- Written by John E. Johnson, Jr.
- Published on 17 February 2014
Horn speakers have compression tweeters and midrange drivers, but conventional woofers. Sometimes all the drivers are arranged in a horn design, which means the driver is at the small end of a megaphone.
A compression driver has a small dome with a voice coil, sitting within a chamber that has permanent magnets on the sides. The dome moves with the musical current, producing compression of the air within the chamber, and this forces air between the magnets out of the chamber into the horn.
The megaphone for a horn speaker does not have two simple straight lines that converge at the driver end and open wide at the output end. There are complicated mathematics involved to design the horn so that it does not sound like the powered megaphones that police use, or the sound that occurs when you cup your hands together to yell something at someone down the street. The result is a design that curves in very specific ways as the horn goes from a small opening where the compression driver is, to the wide opening where the sound emerges. If the curve of the horn is not properly designed, it can have a sound called "shouting" at high volume.
In the photo below, the tweeter with its horn is shown. At the center is the compression driver.
The Athenaeum is very big, and very heavy (225 pounds), with about 2" of layered MDF as the enclosure. The sides of the enclosure are non-parallel, to reduce standing waves.
The crossover is second ordered, with a crossover frequency of 2 kHz. The compression tweeter is actually sold as a midrange driver, but the frequency response extends to 20 kHz, so David Linn built the speaker with just the two drivers instead of putting an official compression tweeter (available optionally, extending the frequency response to 40 kHz), as this eliminates one additional crossover frequency (there are unavoidable problems when crossing over the sound at any frequency from one driver to the next).
When David brought the speakers over to our lab, he removed the crossover network board to show it to me (see photo below). You can see how simple it is, and simple is good. It can be connected in two different ways. One goes to both the tweeter and woofer, with resistors (total of 16 ohms) between the input and tweeter since it is much more sensitive than the woofer. The other way bypasses the resistors, and is used when bi-amping the speaker. This yields a 13 dB improvement in efficiency of the tweeter, so that amplifiers such as single-ended pure Class A triodes (e.g., using a 300B as the triode) can be used to drive the tweeter, using only about 5 watts.
Here is the rear panel with the speaker binding posts. It is wired with the resistors in the tweeter path, and is being driven by one power amplifier. The top set of binding posts go to the tweeter without the resistors in the signal path.