Frequency, Velocity, Wavelength

Sound has many different characteristics including frequency (pitch), which may vary from a low rumble to a high whine. Humans can hear sound possibly over a range from around 20 cycles per second (Hz) to 20,000 cycles per second (kHz). Other animals can hear sound at different frequencies beyond this range; dogs, for instance, can hear a whistle at higher frequencies for us to hear. High frequency (ultrasonic) sound can identify obstacles, similar to the way ships use sonar to detect objects under water.

Sound waves travel at the same speed in a medium such as air, regardless of the frequency of the sound. The speed of sound in air does vary with other factors, such as its temperature and density, although this variation is relatively small in comparison with that due to sound travelling though other media instead, such as water.

For example, the velocity of sound in air is around 340 m/sec whereas it may be around 1480 m/sec in water.

Sound waves consist of pressure pulses with a certain distance between each pulse corresponding to the wavelength of the sound. Because every sound wave travels the same distance regardless of frequency or wavelength, this means that the wavelength of sound is inversely proportional to its frequency. This is best explained with a picture. The two kangaroos are travelling at the same speed. One is hopping more rapidly (higher frequency) but each hop is a shorter (wave) length. The other kangaroo is hopping at a lower frequency but with a longer (wave) length, so that the speed of the two kangaroos remains the same.

Kangaroo jump