PA Horns
PA horns do two jobs for a compression driver: they load the diaphragm so it couples efficiently to the air, and they set where the sound goes. Choosing one comes down to coverage, throat and fixing match, and mouth size. Wavemark horns are coming soon.
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Coverage angle
A 90° × 40° horn covers 90° horizontally and 40° vertically, measured to the points where level falls 6 dB from on-axis. Wide horizontal coverage spreads one stack across a floor; narrow vertical coverage keeps energy off the ceiling and floor and throws further. Each halving of coverage area concentrates the same power into less space, so narrower horns are louder on axis.
Throat, fixing and mouth
- Throat: must match the driver's exit, commonly 1", 1.4" or 2". A mismatch creates a step in the air path.
- Fixing: two-bolt, four-bolt or screw-on. Both throat and fixing must match, or use a proper adaptor.
- Mouth size: a horn holds its pattern only down to frequencies where the mouth is comparable to the wavelength. At 1 kHz the wavelength is 343 mm; at 500 Hz it is 686 mm. Below that limit coverage widens and the horn stops controlling it.
Horns in sound system tops
Mid-high boxes commonly pair a large-format compression driver on a horn with bullet tweeters on top. Horn loading makes the HF section far more efficient than the subs, so it needs much less amplifier power. The crossover point must sit comfortably above the driver's lower limit with a steep high-pass. See horn loading explained and anatomy of a sound system.
Related
Sources: wavelength λ = c / f with c = 343 m/s at 20 °C. Coverage defined at the −6 dB points.
Questions
What does 90 x 40 mean on a horn?
It is the nominal coverage angle: 90 degrees horizontally and 40 degrees vertically, measured where the level falls 6 dB below on-axis.
Can I fit any compression driver to any horn?
Only if the throat size and the fixing match. A 1 inch exit needs a 1 inch throat, and the bolt pattern or thread must match, unless you use a suitable adaptor.
What sets the lowest frequency a horn controls?
Mouth size. A horn holds its coverage only down to where the mouth is comparable to the wavelength: 343 mm at 1 kHz, 686 mm at 500 Hz. Below that, coverage widens.