass or 'boomy" bass.

Some woofers work better in sealed cabinets, so you may end-up blocking-off the port. Or, you may need to alter the port dimensions for best performance. If the softwre tells says you need a smaller cabinet it's not too hard to reduce internal volume by inserting something to take-up space.

[yelkenli]

Thanks for the recommendations. The walls are about 5/8 thick furniture grade particle board and veneers. I do not know if that is rigid enough. Maybe you can point me to some literature to read regarding that aspect of speaker design. the same goes for other aspects as well.

The 'vent' is 2 inches inner diameter, and extends 4 inches into the interior, and looks like a piece of plastic inserted into the face of the baffle. Assuming I am using the term 'vent' correctly. Any links to design literature would be great.

I will also check out the design software.

I notice that the cylinder for the mid-range is used to open the back of the mid range to the outside of the cabinet. Any comments on this? Is this a useful feature or marketing?

Any ideas on how to assess the condition of the tweeter, mid-range, and electronics? These are real old as well. I know the woofers are bad as parts of them are missing. The other hardware doesn't display any issues. This whole unit probably should be tossed, but I thought it would be interesting to refit and I might learn something, and I am most interested in obtaining a 'good' quality sound, not necessarily a huge amount of bass.

Thanks,
Ted

[llung]

Hi Ted,

Speaker building can be as rigorous and complicated as you want it to be, or as simple and cookie-cutter as you can hope for. The former will make you an addict and the latter will give you instant satisfaction! You don't have to be an addict to get good sound and chances are that even a simple DIY design will sound better than the old Realistic speaker. So that said, here's my recommendation.

The 5/8 inch particle board cabinet is ok. Lots of commercial speakers have been made with it because it's cheap. Not to say that you can't get nice sound from it. One of my old favorites were EPI 100 from the late 70's and the cabinet was cheap particle board. But the sound was nice, neutral and very good. But DIYers can do better! You have at least 4 holes in the front baffle (3 drivers + 1 port) and 1 in the rear (midrange). Not sure what you have for hookup connectors. Your new drivers will likely be very different so...

I would toss the existing drivers and crossover (or save what works and experiment elsewhere). Pick a new driver complement (more below) and mount to a new baffle of 3/4 MDF and glue (and screw from the inside if you can) that over the existing baffle. You can then make a grill that fits over that. Obviously you'll have to tweak the overall appearance somehow! For the rear, I would do something similar just to get rid of that hole from the midrange. I would also use a round or rectangular terminal cup in the new rear surface. For the 2 sides, top and bottom, glue some wooden ribs on the inside to stiffen those surfaces.

On to the drivers. Parts Express has a simple woofer selection guide here:
http://www.parts-express.com/resources/woofer-selection-guide.cfm
For each woofer from 4 inches on up, there are sealed and vented cabinet alignment volumes and F3 (bass response). Your current box is about 1.4 cubic feet so look in that range. You can go a little larger for a sealed box by adding some stuffing in the box. In fact, if this is your first speaker and you want something simple, I'd go with a sealed design. For an F3, most music doesn't reach much below 60 Hz. Truly low bass is rare in modern music and is more for movie effects where a subwoofer comes into play. For each driver that looks like it would work, look at the rest of the numbers and see if they work for you. Cost is not in the table so you have to look up each driver. It'll be apparent that some brands are more expensive and likely higher quality than others. But there real bargains and gems out there too so don't judge purely by price.

Again for simplicity, I would go 2-way. That will limit your woofer to maybe 8inch max (depends on the woofer and tweeter used). A 3-way is much harder to do well. Perhaps a 6.5 inch or 7 inch would be best.

Don't be limited by what you see at Parts Express. For example, check out Madisound or MCM Electronics for other drivers. Use the posted T/S parameters and sw tools to model the low end F3 response.

Hope this gets you started.
-lou

[DVDdoug]

Maybe you can point me to some literature to read regarding that aspect of speaker design. the same goes for other aspects as well.

Try diysubwoofers.org. (The theories & physics for a subwoofer are the same as a regular woofer.)

You could try looking for a speaker design book at Amazon.com or Parts Express. I can't recommend a specific book... I've got a pretty good book, but it's way outdated (before software made things easier), and I've got another book that has several "cookbook" designs, but not enough "theory".

Playing around with the software should help too. i.e. You can easily plug-in some different port dimensions, or try a sealed box to see what happens to the response curve...

Assuming I am using the term 'vent' correctly. Any links to design literature would be great.

Yes. The terms "vent" and "port" are interchangable. (Maybe there's some technical difference... in either case we're talking about a "hole".)

