Sound consists of density waves or disturbances made up of compressions and rarefactions. When a sound is transmitted through the Earth’s atmosphere, the waves are made up of fluctuations in air pressure. Your ear is very good at catching these fluctuations in air pressure and interpreting them as sound.
Sound is a wave and its frequency is determined by the number of cycles the wave completes in a certain amount of time. This is commonly measured in cycles per second or hertz (abbreviated Hz), in honor of German physicist Heinrich Rudolph Hertz. If a sound wave completes a cycle in a thousandth of a second, its frequency is measured as 1,000 Hz.
Your ears are very in tune to the atmosphere’s normal pressure. Every time there is a fluctuation that deviates from normal air pressure, even with amplitudes of less than a millionth, your ears pick that fluctuation up and interpret it as sound, according to the book, “How Everything Works.”
When a given fluctuation in air pressure is repetitive over a certain amount of time, it creates a tone with a pitch that is equal to its frequency. Lower frequencies or pitches are interpreted by our ears as low, bass sounds, while higher frequencies sound sharper and tinnier. Bass sounds usually extend from 80 Hz to 300 Hz, while a soprano singer, for instance, produces pitches from 300 Hz to 1,110 Hz.
The human ear can hear sounds over a range from about 30 Hz to 20,000 Hz. As we get older, our hearing range narrows. The ear also detects similarities between two tones of different frequencies when there’s a set ratio between both frequencies, the most common being 3/2. This means that two tones playing at 200 and 300 Hz would sound similar to another pair of tones playing at 500 and 700 Hz, for they have the same ratio. This concept, called intervals, is central to most music.