High-frequency speakers

Loudspeakers generally pose no particular problems at mid-range frequencies. Apart from the specific problems of reproducing very low frequencies, loudspeakers have significant limitations at high frequencies. These limitations are mainly due to two causes:

• radiation-induced hyperdirectivity of vibrating surfaces;

• diffraction, which occurs as soon as the wavelength falls below a certain value.

When an enclosure becomes directive, radiated waves no longer propagate with spherical wavefronts. We generally observe an increasingly narrow main lobe supplemented by highly directive secondary lobes (beams). The presence of these secondary lobes leads to frequency distortion, resulting in altered timbres. The larger the diameter of the loudspeaker, the lower the frequency at which this effect occurs.

The only way to limit this effect without compromising low-frequency radiation is to entrust the reproduction of the different components of the spectrum to speakers of different sizes. This is the principle of the two- or three-way loudspeaker, which requires considerable optimization in the choice of connecting frequencies and appropriate crossovers.

Diffraction is the phenomenon of re-emission by an element subjected to an incident wave. As such, it is particularly applicable to enclosures, where it manifests itself in several forms:

• re-emission from all sides of the loudspeaker (especially the front) excited by the loudspeaker vibrations;

• modification of the loudspeaker's radiation under the effect of waves emitted by other components of the enclosure: other loudspeaker, vent, vibrating surface, etc. ;

• re-emission by the edges of the front face of the grazing waves emitted by the sources.

Of these three forms of re-emission, only the first can be easily controlled by the choice of material, its thickness, stiffness and damping. The other forms are linked to unavoidable physical phenomena and can only be reduced by special measures.

Re-emission by loudspeakers can be evaluated from the mutual radiation impedance, which depends essentially on frequency and intersource distance.

Edge re-emission is by far the most disruptive....