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Old 12th March 2007, 07:03 PM   #1
bluegti is offline bluegti  United States
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Default Is this how you calculate the line length?

I'm just trying to figure out if my "intuition" is correct about how to calculate line length.

Lets say I have a speaker with the Fs of 51.5Hz (Fostex FE167E). Do I always tune my enclosure to the Fs or can I choose an arbitrary value such as 56Hz? I will assume I can choose whatever I want and use 56Hz because it make the math come out nice and works with my diagram.

Once I choose my tuning frequency, I figure out what is the wavelength using a lookup table or utility like this one .

According to the utility the wavelength of for a 56Hz frequency is pretty much 20 feet. Then, I would choose 1/4 of the wavelength which is 5 feet or 60 inches.

Once I know my line length, do I calculate my enclosure size measuring the center of the "pipe" from one end to the other. The first picture shows a straight pipe, the second one its folded over once and the third one (sort-of) spirals. In each case, the length is 60 inches.

Do I have it right or do I calculate some other way (The longest wall, the shortest wall, from driver to port bouncing back and forth, whatever)?
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Old 12th March 2007, 09:25 PM   #2
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Would that it were that simple.

OK: first, a warning. Don't tune below 0.707Fs, or you'll run into problems, unless you know exactly what you're doing, and why you're doing it.

Next -you need to go to my friend Martin King's site at www.quarter-wave.com and download his classic TL alignment tables, and his anatomy of a TL articles which are loaded with information.

Briefly: the tuning of a QWR is a function of length and taper, not just length. The old ROT of taking the desired Fc and /4 only works for straight lines (and assumes they have sufficient Vb to provide the gain you desire down to that frequency). An expanding line will always need to be substantially longer than a straight line for the same tuning frequency. Conversely, a narrowing line can be substantially shorter for the same tuning, as you'll see in Martin's tables (which are based on heavy-weight math, and are of proven accuracy -no fudging here).

Hope this helps
Scott
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Old 13th March 2007, 01:10 AM   #3
bluegti is offline bluegti  United States
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Default Yikes!!!

I've never been good with math and my eyes start glazing over looking at all of the formula's and charts. I think I will stick with using other people's designs.
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Old 13th March 2007, 05:39 PM   #4
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Bluegti,

Don't feel bad. I'm in the same boat as you. I was an abject failure at math in school; so bad, in fact, that it's one of the few things in life I gave up on. Lord knows I have tried over the years to go back and learn, but it just doesn't work for me. I can add, subtract, multiply, and divide easily, but anything from simple algebra on up completely baffles me.

The advice to use Martin's worksheets is very sound if you want to design something. My method now is to take an existing design, build it, and endlessly tweak it to see how the changes manipulate the sound. I'm now starting to understand cabinet design and driver parameters and am learning how to measure my own drivers. I set up a very simple but effective A/B testing method so I can instantly hear the differences in various combinations. It's a long but fascinating process understanding these little beasties, but it can be done without a good knowlege of higher math, although I firmly believe that would help a great deal.

Doug
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Old 13th March 2007, 11:43 PM   #5
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Good advice.

I'm rotten at math myself, or I was until a couple of years back. It was only my interest in speakers that forced me to look into it, and physics generally again & I've learned a huge amount from Martin in particular in this respect. His tables look frightening, but honestly, they're not that bad if you work through them systematically. However -building a good existing design is always a good move, at least initially. There's plenty around, so it depends on what you fancy really.
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