Sound can be low-pitched, like the rumble of a large truck,
or high-pitched, like a whistle. The pitch of a sound depends on the frequency
of its wave. Piano keys produce notes that increase in pitch from the left side
of the keyboard to the right. We can hear sounds of different pitches, but
there are some sounds we cannot hear. Our ears cannot detect very low-pitched
noises, known as infrasound, or very high-pitched noises, called ULTRASOUND.
Bats can hear frequencies up to 120,000 Hz. Other animals cannot
hear such high-pitched sounds. Mice can hear frequencies up to 100,000 Hz, dogs
up to 35,000 Hz, and cats up to 25,000 Hz. Humans hear sounds only up to about
17,000 Hz, but children can usually hear higher-frequency sounds than
adults.
A racing car coming towards you bunches up the sound waves made by
its engine. This makes them travel slightly faster. When the car passes by, the
sound waves spread out and sound lower in pitch. This change in pitch is called
the Doppler effect.
Sound waves with a frequency of 20,000 Hz or more are
known as ultrasound. Humans cannot hear ultrasound, but bats, dogs, porpoises,
and many other animals can easily detect it. Caused by things vibrating
extremely quickly, ultrasound has many uses, from toothbrushes that clean your
teeth with sound to submarine navigation.
Ultrasound allows doctors to check the health of a baby while it
is still in the womb. High-frequency sound waves are sent into the
mother’s body, where some reflect off the baby and bounce back to a
receiver. A computer uses the reflected sound waves to create a scan (picture)
of the growing baby.
