So why would anyone want to use horn speakers today, when we can have 100W
amps at low costs? As I hinted in the previous paragraph, horns DO have
other special qualities than just high efficiency. Okay, it's necessary to
understand a few basic things about HOW HORNS WORK in order to say WHY they
do some things better than other speakers. (You'd be amazed at how many
people who simply refuse to believe that horns can be used for serious music
listening because somebody told them so or because they think horns all
sound like megaphones.)
A horn can be viewed as an acoustic impedance transformer. Turning mechanical
motion (a vibrating diaphragm) into sound waves in air is in many ways a
difficult thing to do. The most fundamental problem, which has a lot to do
with the issue of efficiency, is that the difference in DENSITY between a
paper or metal diaphragm and AIR is huge. There is a tremendous impedance
mismatch. This fact explains that sound travels very far through denser
media like metal, water or rock. In a speaker-air situation, the speaker
diaphragm can be seen as a high impedance source (solid material) and the
air being a low impedance medium (the air does not easily load down solid
moving objects). There is a reason why humans can't fly by waving their
What the horn does is to help the transducer couple its radiated energy into
sound waves in air by means of an impedance transformation. What this means is
that it creates a higher acoustic impedance for the transducer to work into,
which means that more power is transfered. (Analogous to putting an antenna
on a radio transmitter, which seems like an obvious thing to do!) Basically,
a horn is a tube or conduit with increasing cross-section along its axis. The
narrow end (where the driver sits) is called the horn throat, and the large
end (which opens into the room) is called the horn mouth.
Sound pressure is defined as pressure change per unit area. In a horn, the
wave front is restricted by the inner walls of the horn, and the area across
the horn increases as the wave front approaches the horn mouth. So what
happens here is that at the throat we have a small area and high pressure
with small amplitudes, efficiently loading the diaphragm. As the wave fronts
travel towards the horn mouth, the pressure drops, while the amplitude and the
A horn also has the property of directing the sound into a narrower beam,
which increases the on-axis sensitivity (SPL/1W/1m). Increased directivity
combined with high electric-acoustic conversion efficiency means that horn
speakers are very easy to power, even with very small amplifiers.
What does all this really mean, then? In what ways does the horn 'help' the
driver/transducer. And how does all this make horn drivers a bit different
from direct radiators? I will try to sum this up in a few points:
-Improved energy conversion means that for a given SPL, a horn loaded
diaphragm will have to move less than a direct radiating diaphragm of equal
size. For any electromechanical transducer, the distortion generated by
the driver itself will be proportional to diaphragm excursion magnitude.
Thus, for any given SPL, the horn loaded speaker will have lower distortion
than the same size direct radiator.
-A smaller diaphragm on a horn can be used to generate the same SPL as a
larger direct radiating diaphragm for the same excursion amplitude. This
means that you have a smaller mass to accelerate for the same acoustic
output when you horn load a driver. This helps the transient response of
the speaker regardless of what Fourier said. Subjectively, horns will be
noted for their effortless, snappy handling of transients.
-The smaller diaphragm excursions allow the use of short, underhung voice
coils (reduced mass again) taking full advantage of the flux in the pole
piece gap. This increases the efficiency of the transducer, allowing the
amplifier to work with more headroom and greater ease. Horn drivers need to
have powerful magnets and tight magnetic coupling because of the high
pressure they are asked to produce when sitting in a horn throat.
-Because the amp has more headroom, and because the driver handles signal
peaks and high outputs more ideally, horns will be able to produce much
higher SPLs than comparable direct radiators before distortion becomes
objectionable. In short, there will be room for more dynamics, at lower
distortion, with better transient response, with less stress on the amp.
Since this is an enthusiastic pro-horn text, I have not emphasized the
problematic aspects of horns. One thing I haven't mentioned is that the
lower the frequency one wants to reproduce through a horn, the larger the
horn must be. The size of a horn quickly multiplies when you go down a few
octaves. Bass horns can be next to impossible to fit into a normal home.
This really shouldn't be a problem to a true enthusiast, but even I have
had to postpone any dream of a bass horn until I get a bigger place to live.
Some people say that horns have 'horn sound'. I'm not sure but I think what
they mean is a sort of megaphone-like quality to the sound. A good horn
should not have any of this. Unfortunately, many bad horn designs have led
people to think that this is how horns 'are supposed to sound'. To the true
horn fan, 'horn sound' will be a compliment that means, clean, dynamic,
'fast', physical, detailed and present sound.
Admittedly, there are a few things that horns don't always handle quite
as well as your typical small direct radiator speakers. Particularly the
much hyped concepts of 'imaging', 'neutrality' and 'transparency'. Horns
will often lean towards a more 'solid' (as opposed to 'transparent')
presentation with more 'body'. (Less ghost-like if you will!!) And horns
will definitely not sound laid back! The music will jump at you rather than
shyly hiding some place far behind the speakers.
Anyhow, horns have their problems like any kind of speaker, and can be
extremely sensitive to the room and the rest of the system. The enthusiast
would say that the advantages outweigh the disadvantages and that the problems