Vff, Vof, Ffb, Fob parameters

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jimmyjazz

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Joined 2009
#1
Months ago I saw a reference to 4 calculated parameters (Vff, Vof, Ffb, Fob) that had potential merit as "goodness" factors for loudspeaker drivers. I can't find that reference. I don't know if they were ever part of T/S calculations or rather subsequent work by others.

Anyone know where I might find the derivation or other papers showing these parameters? Two volumes (e.g. liter), two frequencies (Hz).

Thanks in advance . . .
 
#6
bjorno said:
PS:
A New Approach to Box Design and Woofer Selection
Speaker Builder #1 1992 by Mark Rumreich.
Note: There are more Graphical examples in the Jornal than in the Url I've posted hereabove.

b 🙂
Click to expand...

Hmmm, I have not seen that before. Interesting. Thanks for posting the link, Bjorno

I read the article available at the link in post #2. What struck me is that the author, at least by the first example given, is prioritizing bass extension over all other aspects. In that example, The system using woofer "B" is made to have the same low frequency extension as system with woofer "A", however, it has a peaking response that will result in hangover/ringing at 100Hz. I'm not really sure that is all that good. But maybe that is not a good example of what this design method offers up in terms of possibilities.

Can someone who is familiar with the approach of Mr. Rumreich comment on its pros and cons?

Also, I very much liked the sentiment he shared in this section:

The Myth of Discrete Alignments
Among speaker builders, there is the popular notion that for each driver there is a discrete set of correct box designs. This belief is most prevalent with ported systems where designers feel compelled to use specific alignments such as SBB4, SC4 and QB3. Some go as far as to put bricks in a constructed cabinet to tweak it to a particular alignment.

Figures 3-8 tell a different story. They show that the relationship between response and box volume is continuous, and that a wide range of box volumes can provide acceptable results. The fundamental tradeoff in box design is between box volume and cutoff frequency.

Just like for the design of the speaker+box, the filters used in crossover design have responses that are points within a continuum (well, curves within a surface would be more accurate I guess). This is best illustrated by the second order filter response (but can apply to other orders as well) where there are only two parameters that control the response shape: Fc and Q. For a given corner frequency Fc, as Q varies from 0.5 to 0.577 to 0.707 you get the Linkwitz-Riley, Bessel, and Butterworth second order responses. But these are just particular responses that fulfill some property in the time or frequency domain (e.g. best transient response or maximally flattest passband) and there is a continuum of completely valid second order filter responses in between and outside these values of Q. You need to consider the phase and amplitude of the driver responses being mated by the crossover to really discover which is better for your application.

The same conclusion can be said for the speaker design. The greatest amount of low frequency extension might not be optimal for your application. You might want to match Fb and Q (e.g. the response shape) to the response of your room at the low end, or you may have a limited box size because you already own or bilt the enclosure, or you may want the transient response to be without ringing, etc. I caution DIYers to not try to make their woofer fit a particular response (e.g. Butterworth) like many design programs will do, but instead look at the whole picture when doing the design.
 
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#7
CharlieLaub, thanks for your detailed thoughts. It all makes sense to me. I've always tweaked designs off a standard configuration to better suit my needs. (In my case, I'm valuing bass extension over pure hifi performance, all from a very small box.)
 
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