Can Loud Sounds Cook Things?

Posted on Categories Discover Magazine

Yes, sound can definitely produce heat. But you can’t cook food from yelling at it (sorry Gordon Ramsey). What we call sound is really a patterned, regular, ordered movement of individual particles. When we speak to each other, we force air through the small opening of our throat and shape it with our mouths. This makes the air molecules bunch together in specific patterns on their way out. Those air molecules then expand again, pushing against their neighbors, which makes them contract, and so on. This repetition of squeezing and expanding creates a wave that travels through the air, eventually hitting your eardrum and forcing it to vibrate in the same pattern, which your brain helpfully interprets as sound. Heat and Movement Motions of particles also creates heat, but in that case the movement is disordered and random. If you sit by a nice, warm fire, the infrared radiation emitted by the flames strikes your skin. The energy of that radiation causes the molecules on your skin to vibrate, rotate and generally wiggle around. Read More: First Demonstration of Energy Teleportation The more movement that a collection of particles exhibits, the higher its temperature (in fact, this is the physical definition of temperature). If the radiation from the fire transfers too much energy to your skin, the motion of your particles becomes so violent that it damages your cells, and you burn. Sound can create heat because much like radiation, the waves of pressure traveling through the air also transfer energy from one object to another. When sound waves hit your skin, some of the particles in your skin respond by vibrating with the same frequency or pattern – if you’ve ever felt yourself vibrate during that sick bass drop, then you know what I’m talking about. Conductors of Sound But your skin isn’t perfect. It has all sorts of microscopic nooks and crannies and bumps and wrinkles. The sound wave doesn’t hit all parts of your skin at the same time in the same way – some of the sound wave gets absorbed over here, but reflected over there, for example. So the orderly motion of the sound wave quickly deteriorates into random, chaotic motion, which becomes heat. The same thing happens to air itself. Air and water are excellent conductors of sound, because air and water molecules are good at smoothly gliding past each other without a lot of friction, and so the sound wave just keeps repeating itself. But, occasionally the air or water molecules do crash into each other instead of maintaining the rhythm, and this adds up over time. If someone calls out to you from far away but you can’t hear them, the energy of the sound wave didn’t disappear into nothing, it just made the air between you slightly warmer. Sound Waves into Heat The best way to convert sound into heat is to transform the orderly motion of the sound wave into random movements as quickly as possible. Sound absorbing materials do just that by containing as many nooks and crannies of various sizes as possible, and by filling themselves with small pockets of air. The combined action of the variations in the surface and the air pockets breaks apart the sound waves. Read More: Is There a Particle That Can Travel Back in Time? While it’s technically possible to heat something with just sound, in practice it’s incredibly difficult. Sound waves carry almost no energy at all. When you hit your hand on a table, the loud smack you hear represents less than 1 percent of the energy of the collision – the rest goes directly into your now painful hand. The sound produced by a stretch of ocean wave the length of a football field crashing onto a beach is less than the kinetic energy of a car traveling down the freeway. In other words, blasting your most powerful speaker at a pot of water wouldn’t raise its temperature enough to even register on a kitchen thermometer. That said, scientists and engineers have experimented with creating and focusing powerful sound waves and efficiently turning those sound waves into heat (and vice-versa) using a process called thermoacoustic energy conversion, but those techniques are still highly experimental. However, these techniques are interesting because it could represent a way to transfer heat without using any moving parts, and with high efficiency. So, in the far future we may indeed cook our nightly dinner with our favorite tunes.

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