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Interference

Constructive Interference​

Constructive interference occurs when two sound waves of the same frequency align in such a way that their peaks and valleys coincide. When this happens, the two waves are said to be in phase. The resulting wave has a greater amplitude than either of the original waves, leading to a louder sound. In essence, the energy of the waves is combined, amplifying the sound.

Module 1
A wave and it constructively interfering with itself.

A standing wave (or stationary wave) is a type of wave that vibrates over time but doesn't travel through space. Its amplitude (the height of the wave) remains fixed in position. At any given point along the wave, the amplitude changes over time in a predictable way, and the wave's motion at different points stays synchronized. One way that this situation can occur is when a wave and its "echo" overlap constructively. See this YouTube video for a demonstration with a simple rope.

Destructive Interference​

Destructive interference is the opposite of constructive interference. When two sound waves of the same frequency are out of phase, they cancel each other out. For waves that are perfectly out of phase, the result is silence.

Module 2
A wave and it destructively interfering with itself.

Partial Interference​

What happens if two waves with the same frequency are partially out of phase? This can occur because of a slight delay between two sound sources, such as the right and left speakers in a stereo speaker setup. In this case, there is either partially constructive interference or partially destructive interfere.

Module 3
A wave and it partially destructively interfering with itself.

Comb Filtering​

When a sound with multiple frequency components is added to a slightly delayed version of itself, then comb filtering occurs. Comb filtering is when two sound waves of the same frequency interfere with each other, but are partially out of phase. This creates a series of peaks and troughs in the resulting sound wave, giving it a "comb" appearance when the frequency response is visualized. Some frequencies will combine constructively, while others will combine destructively. The end result is that the audio will sound distorted.

Module 4
Comb filtering for a song.

In audio systems, comb filtering is often an undesirable effect because it can distort the sound, making certain frequencies more prominent while attenuating others. However, in certain applications, such as sound design, this effect can be used creatively to produce unique audio textures.

Active Noise Cancellation​

Active Noise Cancellation (ANC) is a technique used to reduce or eliminate unwanted sound by generating a sound wave that is exactly out of phase with the sound. ANC systems use microphones to detect incoming sound and then produce the opposite sound in speakers in the headphones. Although ANC systems are extremely beneficial for the user's listening experience, there are some important limitations to keep in mind.

  • There is a delay between when the microphones on the outside pick up external sounds and when the spekaers in the headphones produce the destructively-interfering waves. This delay can vary between 1 and 10 milliseconds, which means that the highest frequency that the headphones can cancel is 100 Hz to 1000 Hz. This is why ANC headphones cancel low-pitched sound but fails to block out high-pitched sound.

  • Full-room active noise cancellation is not feasible because the microphones would need to be placed far away from the listener. Only very-low pitched sounds could be blocked from a distance.

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