In the past ten years, many new wave guide technologies have been produced. The output of the line array can be adjusted by changing the arc of the line array can be roughly summarized into the following four types: ribbon tweeter, horn, acoustic mirror, and acoustic lens.
(1) Ribbon treble
Divide the circular outlet of the compression driver into equal parts, and reach the waveguide outlet through several equal-length paths. A typical high-output band tweeter is about 6 inches (0.15m) high, and it can meet the working conditions of line sound sources within the range of no more than 4.5kHz. After this frequency point is exceeded, the influence of side lobes needs to be considered.
The horn can provide continuous vertical coverage, but be aware of the limitations of geometric applications. The theoretical basis for changing the circular outlet to the square outlet is that the maximum difference of each path does not exceed λ/8, which is approximately in phase. Therefore, the diameter and depth of the horn must have a good ratio.
At the same time, for a line array module with a 12-inch (0.3m) paper cone, multiple compression drives and horns must be installed in a box about 14 inches (0.36m) high to reduce the distance between adjacent units. Try to conform to the working principle of the line sound source.
(3) Acoustic mirror
Reflective waveguides have been used for decades, but mainly in the microwave field, and have also been introduced to audio in recent years. It appears because the wavelengths of microwaves and sound waves are the same.
For example, the sound velocity of 13.76kHz sound waves is about 344 meters per second and the wavelength is 0.25 meters. The speed of light of 12GHz microwave is about 300,000 kilometers per second, and the wavelength is also 0.25 meters. Whether it is microwave or sound, the mirror works in "radiation" mode: due to the short wavelength of the impact energy, all the energy is reflected.
Acoustic mirrors complete the waveform transformation from the exit of the compression driver to the exit of the horn through the parabolic mirror. This design must pay attention to the effective frequency range of its "radiation" mode. The low frequency band has a longer wavelength and will cause refraction when encountering obstacles Or diffraction, not reflection.
(4) Acoustic lens
Similar to mirrors, lenses have also been used in the field of microwave and acoustic control in recent decades.
There are two main types: obstacle type and equal length refraction type. The working principle of the barrier type is like focusing light on a glass lens, similar to a lens. Its baffle (also called a phase plug, can be spherical, disc-shaped, ribbon-shaped or other irregular shapes, as long as it is different for the frequency involved. The size is small enough) when the sound passes through it, the speed of sound will be reduced.
An obstructive acoustic lens can produce acoustic focusing, acoustic diffusion or plane waves depending on its shape. The plane wave is used in the online array system. For this type of lens, a specific material must be selected in a specific frequency band. Above a certain frequency band, the material will absorb sound energy and convert it into heat and kinetic energy.
Below a certain frequency band, sound waves will pass without obstacles. Of course, these frequency points are very special for individual materials. Typical representative brands are L-Acoustics, ADAMSON and so on. The equal-length refraction type uses a dish-shaped baffle (compared with the wavelength, the space between the spacing is smaller) to strengthen the sound wave to be transmitted to a longer distance. For places where sound waves need to be refracted, the baffle can be placed in a "z" shape or simply inclined. The equal-length refractive acoustic lens can generate plane waves in a wide frequency band.
When higher frequencies pass, it works as a "beam effect". When the wavelength is longer, the "wave-like effect" of the acoustic lens will work. At this time, the equal-length refractive acoustic lens will act like closely spaced diffraction grooves. Typical representative brands are Renkus-Heinz and so on.
All waveform control techniques borrowed from microwave research are very useful for generating continuous high-frequency wavefronts. Because the spread angle is independent of the path length from the driver to the waveguide exit and the geometry of the horn. The line array loudspeaker using acoustic mirror or acoustic lens waveguide technology can well control the spread angle in the frequency range of 4-6 octave bands.