"A sound transducer as a symbiosis of art and science" 

At this point, we would like to offer you a small excursion into the theory of electroacoustic conversion, especially by horn systems:


Horn loudspeakers still represent the pinnacle of what is physically possible today. In particular, the enormous efficiency even at the lowest power levels provides an incomparable sound experience.


The term horn is used in sound reinforcement technology to describe a type of loudspeaker in which one or more drivers are coupled to the environment via a precisely defined sound channel with a constantly increasing cross-section. One type of construction is also called a pressure chamber loudspeaker.



Terms

In order to be able to clearly separate the terms, we will first agree on the following nomenclature:

  • Driver is the actual loudspeaker chassis that is built into the cabinet construction.
  • Horn is the sound channel from the neck opening (at the driver) to the mouth opening (to the outside world). Geometrically and acoustically the horn is determined by its contour, i.e. by the course of the cross-sectional area over the horn length.   
  • The horn throat is the smaller end face of the horn to which the driver is mounted directly or by means of a phase correction body.  
  • Horn mouth is the larger end surface of the horn, which emits the acoustic power to the environment. 
  • Solid angle depends on the location of the loudspeaker. A distinction is made between completely free suspension (4-Pi), installation on a surface (2-Pi), installation on a surface in front of a wall (Pi) and installation in a corner (Pi/2).  
  • The cabinet is the construction in which both driver and horn are installed. Of course, parts of the enclosure can also be components of the horn. Occasionally the cabinet is used to direct some of the acoustic energy radiated from the driver back into the cabinet to the outside. For example, a combination of bass reflex cabinet and horn loudspeaker is created.   
  • After all, the loudspeaker is the entire structure.

     


Basics

The task of a loudspeaker is to transmit the electrical energy supplied to it as efficiently as possible to the room surrounding it. There are four requirements in particular:

  • high efficiency (the supplied energy should be used to achieve the highest possible volume (keyword: characteristic impedance).   
  • high fidelity (the sound should not be distorted).  
  • small size, if the speakers are to be portable. In the case of fixed installations (e.g. in theatres, cinemas or discotheques), size no longer plays such a decisive role.   
  • the largest possible bandwidth (ratio of usable upper and lower frequency).


    These requirements influence each other. The difficulty in designing a horn is to find the best possible compromise between these requirements.

    Since there is always talk about frequencies and the corresponding wavelengths, here are some typical sounds and the corresponding frequencies and wavelengths (based on a sound velocity of 340 m/s):

       
  • Concert pitch a: 440 Hz or 0.77 m
  • Lower limit of the human hearing spectrum: 16 Hz or 21.25 m
  • Lowest tone on a modern bass guitar (low B): 30 Hz or 11.33 m
  • Lowest tone on the piano: 27.5 Hz or 12.36 m   
  • Highest tone on the piano: 4.22 kHz or 0.08 m


    A direct-radiating loudspeaker, i.e. a loudspeaker chassis, for example in a baffle, has, like any other acoustic radiator, an acoustic impedance which depends mainly on its geometry (here mainly diameter) and on the specific density and compressibility of the ambient air. If the wavelength of the signal to be transmitted rises above the circumference of the circular radiator, a mismatch occurs, which significantly reduces the efficiency of the electroacoustic transducer. One solution would be to increase the diameter considerably. However, this is regularly ruled out due to the tendency of a very large loudspeaker membrane to generate phase-rotated partial oscillations. In addition, there are often constructive reasons for not doing so.

    Especially in large sound reinforcement systems it is desirable to direct the sound energy to where it is needed; on the other hand, it is often necessary to avoid sounding other areas in order to reduce room reflections. The sound should therefore be directed. The easiest way to do this is to use a radiator (this always refers to the active part of a loudspeaker, i.e. the membrane parts that vibrate in phase) with a wavelength equal to or greater than the largest wavelength transmitted. At very low frequencies, this is only possible by using a sound guide (e.g. a horn) or loudspeaker arrays.

     


Horns as sound amplifiers

A characteristic feature of a horn as a sound amplifier is that a funnel-shaped device, which is in the broadest sense of the word, is attached to the small end of the horn, the diameter of which increases continuously from one end to the other. This horn principle is not a modern invention. Even in antiquity, the special shape of animal horns was used to produce the loudest possible signals. Further examples of the application of the horn principle outside of loudspeaker technology are:

  • Horn of a funnel gramophone
  • Brass instruments such as trumpets, trombones, tubas or alphorns
  • "Whispering bags", the predecessors of the megaphones, consisting of a funnel-shaped sheet metal tube with a speech opening at the small end (known e.g. from the helmsman in the rudder aft, who can strengthen his commands in this way)
  • Typhon or Makrofon, a particularly loud compressed air horn

The operating principle of an acoustic horn is that of an acoustic impedance transformer. Roughly simplified one could say that the horn increases the neck area (usually that of the driver) to the mouth area. With the increase in surface area, the acoustic impedance of the loudspeaker adapts much better to that of the surrounding medium, which, among other effects, results in a greatly improved efficiency. The principle can also be applied the other way round, e.g. at the bell of old telephone receivers (microphone side) at the Edison apparatus.

