Sound is a form of mechanical energy that travels through a medium, such as air, water, or solids, via vibrations. These vibrations create pressure disturbances that propagate outward from the source, forming what we recognize as sound waves. Understanding how these waves behave requires examining several measurable properties, one of the most fundamental being frequency. The frequency of a sound wave is a physical attribute that directly influences how we perceive the sound, serving as a basis for distinguishing one noise or musical note from another.
Defining Wave Frequency
Frequency is a precise measurement defining the rate at which a sound wave oscillates. It quantifies the number of complete cycles a sound wave completes within a single second. A single cycle is composed of one full pattern of compression (increased pressure) and rarefaction (reduced pressure).
The standard unit used to measure frequency is the Hertz (Hz), named after the German physicist Heinrich Hertz. One Hertz is defined as one cycle per second. For example, a tuning fork vibrating at 440 Hz produces a sound wave that completes 440 full cycles every second. Frequency is an objective physical property that can be measured with instruments.
Frequency and Pitch
Frequency is directly linked to the perceptual quality known as pitch. Pitch is the auditory sensation that allows a listener to organize sounds on a scale from low to high, which is the basis for musical melodies. A higher frequency corresponds to a higher pitch (treble), while a lower frequency results in a lower pitch (bass).
The physical mechanism of sound production, such as the vibration of a guitar string or a person’s vocal cords, determines the frequency. When a musician shortens a string or tightens a drumhead, they increase the rate of vibration, which increases the frequency and raises the perceived pitch. Conversely, a longer or looser vibrating element produces fewer cycles per second, resulting in a lower frequency and a deeper pitch.
How Frequency Relates to Other Wave Characteristics
Frequency is mathematically related to the sound wave’s speed and its wavelength. This relationship is described by the fundamental wave equation: the speed of the wave equals the frequency multiplied by the wavelength. The speed of sound is largely determined by the medium it travels through and the temperature.
Since the speed of sound is relatively constant in a given medium, such as air at a specific temperature, a change in frequency must be balanced by an opposite change in wavelength. Frequency and wavelength are inversely proportional. High-frequency sounds have a short wavelength. Conversely, low-frequency sounds possess a longer wavelength.
The Range of Human Hearing
The typical range of frequencies a healthy young human ear can perceive spans from approximately 20 Hz to 20,000 Hz (20 kilohertz or kHz). This range tends to decrease with age, particularly at the higher end of the spectrum.
Sounds below 20 Hz are classified as infrasound. These low-frequency waves are often felt as vibrations rather than heard and can be produced by natural phenomena like earthquakes or by animals such as elephants. Frequencies above 20,000 Hz are known as ultrasound. Ultrasound has very short wavelengths and is used extensively in medical imaging, such as sonography, in industrial applications, and by animals like bats and dolphins for navigation and communication.