"Buyout" Transmission Line? - AVS Forum
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post #1 of 8 Old 08-26-2013, 04:09 AM - Thread Starter
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Quick disclaimer - I'm new to transmission line design - I intend to buy Martin J. King's mathcad worksheets, but I thought I'd come here for advice whether I had the sheets downloaded or not...

Would this parts express buyout be suitable for a transmission line? And what would the output look like below Fs (advertised 52hz)?
http://www.parts-express.com/pe/showdetl.cfm?partnumber=299-609

PRODUCT SPECIFICATIONS
Power Handling (RMS) 50 Watts
Power Handling (max) 100 Watts
Impedance 4 ohms
Frequency Response 50 to 6,000 Hz
Sensitivity 88.3 dB 2.83V/1m
Voice Coil Diameter 1"

THIELE-SMALL PARAMETERS
Resonant Frequency (Fs) 52 Hz
DC Resistance (Re) 3.37 ohms
Voice Coil Inductance (Le) 0.48 mH
Mechanical Q (Qms) 2.99
Electromagnetic Q (Qes) 1.40
Total Q (Qts) 0.95
Compliance Equivalent Volume (Vas) 0.65 ft.³
Maximum Linear Excursion (Xmax) 3.5 mm

Another really cheap option would be the MCM 55-3231 (8" with a claimed Fs 28.8hz)
Thiele-Small Parameters:
Resonance Frequency Fs: 28.8Hz
DC Resistance Re: 3.4Ohm
Mechanical Q Factor Qms: 3.4
Electrical Q Factor Qes: 0.5226
Total Q Factor Qts: 0.4529
Equivalent Cas air load Vas: 98.26 L
Surface Area of Cone Sd: 0.0216 m2
Efficiency Bandwidth Product EBP 55.1
Voice Coil Over Hang X-max 3.5 mm

The theme here is cheap drivers. If the geometry works, I'd like to stack perhaps 8 drivers or so per side in a stereo transmission-line-array (or would it be a transmission line line array? biggrin.gif )
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post #2 of 8 Old 08-26-2013, 06:05 AM
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I am not an authority either but the Q parameters for the 6.5 look high. Could be normal for such a driver. My personal notes from the build note that Qts is one of the most important parameters to consider when building a TL.

A 52hz line would be short and easy to make but I would suggest to measure Fs directly. This means a free air break in for 24 hours and using lab equipment.

I have a thread that's a year old about TL. My speakers were spec'd at 24hz but measured 30Hz.

'Bullock on Boxes' is a book or compilation that detailed how to make these measurements.

I did not get the Mathcad files either. My box was a bit experimental.

You are in the right place by checking out Mr. King's page.

http://www.quarter-wave.com/

The Keith Webb sheet was very helpful too.

http://www.quarter-wave.com/TLs/TL_Theory.html

The Loudspeaker Design Cookbook has a chapter dedicated to Transmission Lines. Only about ten pages but plenty to get you started. Also discusses most other types of enclosures so you can judge the pros and cons.

Good luck!

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post #3 of 8 Old 08-27-2013, 05:10 PM - Thread Starter
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Thank you, Gaugster. I saw you've refined your post at least 3 times now.

Is the dayton test set worth it? I've always relied on the manufacturer's T/S parameters being close enough (haven't taken a shot at high quality bass yet). Doubt they would be for these cheap drivers in a Transmission Line.

http://www.parts-express.com/pe/showdetl.cfm?partnumber=390-806


I actually have another friend interested in building a set of speakers, so if those 6.5's model at all decent... maybe a WWWWMTM tower?

I'll read through Keith's sheet
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post #4 of 8 Old 08-27-2013, 08:02 PM
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Lol! Made a few updates to my original post... no experience with dayton tester. My work has all sorts of electrical test equipment. kinda spoiled in that sense.

since line length is calculated from Fs it is best to measure and confirm. One less variable.

i used a matcad file to convert one parameter for input into Keith's sheet. Bl or something.

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post #5 of 8 Old 08-27-2013, 08:42 PM
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Usually one of the guys at PE test buyout woofers there, the buyouts I've purchased and measured are always pretty close to what they post.
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post #6 of 8 Old 08-28-2013, 04:47 PM - Thread Starter
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I've read a similar post to what I'm about to ask, but don't think there was much in the way of an answer...

If I folded the Transmission line into 4 sections (3 180 degree bends, with the driver at the end of one row and the terminus at the other end of the line), should I keep the cross sectional area of each chamber constant throughout? Or could I achieve a sort of quantized-taper by reducing the width of each section (say 1.25 Sd in the driver section, 1.0 in the second, .75 in the 3rd, and .5 in the terminus section)? Length of each section would be fixed and determine the "depth" of the cabinet, with the driver and terminus on the front.
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post #7 of 8 Old 08-28-2013, 07:03 PM
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Traditional taper (angled cross section) makes the effective line length longer than its phyical length when the area is reduced.

I think you intend to make a stepped reduction. I don't think its the same. Given the speaker diameter, most all TL start with a large cross section and step down to match the speaker.

More reasearch for you...

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post #8 of 8 Old 08-29-2013, 04:16 AM - Thread Starter
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A stepped reduction for me would be much easier to construct (I'm not the best at wood working biggrin.gif )

It wasn't clear to me whether the increase in apparent line length due to taper was a result of the reduction in cross sectional area, or the walls of the tapered section being angled (not parallel).

One of the rules of thumb I read said it would be ideal to start the line cross section at the area of the driver, but due to mounting it always ends up being larger. Therefore, starting somewhere above 1 Sd and preferably below 1.25 Sd, the line would taper down to Sd or less.
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