When thunder and lightning occur in a thunderstorm, we first see the light followed by the sound. Sometimes early in the morning we can hear the sound of the train even though the train station is quite far from our house. At times, even the chirping of birds sitting on trees can be heard from far. How does this sound travel? Sound travels in the form of waves and we can hear the sound when these waves hit our ears. There is constructive and destructive interference of these waves, affecting the sound in a particular manner. What is Constructive Interference? Constructive interference occurs, when two or more waves pass through each other, creating a new wave. It happens when two or more waves have a similar frequency, or are formed by the same source, and have the same phase of oscillation. Sound waves have a certain frequency, which is measured in hertz. Each wave has two components, the upper half and the lower half. The upper half is known as the crest, while the lower half is known as the trough. The crest and trough are the part of one cycle, and the frequency is calculated as cycles per second. Waves have a unique character of not getting reflected, that is, waves never collide with each other and refract, but they merge or combine to form another wave pattern. When the crests of two or more waves combine together, constructive interference occurs. The same interference occurs when the troughs of two or more waves combine together. Now you must be wondering, what is the result of this? Well, it creates waves with larger amplitudes, or in simpler words, louder volume. Amplitude is used to measure how strong the wave is. Waves with larger amplitudes create a louder volume, while waves with smaller amplitudes are softer in volume. When the crest of one wave combines with the trough of another wave, they interfere destructively, resulting in reduced amplitude, that creates softer sound. Examples It is essential to study constructive interference before building auditoriums or concert halls where everyone needs to listen to the speaker or the performer clearly. The walls, ceilings, and baffles, of the auditoriums need be designed in such a way, that they absorb the sound waves and they are not refracted. Refracted waves can interfere destructively and create sound waves with lower amplitude, that will produce softer or no volume, which cannot be heard clearly by human ears. Musical instruments would be good examples to understand sound wave interference, as sound waves play a major role in the world of music. In fact, it is the wave pattern that differentiates music from noise. Generally, musical waves have frequencies that can be mathematically calculated, whereas noise is created by mixing waves of different frequencies, whose mathematical relationship is difficult to establish. Sometimes in musical instruments, when lot of sound waves are produced at the same time, part of the waves are interfered constructively, and some are interfered destructively, creating a beat in the music. This can occur when the waves travel with different speeds, and do not add up the same way to each other. The beat is used to tune the instrument. For example, a guitarist strikes the strings and then if the beat is heard then they tighten or loosen the strings to get the right note. Hearing is one of the important sensory processes in humans. Every physical phenomenon on the Earth, including air, fire, water, has a unique sound. Sounds are used by many species, along with humans, for communication, predation, and navigation.