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What happens to the low frequency output when you combine short horns?

If a horn is too small for the frequency it is operating at (an acoustic mouth circumference much less than about 1 wl), then placing it next to an identical unit will normally extend the low cutoff.

When the area ratio between the mouth and throat is not large, the low cutoff of the horn is set by the wavelength resonance and in this situation the resonance is roughly set by the length of the horn path.

While a horn is not at maximum efficiency until it is wl long or longer, it can provide usable output (significant gain over the same driver not horn loaded) down to about the wl resonance and at much lower frequencies, only has the output of a sealed box (the driver, its compliance and back volume).

The quarter wavelength resonance is the lowest mode a pipe closed at one end and open at the other resonates at, for a pipe to resonate at the same frequency with both ends open ( wave length resonance) it has to be about twice as long.

In a horn, as the mouth area becomes larger, some of the air out in front of the horn is still governed by (highly coupled to) the pressure gradient at the mouth and so is an acoustic length added to the horn path length. How much it adds to the length depends on its shape with round or square (including the mirror image) being the best and typically adding about .6 to .8 times the horn mouth radius. A large mouth can add significantly to the path length and can have a significant effect on the low cutoff.

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