In sound reinforcement technology, the horn is 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 . The aim is to match the wave impedance of the loudspeaker to the characteristic acoustic impedance of the surrounding air .
In order to be able to clearly separate the terms used in this article, the following nomenclature is used:
- The driver is the actual loudspeaker chassis, which is built into the speaker structure.
- Horn is the sound channel from the neck opening (on the driver) to the mouth opening (to the outside world). The horn is geometrically and acoustically determined by its contour, ie by the course of the cross-sectional area over the length of the horn.
- The horn neck is the smaller end surface of the horn to which the driver is mounted either directly or by means of a phase correction body.
- The mouth of the horn is the larger end face of the horn that 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).
- Housing is the construction into which both driver and horn are built. Of course, parts of the housing can also be components of the horn. Occasionally, the housing is used to conduct part of the acoustic energy radiated into the housing by the driver to the outside. It arises z. B. a combination between bass reflex box and horn loudspeaker. Further housing components such as connections, protective edges, transport or assembly fittings will not be discussed here.
- Loudspeaker or box is ultimately the entire structure.
The task of a loudspeaker is to deliver the electrical energy supplied to it as efficiently as possible to the space around it. There are four requirements in particular:
- high efficiency (the highest possible volume should be achieved from the supplied energy, see also wave impedance )
- high fidelity (the sound should not be distorted if possible)
- small size, if the loudspeakers are to be portable. In the case of fixed installations (e.g. in theaters, cinemas or discos), size no longer plays such a decisive role.
- As large a bandwidth as possible (ratio of usable upper and lower frequency)
These four requirements influence each other. The difficulty in designing a horn is to find the best possible compromise between these requirements.
Since frequencies and the associated wavelengths are mentioned again and again in this article, here are some typical tones and the associated frequencies and wavelengths (based on a sound speed of 340 m / s):
- highest note on the piano: 4,220 Hz or 0.08 m
- Concert pitch A: 440 Hz or 0.77 m
- Lowest note on a modern bass guitar (low B): 30 Hz or 11.33 m
- lowest note on the piano: 27.5 Hz or 12.36 m
- Lower limit of the human hearing spectrum: 16 Hz or 21.25 m
A direct radiating loudspeaker, i.e. a loudspeaker chassis, for example in a baffle, has, like any other acoustic radiator, an acoustic impedance that is primarily dependent on its geometry (especially diameter) and the specific density and compressibility of the ambient air. If the wavelength of the signal to be transmitted increases beyond the circumference of the circular radiator, a mismatch occurs that significantly reduces the efficiency of the electroacoustic transducer. One solution would be to increase the diameter considerably. However, this is regularly ruled out because of the tendency of a very large loudspeaker membrane to generate phase-rotated partial oscillations. In addition, constructive reasons often speak against it.
With large public address systems in particular, it is desirable to direct the sound energy to where it is needed; on the other hand, one should avoid covering other areas with sound. So the sound should be directed. The easiest way to do this is if the radiator (meaning always the active part of a loudspeaker, i.e. the membrane parts vibrating in the correct phase) has a dimension that is the same or greater than the largest transmitted wavelength. At very low frequencies, this is only possible by using a sound guide (e.g. a horn) or loudspeaker arrays.
Horns as a sound amplifier
It is characteristic of a horn as a sound amplifier that a sound generator is attached to the small end of a device that is funnel-like in the broadest sense and that constantly increases in diameter from one end to the other, the sounds of which are bundled and radiated by the horn. This horn principle is not an invention of modern times. Even in ancient times, the special shape of animal horns was used (such as the shofar used in the Middle East ) in order to generate the loudest possible signals. Further examples for the application of the horn principle outside of loudspeaker technology are:
- Brass instruments such as trumpets , trombones , tubas or alphorns
- " Whisper bags ", the forerunners of the megaphone , consisting of a funnel-shaped sheet metal tube with an opening at the small end (known, for example, from the helmsman in the helm , who can reinforce his commands in this way)
- Horn of a funnel gramophone
- Typhon or Makrofon , a particularly loud air horn
The operating principle of an acoustic horn is that of an acoustic impedance transformer. Roughly simplified, one could say that the horn enlarges the area of the neck (usually that of the driver) to the area of the mouth. The increase in area results in a significantly better adaptation of the acoustic impedance of the loudspeaker to that of the surrounding medium, which, in addition to other effects, results in a greatly improved degree of efficiency. The principle can also be applied the other way around:
- Horn on old telephone receivers (microphone side)
- on the Edison apparatus
Basically, the considerations in the article apply to so-called "frontloaded horns", in which one side of the driver membrane works on the horn (the other in a closed box), i.e. sound is emitted exclusively via the horn. In the PA and musician area or with broadband drivers, however, "backloaded horns" are also built, where one side of the membrane radiates freely into higher frequency ranges. Interferences and runtime effects between the sound components emitted directly and via the horn lead, however, to barely calculable cancellation and exaggeration. In the ideal case, the horn puts such a load on the driver in the low frequency range that the membrane practically does not perform any bass strokes, i.e. it does not emit directly. (High frequency components are sometimes emitted by a buzzing cone.) A mechanical low-pass filter (pre-chamber) dampens the feeding of higher frequency components into the horn, just as the horn is often partially filled with damping material. The loudspeaker data according to Thiele and Small also cause a level drop at higher frequencies.
