Back-loaded horn: a horn that boosts the bass

A back-loaded horn guides the sound from the back of the cone into a horn that widens gradually and is folded several times. At the end of the horn there is a wide mouth. Such a horn works as a horn and raises the efficiency in the bass, so a small full-range driver plays louder and fuller.

Section through a back-loaded horn. Behind the driver there is a small chamber, from it a narrow throat into the horn, which widens at every fold: along the top, down along the rear panel and along the bottom to the large mouth at the front bottom.

How it works

A horn matches the small cone to the large volume of air in the room, much like a megaphone. The cone then passes more energy to the air, and so the efficiency rises. Behind the driver there is a small chamber which, together with the narrow start of the horn, filters out the higher frequencies. The horn therefore plays only the bass, and the driver plays the midrange directly.

A horn only works above the frequency at which the circumference of the mouth is at least comparable to the wavelength. For bass below 50 Hz the mouth would need the area of a door. That is why back-loaded horns are placed in a corner, where the walls make the mouth acoustically larger, and in the bass the gain is more about higher efficiency than the deepest bass.

Pros and cons

Pros: high efficiency, a dynamic and lively sound from a single full-range driver, suitable for valve amplifiers with a few watts.
Cons: a large and complex box with many folds, a demanding build. The response tends to be ragged and the horn can sound coloured. The result is hard to predict with a simple calculation.

Which driver and which room

For full-range drivers with a low Qts (about 0.2 to 0.35), a strong magnet and a light cone, for example Fostex FE or Lowther. Best in a corner of the room.

Well-known designs

Frugel-Horn by Dave Dlugos and Scott Lindgren: a corner horn for a 4" full-range driver, built more than a thousand times.
Nagaoka (for example the D-101, D-55): Japanese back-loaded horns by Shuzo Nagaoka for Fostex, a legend among full-range speaker kits.
Martin J. King: a compact back-loaded horn for the Fostex FF125WK with a rear mouth, published together with its calculation.

In Loudsmith

Loudsmith does not simulate the back-loaded horn, which is why this article has no charts. The best tool for calculating horns is the free program Hornresp. You will find more about it in the article Loudspeaker design software. For tweeters, Loudsmith has a waveguide and horn calculator in the Horn calculator tab.

Enclosure types series: sealed · bass reflex · passive radiator · aperiodic · transmission line · TQWT · ML-TL · open baffle · back-loaded horn. You will find the formulas in the article Speaker box calculations step by step.