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MultiWay Conventional loudspeakers with crossovers 

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30th June 2007, 03:26 PM  #1 
diyAudio Member
Join Date: Jun 2007

Impossible Task? RePorting Speakers?
I realize that what I am asking is difficult, but if anyone would like to take a shot at it, or direct me to some references, it would be appreciated.
And, I apologies in advance for being so longwinded. First, my speakers  Woofer  CTS 12W38C 12" 60 watt (1377828) fs: assumed 23 hz I think the '38C' means 38 ounce Ceramic Magnets Mid 3"x9" Horn (assumed 40 to 100 watt) HEP40TM Similar to http://www.partsexpress.com/pe/showd...number=280050 High Piezo 3.5"x3.5" (assumed 40 to 60 watt) connected to high freq output of crossover with added 10 ohm resistor across terminals to stabilize the apparent impedance load to the crossover. Similar to http://www.partsexpress.com/pe/showd...number=270011 Crossover SPECO HN3100 100 watts 3way Similar to (visually identical to) http://www.partsexpress.com/pe/showd...number=260210 Cabinets Ext. 17.5"w x 13"d x 26"h Int. 16"w x 11.5"d x 24.5"h 4508 Cu.In or 2.61 Cu.Ft. Bass Ports  (2X)  3" x 3" Ports 3/4" deep 18 sq.in. port area Reference book used in the design process "How to Build Speaker Enclosures" by Alexis Badmaieff and Don Davis. I'm hoping to increase the bass response by reporting the cabinets; that's what I need help with. See attached photo. Unfortunately, I built these speakers a long time ago. So long ago that I can't find any specs by searching the internet. In fact, I can barely find any reference to any of these components at all. I don't remember what the crossover points are, but logically, I wanted the low in the 700 to 800 range, and that would make the high in the 3,000 to 5,000 range. Though since I'm using a Horn Mid, I would want to use it and would expect the high cross over to be high, in the 4000 to 5600 range. [700/5000 seems very reasonable) Performance Nice, exceptionally clear, naturally with the Horn Mid I have tons of clear midrange which I dial back with an LPad. The Piezo tweeter is a direct connect to the XO, with a 10 ohm resistor across the Piezo terminals. The logic was that attaching it to the XO would protect it from any low freq. surges, and the 10 ohm resistor would present a stable and predictable load to the crossover. The Piezo's are fine, though I'm considering replacing them with a regular horn tweeter. Now we are getting close to the heart of the matter. The CTS woofer is clear, but not especially loud. That is, I don't get a full powerful bass. Again, what I do hear is crystal clear, and I've been satisfied with it for years, but now I'm starting to doubt my Port calculations. Though I remember spending countless hours determining the port size, now I see no logic in arriving at 18 Sq.In. Though I do vaguely remember I chose a port size that would have a port depth between 3/4" and 1" for my cabinet volume. Now, as I review the book, it seems that a single 2" port 4.5" deep would have made more sense. Impossible to find specs on the woofer, but based on extremely vague recollection and the margin notes in the design book, it looks like it is 23 hz. So,  2.61 Cu.Ft. internal cabinet size 23 hz Fs How do I determine a more reasonable port size with an eye toward improved bass response? If you can help in any way, with specs, with advice, pointing me toward a reference, whatever... it would be appreciated. It's a long shot, but I had to ask. Steve/BlueWizard 
30th June 2007, 08:55 PM  #2 
diyAudio Member
Join Date: Mar 2007
Location: Canandaigua, NY USA

IMO, you need to start with the basic TS parameters for the woofer. Since it's old, even having the published specs might not be too useful. It isn't hard to measure a woofer, if you have a sig gen and meter, or a sound card and suitable software (there's freeware to do this), plus some simple cables and a resistor. This will be way better than any second hand numbers. Might be best to pick up a copy of Vance Dickason's book, measure your woofer, and recalculate from there. Another recent thread discusses the aging of woofers, and it's possible your original tuning was right, but the woofer has degraded. Symptom would be high Fs. Unfortunately, they just don't last forever.
edit you might want to use a sound card to do an SA of the speaker response to see where it's at. Maybe adjust the porting based on that and see what the result is. It would also be wise to check the crossover components. 
1st July 2007, 03:13 AM  #3 
diyAudio Member
Join Date: Jun 2007

