To answer this question we need to understand how a loudspeaker 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, creating the sound wave that we then hear. Let’s see how. If you think about the front of the membrane, the one in view, as it moves forward it pushes the air molecules that are in contact with it, causing the air in that area to have a density greater than that of air. at rest, that is, in silence.
Since air is an elastic medium, the molecules try to regain their equilibrium state by expanding again. This causes the area that previously had a higher density, or pressure, than normal, to now have less than normal. In addition, when the air molecules expand, they compress the ones next to them, which immediately try to expand again, 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 ears.
The rear part of the membrane does the same, but in antiphase, that is, when the front part generates an overpressure, the rear part generates a rarefaction (pressure below the resting level), so we can say that the rear part of the membrane generates another sound wave of opposite sign to the front one. To prevent both from interfering and spoiling the sound field that we want the loudspeaker to generate, we enclose the membrane in a box that prevents the back wave from propagating. So even if you turn the speaker around, it still radiates sound forwards only, because it’s designed to work that way. You would have to drill a hole in the back of the box to change how it works.
An interesting, and certainly less destructive, experiment might be to aim the speaker at a nearby surface, and listen for the sound that that surface reflects. If the surface is hard, for example the wall or the floor, it will be very reflective, and the sound that it reflects will be quite similar to the original, that is, what you would hear with the speaker facing you. But if the surface is soft, for example a sofa, it will not reflect all sounds equally, because it will absorb the highest pitched sounds. Then the reflected sound, the one you can hear, will be lower.
And put to experiment, I leave you another idea: if the speaker does not have rubber feet, or you can remove them, try putting it on top of a piece of furniture that is not very heavy, or a wooden box. With any luck, the vibration from the speaker is transmitted to the surface you place it on, and it behaves like the soundboard of a piano, amplifying the sound.
And when you’re done with the experiments, if your speaker is part of a stereo (which is old, but we still have it in many houses), don’t 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 chair for listening to music.
Soledad Torres Guijarro She is a PhD in Telecommunications Engineering, professor and researcher at the School of Telecommunications Engineering at the University of Vigo.
Question sent by José Félix Pereira Ríos
Coordination and writing:Victoria Bull
we answer is a weekly scientific consultancy, sponsored by the Dr. Antoni Esteve Foundation and the program L’Oréal-Unesco ‘For Women in Science’, which answers readers’ questions about science and technology. They are scientists and technologists, members of AMIT (Association of Women Researchers and Technologists), which answer those questions. Send your questions to [email protected] or by Twitter #werespond.
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