- Front loaded horn (FLH)
- Back loaded horn (BLH) - sometimes referred to as a scoop bin
- Tapped horn (TH)
Front loaded horn
Back loaded horn
The only physical difference in theory is the rear chamber. A front loaded horn is the best choice for efficiency and accuracy with limited bandwidth woofer or subwoofer applications. The sealed chamber helps with controlling the behaviour of the bottom end. It restricts cone motion so that the cone is loaded on both sides - behind with the air spring from the chamber, and in front with the mass of air on the horn. This causes the driver to perform in a more linear fashion than if loaded only on one side as a direct radiator.
A back loaded horn lacks the sealed chamber and so may require a high pass below cutoff to protect the driver. The sound from the rear of the driver causes cancellation and is problemmatic. However, a back loaded design is appropriate for a wider bandwidth and some drivers are made for this type of design. If you see a 15" pro driver with 100db sensitivity or higher, and an early drooping response, it's probably intended for a BLH where the horn brings the sensitivity up. Some full range hifi drivers perform well in this type of horn.
A tapped horn is very different. There is some debate whether it should be classed as a horn and many have described it as a bandpass or transmission line. While there are similarities, there are also differences so I consider it best to simply accept it as a special type of horn with it's own set of characteristics.
A tapped horn can achieve high efficiency but is even more difficult to design. The size is smaller and they can as a result be used practically for a lower cutoff. Often the bandwidth is less than a conventional horn and many drivers don't suit a TH. The response is also peaky.
The best of both
Tapped horns and conventional horns can work together well. Start with a tapped horn for the bass and it will provide as much extension and low frequency output as you want, but over a narrow bandwidth. You might aim for 15 - 50 Hz for a home theatre system or 35 - 120 Hz for a music system. Then you can cross to a FLH which can now be much smaller. Ideally the TH would have more power and a driver with more excursion, and the FLH would use a driver designed for midbass efficiency.