The TS, which I assume means 'Technical Specs' are nonexistent.
I did email CTS and request a Tech Sheet, but I'm not holding out much hope. I also emailed SPECO in hopes of getting the XO specs on the crossover, but again, hope is extremely dim. This is all ancient history to them. I am confident of the Fs (free air resonance) numbers as they appear in several places in the margin notes in the book, and the number 23hz is consistent with my memory. So, for now, I'm going with that. Other specs like Le, or the various Q numbers simply aren't available unless you happen to have a spec sheet laying around (or some one else). In another old thread here, I discovered this formula  fp = (c/2pi) * sqrt(S/VL) Post #6 in this thread by Svante in Stockholm Feb 2004 Why big port diameter? where fp = resonance frequency of the cabinet c = speed of sound = 345 M/s = 1132 ft/s = 13,583 in/s pi = 3.1416 S = opening Size of Port (inch^2 or ft^2) L = Length of Port V  Volume of the Cabinet Assuming a fixed size port opening for 2", 3", and 4" PVC pipe, and knowing the Speaker Resonance, the Cabinet Volume, choosing a desired Cabinet Resonance, and choosing a pipe diameter, I can solve for pipe/port Length. L= ((c/2pi)/fp)^2 * S/V ...assuming my Algebra is correct. However, when I run these calculations for various values, the result doesn't come up consistent with my book. The book says I should have a 2" dia. port that is 4.5 inches long. As a side note; when I was designing the cabinet, I think I was operating under the idea that if I double the port area and can halve the length of the tube and keep the same resonant frequency. Starting with 2" x 4.5" port tube, I shrunk the length to .75" and that expanded the port area to 18 sq.in. Whether that was a correct assumption or not, I don't know. Anyway, using the formula, I calculate my cabinet is resonating at 157.7hz. If the formula can be trusted, that is a little high. So, I used a desired resonance of 40hz and 35hz and ran the formula, and came up with a 3" pipe of 5.98" in length or two 2" pipes in a length of 5.3" (@35hz). For a 23hz cabinet resonance, it worked out to a single 2" pipe with 7.9" length. But again, do I trust this formula; can anyone verify or deny it. I think at this stage, a formula like or similar to this is what I need. Someway to make the cabinet/port conbination resonate at a desired frequency, then choosing a desired frequency that will give the effect I want. I've been lead to believe that choosing a cabinet resonances that is slightly higher than the speaker resonance is desirable, and that is why I chose 35hz. So, can anyone verify this formula, or direct me to a similar formula, or to a website with information regarding cabinet/port tuning? For the record, I am familiar with how to determine the freeairresonance of a speaker. Keep in mind though, that I have no money, and very limited homemake test equipment. "pick up a copy of Vance Dickason's book" do you happen to have the title of this book? Steve/BlueWizard 
1st July 2007, 04:16 AM  #4 
diyAudio Member
Join Date: Mar 2007
Location: Canandaigua, NY USA

TS=ThieleSmall parameters. Google will bring up a plethora of stuff, including a large database, but start with Elliot . Dickason's book is The Loudspeaker Design Cookbook (I may not have it exact, but close) and it covers various tuning methods. There are various philosophies to tuning a ported enclosure, and it's a continuum, not necessarily a single "correct" point. What have you got for test equipment, self made or otherwise?