I notice that the cylinder for the mid-range is used to open the back of the mid range to the outside of the cabinet. Any comments on this? Is this a useful feature or marketing?

It's unusual, but it's something like the Bose Direct-Reflecting speakers, designed to reflect some sound off the walls to give a sense of space... From RadioShack.com:

The back portion of the midrange element is left open to radiate sound to
the room rather than absorb within the system or element enclosure. This
allows an increase in spaciousness to the sound which contributes to the
life-like sound quality of Realistic Optimus-50.

The effectiveness of something like that will depend on the acoustics of the room, placement in the room, etc. Again, you can experiment with plugging-up the midrange "port".

Any ideas on how to assess the condition of the tweeter, mid-range...?

If the midrange and tweeter are not physically damaged, if you're getting sound out of them, and if the sound is OK*, they are probably fine. (I assume that your woofers are either physically damaged or the foam surround has rotted-away.... Woofers with rubber or cloth surround can last "forever", but foam seems to deteriorate.)

... and electronics?

The electronics are probably in good shape. Assuming nothing drastic like corrosion... Coils last forever. Resistors (if any) last forever. Most "film" capacitors last forever.

However, your crossover may use non-polarized electrolytic capacitors, and they can change in value (microfarads) as they age. Electrolytics are cheaper and they have looser tolerances than other capacitors.

Electrolytic capacitors have sort-of a crimp around each end, and a shrunk-on plastic insulating wrapper. A film capacitor (or other non-electrolytic) will usually have a smoother, "cleaner", appearance. If your crossover uses something other than electrolytic capacitors, that's a sign of a good-qualtity speaker system.

If you have electrolytics and you decide to change them, just get the same value (uF) and the same or higher voltage. If you get an electrolytic, make sure to get a non-polarized electrolytic. (Most electrolytics are polarized... Non-polarized electrolytics are special and only used in crossovers.) If you upgrade to non-electrolytics, they will be larger and it might take some creativity to install. You probably won't hear a difference if you replace the capacitors, but it wouldn't hurt. (I'm assuming you have a soldering iron.)

Your speakers also have mid & tweeter level controls. These can get "dirty", and might have dead spots. If these are not operating "smoothly", you can spray some Tuner/Control Cleaner into the controls and work them back-and-forth. That usually makes the controls "good as new".


* You'll probably have to replace the woofers before you can really evaluate the sound quality of the mid & tweeter.

[llung]

Oh I forgot to mention some reading material.

For web sites, look up the excellent compilation by Bob Stout - the Loudspeaker Design Selection Guide or LDSG
http://ldsg.snippets.org/
Last time I checked, some of the links in the pages were dead but others worked just fine. You can learn a lot those pages, not to mention the links to other useful sites.

If you want a physical book to read, get the bible for DIY speaker building - Vance Dickason's _Loudspeaker Design Cookbook_ Google it to see who carries it. Madisound and Parts Express should. Despite its name, this book is not a "cookbook" per-se, that is it does not provide recipes. Rather it gives all sorts of technical details on how speakers work and what you need to know to make them work for you.

hope this helps

-lou

[lvw]

You mention that parts of the woofers are missing. Do you mean the cone surrounds (the outer part that joins the cone to the speaker frame) is damaged? If so, that's pretty common with older drivers having foam surrounds. Often all that's needed is a replacement surround, and kits for that job are fairly cheap and easy to use.

That repair would not, of course, improve the speakers from the original. Easy though, and may match what you want to do...

Larry Van Wormer

[yelkenli]

The damage to the woofer is the surround. Every time I take the fabric grill of the front, more of it falls out. The remainder looks undamaged (amazing they have survived my kids).

I will pick up some of the literature and look into some rebuild. I have all the tools and skill to tackle something like this. I just need to set a plan.

Thanks for the recommendations. This is a great help.

-Ted

[DVDdoug]

The damage to the woofer is the surround. Every time I take the fabric grill of the front, more of it falls out. The remainder looks undamaged (amazing they have survived my kids).

I will pick up some of the literature and look into some rebuild. I have all the tools and skill to tackle something like this. I just need to set a plan.

Larry makes a good point! Here is a repair kit. There are also companies that do this for you, or you can get the speaker reconed. When a speaker is reconed, essentially everything except the magnet & frame are replaced and as long as the frame isn't' bent, the speaker is truly "like new".

Re-edging and reconing are good ways to restore the original specifications. (If the reconing company doesn't have the exact replacement parts, they usually have something close.) The downside is that reconing can be expensive, especially you can't find a local shop and you have to pay for shipping both ways. It's usually only cost-effective when you're repairing an expensive speaker.