Good Vibrations, Sympathetic Vibrations

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Author: Janice VanCleave

As soldiers walk across a bridge, they cause the bridge to vibrate. If the soldiers march in rhythm with the natural frequency of the bridge, each step will cause the bridge to vibrate at a higher amplitude. If the amplitude is great enough, the bridge could actually collapse. This phenomenon, in which a small repeated force causes the amplitude of a vibrating object to become very large, is called resonance.

In this project, you will demonstrate sympathetic vibration, or resonance, and determine how distance affects sympathetic vibration. You will investigate how building size and stiffness affect the resonance caused by earthquakes. You will also learn how the application of a force at the same natural frequency of an object affects the amplitude of the object's motion.

Getting Started

Purpose: To demonstrate sympathetic vibration.


two identical empty 1-liter plastic soda bottles


  1. Blow across the mouth of one soda bottle to produce a constant sound. Note the pitch and the loudness of the sound.
  2. As you blow across the mouth of the bottle, place the mouth of the other bottle near your ear as shown in Figure 29.1. Note any changes in the pitch and the loudness of the sound


When you blew across the mouth of the first bottle alone, you heard a sound. When you blew across the mouth of the first bottle while the second bottle was next to your ear, you heard a sound that had the same pitch as the first sound but was louder.

Resonance Sympathetic Vibration


Since the two soda bottles are alike, they have the same natural frequency. Blowing across the first bottle causes the air in the bottle to vibrate, which makes the air around the bottle's mouth vibrate. This vibrating air moves outward and causes the air in the second bottle to start vibrating. Frequency is a term used in physics to denote the number of times that any regularly recurring event, such as vibrations or oscillations (swings or back-and-forth movements), occurs in one second. Resonance is the condition of starting or amplifying vibrations in a body at its natural vibrating frequency by an outside vibrating force having the same frequency; also called sympathetic vibration. Resonance occurs when the natural frequencies of two objects are the same or if one has a natural frequency that is a multiple of the other. The second bottle vibrated without air being blown across it because the vibration of the air entering it was the same frequency as the bottle's natural vibration. Since the rate of vibration for the two bottles was the same, there was constructive interference, meaning the superposition of sound waves produced a sound wave with a larger amplitude (the farthest displacement of an object from equilibrium) and thus a louder sound. The vibrating air makes a sound that you can hear when the sound waves reach your ears. So the sound waves from the two bottles together produced a louder sound but at the same pitch (how high or how Iowa sound is).

Try New Approaches

How does the distance of a vibrating source affect resonance? Repeat the experiment, asking a helper to hold a bottle near his or her ear while you blow across your bottle's mouth. First repeat the experiment standing about 3 feet (1 m) from your helper, then standing about 6 feet (2 m) away. Reverse positions and ask your helper to blow across the mouth of one of the bottles while you hold the other bottle near your ear.

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