• Increased Efficiency
• Controlled Directivity
• Less Distortion
• Bang for the Buck

The ultimate advantage to horns is their huge increase in efficiency. The typical direct-radiating loudspeaker will have a sensitivity somewhere around 90-dB. This means that the speaker will output 90-dB measured one meter away with an input of 2.83V, which for a nominal 8-ohm speaker is 1 watt. The equivalent acoustic power of 90-dB is 0.001W, which means the speaker is only converting 0.1% of its power into sound! The remaining 99.9% of the power is being wasted as heat inside the voice coil. In fact, it isn't uncommon for voice coils to achieve temperatures in excess of 500 degrees Fahrenheit!

The typical horn loaded speaker generally has a 100-dB sensitivity. So for 1W of input you are seeing 0.01W of acoustic power. It's still a lousy efficiency of 1%, but still a huge improvement. Some horn loaded speakers will boast sensitivities as high as 110-dB, which is an efficiency of 10%! Now we're talking.

So what does this mean for the end-user? For starters it means you can use an amp with 1/10 to 1/100 the power capabilities and achieve the same SPL. Or you can use an amp with the same power capabilities and go 10-dB to 20-dB louder. As will be discussed later it also results in much less distortion.

The ideal directivity for any loudspeaker is a debated topic that is also very dependent on the application in question. Directivity (a.k.a. Polar Response) is a term used to describe the direction that the sound travels when it leaves the speaker. For any type of loudspeaker the directivity is actually a direct result of its shape. The typical direct-radiator usually resembles a concave circular prism. At lower frequencies where the wavelength is much larger than the diaphragm the sound is omnidirectional. At higher frequencies where the diaphragm is much larger than the wavelength, the sound starts to beam. Every direct-radiator will have similar behavior and this is the reason that tweeters tend to be much smaller than woofers.

Horns on the other hand can have whatever directivity the designer wants. It is still a function of the geometry of the transducer, but the designer can choose within limits any geometry for the horn.

Why worry about directivity? What if I sit directly on-axis with my speakers? The fact of the matter is that we hear both the on- and off-axis response. Our speakers reside in a room and there are always going to be reflections that make their way to the listening position. And in home theatre situations we are usually targeting multiple listening positions so it becomes even more critical.

As mentioned earlier, horns will have way less distortion for the same output. It is not within the scope of this article to cover every kind of distortion, but here are two of the more important ones:

Intermodulation Distortion (IMD) or Frequency Modulation Distortion (FMD), commonly referred to as "Doppler Distortion," is the result of low and high frequencies being reproduced by the same transducer. Low notes require the diaphragm to move further and when high notes are being reproduced at the same time there is a Doppler Shift. The higher frequencies change in pitch because they are riding along the larger movements of the lower frequencies.. The audible effect will be very similar to that of a tremolo. Since a horn reduces the cone excursion necessary for a given SPL, the horn is going to exhibit much less IMD.

Power Compression is the speaker's inability to linearly increase in output. Ideally, a doubling of the input power should result in a 3-dB increase in output. So a speaker that is 90-dB sensitive should output 93-dB with 2W from the amplifier. 4W would then be 96-dB, 8W 99-dB, 16W 102-dB and so on.. But as the output level increases, there will come a point where a doubling of the input power results in less than a 3-dB increase. This can be due to all sorts of things: nonlinear Bl curves, suspension compliance, voice coil temperature, etc etc.. Horns still suffer from the same nonlinear behavior, but it starts occurring at much higher SPL's. So when compared to direct-radiating systems, horns will exhibit much more realistic dynamics.

In the search for the utmost in sonic fidelity, the pocketbook is always the final limitation. So the real question is can better performance be achieved for the same dollar. Or in light of current society it seems the goal is the same performance being achieved for cheaper. Due to the 10-20dB advantage of horns, the drivers implemented can be much cheaper and don't need all the latest expensive driver technologies. On top of it all you don't need to spend as much on your amplifier either and you'll still end up with less distortion.