DIY - China Syndrome
The Repository->DIY->China Syndrome
ATTENTION: the Vifa Logic AC25 tweeter is No Longer Available. This project is displayed for reference and archival purposes. Please do not ask me to substitute in another tweeter at this time.
Silver Flute W14RC25-S + Vifa Logic AC25SG05
8"w x 14"h x 10.5"d
11 Liters (.375 cu ft) vented | F3 @ 49 Hz
Total cost: between $90-105 per pair
Well, it seems that as soon as I published the design for the Orient Express, which used the unshielded version of this woofer, people were immediately asking me for a version using the shielded woofer. Why? Because Madisound is blowing them out for a steal of a price at $12.50! (Granted, the unshielded model is only $20)
Is the shielded woofer a drop-in replacement in the Orient Express? Unfortunately, no. Not at all. And since I had already delivered the Orient Express to their final owner, I couldn't re-measure and tweak a new crossover. However, a very generous member of the DIYAudio forum sent me a pair of the Silver Flute shielded woofers as a bit of a "nudge," which finally got me off my lazy arse to do a new design.
So here we are... very similar lineage to the Orient Express, but a slightly new design: the China Syndrome. This was designed to work equally well as a bookshelf speaker or Home Theater satellite. They were voiced to sound best up near a wall or on a bookshelf, and to have a very clear and distinguishable midrange. It also has a comparable parts cost of around $100.
Matching Center Channel Design here
I tried to keep things very sensible with this cabinet design. To make the speaker as modular as possible within a room, everything is centered horizontally on the baffle. The enclosure itself is the exact same size as the Parts Express pre-built .375 cu ft cabinets (which are beautiful, by the way). Everything is made from 3/4" stock, and you will probably want some sort of horizontal brace (3" wide piece of scrap, thick dowel, windowpane, etc.) tying the sides of the enclosure together.
I put the vent on the front because I designed the speaker to sit near a wall boundary. I used 1.5" dia PVC pipe with a 1/4" roundover. The total vent length is 3.5" Since the internal volume is about 11 Liters, this tunes the box to 50 Hz. With the walls lined with two layers of carpet padding, I get an F3 of about 49 Hz. If you want to use a pre-fabricated vent instead of PVC, that's fine. We can easily plug the numbers into Unibox or somesuch box modeling program and find the correct length.
Also, the design can be built sealed. I'd recommend an enclosure between 7-9 Liters, heavily stuffed. This should give an F3 around 70 Hz.
The crossover for the China Syndrome uses the same basic topology as the Orient Express. But values are all different because I had to cross lower.
Here's a bit of help with component layout.
Piece of cake, right? Ha!
This design basically proves the case that getting a flat Frequency Response in simulation doesn't mean you will have a listenable speaker. Right now, my hard drive has about 20 different versions of the crossover--each with a flat FR--most of which are very difficult to listen to. This speaker took a lot of critical listening, re-evaluating, and humility to get right.
What went wrong? Well, for starters, let me say that just because a driver is really inexpensive doesn't mean it's necessarily bad. In fact, I believe that at least half of the DIY community has done speakers using "budget" drivers, and the results are often mind-blowing in terms of sound quality. The thing to be wary of in regards to inexpensive drivers, though, is that the manufacturer's specs of them are often very... shall we say "glossy"? Many Chinese companies will smooth the FR graph so much and crunch the dB scale to the point where you could swear it's a Vifa P13. Don't be deceived!
Let's take the Silver Flute woofer as an example. Here is the FR graph Silver Flute publishes for the woofer:
Looks pretty flat, right? Sure, maybe a bit of wiggle between 1 and 2 KHz, and a breakup peak out at 7 KHz... but still, doesn't look too daunting.
Now, here is my measurement of the W14RC25-S. (Don't panic that the response falls off after 200 Hz; that's a normal artifact of using MLS measurements. The only reason Silver Flute's graph has "bass" measurements is because they pasted on a simulated response of the woofer in a box up until 700 Hz.)
Similar? Yes. But that breakup mode is looking a bit more challenging now that we look at it up close (and it should!). A bit less obvious of a problem is the 5 dB hump in response centered around 1 KHz. This actually posed a bigger problem than the breakup, because it meant that the woofer will have an inherent "barky" quality.
I honestly didn't notice the midrange hump for a while. It wasn't until about a week of listening to these things, and never quite feeling happy with the overall sound. I found the speakers to be sort of annoying and fatiguing, despite a relatively flat overall System Response. I spent day after day adjusting the tweeter's filter, thinking that was the cause of the obnoxiousness. But it seemed no matter what I did, I just wasn't liking them. Finally--perhaps by accident--I listened to the woofers on their own without the tweeters. Instantly I thought, "Man, this sounds awful! No one should pay good money for something that sounds this harsh. I dont even want to hear it with a tweeter." That midrange hump had to be addressed, and to my knowledge, there are only really ways of dealing with it. One would be to use an LCR shunt filter to EQ the midrange down to an acceptable level. But that would cost more money in crossover parts than the woofer was even worth! So instead, I went with the simpler answer, which was to cross lower. After lowering the XO point, I listened to the woofers again on their own and found them to be much more natural (in fact, compared to what they used to sound like, it was downright soothing!).
Long story short, the shielded and unshielded Silver Flute were far from being "drop in replacements" for one another. In fact, if you tried to use the shielded woofer with the Orient Express' crossover, you might just want to rip your ears out, because not only will the midrange hump bark at you, but the breakup mode is at a different frequency and the notch filter wouldn't catch it.
But here's the rub: I had to cross lower than I did in the Orient Express, which means component values are larger, which means that the crossover for the China Syndrome costs a few dollars extra. Sorry! Seems you can't win sometimes; you pay less for a driver, only to pay more for the necessary crossover components.
I have really put these things through the ringer to make sure I got it right. The problems I encountered along the way scared me into making sure the final product was as robust and listenable as I could possibly milk out of these drivers. I've tested these speakers out on all styles of music (and movies) at all volumes. They do have a clear and articulate midrange, and throw out a pretty big overall sound despite being rather small. A few times I actually forgot I had them hooked up and mistook them for some of my larger speakers. The China Syndrome sound pretty good cranked, too, as I found out while vibrating the walls with Incubus' breakthrough album "SCIENCE."
It's time to roll this project out. If you're looking for a very inexpensive set of bookshelf speakers to fill a medium-sized room with sound, or build an exciting HT on a tight budget, then this project should be a blast!
(c) Paul Carmody | this page was last updated October 14, 2009