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I have been doing a lot of reading and researching to understand more about paramters so I can build my own speakers for my home theater. I bought Vance's book and read it and I am still a little confused on some things. Would someone read my understandings and correct what is wrong, or explain in a better way the idiot guide to paramters?

1. Fs: the resonant frequency of a speaker, it is the frequency at which the speaker hits the peak of impedence and determines what type of speaker it will be. For subs the Fs needs to be as low as possible. So a sub with an Fs of 22 will be better than one with 38 right? I have read that below the Fs point the speaker isn't very good, so I would want the lowest possible Fs I could find correct? Also you want to use a crossover 2 octaves above the Fs for mids and tweeters according to what I have read, since most distortion will happen the closer you get to the Fs, but for lows that isn't much of a problem.

2. Vas: the volume of air equal to the compliance of the drivers suspension. This means what? I am looking at 2 15" subs in a magazine and one has a Vas of 9.6ft^3 and one is 11.11. Does this mean that the second one has 2 cubic feet more volume taken up by the spider and surround? I have read that the smaller the Vas the stiffer the suspension, which makes a tighter speaker, which can then handle a smaller enclosure. So it would seem that smaller Vas = better quality?

3. Q/Qts/Qes/Qms: These are the numbers that refer to the speaker loss, electrical and mechanical, and total. A smaller Qts results in a smaller enclosure I know from practicing with box building software. But why? Does a smaller Qts= a better quality? The bigger the Qts, the lower the F3, but the bigger the enclosure.

I ask these questions here because I have been to many sites and done many searches to find new sites that explain in easy to understand terms what all these numbers mean. I just want to make sure I fully understand them so I can tell better quality. For instance here's one of my problems, I am looking at 2 speakers, both are 7" and here's their stats:

Speaker 1: Speaker 2:

Fs 36hz 40.6

Vas .75ft^3 1.4

Qts .42 .33


When they are both plugged into the formulas for a ported box I get 1 being .93ft^3 with F3 of 31.5hz, and #2 being .95 with F3 of 42hz. This would make 1 seem better than 2 because of a lower F3, but when you consider #1 is a $17.00 Dayton and #2 is a $117.00 Focal how is that possible? Then I look at the sample respons graphs and the Focal is more efficient and much more flat than the Dayton. So the price makes a little more sense, but how can you tell if you didn't have the response graph which will be a better speaker?

OK, this was a long post, but I hope it makes sense and someone can help me understand what I am missing.
 

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Just the technicals right now. While these are not classical definitions, they give a basic understanding of what each is and why it matters/changes/etc.


1. Fs is the free air resonance of the driver. ie, the frequency that will play the loudest with the same input power if the speaker was hanging out in free air. Lower is not necessarily always better. I will take a driver with Fs of 25Hz but flat +/- 2db over a driver with an Fs of 20Hz but with wild dips/peaks.


2. This means how much air can be displaced by the driver at max excursion. In your example, #2 'moves' more air. This is not the classic definition but it is effective for evaluation.


3. Q is interesting. Q basically says how tight a hold are you putting on the driver. A smaller box has less air - therefore less compliance - therefore, more control over the driver (usually at the expense of extension). A larger Q will allow the driver to 'breathe' a little more.


Finally, without a plot, you CAN'T tell. Even with one, it is hard since the modelling is only theoretical. Anyone who has built a lot of subs will tell you that the model and the actual outcome rarely are exactly the same.


You might also want to check out the speaker building forum over at AudioAsylum. There are a lot of guys over there that are deeply engrossed in the 'black art' of speaker design.
 
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