Frequency: Sounds can be low or high.

Sounds can be low like a growling tiger or high like a chirping bird. This characteristic of sound is called pitch or frequency. Objects which vibrate faster produce a higher frequency, and objects which vibrate more slowly produce a lower frequency. The frequency of a sound is equal to how many times it vibrates each second. Vibrations per second are measured in Hertz (Hz).

 An object that vibrates 1 time each second would have a frequency of 1 Hertz (Hz). An object that vibrates 5 times each second would have a frequency of 5 Hertz (Hz).   In music we say “bigger is lower”, and this is true with guitar strings, drums of different sizes, and every other instrument that produces different pitches, or notes.

Water Xylophone

When you add water to a bottle, the pitch or frequency gets lower. This happens because as you add water, you also add more mass to the bottle — the combined mass of the bottle plus the water.  More mass results in a slower/lower vibrating frequency, and less mass produces a faster/higher vibrating frequency. 

Using 6 12 oz glass bottles, use the following measurements of water to make your own water xylophone.

9.5 oz / 6.5 oz / 5.5 oz / 5 oz / 3 oz / 0 oz

Line up the bottles in order with the empty bottle on the right side.

Number the bottles 1 to 6.

Play “Twinkle Twinkle” like this, use a spoon or other tapping device.

1 1 5 5 6 6 5

4 4 3 3 2 2 1

5 5 4 4 3 3 2

5 5 4 4 3 3 2

1 1 5 5 6 6 5

4 4 3 3 2 2 1

 

Now try this:

Blow across the bottles and discover that blowing produces the OPPOSITE effect.  (The empty bottle will be the LOWEST pitch and the fullest bottle will be the HIGHEST pitch.)  Why?  Because instead of the bottle/water mass vibrating, the air INSIDE the bottle is vibrating.  The empty bottle contains the most air, and therefore will produce the lowest pitch.