Flat Diaphragm Electrodynamic Heads

The development of flat honeycomb loudspeaker heads began in the 1980s, but loudspeakers using them have only recently become available. Last year, our magazine has repeatedly introduced radio amateurs to acoustic systems of various classes based on cellular heads, which are produced by the St. Petersburg firm “Sound”. This information interested many readers who asked to tell in more detail about the advantages of such heads, to give the parameters. Meeting their wishes, we publish an article by former employees of VNIIRPA im. A.S. Popov, engaged in the development of heads with honeycomb diaphragms.

Many factors are known to affect the sound quality of a speaker, but it primarily depends on the speaker heads used in them. Considering these circumstances, experts in electroacoustics pay close attention not only to improving the designs of moving head systems, but also to the materials of the emitting elements.

As a result, flat-diaphragm heads have become widespread in recent years along with traditional cone diffusers.

The main requirements for the physical and mechanical parameters of the materials of the radiating elements are, as you know, high bending stiffness, low density, and high internal losses. The higher the first of the named parameters, the wider the frequency range of the head and the less the amplitude-frequency distortions it introduces into the signal.

The density of the material of the radiating element largely determines the sensitivity of the head and, finally, internal losses contribute to the damping of oscillations at resonant frequencies.

For decades, paper pulp has been used to make cone cones for drivers. With the development of Hi-Fi technology, leading manufacturers of high-quality speakers in order to increase the modulus of elasticity and losses of head diffusers began to use for their manufacture all kinds of composite materials based on cellulose (for example, cellulose with carbon or metal fibers).

However, it was not possible to significantly increase the rigidity of such materials because of the small internal elastic forces connecting the components.

For this reason, polymeric materials such as mylar, polyamide, polypropylene, polyvinyl chloride, olefin films, tergal, supronil, ceramic olefin, polymer graphite, etc. have begun to be used for the manufacture of low-frequency, mid-frequency, high-frequency heads of high-quality speakers. for these purposes composite (bextren, kobex, kapton), as well as laminated materials mylar and polyester with aluminum spraying, two-layer polypropylene).

The development of the technology of chemical vacuum deposition made it possible to obtain a number of layered metals (titanium-boron carbide, aluminum-magnesium, aluminum-sapphire, etc.). For the dome-shaped diaphragms of the midrange and high frequency heads, simple metals are also used: aluminum, titanium, beryllium alloys, porous nickel.

Tab. 1. Main technical characteristics of flat diaphragm heads

However, the production of many of the above materials requires very complex and expensive technological processes. In addition, they are not universal, that is, they cannot be used for the manufacture of radiating elements of all speaker links (LF, MF and HF heads).

For this reason, heads with radiating elements made from the materials listed above have not been able to replace heads with paper cone diffusers, and until recently remain just separate successful achievements of leading foreign companies.

The analysis of the properties of the materials of the radiating elements and the design directions of the loudspeaker heads for the Hi-Fi class speakers were constantly engaged in by the specialists of VNIIRPA them. A. S. Popova^{[1–3, 4]}… The results of fundamental research carried out by them in 1980-1990.^[5–8], have shown that a very interesting and promising direction in the design of dynamic loudspeaker heads is the use of flat honeycomb diaphragms as a radiating element.

One of the main advantages of this area is the suitability of such diaphragms for the design of bass, midrange, treble and even broadband loudspeaker heads, as well as the ability to create acoustic systems on them for all purposes, from automobile to Hi-Fi and High End speakers.

It is known that a flat honeycomb diaphragm has a three-layer structure: a honeycomb base made of aluminum foil, covered on both sides with sheathing made of sheet materials.

Three-layer honeycomb-based materials have been used in the aviation industry for many years. However, the specifics of the operation of the loudspeaker heads required the creation of new technological processes and special equipment for the production of honeycomb diaphragms.^[9]…

Many years of experience in the design of flat honeycomb diaphragm heads have revealed a number of their advantages over traditional paper cone heads.

First of all, heads with honeycomb diaphragms reproduce a wider frequency range with minimal amplitude-frequency distortions of the sound signal, which makes it possible to create speakers on their basis with an uneven frequency response in the operating range of +1.5 dB. The use of honeycomb diaphragms makes it possible to significantly reduce nonlinear distortions.

On their basis, it is possible to create more powerful loudspeaker heads, since the heat from the voice coils is dissipated in them through the diaphragm into the surrounding space, while in heads with paper cones through the parts of the magnetic circuit it enters the speaker.

The flat surface of the honeycomb emitters does not require special measures to align the radiation centers, which greatly simplifies the speaker design.

The electroacoustic parameters of the honeycomb heads are less influenced by the temperature and humidity of the air, and are more stable during mass production.

Currently, the company “Sound” has developed …