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In Reply to: Horns below driver Fs posted by dsm on February 26, 2007 at 08:31:52:
HiThe Fs is set by how much moving mass you have and how stiff the suspension spring is, you can picture the relationship.
In a horn, the suspension spring part is in parallel with the spring force produced by the air trapped in the sealed box, acting on the piston area. This new stiffer spring produces a new resonant frequency, Fb.
To get the most bass out of a given horn, both the driver properties and the rear air volume have optimum values.
In this case, all things being equal, if one lowered the Fs, the ideal rear volume would have to be made smaller to off set that. The up side is the air spring is typically more linear than driver suspension.
Fb, resonance “in box” with no horn attached is normally above the low corner when horn loaded.
When you approach the low cutoff of a proper horn, one has some acoustic reactance, the horn appears to have an increasing mass reactance as the frequency falls.
By choosing the right flare “t” for the driver, the rear volume is sized to off set this reactance, extending the low corner f (reactance annulling). It is entirely possible on many horns to end with more bass by making this volume smaller than intuition might suggest.
Where to go now?
Look into Marshal Leach’s paper on matching moving coil drivers to horn parameters.
Some one here will likely have a link to it handy.
Look in to the McBean horn simulator and the other free or near free modeling programs for horns, nothing saves sawdust and sweat like a good computer model.
Best,
Follow Ups:
Thanks Tom:Always helpful to understand things rather than just model with programs!!
Thanks Tom. Creating sawdust has always worked well for me as my learning abilities tend to be at the "hands on pace" these days.
I have a few drivers here I'd like to play with in various "horns" to kinda get a feel for how the horn equates to a 1/4w piece of coaxial transmission line which I am somewhat familiar with.
Here's the link to the paper Tom mentioned.
I was VERY frustrated crunching the numbers until I found the corrections for the typos.
Jeff Robinson
Are these the same as the corrections on page 11 and 12 of the pdf?
Ahh!The copy I purchased from the AES didn't have that addendum. The errors in the equations are described in the bottom left paragraph on page 12 of the copy on the Georgia Tech page of Leach's. I've updated the copy I put on my geocities page. The 4 equations in the body of the paper are still wrong, you just need to carefully read the addendum to find that out.
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