Sound is a wave traveling through the air, and the reason we can hear someone speaking over the radio from a distance is because of the principle of modulating sound information into radio waves, which are then transmitted and demodulated by the receiving radio to restore the original sound. This process, which utilizes frequencies, allows multiple sound signals to be transmitted simultaneously, and the radio can tune into specific frequencies to select the stations it wants to hear.
Turn on the radio, tune in, and you’ll hear speech. It’s something we take for granted in a generation that has been exposed to radio, television, and telephones since birth, but a hundred years ago, it was unimaginable to hear a person’s words verbatim from a distance. People talking dozens of kilometers away sound as real as if they were talking in the room. How is this possible?
Sound is a wave in the medium of air, meaning that it travels as vibrations in the air that originate at the sound source spread out. As the air vibrations rattle our eardrums, our brain interprets them appropriately to get information about the sound. Depending on how the air vibrates, the sound we perceive is different. For example, if a sound has a high frequency, we perceive it as high, if a sound has a low frequency, we perceive it as low, and waves of different frequencies mix to form a single sound. Two sounds that we perceive as different are waves made of different frequency combinations. Conversely, two air waves made with the same combination of frequencies sound the same.
Another characteristic of sound is its speed of travel. Sound travels at different speeds depending on the medium, and in air, it travels at about 340 m/s. This puts a natural limit on the ability to transmit sound over long distances. If a machine could mimic the vibrations in the air that occur when a person speaks, the listener would not be able to tell if the sound was coming from a machine or a person speaking. We can use this to our advantage by transmitting the vibration shape of the sound when a person speaks in region A to region B, so that the machine can mimic the sound of the person’s speech.
However, you can’t transfer sound information from one region to another in its original wave form. As a simple example, the sound of Z speaking here would be hard to hear 100 meters away. This is because the intensity of the waves decreases as the distance increases, and the original shape of the air vibrations is not maintained at 100 meters away. In addition, the exponential decrease in sound intensity with distance makes it practically impossible to have a direct conversation with someone who is far away.
Therefore, in order to transmit sound information from area A to area B, the sound information must be processed into a form suitable for transmission. The process of processing sound information into a form that is easy to transmit over long distances is called modulation. Modulation adds information by changing one of the amplitude, frequency, or phase of the carrier wave, which is the simplest shaped wave such as a sin wave. There are many different modulation methods, but the two most common are amplitude modulation (AM) and frequency modulation (FM). AM transmits sound signals by changing the amplitude, while FM transmits sound information by changing the frequency. Each method differs in noise immunity and transmission distance and is utilized for different purposes.
In contrast, in Area B, the received signal must be reverse-processed to restore the original sound, a process called demodulation. Demodulation is the process of removing the carrier wave from the radio waves, and only the changed part is extracted to obtain the desired information. To summarize, we modulate and transmit a sound from area A, receive a signal from area B, and demodulate it so that the sound from area A can be heard in area B. This is the basic principle of radio operation. This is the basic principle of how radio works. A station modulates a voice signal and sends it out into the airwaves, and a radio device picks it up, demodulates it, and reconstructs the voice signal so you can hear the radio broadcast.
By using multiple frequencies of carrier waves, it is possible to transmit several different pieces of sound information at once. By modulating carrier waves of different frequencies, multiple sound signals can be transmitted at once without mixing. Radio broadcasts are made by sending sound information on carrier waves at frequencies assigned to each broadcaster. The frequency we tune in to when we listen to the radio is the frequency of the carrier wave. By taking in all the frequency components, then reducing the other frequency components and amplifying the target frequency components, the radio can obtain only the sound information corresponding to a particular broadcast.
This technique is not limited to just voice signals. The same principle applies to TV broadcasts and modern digital communications. Digital signals use more complex modulation schemes, but ultimately, the process of carrying and restoring information is the same. This is how we are able to communicate with people far away in real time. Advances in the technology of transmitting information over radio waves have led to the emergence of radio and other communication devices that have changed our lives dramatically.
