High and low
Zzzeee goes the tiny mosquito as it zips past your ear. HROOM growls
a big truck as it rumbles by on the road. The sound the mosquito makes
is much higher than the sound of the truck. Why are the sounds
different?
When something vibrates, sound travels outward from it in waves. Each
vibration— each complete back-and-forth motion—makes a single wave.
The faster something vibrates, the more waves it makes.
A mosquito makes high-pitched sounds because its wings vibrate very
fast—about a thousand times a second. So every second, a thousand
sound waves travel through the air—and you hear a high-pitched
zzzeee.
A truck makes low-pitched sounds because the heavy metal parts vibrate
slowly. The slow vibrations make only a few sound waves every
second—the low rumble that you hear.
The number of times a sound wave vibrates in a second is called the
frequency [(free]{.smallcaps} kwuhn see) of the sound. The higher the
frequency is, the higher the sound you hear.
Most people can hear sounds with frequencies about twenty times greater
than a mosquito’s whine. And some animals can make and hear even higher
sounds—sounds that people can’t hear at all.