Created on 05/04/2015 10:23 PM.
Updated on 16.02.2020 16:05.
Author: A. Grudinin.
The most important structural elements of loudspeakers have remained unchanged since their invention at the beginning of the last century. Modern electroacoustics entered the market with the invention of the telephone by A.G. Bell and T. Watson in 1876. And although since then the improvement of electroacoustic transducers (that is, loudspeakers) has been the topic of an endless series of scientific studies and articles, much more than those devoted to any other element of the sound amplifying path, there are practically no fundamental changes.
Speaker history began in the distant 19th century.
The first patent application for a moving coil electrodynamic structure was filed in 1877, and for an electrodynamic loudspeaker in 1898.
However, these inventions did not receive practical application at that time – there was not yet a sufficiently powerful source that would allow shaking the head of a moving coil loudspeaker.
Commercial models did not appear until the 1920s when tube amplifiers became available. The first electrodynamic loudspeakers had high impedance coils, woven suspension and DC powered electromagnets. Some technical historians point out that the first electrodynamic head, as close as possible to its modern design, was patented in 1925 by the company General electric…
Externally, the speaker designs for bass and treble are different, but contain the same components. The woofer has a metal (less often plastic) frame, which is also called a basket for its shape or a diffuser holder – this is already its purpose.
Diffuser windows allow free air flow at the rear of the diffuser. In the absence of windows, air could act on the moving system as an additional acoustic load, reducing the low frequency response.
The technology for manufacturing the diffuser is determined by the power and dimensions of the head.
The main requirement is to provide a rigid structure, free from vibrations that can cause overtones. From this point of view, it is better to use cast structures made of metals or composite materials.
A conical diffuser is fixed to the frame, usually made of paper (in fact, from shredded wood), clean or filled with plastic and, less often, metal or ceramic.
A sleeve (impregnated paper or metal) is attached to the rear (narrower) part of the cone, onto which the voice coil is wound.
The voice coil is usually wound in two (less often – four) layers of copper or aluminum wire in enamel insulation on the frame (sleeve) and fixed on it with varnish. Typically, standard round wire is used, but for very heavy-duty heads, rectangular wire is used to provide almost 100% gap filling. Modern materials are widely used in the assembly of the head moving part.
For example, UV-cured polymer adhesives are used to glue the voice coil bobbin to a ceramic or metal dome.
The coil leads are connected to the contacts on the connection board using special, very flexible wires.
Despite ongoing research in materials science, most woofer and midrange drivers, which are similar in design but differ in size, use paper pulp cone diffusers. In addition, materials such as polypropylene, bextrene, and more recently light metals (aluminum, titanium, magnesium) are used.
Firms with a name and a history of speakers, having their own research centers or ordering development, are actively experimenting with various fillers and composite materials, creating combined diffusers.
Here, the most famous example is the B&W midrange heads with impregnated woven Kevlar cone.
Straight-line cones were used in low-frequency drivers only in the very first heads. The rigidity of such a structure is not enough for the entire operating frequency range, and above a certain frequency, the radiation acquires a bending character: only its central part actually works.
The diffuser is too heavy and too soft to accurately follow the movement of the coil. It simply does not have time to completely deviate and return, and bending vibrations generate overtones and additional coloration of the sound.
The simplest and most ancient way to combat this phenomenon is to form a series of concentric grooves on the surface of the cone during the manufacturing process. Modern loudspeakers use a whole range of measures to suppress parametric resonances.
Firstly, almost all diffusers have a curved generatrix.
Secondly, more and more of them are made of materials that effectively damp longitudinal vibrations and, in addition, they have a variable cross-section: the coil has a larger section, and the suspension has a smaller one. Of course, it all depends on the material chosen. For a paper diffuser, a special impregnation is suitable, and for a layered or composite structure, it is important to combine the physical and mechanical properties of its constituent materials.
Since the range of reproducible frequencies of a loudspeaker head is determined by the area of the piston movement of its cone, it is important that it is as rigid as possible, while still having a minimum mass.
The external suspension of the diffuser, which ensures its translational movement during operation, can be made as a whole with the diffuser (in the form of a corrugation with one or more grooves) or as a stand-alone ring made of rubber, rubber, polyurethane and other materials with similar properties, which is then glued to the outer edge of the diffuser. The suspension, especially of the low-frequency driver, must be very flexible: this provides a low natural resonance frequency.
Almost immediately below this frequency, the efficiency of the head drops sharply, that is, its own resonance determines the boundary of bass reproduction.
The second main requirement for the suspension is that the elastic properties must remain linear over the entire range of displacement of the loudspeaker’s moving system.
For a long time, the high-frequency heads had the same conical diffuser, only of a smaller size. However, the most common tweeter today is the domed diffuser. It can be soft (from textiles, for example, impregnated silk) or hard – from metal or ceramic.
The design of a typical tweeter differs in more than just the size of the cone. Typically, a suspended dome diffuser is manufactured in one piece, to which the voice coil sleeve is glued. At the same time, there is no flexible centering washer in the design. The magnetic system, like the diffuser, …