The Folded Horn

Author: N.H. Crowhurst

The horn is a wonderful way to improve acoustic efficiency, but where we really could use this help - at the low frequencies - it has to be very large. For example, to get down to 100 cycles, the mouth needs to be 5 feet across, and the length, starting from a throat 4 inches across, needs to be 64 inches, (over 5 feet). To work from a 3/4-inch throat, using a regular horn drive unit, we would need a horn over 9 feet long.

To show that the principle works, horns about 30 feet long have been built, getting down to 40 cycles. But these are hardly practical for everyday use. The 100-cycle horn however is not so very big, although it is somewhat awkward. This can be overcome by "folding" it.

Low frequency horns can become quite large, but folding helps.

For low frequencies, the folding of a horn has no disadvantages - in fact, any horn can be folded without disadvantage to frequencies near the lower end of its frequency range. However frequencies that are well above the cutoff frequency (the lowest it will handle) tend to get "lost" due to folding. Different paths down the horn have somewhat different lengths, and the higher frequencies can get to a condition in which the wave following a longer path cancels one taking a shorter path.

Avoiding high frequency cancellations

One way in which to compensate for this problem is to design the bends of the horn so as to "invert" the waves at higher frequencies, thereby equalizing the path lengths. Instead of following a smooth contour at the corners, the shape is arranged to reflect the complete wave.

For horns designed for higher frequencies, a very convenient form of construction is the re-entrant type horn, in which the whole structure is concentric.

The re-entrant horn