The lower cut-off frequency is determined by the momentum of the opening function (in the case of the exponential horn by the horn constant) and in a very important sense also by the mouth opening area.

A horn that fills a 4 Pi room (free placement, without adjacent walls at a significant distance) requires a mouth opening whose circumference corresponds to the lowest wavelength to be transmitted. Smaller solid angles allow the mouth opening to be reduced by the same amount, reducing the required mouth opening to 1/8th in a corner installation (as the Klipsch horn impressively and successfully demonstrates). However, practically realized horns - especially for the low frequency range - are often realized with mouth openings that are clearly too small. Although this drastically reduces the size of the instrument, it has proportional disadvantages in the ripple of the frequency response and in the drastic deterioration of the impulse response. Many "horns" thus turn out to be transmission line boxes after close examination and recalculation - with all their advantages and disadvantages. By "stacking", i.e. arranging identical horns with individually too small mouth openings into arrays (as known from large concerts) these problems are successfully eliminated, while the modular horn remains easily transportable.

Seriously, therefore, the construction begins with the mouth opening; the length or construction volume of the entire loudspeaker is then determined by the neck area and the momentum of the opening function. This means that the larger the neck area, by increasing the membrane surface, using several drivers or reducing the ratio of membrane surface to neck opening, the shorter the horn will be. As an extreme case, therefore, a loudspeaker is created whose circumference has the wavelength of the lowest frequency to be transmitted, the horn length zero.

This is why bass horns are usually built as so-called folding horns, i.e. the horn axis, which is straight in theory, is bent once or several times by 90° or 180° in order to make optimum use of the (e.g.) cuboid cabinet volume. If there are no standing waves in the cabinet, there is no negative effect on the linearity of the frequency response; according to Bruce Edgar even the distortion factor is improved by damping the harmonics. However, a negative influence on the upper frequency limit is to be expected.

During construction, great attention must be paid to mechanical stability, since high alternating pressures (especially when the intermediate walls of a folding horn are in phase opposition!


 

Advantages.

By coupling a horn to a driver, the radiation resistance increases earlier in frequency than when the same driver radiated freely. However, the final value of the radiation impedance is the same in both cases and depends only on the diaphragm diameter of the driver. In the case of a (high-frequency) complete adaptation of the driver, the addition of a horn does not result in a higher efficiency! Thinking the other way round, horn operation beyond the matching frequency of the free driver makes no sense. For a given diaphragm diameter this limits the useful upper frequency of the horn.

Nevertheless, many horns have a superior efficiency that clearly surpasses all other concepts (closed box: 0.1 to 2%, horn up to 50%). Certain amplification principles with low efficiency or low power output (e.g. Class A amplifiers, even with electron tubes) can only be operated sensibly with horn loudspeakers.

In broadband horn operation (over about a decade), however, this gain is only possible by using much more efficient drivers than those usually used with free radiators. On the one hand, this is possible because the driver diaphragm of the horn is much more heavily loaded and therefore deflected much less. The air gap can therefore be designed with a very small surface area, the magnetic field is thus highly concentrated. However, this alone is not enough; instead of ferrites, the typical horn driver also uses high-quality Alnico or neodymium magnets. The typical horn driver thus achieves characteristic sound pressure levels of 100 dB and more with free adaptation. Seen in this light, the horn only serves to push the matching limit further down, so that the driver can be adjusted over a wide lower frequency range.

Conversely, an average driver with a horn will disappoint, high efficiency will only be achieved in a narrow band far below the free matching (nasal characteristic), if broadband radiation is attempted, the efficiency will be close to the free beam values.

Due to the lower excursion of the horn system, the linear distortion will be lower and, what is more important, the intermodulation distortion will be much lower. Their system-related directivity plays a decisive role, especially where sound is to be addressed in a targeted manner (long throw) and/or where certain areas are not to be exposed to sound or are to be exposed only slightly. Horn loudspeakers are indispensable for professional sound reinforcement of large areas (stadiums) or volumes (halls).


A typical tweeter without horn can only achieve a continuous sound pressure level of 100 dB SPL, whereas a tweeter with horn can achieve about 115 dB. This is more than 30 times the sound pressure level (L=10*log(31.5)dB=15dB ) and therefore clearly perceptible. The larger the room to be sounded (up to an open-air situation) the more the drivers for lower frequencies have to be equipped with horns from the same argumentation.

Outdoors, one has to use the directional characteristic additionally to be able to realize the required sound levels at all, one is therefore even forced to combine whole batteries of 20 or more horns, whereby vertical towers or stacks are preferred. However, it can be observed that at the lowest frequencies, even outdoors, direct radiators are still used because of their compact dimensions, often massively parallel, e.g. 40 or 80 drivers with 18 inch diameter. The directional characteristic of such a large number of radiators can be additionally shaped by electronic delay circuits to improve the sound pressure in the desired range and to minimize radiation into undesired areas. This works in analogy to the figure-of-eight or cardioid characteristics of microphones.

 

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