Horn loudspeaker
A horn loudspeaker is a loudspeaker or loudspeaker element which uses an acoustic horn to increase the overall efficiency of the driving element. A common form consists of a compression driver which produces sound waves with a small metal diaphragm vibrated by an electromagnet, attached to a horn, a flaring duct to conduct the sound waves to the open air. Another type is a woofer driver mounted in a loudspeaker enclosure which is divided by internal partitions to form a zigzag flaring duct which functions as a horn; this type is called a folded horn speaker. The horn serves to improve the coupling efficiency between the speaker driver and the air. The horn can be thought of as an "acoustic transformer" that provides impedance matching between the relatively dense diaphragm material and the less-dense air. The result is greater acoustic output power from a given driver.
The narrow part of the horn next to the driver is called the "throat" and the large part farthest away from the driver is called the "mouth". The angular coverage of the horn is determined by the shape and flare of the mouth. A major problem of horn speakers is that the radiation pattern varies with frequency; high frequency sound tends to be emitted in narrow beams with poor off-axis performance. Significant improvements have been made, beginning with the "constant directivity" horn invented in 1975 by Don Keele.
The main advantage of horn loudspeakers is they are more efficient; they can typically produce approximately 10 times more sound power than a cone speaker from a given amplifier output. Therefore, horns are widely used in public address systems, megaphones, and sound systems for large venues like theaters, auditoriums, and sports stadiums. Their disadvantage is that their frequency response is more uneven because of resonance peaks, and horns have a cutoff frequency below which their response drops off. To achieve adequate response at bass frequencies horn speakers must be very large and cumbersome, so they are more often used for midrange and high frequencies. The first practical loudspeakers, introduced around the turn of the 20th century, were horn speakers. Due to the development in recent decades of cone loudspeakers which sometimes have a flatter frequency response, and the availability of inexpensive amplifier power, the use of horn speakers in high fidelity audio systems over the last decades has declined.
Operation
An acoustic horn converts large pressure variations with a small displacement area into a low pressure variation with a large displacement area and vice versa. It does this through the gradual, often exponential increase of the cross sectional area of the horn. The small cross-sectional area of the throat restricts the passage of air thus presenting a high acoustic impedance to the driver. This allows the driver to develop a high pressure for a given displacement. Therefore, the sound waves at the throat are of high pressure and low displacement. The tapered shape of the horn allows the sound waves to gradually decompress and increase in displacement until they reach the mouth where they are of a low pressure but large displacement.Technology history
The physics of horn operation were developed for many years, reaching considerable sophistication before WWII. The most well known early horn loudspeakers were those on mechanical phonographs, where the record moved a heavy metal needle that excited vibrations in a small metal diaphragm that acted as the driver for a horn. A famous example was the horn through which Nipper the RCA dog heard "His Master's Voice". The horn improves the loading and thus gets a better "coupling" of energy from the diaphragm into the air, and the pressure variations therefore get smaller as the volume expands and the sound travels up the horn. This kind of mechanical impedance matching was absolutely necessary in the days of pre-electrical sound reproduction in order to achieve a usable sound level.Megaphone
The megaphone, a simple cone made of paper or other flexible material, is the oldest and simplest acoustic horn, used prior to loudspeakers as a passive acoustic amplifier for mechanical phonographs and for the human voice; it is still used by cheerleaders and lifeguards. Because the conic section shape describes a portion of a perfect sphere of radiated sound, cones have no phase or amplitude distortion of the wavefront. The small megaphones used in phonographs and as loudhailers were not long enough to reproduce the low frequencies in music; they had a high cutoff frequency which attenuated the bottom two octaves of the sound spectrum, giving the megaphone a characteristic tinny sound.Exponential
The exponential horn has an acoustic loading property that allows the speaker driver to remain evenly balanced in output level over its frequency range. The benefits of the design were first published by C.R. Hanna and J. Slepian in 1924 for the American Institute of Electrical Engineers. A major drawback is that the exponential horn allows for a narrowing of the radiation pattern as frequency increases, making for high frequency 'beaming' on axis and dull sound off axis. Another concern is that a throat of small diameter is needed for high efficiency at high frequencies but a larger throat is best for low frequencies. A common solution is to use two or more horns, each with the appropriate throat size, mouth size and flare rate for best performance in a selected frequency range, with sufficient overlap between the frequency ranges to provide a smooth transition between horns. Another solution tried in the late 1930s by Harry F. Olson of RCA was to use multiple exponential flare rates, either by connecting increasingly larger horns in series or by subdividing the interior of a single horn. Exponential horns continue to be used by some designers, and in some applications.Multicell
A number of symmetrical, narrow dispersion, usually exponential horns can be combined in an array driven by a single driver to produce multicell horns. Patented in 1936 by Edward C. Wente of Western Electric, multicell horns have been used in loudspeakers since 1933 to address the problem of directivity at higher frequencies, and they provide excellent low frequency loading. Their directional control begins to beam both vertically and horizontally in the middle of their target frequency range, narrowing further at high frequencies with level changes as great as between lobes. Multicell horns are complex and difficult to fabricate and thus have a higher associated expense. They persisted in public address applications for many years because, even with their faults, they sounded very good, and still do with competent design. The revolutionary coaxial driver, the Altec Lansing Duplex 601 and 604, used a multicell horn for its high frequency component from 1943 to 1998.Radial, sectoral, and diffraction
Radial horns have two surfaces based on an exponential flare rate, and two straight walls that determine the output pattern. The radial horn exhibits some of the beaming of the exponential horn. Altec sectoral horns were radial horns with vanes placed in the mouth of the horn for the stated purpose of pattern control. For ease in mounting to loudspeaker cabinets, flat front radial horns have been used, for instance by Community in their SQ 90 high-frequency horn. JBL's diffraction or "Smith" horn was a variation on the radial design, using a very small vertical dimension at the mouth as a method of avoiding the mid-range horizontal beaming of radial horns that have a larger vertical dimension at the mouth.The diffraction horn has been popular in monitor designs and for near-field public address applications which benefit from its wide horizontal dispersion pattern. Counterintuitively, the narrow vertical dimension provided for an expansive vertical output pattern approaching 90° for frequencies of a wavelength equal to the narrow vertical dimension. A very small version of the diffraction horn was designed in 1991 into the JBL model 2405H Ultra-High Frequency Transducer, yielding a 90° x 35° output pattern at.