Straw Trombone

How does changing the length of an instrument change the pitch?

At its heart, sound is vibration. Pitch and volume depend on how fast and how large those vibrations are. When something vibrates, it vibrates the air (or water or whatever it’s in) around it at the same frequency. The vibrating air vibrates the air next to it and so on all the way till it hits your eardrum, which vibrates and your brain interprets that vibration as sound. The faster the vibration, the higher the pitch, the greater the amplitude, the louder the volume.

But how do musical instruments make different notes? Sound is a compression wave. This means it’s not a wave that goes up and down like a water wave, but a wave of compressions and rarefactions. During the compressions, the vibrating molecules are closer together and during the rarefactions they are farther apart. The movement of the molecules is in the same direction the wave is traveling, meaning it’s a longitudinal wave.

Water waves and light waves are called transverse waves because the motion of the particles is perpendicular to the direction the wave is traveling. When the students blow on the straws in this activity, they are making the air in the straws vibrate. When they do the activity they’ll learn that they have to blow quite hard. When they blow on the little “v” at the top of the tube, they are making the “v” vibrate which causes the air in the tube to vibrate at the same frequency.

But when they blow, they aren’t making it vibrate at any specific pitch — they are making it vibrate at a lot of different frequencies, meaning a lot of different pitches. They will naturally, without even realizing it, blow in different ways to make the straw vibrate differently until they hear a loud note.

This vibration (note) is called the resonant frequency of the tube. When the straw and air vibrate from the student’s air, the compression wave travels down the tube. When it hits the bottom, it is reflected back and then reflected back again when it hits the top. When reflected back and forth, the waves interact with each other. When the waves interact, sometimes they interact constructively, meaning they amplify each other, and sometimes they interact destructively, meaning they cancel each other out.

When waves are interfering in a tube there are specific frequencies that interact constructively and these are amplified by the constructive interference. These are called the resonant frequencies of the tube. The longer the tube, the lower the resonant frequencies. The shorter the tube, the higher the frequencies. When the length of the straw trombone is changed, the resonant frequencies of the tube change. Because the air is vibrating at a different frequency, your ear hears a different sound.

You can use this principle to make instruments out of all sorts of tubes. Try it next time you are drinking a bottle of soda. The less soda you have in the bottle, the longer the column of air and the lower the note. It also works well with PVC pipes of different lengths.

"Boomwhackers" are also quite fun. You can just bang them on a table to get the air vibrating and hear different notes with different lengths. The straw trombone is one of the most fun ways to explore this topic — though you might want to warn the teacher next door that you’ll be doing this activity.

Real world connections -

• Based on what you learned, why do you think a piccolo sounds different? Listen Here What can you say about the length of the different resonant frequencies?

Suggestions for drawing, illustrating, presenting content in creative ways

• Try a cross-curricular study with Music using Boomwhakers

Engineering and design challenges connected to the content

• Now that students know about resonant frequencies and how they can be adjusted, have students try and design their own instrument using