Electromagnetic Speakers History
Back in the early part of the 20th century, people were still exploring the basic knowledge of math and frequencies. While the electromagnet part of the speaker originates from the 1860s, it took 40 more years to develop a knowledge of acoustics and materials. C.W. Rice and E.W. Kellogg finally invented it by solving the final part of the puzzle. The final work was on how to shape the diaphragm and what materials to use. The then-recent developments in vacuum tubes in the 1910s helped to do the job of sophisticated control of frequencies and power regulation and amplification. Today an engineer gets training in these areas if they choose after a few years of college.
Applying steam and pressure to the paper is a construction process that people still use today in low-cost loudspeakers. The more common approach is felting. Most paper-coned loudspeakers are felted. A screen mold is made in the shape of the desired cone, and wet pulp is pressed into it. By varying the pressure, heat, and the amount of pulp, engineers can make many different cones from tweeters to subwoofers. Paper cones can change weight in relation to humidity, prompting the construction of cones from different materials.
Polypropylene cones are a common type. Humidity does not affect them and they also have a flatter frequency response. However, it is difficult to bond the surrounding material to the polypropylene cone. Another disadvantage is the low melting point of polypropylene, which makes it unsuitable for continuous high power use. Kevlar and Carbon-fiber cones are popular in that these materials are extremely light and stiff, and they have a faster response than paper or polypropylene.
The base for electrodynamic speakers, as its name suggests, is an electromagnet. In order to provide a very strong magnetic field for high wattage speakers, engineers use an electromagnet instead of permanent magnets. The working principle of an electrodynamic speaker is the same as that of the permanent magnet type. Loudspeakers of more than 25 watts up to a few hundred watts of electrodynamic type. A large field coil wrapped around a core produces a strong and steady magnetic field. The shape of the magnet is pot type with South pole in the center and North pole in the periphery.
The special shape of the core allows magnetic flux to remain concentrated in the annular gap between pole pieces. The voice coil is wound on fiber or aluminum (to keep it lightweight). It is further placed in the annular gap. The audio signal from the amplifier’s output transformer applyes to the voice coil. The signal causes a varying magnetic field. The resultant introduction between two magnetic fields (one due to the electromagnet and the other due to audio current in the voice coil) produces mechanical vibrations (motor action) in the coil assembly, which corresponds to the audio signals. The vibrations of the coil transmit to the attached cone which creates sound waves in the air in listeners area and hence radiates energy sound energy directly.