Sonist Concerto 2

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.

5. How the benefits of horn design have been incorporated into the design and fabrication of Sonist 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.

Waveguides are Specialized Horns

Technically speaking, all waveguides are horns, with a few specialized features which differentiate them from the generalized definition of a horn.  Waveguides are used to shape the wavefront of tweeters, narrowing the spread of the wavefront and providing a constant SPL level and frequency response within the included angle of the waveguide.   A waveguide can also be used on midrange and low frequency drivers, although at frequencies below 200Hz the wavefronts are omnidirectional.  The benefits of the directivity of a waveguide are improved frequency response and SPL levels within the included angle of the waveguide within the operating frequency band of the waveguide.  In addition, sidewall and floor bounce reflections are reduced by the controlled directivity.  These benefits are achieved in the Sonist Concerto 2 standmount and Concerto 3 floorstander through the use of integral waveguides formed in the 2" thick solid poplar baffle for both the ribbon tweeter and 8" woofer.  In addition to providing directional focus, the tweeter waveguide provides between 3 - 4dB of gain between 2500Hz and 8000Hz, so the signal level to the ribbon tweeter can be reduced, reducing distortion, since the ribbon tweeter diaphragm doesn't have to move as far for a given SPL level.  This improves soundstaging and transparency, and provides very low distortion at high SPL levels.  The woofer waveguide narrows the midrange wavefront of the 8" woofer between 1000Hz and 1400Hz, which makes room placement easier by reducing sidewall and floor bounce early reflections, thereby reducing notch filter effects caused by the out-of-phase reflections arriving at the ear later than the original wavefront.  The 8" woofer naturally narrows the frequency response starting around 1200Hz, where the dispersion angle is around 60 degrees, narrowing with increased frequency above 1200Hz.  The woofer waveguide also provides 1-2dB of gain between 1100Hz and 1400Hz, providing increased liveliness and dynamics.  The woofer crossover is designed to compensate for the increased output in the operating range of the woofer waveguide, providing a flat frequency response.  Here is a picture of the Sonist Concerto 2 standmount, showing the tweeter and woofer waveguides:

Sonist Concerto 2

Note: Please visit the Sonist website for additional design and construction information.  

 

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.

Sonist loudspeakers incorporate the benefits of horns by using integral waveguides for both the tweeter and woofer, reducing sidewall and floor bounce early reflections and providing increased dynamic range with lower distortion.  

Copyright 2006 by Randy Bankert, O. S. Services, Inc., all rights reserved.