Electromagnetic speakers were invented around 1860, but it took over forty years to develop the knowledge of acoustics, materials, math and frequencies that gave birth to the electrodynamic speakers that we know today. The then-recent developments in vacuum tubes were extremely helpful regarding control of frequencies and power regulation, as well as amplification.
Nowadays engineers still apply stream and pressure to the paper when creating low-cost loudspeakers. The felting process is typically used since paper-coned loudspeakers are felted. A screen mould is made in the shape of the desired cone, and wet pulp is pressed into it. By varying the pressure, the heat, and the amount of pulp, engineers can create a variety of cones, ranging from tweeters to subwoofers.
It’s worth mentioning that paper cones can modify their weight when in humid environments, so cones can be made of different materials. The most used cone material is polypropylene since humidity is not an issue. These cones tend to have a flatter frequency response, but they also have downsides. The downsides of polypropylene is that it is difficult to fold and has a low melting point, which makes this material inappropriate for continuous high power use. There are also Carbon-fiber and Kevlar cones which are quite popular because these materials are extremely light, rigid and have a fast frequency response.
Electrodynamic speakers use electromagnets to provide a strong magnetic field for high wattage speakers. Electrodynamic speakers work by wrapping a large field coil around a core that produces a strong and steady magnetic field. The magnet is pot type shaped and includes a south pole in the center and a north pole in the periphery. The shape of the core allows the magnetic flux to remain concentrated in the annular gap between the poles. The voice coil is made of fiber or aluminum and located in the annular gap.
The audio signal from the amp’s output transformer affects the voice coil; this signal causes a varying magnetic field. As a result, both magnetic fields generate mechanical vibrations in the coil assembly. These vibrations run through the attached cone creating sound waves in the air and radiating energy and sound.