More on Resonance: Standing Waves

M ore on resonant frequency. When you find resonance, you are making standing waves. Here is a video that shows what standing waves are for a vibrating string:

The important thing to see here is that a single shake of the rope makes a single wave that travels down the rope, bounces, and comes back. If many of these shakes happen, there will be lots of little waves, moving back and forth along the rope simultaneously – a real mess.

That is, it’s a real mess until you time the shakes just right. As soon as that happens, the waves start lining up with each other. What you are doing is matching the resonant frequency of the rope, and you’ll get what is called a standing wave. If you shake the rope twice as fast, you’ll see two smaller standing waves (the octave pitch), shake it three times as fast, and you’ll get the next harmonic, and so forth.

If the rope is shorter, then the waves have less distance to travel, and so they make the trip quicker. You have to shake the rope more rapidly to get those standing waves. This is why shorter strings vibrate at higher frequencies, and result in higher pitches. Waves travel  more slowly through thicker ropes, so thicker strings give lower frequencies, and so forth.



One focus of the IHTP training is learning how to recognize a “resonant tone,” or a tone that matches a sound in the environment. Ideally we match the pitch to the patient by listening to the patient’s voice, but there are times where it is useful to match another pitch in the environment, such as a noisy machine, in order to mask the sound.

Resonance is when one vibrating object acts on another object at the same frequency, as in this demonstration. We’ve all seen or heard this happen, usually with something annoying, like when something on our car vibrates at a certain road speed.

When we achieve resonance, the vibrations in the two objects play off of each other to make the sound louder. This is usually when things sound in tune, and the sound we hear is usually richer and fuller, because we have vibrations that work together, instead of fighting each other.

Resonance is NOT always good, though. Here is a video of the famous Tacoma Narrows Bridge, which succumbed to resonance just 4 months after it opened in 1944:

Fortunately, engineering has come a long way since then.

Tomorrow, more on how strings vibrate.

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