Bass Horn: How a Horn Increases Efficiency

A horn is a flaring sound channel between the cone and the surroundings. It does not amplify anything, but matches the acoustic impedance: the cone transfers more of its energy to the air. As a result, the driver appears considerably more efficient – a gain of approx. 10 dB compared with a driver without a horn is typical.

The lower limit of a horn is related to the horn mouth: the cut-off frequency lies where the wavelength equals the circumference of the mouth. For low frequencies, a horn therefore needs a large mouth and a long path. To fit this into a transportable enclosure, the horn path is folded inside into a zigzag channel.

Rog Mogale adds in his FAQ: a horn only reaches its full efficiency from a path length of approx. half a wavelength. Shorter horns still deliver considerably more level than the same driver without a horn – down to the quarter-wave resonance of the horn path. If you place several short horns side by side, the combined mouth area grows – the lower limit drops.

Horn drivers need a strong motor. As a guideline, Rog Mogale gives an EBP above 130, but himself stresses that this is only a rule of thumb. For the 1850 Horn he requires “EBP above 125 and BL above 28”.

• 18 Sound 18H, B&C S18H, PD-H118A – manufacturer bass horns
• Speakerplans 1850 Horn and 186 Horn – folded 18″ horns
• Speakerplans HD 15 Horn – short 15″ horn for 52 to 180 Hz

• large and heavy, especially for low frequencies
• uneven frequency response due to resonances in the horn path
• complex construction with many angles and mitres