HORN LOUDSPEAKER DESIGN - THEORY AND PRACTICE

by Randy Bankert - O. S. Services, Inc.

Purpose of this article

1. To briefly explain the history of horn loudspeakers.

2. How and why horn loudspeakers were replaced by conventional direct
radiator loudspeakers in most consumer applications.

3. How and why horn loudspeakers are becoming popular once again.

4. The benefits of horn loudspeakers over conventional direct radiator
loudspeakers.

State of the horn

Recent developments in vacuum tube amplifier design and improved components
have renewed interest in single-ended triode amplification, and many different
brands and models of low powered tube amplifiers are currently available. The
resurrection of the directly heated triode vacuum tube for audio usage has been
accompanied by renewed interest in highly efficient loudspeakers, so that the
magical characteristics of accurate harmonic tonality, immediacy, transient
detail and micro-dynamics, and involving presentation of single-ended directly
heated triodes can be heard at higher levels than would otherwise be
possible.

Nutshell history of triodes and horns

The invention of the triode tube in 1923 enabled an electrical signal to be
amplified to greater levels than had been possible, and the triode could produce
an audible output when used to drive a rudimentary headphone device known at the
time as a "receiver." The receiver consisted of a fixed coil on an iron anchor
which produced vibrations in a metal diaphragm close to the coil. The first horn
speaker was basically a horn attached to a receiver, and although the sound
quality was quite bad it was better than what had been possible prior to the
invention of the triode output tube.

Due to the low power output of the triode, high efficiency loudspeakers were
required to produce sound pressure levels sufficient to be heard in a room,
which happened in 1927 when two Bell Laboratory engineers, Wente and Thuras,
invented the compression horn driver, commonly known as a driver. Their driver
used a field coil to magnetize the pole pieces (permanent magnets at that time
did not have a practical strength/size ratio to be of practical use) and
incorporated an under-hung edge wound aluminum ribbon voice coil.

The diaphragm was an inverted aluminum dome attached to a self supporting
voice coil. The Wente and Thuras driver also had a phase plug; a device placed
between the diaphragm and horn throat which enabled the sound waves from the
diaphragm to merge into a coherent wave front in the horn throat.

Direct radiator drivers

As time went on, Rice and Kellogg "invented" the direct radiator speaker
(conventional driver). Horns began to disappear from the scene, being big and
unwieldy. Soon after World War II, more powerful push-pull pentode amps began
relieving the humble triode of its duties. Later on, the ultimate power/dollar
rating transistor amps arrived. As the need for high efficiency loudspeakers
diminished, manufacturers began cutting costs by using low efficiency smaller
magnets in their drivers. Although horns no longer had a major share of the
market, they survived in movie theatres, PA applications, and other applications
where their high efficiency and low distortion outweighed their typically large
cabinet volumes.

Drawbacks of conventional direct radiator loudspeaker
designs

While horns offer a dramatic increase in dynamic capability, image size, and
presence, with harmonic distortion less than one quarter of the value found in
audiophile direct radiator systems, most direct radiators severely compress
dynamic contrasts and reduce image size. In addition, many direct radiator
designs suffer from dynamic compression coloration, where the highs and lows are
rolled off at high SPL's, resulting in a perceived midrange boost. Many direct
radiator designs are not efficient enough to be used with low-powered
single-ended triodes, and even with high-wattage and high-current amplification,
these loudspeakers sound polite and uninvolving compared to high efficiency
loudspeakers. Single-ended tube amplifiers need not apply, for obvious reasons
(low wattage).

Special qualities of horns

What special qualities do horns have that endear them to enthusiasts besides
having very high efficiencies? The horn may be viewed as an acoustic impedance
transformer. When a diaphragm vibrates, pressure waves are created in front of
it. This is the sound we hear. Coupling the motion of the diaphragm to the air
is not an easy thing to do due to the very different densities of the vibrating
diaphragm and air. This can be viewed as an impedance mismatch. We all know that
sound travels better in high density materials than in low density materials,
and in a speaker system, the diaphragm is the high density (high impedance)
medium and air is the low density (low impedance) medium. The horn assists the
solid-air impedance transformation by acting as an intermediate transition
medium. In other words, it creates a higher acoustic impedance for the
transducer to work into, thus allowing more power to be transferred to the
air.

A horn is a tube whose cross-section increases exponentially. The narrow end
is called the throat and the wide end is called the mouth. The transducer is
placed at the throat. When the diaphragm moves near the throat, we have a high
pressure with a small amplitude in a small area. As the pressure wave moves
towards the mouth, the pressure decreases and the amplitude increases. Excellent
natural efficient amplification.

As mentioned earlier, horns have very special properties, including lower
distortion than conventional drivers, faster transient response than
conventional drivers, and are easier to drive at high SPL's than conventional
drivers.

Lower distortion at a given SPL: For an equivalent SPL, horns require a
smaller diaphragm, and since distortion is directly proportional to the size of
the diaphragm, a large diaphragm electromechanical transducer (conventional
driver) has to move much more than a horned diaphragm in order to create the
same SPL (sound pressure level). The larger the excursion, the worse the
distortion. So, for a given SPL, a horn loaded system will generate much lower
distortion than an electromechanical transducer.

Faster transient response: Since the diaphragm is smaller, it is lighter
and thus it accelerates and decelerates faster. This, in the real world means
superb, fast snappy transients. As the excursion of the diaphragm is very small
as compared to an electromechanical transducer, the voice coil is much smaller
and again, this translates to a lower moving mass and again, results in fast
transients.

Higher SPL's with a given input wattage: Small voice coils also take full
advantage of the flux in the pole piece gap. This increases the efficiency of
the transducer allowing the amplifier to work with greater ease. Since the
amplifier has more headroom and the driver handles peaks and high outputs more
efficiently, horns are able to produce much higher SPL's before they
distort.

Thus, in the normal operating range, horn designs are faster, more dynamic,
have a better transient response, have less distortion, and are easier for an
amplifier to drive than conventional driver designs.

Horns have impact!

You feel the music, you become part of the music, and the music becomes part
of you. The full-range phase coherent wave front of horns produces a lifelike
presentation with tremendous dynamic range, as opposed to "polite", compressed
presentation of low efficiency conventional driver designs. Horns will never
sound veiled or compressed.

Due to their inherent benefits of low distortion, high efficiency, fast and
accurate transient response, and wide dynamic range, horn loudspeakers provide a
pure, unadulterated musical presentation, a more organic and natural recreation
of the acoustic event.

Synopsis

Lifelike and involving music is made possible by the combination of highly
efficient and low-distortion horn loudspeakers and low-powered single-ended tube
amplifiers.

Modern horn loudspeakers have utilize classic horn design principles and
improved upon them to provide a harmonically accurate, dynamic presentation
which enables the music to come to life, in an emotional, involving way.

Modern horn loudspeakers are capable of handling hundreds of watts if
desired, making the line versatile and easy to use with a variety of associated
equipment.

There is virtually no dynamic compression coloration with modern horn
loudspeakers, the correct frequency response is obtained at all volume levels,
unlike many conventional multi-driver dynamic loudspeakers, which roll off the
treble and bass at high volumes.