The lower limit 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 sounding in the 4-pi room (free installation, 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 to the same extent, which in a corner installation reduces the required mouth opening to 1/8 (as the Klipsch Horn impressively and successfully demonstrates). Practically realized horns - especially for the low frequency range - are often made with mouth openings that are clearly too small, which drastically reduces the size, but has proportional disadvantages in the waviness of the frequency response and in the drastic deterioration in the impulse behavior. Many "horns" turn out to be transmission line boxes after closer inspection and recalculation - with all their advantages and disadvantages. These problems are successfully eliminated by “stacking”, that is, arranging the same horns with individually too small mouth openings in arrays (as known from large concerts), while the modular horn remains easy to transport.
Seriously, therefore, the construction is started from the mouth opening; The length or the structural volume of the entire loudspeaker then results from the neck area and the momentum of the opening function. I.e. the larger the neck area, by increasing the membrane area, using more drivers or reducing the ratio of the membrane area to the neck opening, the shorter the horn will be. As an extreme case, there is a loudspeaker whose circumference has the wavelength of the lowest frequency to be transmitted, the horn length zero.
That is why bass horns are usually built as so-called folding horns, i.e. H. The horn axis, which is straight in theory, is bent one or more times by 90 ° or 180 ° in favor of an optimal use of the (e.g.) cuboid housing volume. If there are no standing waves in the housing, no negative effects on the linearity of the frequency response are to be feared; According to Bruce Edgar, this even improves the distortion factor by attenuating the harmonics. However, a negative influence on the upper limit frequency is to be expected. It must be stated, however, that great attention is paid to the mechanical stability during the construction, since high alternating pressures (especially in the case of intermediate walls of a folding horn that may be acted upon in antiphase!) Stress the construction.
When a horn is coupled to a driver, the radiation resistance increases earlier in frequency than when the same driver was radiant. However, the final value of the radiation impedance is the same in both cases and only depends on the diaphragm diameter of the driver. In the case of the (high-frequency) complete adaptation of the driver, the addition of a horn does not result in any greater efficiency. Conversely, horn operation beyond the adaptation frequency of the free driver makes no sense. For a given membrane diameter, this limits the useful upper frequency of the horn.
Nevertheless, many horns have a superior level of efficiency that more than clearly exceeds all other concepts (closed box: 0.1 to 2%, horn up to 50%). Certain amplifier principles with low efficiency or low power output (e.g. Class A amplifiers, also with electron tubes ) can only be operated sensibly with horn loudspeakers.
With broadband horn operation (over approx. A decade) this gain is only possible by using much more efficient drivers than is usual with free radiators. On the one hand, this is possible because the driver membrane in the horn is subjected to a much greater load and is therefore much less deflected. The air gap can therefore be designed with a very small surface, the magnetic field is thus highly concentrated. This alone is not enough, on the other hand, with the typical horn driver, higher quality alnico or neodymium magnets are used instead of ferrites . The typical horn driver thus achieves characteristic sound pressures of 100 dB and more with free adaptation. Seen in this way, the horn only serves to push the adaptation limit further down, so that work is carried out in an adapted manner over a wide lower frequency range. Conversely, an average driver with a horn will be disappointing, a high level of efficiency can only be achieved in a narrow band far below the free adaptation (nasal characteristic), if you try to achieve broadband radiation, the level of efficiency will be close to the free radiation values.
Due to the lower deflection of the driver membrane, the horn system causes lower non-linear distortions and what is more important, significantly lower intermodulation distortions. Their system-related directivity plays a decisive role where sound is to be specifically addressed (long throw) and / or where certain surfaces are not to be exposed to sound or only to a limited extent. Horn loudspeakers are indispensable for the professional coverage of large areas (stadiums) or volumes (halls).
Horn loudspeakers, regardless of the frequency range, are complex and generally expensive to develop and manufacture. Especially horns for low frequencies are either extremely large (e.g. 3 m length with 10 m² mouth area) or require a wall or corner of the room as an extended sound guide, which allows the required dimensions to be reduced (see above under solid angles). However, this limits the choice of location and can lead to problems with room modes .
The increased acoustic coupling and increased radiation of active acoustic power due to the improved radiation resistance also works the other way round (reciprocally): Room resonances have a strong influence on the horn driver, while directly radiating chassis are practically not influenced by the room, so that electrical or mechanical measurements can usually be made on them without an absorber chamber and without evading into the open field .
Any kind of sound guidance allows standing waves and thus resonances. Such resonances can often be observed in horns and are very difficult to avoid, especially in the high frequency range. That is why so-called multicellular horns were built. Nevertheless, with direct radiators, significantly fewer resonances can usually be observed.
Horns form an acoustic high pass with z. T. extremely steep sound pressure drop. The group delay effects of this drop are far stronger than those of direct emitters.
