Along with light, sound is one of the most important ways in which we experience the world. For most people, sound is a fundamental part of every moment. You might not hear the terms frequency and amplitude in everyday conversation but almost everyone knows these same properties by the names low and high pitch and loudness. For example, musical notes are different frequencies of sound. This section describes the basic properties of sound and how we perceive and understand voices and music.
Basic properties of sound
What are sound waves?
Sound is a longitudinal wave, like the compression wave on a Slinky™ spring. Sound waves are similar except that they are much higher frequency and it is air that is being alternately compressed and expanded rather than the coils of a spring.
The range of sounds we can hear
We perceive different frequencies as having different pitch. The lowest frequency humans can ordinarily hear is a deep hum at a frequency around 20 Hz. Even this frequency is so fast that you cannot see the vibration with your eye; instead, you can only see a slight blur. The highest frequency that a young, healthy human ear can ordinarily perceive is a high-pitched whine at a frequency near 20,000 Hz.
Age and species effects
Physically, sound is a pressure oscillation, usually in air but also in water and even solid rock. However, sound also describes the perception of the physical wave. Perception varies a great deal among species and even among individuals of the same species. For example, to a human ear, a 50,000 Hz sound wave is perceived as no sound at all—totally quiet. Few adults can hear sounds at frequencies higher than about 15,000 Hz whereas young children can often hear sounds higher than 20,000 Hz. Most of the information content of sound in human voices falls in the frequency range from 100 to about 2,000 Hz. Large animals such as whales can hear lower frequencies than humans, as low as a few hertz. Small animals such as bats can hear frequencies higher than 100,000 Hz. Bats “see” with sound as well as we see with light.
Loudness and
pitch
We also perceive sound to have a loudness. The loudness of a sound depends on the amplitude of the wave. A loud sound has a larger amplitude than a soft sound of the same frequency. A stereo’s speaker moves back and forth a greater distance when producing a loud sound than when producing a soft sound. The larger amplitude of the speaker’s motion causes larger amplitude pressure variations in the air.
Multiple frequencies
Almost all sound you hear contains many simultaneous frequencies at once. Even a “clean” musical instrument sound contains a dominant frequency, called the fundamental, and many overtones, which are additional frequencies that give the sound its characteristic “piano-ness” or “guitar-ness.” The fundamental frequency is also called the first harmonic; the higher frequency overtones are called the second harmonic, third harmonic, and so on. The graph on the right is a frequency spectrum, which shows a range of frequencies up to around 2,500 Hz with the loudest peaks at 400, 800, 1,600, and 2,000 Hz.
Harmonics and overtones
The naming convention for fundamental frequencies, overtones, and harmonics can be confusing. Here is a table that shows the correspondance between the fundamental frequency and overtone terminology and the harmonics terminology:
Harmonics and overtones
Fundamental and overtone terminology
Harmonics terminology
Fundamental frequency
First harmonic
First overtone
Second harmonic
Second overtone
Third harmonic
Third overtone
Fourth harmonic
Test your knowledge
A dog whistle is used to train dogs. It gets the attention of dogs but is silent to humans. Why is this?
A dog whistle creates a sound with a frequency that is higher than the human hearing range. Although the frequency is beyond the human hearing range, it is within the range of dogs.
Sound is a longitudinal wave, like the compression wave on a Slinky™ spring. Sound waves are similar except that they are much higher frequency and it is air that is being alternately compressed and expanded rather than the coils of a spring. 
Almost all sound you hear contains many simultaneous frequencies at once. Even a “clean” musical instrument sound contains a dominant frequency, called the fundamental, and many overtones, which are additional frequencies that give the sound its characteristic “piano-ness” or “guitar-ness.” The fundamental frequency is also called the first harmonic; the higher frequency overtones are called the second harmonic, third harmonic, and so on. The graph on the right is a frequency spectrum, which shows a range of frequencies up to around 2,500 Hz with the loudest peaks at 400, 800, 1,600, and 2,000 Hz. 