1st July 2007, 02:20 PM  #5  
diyAudio Member
Join Date: Feb 2004
Location: Stockholm

Quote:
Just remember that the L in that formula is the effective length of the vent. The actual physical length is a bit shorter due to that the air just outside the tube also has a mass. You should subtract two end corrections from it, one for each end of the vent. This end correction is roughly 0.85 times the radius of the vent. So if the diameter of the vent is 5 cm, you should make the vent 2*0.85*2.5 cm shorter than what the formula says. Also, the volume V of the box will seem slightly larger than the physical size if you have any damping material inside it. This is due to heat exchange between the air and the stuffing (also called isothermalization). For a box completely filled, the volume increase may become 2030% with a theoretical maximum of 40%. If less volume is occupied with stuffing, the number decreases accordingly. And last; it is not nessecarily so that fs and fp should be the same. In order to sort that out you would need the T/S (Thiele/Small) parameters of the driver and a simulation program. 

1st July 2007, 06:42 PM  #6 
diyAudio Member
Join Date: Jun 2007

Conrad Hoffman, thanks, I'll search for "ThieleSmall" and see what I leads me too; should be helpful.
For test equipment I have 2 digital multimeters of the very cheap variety, 1 cheap analog volt/ohm/amp meter, and a two channel oscilloscope. Plus, a function generator which produces a nice sinewave, but the output voltage shifts as I adjust the frequency. The frequency ranges are (1100hz), (101k hz), (10010k hz), and (1k100k hz). No frequency counter and the dial simply sweeps the range with no calibrated marks. It's been a while since I used it, but I don't think the dial is all that linear either. It is based on the EXAR XR2206CP. Chip details for those interested  http://www.chipdocs.com/pndecoder/da...XR2206CP.html Svante, So glad you responded, you seem to have the minimalist approach I need. First, did I understand your original formula correctly? Second, is my algebra correct in resolving the equation to find 'L'? As to the new information  This end correction is roughly 0.85 times the radius of the vent. So if the diameter of the vent is 5 cm, you should make the vent 2*0.85*2.5 cm shorter than what the formula says. Small point of confusion on this part, though it is not critical, the formula seems to be 2Kr, where K is the constant (0.85) and 'r' is the radius. Since you are taking 2r, why not use the diameter instead of the radius, or does that '2' come from some place unrelated to the Radius, like one correction factor for each end of the tube? the volume V of the box will seem slightly larger than the physical size if you have any damping material inside it. Based on the book I have, the typical internal displacement of a 12" speaker is about 0.4 cu.ft., so I've been debating whether to use 2.2 cu.ft. as my box volume, or to use 2.6 cu.ft. as the volume. The difference is about 18%. The cabinet has five sides covered with roughly an 1 inch to 1.5 inches of standard house fiberglass insulation. Based on what you are saying, I can reasonably use 2.6 cu.ft. for my calculations? By ignoring the internal displacement of the speaker itself, I should be able to roughly compensate for the dampening material? And last; it is not necessarily so that fs and fp should be the same. I've been lead to believe from a variety of sources, that a box resonance slightly higher than the speaker resonance is desirable. Especially when my speaker resonance is so low (assumed 23hz). While, the TS parameters may turn up something different, I have been trying for a box resonance of 35hz. Does this make sense, or am I completely lost? Next, how confident are you of this formula  fp = (c/2pi) * sqrt(S/VL) Have you used it with good results? Is if from a specific reference book or based on general knowledge? If a reference book, what book? I found another online Port Length calculator that uses similar parameters, but the results aren't the same. Though I haven't checked them against the new information you've given me here. http://www.linearteam.dk/default.asp...ventcalculator If I could find two sources that agree or somewhat agree, I could feel confident working from there. But every formula or resource I find produces different numbers. Though I am starting to see a trend. It looks like two 2 inch tubes or one 3 inch tubes with a length of 4 inches to 6 inches for a box resonance of 35hz. At this stage, I simply want to pick a reasonable box resonance relative to my speaker resonance, and calculate the length and crosssectional area of the tubes based of on available parameters. True I could get it perfect with a ton of work and a lot of expensive equipment, but for what I am trying to do, that seems like overkill. So, in short, is this a theoretical formula, or is it one you have applied successfully in the past? Thanks for the update, I will run your formula again using the correction factor you mentioned, and see what I come up with. I assume the correct way to apply it is  L= {((c/2pi)/fp)^2 * S/V}  2Kr or more simply Lf = Lc  (2Kr) Lf = Final Length Lc = Calculated Length Thanks again for everyone's help, it is greatly appreciated. Steve/BlueWizard 
1st July 2007, 08:20 PM  #7 
diyAudio Member
Join Date: Apr 2004
Location: new jersey

