Main Difference – Radio Waves vs. Sound Waves
Sounds are made of waves and radios produce sound. However, when we talk about radio waves, we are not talking about the sound waves produced by the radio. Rather, we are talking about the waves that transmit radio signals. The main difference between radio waves and sound waves is that radio waves are a type of electromagnetic wave that can travel when there is no medium, whereas sound waves are a type of mechanical wave that cannot travel if there is no medium.
What are Radio Waves
Radio waves are electromagnetic waves. These are waves composed of electric and magnetic fields, which are oscillating at right angles to each other. The energy of an electromagnetic wave propagates at a direction at right angles to the oscillations in both the electric and magnetic fields. In the diagram below, the black arrows show the oscillations in electric and magnetic fields. The direction of propagation of the wave is indicated by the grey arrow.
Oscillations in an electromagnetic wave.
Since the actual oscillations take place at right angles to the direction of wave propagation, radio waves are transverse waves. Since radio waves are not mechanical, they do not need a medium to travel in; they can travel even in a vacuum. Like all types of electromagnetic waves, radio waves travel at a speed of about 300 000 km per second in a vacuum. When radio waves enter into other material, they slow down a little.
When you tune a radio to listen to a particular frequency, the radio takes in the signals at that frequency. Then, the radio’s circuits convert the electric signals into the movements in the speaker. These movements create movements in the air in front of the speaker, recreating the sound.
An old radio
What are Sound Waves
Sound waves are mechanical longitudinal waves. “Mechanical” means that sound waves must have a medium to pass through. The sound is really composed of the back-and-forth motion of molecules that make up the medium. These back-and-forth motions of molecules cause them to come towards each other, forming compressions. Then, the molecules move away from each other, forming rarefactions. This happens over and over again. Humans can “hear” sounds when a molecule undergoes this back-and-forth motion about 20-20 000 times per second. We say that sound waves are “longitudinal” because the movement of molecules take place parallel to the direction that the sound travels in. The speed of sound in a medium depends on the density of the material. Sound travels through air at room temperature and pressure at a speed of about 340 m per second. Typically, sound can travel faster in liquids and even faster in solids. Sound can travel through diamond at a speed of about 12 km per second1.
Sound is really a pressure wave, composed of compressions and rarefactions in the molecules that make up the medium.
Difference Between Radio Waves and Sound Waves
Medium
Radio waves are electromagnetic waves that can travel through a vacuum.
Sound waves are mechanical waves that require a medium to travel through.
Wave classification
Radio waves are transverse waves. They can be polarised.
Sound waves are longitudinal waves. They cannot be polarised.
Speed
Radio waves are much faster, typically travelling millions of metres per second.
Sound waves are much slower, typically travelling a few hundred or a few thousand metres per second.
References
1. Nave, R. (2012). Speed of Sound. Retrieved September 17, 2015, from HyperPhysics Concepts
Image Courtesy
“Electromagnetic wave” by User:LennyWikidata (Own work) [CC BY-SA 3.0], via Wikimedia Commons
“Dad’s Radio” by PROAlan Levine (Own work) [CC BY 2.0], via flickr
“Diagram showing physical manifestation of a sound wave through air from a speaker to a human ear” by Pluke (Own work) [CC0 1.0], via Wikimedia Commons (modified)
