Why doesn’t the sound come out backwards when I put the speaker upside down? | Scientists respond | Sciences



To answer this question, we need to understand how an amplifier works. The core part of the amplifier is a diaphragm that moves according to the voltage changes of the electrical signal. This membrane moves back and forth, pushing air molecules into contact with it, which creates the sound wave we hear next. Let’s see how. If you think about the front of the membrane, the visible membrane, as it moves forward, pushes off the air molecules that come into contact with it, causing the air in that area to be denser than the air. That is, in silence.

Since air is an elastic medium, the molecules try to restore their equilibrium state by stretching again. This causes an area that was once of higher density or pressure than normal to now be less than normal. In addition, when air molecules expand, they compress neighboring molecules, which immediately try to expand again, pushing out the next molecules, and so on. This is how sound propagates, which as you can see is just a pressure disturbance that propagates through an elastic medium, air in this case, and reaches our ears.

The back part of the membrane does the same thing, but in antiphase, that is, when the front part generates excessive pressure, the back part generates rarefaction (pressure less than the resting level), so we can say that the back part of the membrane generates another sound wave opposite to the forward signal. To prevent interference and spoil the sound field we want the loudspeaker to generate, we put the diaphragm in a box that prevents the back wave from propagating. So even if you turn the speaker, it still only radiates sound forward, because it’s designed to work that way. You will have to drill a hole in the back of the box to change how it works.

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An interesting, and certainly less disruptive, experience might be to point the speaker at a nearby surface, and listen for the sound that surface reflects off. If the surface is solid, for example a wall or floor, it will be very reflective, and the sound it reflects will be exactly the same as the original, that is, what you might hear with the speaker facing you. But if the surface is soft, like a sofa for example, it will not reflect all the sounds equally, because it will absorb the sharpest sounds. After that, the reflected sound, which you can hear, will be less.

And experiment, I leave you another idea: if the speaker does not have rubber feet, or you can remove it, try placing it on top of a piece of furniture that is not too heavy, or a wooden box. With any luck, the vibration from the speaker is transmitted to the surface you place it on, acting 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 homes), don’t forget to put it in the right place, that is, at the height of your head and forming an equilateral triangle with the other speaker and your favorite listening chair to music.

Soledad Torres Guijaro She holds a PhD in Telecommunications Engineering, and is a professor and researcher at the School of Telecommunications Engineering at the University of Vigo.

Question posted by José Félix Pereira Ríos

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Formatting and writing:Victoria Paul

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