To answer this question we need to understand how a speaker works. The fundamental part of the loudspeaker is a membrane that moves following the voltage variations of an electrical signal. This membrane moves back and forth pushing the air molecules that are in contact with it, and thus the sound wave that we then hear is created. Let's see how. If you think about the front of the membrane, the one in sight, when moving forward it pushes the air molecules that are in contact with it, which causes the air in that area to acquire a density greater than that of the air at rest, that is, silently.
Since air is an elastic medium, molecules try to regain their equilibrium state by expanding again. This causes the area that previously had higher density, or pressure, than normal, now has less than normal. In addition, as they expand the air molecules compress those next to them, which will immediately try to expand again by pushing the next ones and so on. This is the way in which sound propagates, which as you can see is simply a pressure disturbance that propagates through an elastic medium, air in this case, and reaches our ear.
The back of the membrane does the same, but in counterphase, that is, when the front generates an overpressure, the rear generates a rarefaction (pressure below the resting level), so we can say that the back of the membrane generates another sound wave of opposite sign to the front. To prevent both from interfering and damaging the sound field that we intend the speaker to generate, we enclose the membrane in a box that prevents the back wave from propagating. That's why, even if you turn the speaker over, it still radiates sound only forward, because it's designed to work that way. You would have to make a hole in the back of the box to change its operation.
An interesting, and certainly less destructive, experiment might be to point the loudspeaker toward a nearby surface, and listen to the sound that surface reflects. If the surface is hard, for example the wall or the floor, it will be very reflective, and the sound it reflects will look quite similar to the original, that is, the one you would perceive with the speaker facing you. But if the surface is soft, for example a sofa, it will not reflect all the sounds equally, because it will absorb the highest sounds. Then the reflected sound, the one you can hear, will be more serious.
And put to experiment, I leave you another idea: if the speaker does not have rubber feet, or you can remove them, try to put it on top of a not very heavy piece of furniture, or a wooden box. With a little luck, the vibration of the speaker is transmitted to the surface where you put it, and this behaves like the soundboard of a piano, amplifying the sound.
And when you finish with the experiments, if your speaker is part of a stereo equipment (which is old, but we still have it in many houses), do not forget to put it back in the right place, that is, at the height of your head and forming an equilateral triangle with the other speaker and your favorite armchair to listen to music.
Soledad Torres Guijarro is a PhD in Telecommunications Engineering, professor and researcher at the School of Telecommunications Engineering of the University of Vigo.
Question sent by José Félix Pereira Ríos
Coordination and writing: Victoria Toro
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