Musical Pipes

Age

Elementary School, Middle School, High School

Format

Stage Show,Hands-on

Materials

   Musical pipe instrument
   Posters (optional)
   “Soft” object to hit top of pipes with - flip flop (optional)
   Slinky (optional) 

Safety Precautions

Science Theatre demonstrators must keep the safety of themselves and their audience in mind at all times. All Science Theatre demonstrators must have read through the Safety Training page. The ST Safety Box with first aid kit, fire extinguisher, etc. should always be available to demonstrators. Always wear safety gloves, glasses, and a labcoat if handling chemicals; always perform potentially dangerous demonstrations at a safe distance from the audience; and always keep a very close eye on any volunteers you call from the audience.

Keep ears away from tube openings. Don’t hit the tubes too hard with your hands.

Preparation

Bring the instrument to where the demo will take place.

Demonstration

Hit the tops of the tubes with your hands or the object to make sounds. When you hit the tube, keep your hand on top of it until the sound stops. By covering half the tube with your hand, you can play pitches that are an octave higher. (This is kind of difficult to do, but it is easiest with the shortest pipe.) You can play songs if you like. (See section below) The slinky can be used to help describe/show what a compression wave is (it may be easier to do this on a table or the ground rather than in the air). You hold one end and have someone else hold the other. Move your end a little toward the other person and then back to its initial position. A compression wave should move along it. You can have older audiences calculate the wavelength and/or the frequency of the waves created in each pipe (the equations can be found below).

What to Say

When you hit the top of the tube, your hand causes the air to compress inside the tube. The tube helps to focus and direct the air compression. This air compression is known as a sound wave, which is a type of compression wave. (You can use the slinky demo here if you want. Try holding it vertically next to the pipe to show a perspective like the pipe. This can also be a bit tricky.) Every wave has a certain wavelength. For sound waves, each different wavelength has a different pitch associated with it. Musicians assign note names A through G to different pitches. These are determined by what is known as frequency. Frequency is how many wavelengths pass one point per second. The shortest pipe creates the wave with the shortest wavelength and the largest frequency. This means that it is the highest pitch. The frequency of a sound wave moving through it is about 220 Hertz, which corresponds to the note A (3rd octave).

For older audiences, you can explain the difference between an open and closed pipe. When a pipe is closed on one end, in this case by your hand, the wavelength is equal to four times the length of the pipe because a node (area of zero amplitude) is formed on the closed end with an antinode (area of maximum amplitude) on the open end. The amplitude of a sound wave is given by its pressure. A frequency that is twice the magnitude of another has the same note name but is said to be an octave higher. For the shortest pipe, you get the 220 Hz frequency as said above. By covering only half of the top of the pipe with your hand, it is the equivalent of an open pipe. With an open pipe, the wavelength is only twice the length of the pipe because an antinode is formed on each end. For the shortest pipe this would correspond to approximately a 440 Hz frequency which is also the note A (but of 4th octave).

Why It Is

When air is compressed and sent through a tube, it becomes a compression wave with a wavelength determined by the length of the pipe that the air is moving through. Each length tube gives a different wavelength and, therefore, a different frequency. Different frequencies correspond to different musical notes. Different wavelengths are also found in open vs. closed pipes (see above figure). When your hand completely covers the top of the tube, it acts like a closed tube. This would mean that there is a node at the closed end (where your hand is) and an antinode at the other end. There is only 1/4 of a wavelength formed (as seen above), so one wavelength is 4 times the length of the pipe. By only covering half of the top, the tube acts as an open tube, and you get antinodes at each end. This allows for 1/4 of a wavelength to fit in the length of the pipe meaning the wavelength is twice the length of the pipe.

Real Life Examples

Any wind instrument

Open pipe: flute

Closed pipe: clarinet