At the lower limit frequency, the acoustic load on the membrane decreases significantly. Especially the drivers that are optimized for horn operation can be mechanically destroyed at these frequencies. Such horn drivers should never be operated without a horn, not even for test purposes. When amplifiers are switched on or in the event of overload or due to errors, signals with low frequencies or even with a constant component may be present at the terminals of the driver (pops or clicks). An electrical high-pass filter (at least one capacitor in series) and a steep electronic high-pass filter are therefore usually unavoidable, with all the negative effects on the group delay.
The distortion of a chassis or driver is not only dependent on the deflection of the membrane, even with the smallest deflections there is always a residual amount of distortion. Especially high-performance horns with strong compression (pressure chamber) have additional distortions due to non-linear air compression and non-linear deformations of the driver membrane.
The horn transforms the small area of the neck into a large area of the mouth. The situation at the mouth is the same as if an equally large direct radiator were used, the membrane of which has the shape of the wave front at the horn mouth. At wavelengths that are small compared to the mouth of the horn, bundling phenomena occur which, at lower frequencies, change into spherical radiation. This further narrows the usable frequency range. These conditions are largely independent of the horn contour. B. also with the so-called spherical horns. However, it is often misunderstood that such horns on the mouth represent the situation of an actual spherical wave (i.e. a radiator that is very small compared to the wavelength) with its direction-independent characteristic. This is not the case.
So there are a number of problems with using horns. On the other hand, good direct emitters are quite sufficient in normal living spaces and are even able to generate hearing-damaging sound levels. These are chassis that, due to their strong (and therefore somewhat more expensive magnets), generate reference levels of 96 dB SPL, with 1 watt input power at a distance of one meter. It is certainly true that typical products have very poor efficiencies compared to only 86 dB SPL, namely only a tenth of that! With a correct design, especially in the low-frequency range (several paths), structures that are not too large (<0.5 m³) are not obtained that play down to very low frequencies (<20 Hz) without annoying clinking. In addition, it is possible to improve the radiation resistance and at the same time the level stability of direct radiators by means of parallel arrangements (groups). This is limited by the fact that with larger overall dimensions, bundling phenomena set in earlier (but this also applies to horns). The mandatory technical authorization of horns arises in somewhat larger rooms (small halls), first of all with the tweeters, which, due to the structural requirements of the small driver diaphragm mass and the small diameter, can hardly absorb more than 10 watts of power loss (even if always significant larger numbers are given) and thus limit the system in such situations or even fail. A typical tweeter without a horn can therefore only achieve a continuous sound level of 100 dB SPL, but a tweeter with a horn around 115 dB. That is more than 5.5 times the sound pressure ( L = 20 · log (5.65) dB = 15 dB) and is therefore clearly perceptible. The larger the room to be covered with sound (up to the open air situation), the more the chassis for lower frequencies must be provided with horns for the same reasoning. In addition, outdoors you have to use the directional characteristic in order to be able to achieve the required sound level at all. One is therefore even forced to combine entire batteries of 20 or more horns, with vertical towers or stacks being preferred. However, it can be observed that at the lowest frequencies, direct radiators are still used due to their compact dimensions, even outdoors, often massively parallel, e.g. B. 40 or 80 chassis with 18 inch diameter. The directional characteristics of such a large number of radiators can be additionally shaped by electronic delay circuits in order to improve the sound pressure in the desired area and to minimize radiation in undesired areas. This works in analogy to the figure-of-eight or cardioid characteristics of microphones.
The main difference between the various horns is their geometry, i. H. by increasing the horn diameter from the throat to the mouth opening. The determining feature for the attainable lower limit frequency of the horn is the size of the mouth opening. Each of these forms basically represents an approximation to solve the partial differential equation for sound propagation under known boundary conditions . The wave equation is effectively reduced to a one-dimensional problem. In all analytical approaches, the practically finite length of the horn remains an open problem, so that the transition of the sound radiation from the horn funnel into the surrounding space must be postulated ad hoc .
The oldest and most common type of horn is the exponential horn. The cross-sectional area of the horn from the neck to the mouth expands according to the exponential function:
- = Mouth cross section of the horn
- = Cross section of the neck of the horn
- = Euler's number
- = Funnel length
- as the funnel constant results from the function , where the desired lower limit frequency of the horn and the speed of sound denotes.
Many classic horn loudspeakers use this method. The exponential horn assumes that the sound propagates in the horn as a plane wave and thus detaches from the horn mouth.
Spherical wave horn
The contour of the spherical wave horn is a tractrix , which means that a spherical wave shape is required in the design. This follows the idea that the waveform should always be perpendicular to the horn walls.
The acoustic impedance at the horn neck of the conical horn, which is largely proportional to the frequency response in the lower frequency range of a horn, shows a premature drop towards lower frequencies. Depending on the selected geometry of the conical horn, which is identical in length, neck and mouth diameter, and of the exponential horn, the lower limit frequency of the conical horn is at least two octaves above that of the exponential horn. However, the ripple in the lower frequency range is significantly lower.
Other horn contours or rules for determining the cross-sectional areas cause either less favorable impedance adjustments or a far more wavy frequency response.
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