Interference Patterns of Sound Waves
Let’s conduct a scientific experiment. Take your hands and hold them about six inches apart. Now, bring them together really quickly. Continually repeat this action again and again, faster and faster. Thank you for the applause! Really, you’re too kind. Okay, so what happened there? You hit your hands together, and they produced a sound. If we imagine that we could see sound, this is what that would look like (see video). The vibrations from the contact expand outward at the speed of sound the same way that waves ripple when you toss a pebble into the water.
The fluctuating dispersion of sound from a source is called a sound wave. Like any other sort of wave, it travels through alternating highs and lows known as crests and troughs, which look something like this (see video). So, that’s a basic sound wave. But, what happens when a second source is added? The two sound waves will interfere with each other, and as anyone who’s ever been wakeboarding knows, when waves mix, things can get a bit choppy.
When we’re studying wave interference, the results of two waves of the same medium interacting, there are two essential interference patterns that emerge. To see these, let’s look at those sound waves again (see video). We’ve got two different sound waves here, and for the sake of this lesson, I’m going to separate them out. Notice how these two different waves look similar? The crests and troughs line up with each other. When sound waves interact in the same pattern, we call it constructive interference.
When constructive interference occurs, generally from two waves traveling in the same direction, the result tends to be an amplification of the sound; a super-wave combining the force of both waves, creating what is called a hot spot, or live spot, in acoustics. Now, just because this amplification comes from two different sound sources that are interfering with each other, a louder sound does not always mean a clearer sound. These sounds could be muddled, competing against each other as well.
So, what happens when waves don’t align so neatly? When waves are traveling in opposite directions and sound waves interact in opposite patterns, the result is destructive interference. So, if the trough of one wave meets the crest of another, that’s destructive interference, and the result is a dampening of sound, called a dead spot in acoustics. Basically, the amplitudes of each wave subtract from each other, as opposed to adding to each other as in the constructive interference. A common example used to demonstrate both of these kinds of interference is by using a tuning fork. Tap the tuning fork against a solid object, making it ring. There are two points on it, so there are two different sound sources. If you hold the tuning fork up to your ear and rotate it, it will fluctuate between sounding loud and sounding soft. This is because as you rotate it, you are hearing waves from each prong as they move in the same direction and as they move in opposite directions, depending on which angle the tuning fork is relative to your ear. So, you hear both constructive and destructive interference as moments through the changing volume.
Two-Point Source Interference
When two sources create sound waves, those sounds waves will interfere both constructively and destructively. This creates an interesting pattern of sound displacement called the two-point source interference pattern. Basically, this pattern is characterized by points where sound waves interacting constantly hold the same position.
Let me show you what I mean. Look at this graphic (see video). You’ll notice that as the wave fluctuates, as it moves, the tops of crests and bottoms of troughs return to the same points. These points of maximum displacement are called antinodes and are created by constructive interference. But that’s not all we see here. If we draw a horizontal line through the middle of this image, you’ll see an axis along which the line seems to travel. The points where the wave meets this axis, the points of minimum displacement, are called the nodes. These are created by destructive interference.
Two-point sound interference patterns are created by the consistent alteration between nodes and antinodes. Because these points are always located at the same spot along the wave, this pattern is also called a standing wave. Two waves resonating at the same frequency will always create a standing wave pattern, defined by these fluctuating nodes and antinodes.
When two objects interact, they create vibrations that produce sound waves, fluctuating dispersions of sound from a source. Sound waves, like ripples in water, pulse outwards in a pattern of crests and troughs.
When sound is produced by multiple sources, the sound waves interfere with each other. Two sound waves traveling the same direction interact in the same pattern, which we call constructive interference. Sound waves traveling in opposite directions interact in opposite patterns, called destructive interference. This creates a two-point sound interference pattern, in which points where sound waves interact constantly hold the same position. The points of maximum displacement are called antinodes. The points of minimum displacement are the nodes. Since nodes and antinodes hold the same place within the wave, this is also called the standing wave pattern. And all of this from such a simple scientific experiment.