How does the sound come from the speakers of the magnets

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By Hans-J├╝rgen Humbert

The principle of the electrodynamic sound transducer was invented in 1925 by the American company Western Electric, and around 90 percent of all loudspeakers on the market today still work with this method.

A loudspeaker consists of a strong ring-shaped magnet with an iron core on the inside. There is a small gap between the magnet and the iron core. Depending on the design of the loudspeaker, this is a fraction of a millimeter or a few millimeters. In this gap there is a moving coil with many turns of copper wire. The speaker membrane is suspended from the top of the coil.

If you now send electrical current through the coil, it is either drawn deeper into the gap or pushed out of it, depending on the direction of the current. Since the coil is firmly connected to the membrane, the movements of the coil are transferred to the membrane. This pushes the air molecules, and there is an audible "pop". With an alternating current, the coil oscillates continuously back and forth at the frequency of the alternating current, and the membrane produces an audible tone at the same frequency. With an audio signal fed in, you can now hear music.

So a loudspeaker is built quite simply, but why are high-quality loudspeakers so expensive?

theory and practice

A problem with loudspeaker production is that the membrane has a certain weight. That has to be accelerated or slowed down again. The larger the membrane, the greater its mass. In the event of rapid changes in the alternating current, the membrane can no longer follow properly, and distortions occur. Apart from custom-made loudspeakers, a single loudspeaker is hardly able to reproduce the entire human hearing range from 16 Hz to 20 kHz. That is why the listening spectrum is divided into different areas and fed to different loudspeakers.

The low tones reach woofer or bass loudspeakers, which have quite large membranes. Mid-range or high-range speakers, on the other hand, manage with small membranes. A so-called crossover network - consisting of coils, capacitors and resistors - ensures that each loudspeaker only receives the frequency spectrum that it can process. This reliably prevents distortions during playback.

Problem case housing

For example, if a loudspeaker is operated openly, i.e. without a housing, it sounds tinny and quite quiet. Why is that? To do this, let's take a closer look at the loudspeaker at work. If the membrane moves forward, an overpressure is created in front of the loudspeaker and at the same time a negative pressure behind the membrane. The air now flows around the loudspeaker and immediately compensates for the pressure difference - an "acoustic" short circuit is created.

Solution 1: You artificially extend the path of the air. To do this, the loudspeaker is built into a plate - it remains open at the back. The air now has to travel a long way to compensate for the pressure difference. The sound quality is increased enormously.

Solution 2: The loudspeaker is installed in a closed box. The acoustic short circuit has been eliminated and the loudspeaker sounds much better and louder. But due to the dimensions of the box, resonances can develop and amplify certain tones while others are swallowed up. This resonance behavior can only be brought under control with complex calculations and technical tricks during construction. This also explains why good speakers are expensive.