ALTEC A7-8G special

[jmarkwart]

Dave,
A horn's mouth area determines the lowest frequency it can support as a horn. An A7 with a mouth of 28 X 19.625 inches has 549 in^2 of area, which sets its lower horn loading limit at 163Hz. For A.l.'s horn design to go to 100Hz would require the mouth diameter to increase to 43 inches! Big horn! The A7 horn, (a 32 inch radius actually, not a traditional horn curve) as well as the tractrix, start to drop in output well before their lower loading limits are reached anyway. Good horn design is a challenging task - good luck A.L.
Jeff

[jmarkwart]

Roland and A.L.,
I would agree that the tractrix is not the best choice for a large low freq horn - a hypex does a better job of loading as it nears LF cutoff. For A.L.'s application though, the bass horn will be crossing over with the mid/HF horn around 700-800Hz, and using the same horn curve for both drivers wiould likely be the most transparent to the ear. Is WAF a factor here? I would also agree that you might want to consider other bass reflex loading options with the 515-8G rather than just settling for the 828 response. It would be much easier to design in variable porting and volume options now.
Jeff

[Fitts]

Jeff, your observation about the cut off frequency is astute and correct. I double checked and found I had miss read my reference. Thanks for your correction.

[rolandr]

For full range high efficiency audio coverage of 20-20K you are pretty much stuck with using:

subwoofers with multiple drivers moving lots of air putting out watts of power ideally x oxer about a hundred Hz or below.

Bass/Midrange horn ideally keeping cut off about 100 Hz or less. The ear is very sensitive to midrange and especially accurate with voice. The lower range of male voice is around 125 Hz therefore you want one driver handling the critical midbass/midrange.

Very few compression drivers/horns can really cover 100 hz to 20K. So at this point compromises are usually made. "Best" is probably to cover 100-125 Hz to about 10K in other words single driver midrange then go with tweeters to 20K. In the higher freq the Laws of physics take over and tweeters with the smaller lighter elements can resolve more and are less beamy.

Obviously it is always a compromise but thats the logic behind it. Many outstanding systems do not follow it.

For less efficient speakers the MR becomes a 6-10 inch dynamic driver ideally covering 100Hz to a few thousand Hz. The upper limits dictated by
their increasing beamyness then x over to a tweeter usually between 3 to 8 thousand.

But does this all affect the enjoyment of music??
Perhaps it will help some?

[alhuds]

Jeff,

Are you the Jeff Markwart that wrote the article about the Exemplar Horn in Sound Practices #9? If so, I wish to really thank you because that article was the inspiration for this design "A7-8g Special".

I would like to ask agian, if i wished to cross-over at 500 Hz with 12 db / oct. what should my hi horn cut-off be? Or, how do i calculate this myself?

I am also attempting to move the ports to the front baffle.
Thanks A.L.

[jmarkwart]

A.L.,
Yep...I'm the same Jeff Markwart. John Tucker supplied the other half of that effort.
If you want to cross over at 500Hz I would adjust the horn lower cutoff from 385 to 250Hz, which would give you that octave of headroom I mentioned above.
The cutoff can be calculated using 13560/(2*(SQRT(Pi*Mouth Area))) - with mouth area supplied in square inches. You can plug this directly into Excel if you have it.
Jeff