TS parameters
You may have already tried this:
Madisound and Zalytron are two suppliers who offer replacement drivers for old speaker systems. They likely have a library of old drivers and your CTS TS specs. Failing that, they probably have a low cost replacement that will work in your cabinet; have TS specs and will optimize your situation. By the way, the Speco xover probably is equipped with electrolytic crossover capacitors. I would replace these. best of luck Les 
1st July 2007, 10:09 PM  #8  
diyAudio Member
Join Date: Feb 2004
Location: Stockholm

Quote:
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But you are right, if you just want a formula and don't care about the physics, you could say that the correction should be 0.85 times the diameter. Quote:
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But given the difficulties in estimating the isothermalizing effects of the stuffing, it might be wise to measure the fp after the design is made and possibly adjust the length of the tube accordingly. 

1st July 2007, 11:14 PM  #9 
diyAudio Member
Join Date: Mar 2007
Location: Canandaigua, NY USA

You've got a scope, so you can tell what frequency and amplitude your sig gen is doing, even if the dial isn't great. That, and a resistor, is all you need to get the important parameters of your woofer. Armed with that, there are any number of box tuning programs that will predict the response curve for various tunings. I've got one I wrote from the old Speaker Builder articles on TS parameters that runs in a DOS window (never bothered to update it) if you like, or a search should find you various others that are substantially more modern. I did put in the step by step procedure for measuring driver parameters. IMO, with only the Fs, little can be accomplished.

2nd July 2007, 03:50 AM  #10 
diyAudio Member
Join Date: Jun 2007

Thanks again to Svante for taking so much time to assist me. I realize things are much more complicated than I would like them to be, but I've lived with this speaker for years, and I'm just trying to improve the bass a bit, not design the perfect speaker. I think what you have given me is sufficient.
I already have TWO 3 inch by 3 inch square holes, that should make it easy to insert TWO 2" diameter PVC pipes. I've just run your calculations again in a spreadsheet. Without the corrections, TWO Ports of 2 inches in diameter, would have a length of 5.336 inches. After the correction, it comes out to 3.636 inches. I can't flair the edges, but I might be able to find someone to round them off a bit with a router. I can buy 2 feet of 2 inch PVC for $2.18 or 10 feet for $4.44. That is a lot of pipe to play around with and test. For $8.13, I can get a 2 foot by 4 foot 3/4 inch MDF. That is a lot of 3" x 3" squares to fit with test pipes. And...I understand how you feel about trying to think of things in Metric vs American measurements (we are the last holdout aren't we). Because I'm the same way in reverse. Thinking of cabinet volume in Liters makes me think of cabinets filled with bottles of milk; it's just illogical. But, I found a nice converter online so I can covert between them. I read through the TS spec testing procedure at the website referenced, and while it is technically slightly over my head, there is a spreadsheet that makes all the calculations, and really in the end, there are only a few measurement to make. So, while I'm not sure I will apply that to this speaker cabinet, I use it everyday, it looks like I could learn a lot from it by trying it on a random test speaker. All I really need is a standard box. I've already got several 8 ohm power resistors (25 watt; heatsinked), and a signal generator, sad as it is. I also have a spare stereo amp I'm not using. Unfortunately, I don't have a lot of uncluttered space to work in. I'll either have to do it in my living room or out on the deck. And how do I mount a speaker for free air resonance testing? Do I hang it from the ceiling? Do I set in on a tall sand filled column? On the horizontal plane, I'll be lucky if I can get it 6 feet (1.8 meters) from nearby object in every direction. I do have some other questions, but they are off this topic, so I will make them separate posts. Thanks again to everyone who replied. I'm still open to comments though. Steve/BlueWizard 
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