Like many things in our universe, radio transmissions get more interesting the more you think about them. Without them, we obviously wouldn’t have radio stations or radar, but they’re also behind some of the technologies we take most for granted, like cell phones, satellite navigation, and Wi-Fi (or pretty much anything wireless).
At face value, radio certainly seems to make sense for transmitting sound. After all, we talk about “radio waves” and “sound waves,” so it seems kind of intuitive that they would be compatible. Radio waves = sound waves, right?
However, radio waves are actually on the electromagnetic spectrum, which we generally call “light” within a narrow range of wavelengths that are visible to us. They’re really not the same kind of waves as sound waves at all! (Let’s completely table the discussion of wave-particle duality of light for another post, so that we can just talk about waves for now. Suffice it to say that it isn’t nearly that simple, but we can get away with it here.)
The two kinds of waves are even totally different in how they travel. Light waves are transverse, meaning that the vibrations it creates move up and down perpendicular to the direction the whole wave is moving. Imagine a rope stretched out horizontally, so that you can flick the end of the rope up and down to make waves.
These waves have high and low points (crests and troughs) and the length of a single complete wave (the distance between one crest and the next) is called, oddly enough, a wavelength. The distance of the crests and troughs from the baseline of the straight rope is the wave’s amplitude, or the strength of the wave.
Sound waves are longitudinal, so their waves bunch up and spread out parallel to the wave’s direction of travel. Think of a spring, also stretched out horizontally. Instead of moving the end up and down, you push it in and out, sending a wave of compression down the length of the spring.
For longitudinal waves, the wavelength is measured according to the distance between two points of greatest compression (or stretching). The amplitude is a little tricky—it represents the greatest amount of stretching or compression that happens, relative to the material at rest.
This kind of wave needs a medium to travel through, something it can apply more or less pressure to. That’s why sound doesn’t travel in a vacuum, unlike light.
So how do radios work, then? Is sound traveling at the speed of light? How does sound get translated into light, and then back again (since we obviously hear the result as sound at the other end)?
For our purposes, let’s stick to talking about analog radio, which represents sound waves by physically changing radio waves in sync with the sound. You’ll have heard of both main forms of analog radio if you’ve ever used a car radio. That’s right, I’m talking about good old AM and FM! You may be aware that AM waves have lower frequencies (longer wavelengths with more time between peaks) than FM waves. That’s true, but there are other, much bigger differences between the two types of radio waves.
The names AM and FM tell you a lot about those differences—if you know they stand for “amplitude modulation” and “frequency modulation.” What they have in common is that they start with base or carrier radio waves that have constant amplitudes and frequencies. (Frequencies are just the flip side of wavelengths—the longer your wavelength is, the less often a new wave hits, so the lower your frequency.) The trick is to modify or modulate these base features to carry your sound waves.
Let’s start with AM. As the name suggests, this method changes the amplitude of the radio wave, adding information that a receiver can translate back into sound waves. Keep in mind that we’re talking about translating a longitudinal pressure wave into the peaks and troughs of a transverse wave. How do we transfer that information?
The answer with AM is to shape what we call the envelope, or the overarching shape of the waveform. The frequency of the original carrier wave remains the same, but the carrier amplitude changes to reflect the frequency and amplitude of the sound wave.
(If this sounds like a lot of information to carry, think of it this way: by following the amplitude (the strength of the pressure change) at every moment in real time, we’re also getting the frequency. After all, frequency is just the number of times per second that we go through a whole wavelength between peak amplitudes!
If we just followed the frequency but ignored the amplitude, everything would come out at the right pitch but exactly the same volume no matter what, and I can almost guarantee nobody would ever want to listen to the radio.)
Here’s how a radio wave ends up looking when we change its envelope to fit the amplitude pattern of a sound wave:
The amount that the amplitude varies from the carrier wave represents the strength of the sound wave, but as a proportion of the maximum possible signal strength rather than a literal transfer of numbers.
FM, or frequency modulation, uses the same idea, but—you guessed it—with frequency instead of amplitude. The amplitude of the carrier wave stays the same, but the frequency changes to represent the sound amplitude and frequency, with the strongest amplitudes causing the greatest frequency variations.
Understanding the modulation of radio waves is the biggest conceptual part of radio transmission, but there are a few steps involved in going from sound to radio waves and back again. Here’s what goes on in between:
Transduction: This is essentially translation between different forms of energy, and it happens a few times during this process. First, a microphone translates the pressure from the sound waves into an alternating electrical current, usually by way of a magnetic field from a vibrating membrane. Then this signal is combined with the carrier signal, which is another alternating electrical current, to produce the final signal.
Transmission: The electrical signal travels through an antenna, which amplifies and transmits electromagnetic waves (carrier + information) at a specific radio frequency corresponding to the alternating current.
Reception: An antenna tuned to receive this particular frequency intercepts the radio waves, and the interception (and sometimes amplification again) excites a corresponding electrical current. This passes on to the transducer(s) in the receiving radio setup.
Transduction, part deux (transdeuxction?): Same thing as on the other end, but in reverse. The electrical signal is translated into a magnetic field, and the non-carrier part is then translated into movement of a membrane in the speaker or headphones, producing sound waves.
I think it’s amazing that we’ve figured out such an ingenious way to transmit sound over large distances, not to mention at light speed, which is a huge deal given the difference between the speeds of light and sound (186,000 vs. 0.21 miles per second). It’s crazy that depending on how large your venue is, sound from the front of a stage can reach radio listeners before it reaches listeners in the back of the hall! So much of our modern way of life is built on the assumption of easy, real-time communication with people across the world, and we owe it all to radio.
Oh, and don’t forget to take one with you if you’re going on a space odyssey. You’ll need it, since as I mentioned before, no one can hear you scream out there.
[There are tons of cool details I didn’t even mention in the process of transduction, transmission, and reception, not to mention how digital radio works. AM and FM also have different advantages and drawbacks, which I’ve skimmed over here in favor of talking about the basic concepts. You can read up on all of that with the links in this paragraph, and you definitely should! Radio technology is amazing.]