skip navigation
» McREL: Mid-continent Research for Education and Learning, delivering research and practical guidance to educators
     
 

Whelmer #10: Singing Rod

horizontal line

Illustration of a hand grasping a metal rod at the mid-point, stroking one end with the free hand while the other end of the rod vibrates.Description:
An aluminum rod is stroked, causing it to emit a high pitched squeal.

Science process skills:

  1. observation
Complex reasoning strategies:
  1. comparison
  2. deduction
Standards:
K-4:
  • Employ simple equipment and tools to gather data and extend the senses (Standard A.1.3).
  • Sound is produced by vibrating objects. The pitch of the sound can be varied by changing the rate of vibration (Standard B.2.4).
5-8:
  • Energy is a property of many substances and is associated with sound (Standard B.3.1).
9-12:
  • Waves, including sound, have energy and can transfer energy when they interact with matter (Standard B.6.1).
Above Standards from the National Science Education Standards.

Content topics:
  1. propagation of sound waves
  2. resonance
You will need:
  1. aluminum rod
  2. steel wool pad
  3. rosin powder
  4. pan of water
Instructions:
Obtain a solid aluminum rod from the hardware store. They are usually sold in six foot lengths; 1/4" or 1/2" in diameter. Either diameter rod will work. Rods from 2 feet to 6 feet in length work well. Use steel wool to remove grease and oil from the rod. Use a measuring tape to determine the center (balance point) of the rod. You can also balance the rod on a finger to find the balance point.

Rub some powdered rosin on the thumb and forefinger of one hand. Hold the rod at the balance point with the thumb and one or two fingers of the other hand. Stroke the rod lengthwise, from the balance point to one end, using the rosin coated fingers. The rod will begin to vibrate and emit a high pitched squeal.

It will take some practice to learn the correct technique to create the noise. Too much pressure from the rosin-fingers will muffle the vibrations. Too little pressure will not allow your skin to grab the metal and create the vibrating effect. It must be "just right." The music department can be a source of cracked and broken blocks of violin rosin. Use a hammer to powder the rosin. Dried tree sap can also be used.

Presentation:
This activity is often referred to as the "one-string" violin or guitar. Ask students if they can recall the experience of sliding down a metal playground slide while wearing short pants. ( their skin "grabs" the metal ) Relate that your skin is doing something similar as you stroke the metal rod. Your skin is grabbing and releasing the metal, or slapping it repeatedly as your fingers move along its length. That causes the metal rod to vibrate.

Allow students to gently feel the end of the rod as it vibrates. You can also dip the end of the rod in a shallow pan of water. The rod will create ripples in the water. Flat aluminum bars and aluminum electrical conduit will also sing if stroked with rosin. It requires practice.

To avoid sticky rosin-fingers, you can impregnate a piece of leather or scrap of heavy fabric with powdered rosin. Once you have perfected the generation of sound by holding the rod at the mid-point, you can generate a much higher pitched squeal by holding the bar at the 1/4 length point. Stroking the longer (3/4 length) end will generate a very annoying sound.

Content:
Sound travels through air about 340 meters per second or 750 miles per hour. The temperature, pressure and density of the air alters the speed of sound. Sound travels faster in liquids than it does in gases. It travels even faster through solids. What we normally call sound is produced when objects vibrate in air. The vibrating object causes the surrounding air to vibrate. Our ears detect the waves of vibrating air molecules. Sound must have a medium though which to travel. It will not travel through a vacuum because there are no molecules (objects) to transmit the vibrations.

Assessment:
Type: individual.
Content/Process: propagation of sound waves.
Age/Level: all.

Give each student a piece of heavy cloth that has been impregnated with rosin. They should use this to find another object in their home or at school that vibrates and produces a sound. Some objects to investigate are the baton or high jump pole used in track, or the tambourine. If possible, bring the object to class. Have the student explain how the sound is created. Try to tune the objects to make a scale and form an orchestra or band.

Notes:






  
     
 

back