How do speakers make sound?
Speakers push and pull surrounding air molecules. Specifically, the movement of the diaphragm is what moves the air molecules. Thanks to the back and forth movement of the diaphragm, air molecules bump into each other in a domino effect. Depending upon the movement of the diaphragm, air molecules will bunch together in higher frequencies in some areas, and in lower frequencies in others. Places where the air molecules bunch together in high frequencies are, by definition, high pressure regions, and places where the air molecules bunch together in low frequencies are low pressure regions. These moving “pressure regions” are really just sound waves, and they propagate outwards from the speaker in a radial fashion. When they reach our ears, we perceive them as sound.
How do speakers convert electricity into sound?
At the very front of every speaker is a thin membrane, referred to as either the cone or diaphragm. Interestingly enough, with regards to the study of sound, a diaphragm is a device which converts mechanical motion into sound waves, or sound waves into mechanical motion. And that is just what the cone does: it converts mechanical motion into sound waves. The cone is attached at its outermost edge to the circular rim of the speaker. In some speakers, the cone is suspended and stabilized by the spider and suspension. They serve to keep the cone aligned with the voice coil, and ensure that the cone returns to its neutral position between vibration. The voice coil is a coil of iron wire attached to the underside of the cone near its center. The voice coil rests on top of/inside the permanent magnet, which is usually located at the very rear of the speaker. |
When electric current flows through the voice coil, it becomes an electromagnet thanks to Faraday's Law, which states that any change in the magnetic environment of a coil of wire will cause a voltage to be "induced" in the coil, and vice versa. Hence, the direction of the current passing through the voice coil determines the magnetic polarity of the voice coil. The polarity of the permanent magnet is always the same, so depending entirely upon the direction of current flowing through the voice coil, the voice coil will either be attracted to or repelled from the permanent magnet. And so, every time the current changes direction, the voice coil changes direction too. Now, because the voice coil is attached to the cone, any movement in the voice coil is translated directly to movement in the cone. Thus, the movement of the cone is ultimately determined by the direction of current flowing through the voice coil.
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When the electrical current flowing through the voice coil changes direction, the coil's polar orientation reverses. This changes the magnetic forces between the voice coil and the permanent magnet, moving the coil and attached diaphragm back and forth. |
How do speakers produce varying pitches and volumes?
The faster the diaphragm moves back and forth, the more waves are created per second. The more waves per second, the higher the frequency of the sound waves. And, the higher the frequency of the waves, the higher the pitch. The greater the distance the diaphragm moves back and forth, the more air molecules it will transmit/disrupt per wave. Thus, the greater the displacement of the cone, the higher the volume of the sound produced.
How can the simple back and forth movement of the diaphragm possibly recreate sounds as complex, layered, and numerous as those found in music?
Here's where it gets a little confusing. Sound waves are, by definition, longitudinal waves, meaning that the air they travel through moves back and forth, not up and down as in transverse waves. So, sound waves travel through air as compressions (high pressure regions) and rarefactions (low pressure regions). It is extremely difficult to visualize the complex wave forms of music as longitudinal waves. It is much easier to picture them as transverse waves, or curvy lines with peaks and troughs. Peaks represent high pressure areas (compressions), and troughs represent low pressure areas (rarefactions). The amplitude of the wave determines sound volume, and wave frequency determines sound pitch.
How can a single sound wave relay all the same sound as an entire orchestra? Well, each instrument has its own distinct sound wave. By the Superposition Principle, all the different waves of each instrument in the orchestra combine to form a single, very complex wave. Our ears are sensitive enough to pick out and distinguish the distinct sound of each instrument in the orchestra even when all the waves have been superimposed on one another. Even though we are only hearing one sound wave, we can still perceive and identify the original notes/sounds that combined to form the complex one. The speaker is just as capable of reproducing simple sound waves as it is reproducing complex sound waves. There is no real reason for complex waves to be any harder to reproduce, but to us humans, it just seems like it should be harder. It seems this way to us because we are not accustomed to the idea that a single sound source can simultaneously produce multiple different sounds/notes. It really is largely a matter of perception.
How can a single sound wave relay all the same sound as an entire orchestra? Well, each instrument has its own distinct sound wave. By the Superposition Principle, all the different waves of each instrument in the orchestra combine to form a single, very complex wave. Our ears are sensitive enough to pick out and distinguish the distinct sound of each instrument in the orchestra even when all the waves have been superimposed on one another. Even though we are only hearing one sound wave, we can still perceive and identify the original notes/sounds that combined to form the complex one. The speaker is just as capable of reproducing simple sound waves as it is reproducing complex sound waves. There is no real reason for complex waves to be any harder to reproduce, but to us humans, it just seems like it should be harder. It seems this way to us because we are not accustomed to the idea that a single sound source can simultaneously produce multiple different sounds/notes. It really is largely a matter of perception.