Physics - Fizik

THE PITCH OF A SOUND means how low or high it is. In a music band,
the bass drum's deep boom is low-pitched, while the triangle's shrill tinkle is high-pitched. Pitch depends on how many times the sound source moves to and fro, or vibrates, each second. This
is the same as the frequency - how many sound waves are produced each second. The frequency of a wave is measured in units called Hertz, Hz. For example, the note of middle C, in the
middle of a piano keyboard, has a frequency of 261 Hz. Frequency is related to wavelength, since higher frequencies have shorter waves. The length of a middle C sound wave is 126 centimetres.

We hear many sound frequencies, from the shrill notes of bird song to the deep growl of traffic. But, because of the way our ears work, we do not hear all of the sounds around us. Our ears pick up frequencies from about 20 to 20,000 Hz (Hertz, vibrations per second). We hear sounds below 80 Hz as low, deep booms, thuds or rumbles. Frequencies below about 30 Hz may not be heard clearly, but if they are powerful enough, we can feel them as vibrations in the air and ground. Our ears are most sensitive in the range from 400 to 4,000 Hz. (Human speech tends to be around 300-1,000 Hz.) Sounds above about 5,000 Hz are extremely high-pitched squeaks, hisses and screeches. As people get older, their ears become less sensitive to high notes. So a young person can hear a bat's very high-pitched squeaks, while an older person cannot.