What is Sound?

What Is Sound

In physiology, sound is produced when an object’s vibrations move through a medium until they enter the human eardrum. In physics, sound is produced in the form of a pressure wave. When an object vibrates, it causes the surrounding air molecules to vibrate, initiating a chain reaction of sound wave vibrations throughout the medium. While the physiological definition includes a subject’s reception of sound, the physics definition recognizes that sound exists independently of an individual’s reception. You may recognize this section from our blog post, “What is a Sound Wave in Physics?” Keep reading for a more in-depth look at sound waves.

Sound waves and the brain

Types of Sound

There are many different types of sound including, audible, inaudible, unpleasant, pleasant, soft, loud, noise and music. You’re likely to find the sounds produced by a piano player soft, audible, and musical. And while the sound of road construction early on Saturday morning is also audible, it certainly isn’t pleasant or soft. Other sounds, such as a dog whistle, are inaudible to the human ear. This is because dog whistles produce sound waves that are below the human hearing range of 20 Hz to 20,000 Hz. Waves below 20 Hz are called infrasonic waves (infrasound), while higher frequencies above 20,000 Hz are known as ultrasonic waves (ultrasound).

Infrasonic Waves (Infrasound)

Infrasonic waves have frequencies below 20 Hz, which makes them inaudible to the human ear. Scientists use infrasound to detect earthquakes and volcanic eruptions, to map rock and petroleum formations underground, and to study activity in the human heart. Despite our inability to hear infrasound, many animals use infrasonic waves to communicate in nature. Whales, hippos, rhinos, giraffes, elephants, and alligators all use infrasound to communicate across impressive distances – sometimes hundreds of miles!

Ultrasonic Waves (Ultrasound)

Sound waves that have frequencies higher than 20,00 Hz produce ultrasound. Because ultrasound occurs at frequencies outside the human hearing range, it is inaudible to the human ear. Ultrasound is most often used by medical specialists who use sonograms to examine their patients’ internal organs. Some lesser-known applications of ultrasound include navigation, imaging, sample mixing, communication, and testing. In nature, bats emit ultrasonic waves to locate prey and avoid obstacles.

Frequencies of sound and average range of hearing

How is Sound Produced?

Sound is produced when an object vibrates, creating a pressure wave. This pressure wave causes particles in the surrounding medium (air, water, or solid) to have vibrational motion. As the particles vibrate, they move nearby particles, transmitting the sound further through the medium. The human ear detects sound waves when vibrating air particles vibrate small parts within the ear.

In many ways, sound waves are similar to light waves. They both originate from a definite source and can be distributed or scattered using various means. Unlike light, sound waves can only travel through a medium, such as air, glass, or metal. This means there’s no sound in space!

Sound Waves are Longitudinal Waves

How Does Sound Travel?


Before we discuss how sound travels, it’s important to understand what a medium is and how it affects sound. We know that sound can travel through gases, liquids, and solids. But how do these affect its movement? Sound moves most quickly through solids, because its molecules are densely packed together. This enables sound waves to rapidly transfer vibrations from one molecule to another. Sound moves similarly through water, but its velocity is over four times faster than it is in air. The velocity of sound waves moving through air can be further reduced by high wind speeds that dissipate the sound wave’s energy.

Mediums and the Speed of Sound

The speed of sound is dependent on the type of medium the sound waves travel through. In dry air at 20°C, the speed of sound is 343 m/s! In room temperature seawater, sound waves travel at about 1531 m/s! When physicists observe a disturbance that expands faster than the local speed of sound, it’s called a shockwave. When supersonic aircraft fly overhead, a local shockwave can be observed! Generally, sound waves travel faster in warmer conditions. As the ocean warms from global climate, how do you think this will affect the speed of sound waves in the ocean?

Propagation of Sound Waves

When an object vibrates, it creates kinetic energy that is transmitted by molecules in the medium. As the vibrating sound wave comes in contact with air particles passes its kinetic energy to nearby molecules. As these energized molecules begin to move, they energize other molecules that repeat the process. Imagine a slinky moving down a staircase. When falling down a stair, the slinky’s motion begins by expanding. As the first ring expands forward, it pulls the rings behind it forward, causing a compression wave. This push and pull chain reaction causes each ring of the slinky’s coil to be displaced from its original position, gradually transporting the original energy from the first coil to the last. The compressions and rarefactions of sound waves are similar to the slinky’s pushing and pulling of its coils.

Compression & Rarefaction

Sound waves are composed of compression and rarefaction patterns. Compression happens when molecules are densely packed together. Alternatively, rarefaction happens when molecules are distanced from one another. As sound travels through a medium, its energy causes the molecules to move, creating an alternating compression and rarefaction pattern. It is important to realize that molecules do not move with the sound wave. As the wave passes, the molecules become energized and move from their original positions. After a molecule passes its energy to nearby molecules, the molecule’s motion diminishes until it is affected by another passing wave. The wave’s energy transfer is what causes compression and rarefaction. During compression there is high pressure, and during rarefaction there is low pressure. Different sounds produce different patterns of high- and low-pressure changes, which allows them to be identified. The wavelength of a sound wave is made up of one compression and one rarefaction.

Tuning Fork

Sound waves lose energy as they travel through a medium, which explains why you cannot hear people talking far away, but you can hear them whispering nearby. As sound waves move through space, they are reflected by mediums, such as walls, pillars, and rocks. This sound reflection is better known as an echo. If you’ve ever been inside a cave or canyon, you’ve probably heard your echo carry much farther than usual. This is due to the large rock walls reflecting your sound off one another.

Types of Waves

So what type of wave is sound? Sound waves fall into three categories: longitudinal waves, mechanical waves, and pressure waves. Keep reading to find out what qualifies them as such.

Longitudinal Sound Waves

A longitudinal wave is a wave in which the motion of the medium’s particles is parallel to the direction of the energy transport. Sound waves in air and fluids are longitudinal waves, because the particles that transport the sound vibrate parallel to the direction of the sound wave’s travel. If you push a slinky back and forth, the coils move in a parallel fashion (back and forth). Similarly, when a tuning fork is struck, the direction of the sound wave is parallel to the motion of the air particles.

Mechanical Sound Waves

A mechanical wave is a wave that depends on the oscillation of matter, meaning that it transfers energy through a medium to propagate. These waves require an initial energy input that then travels through the medium until the initial energy is effectively transferred. Examples of mechanical waves in nature include water waves, sound waves, seismic waves and internal water waves, which occur due to density differences in a body of water. There are three types of mechanical waves: transverse waves, longitudinal waves, and surface waves.

Why is sound a mechanical wave? Sound waves move through air by displacing air particles in a chain reaction. As one particle is displaced from its equilibrium position, it pushes or pulls on neighboring molecules, causing them to be displaced from their equilibrium. As particles continue to displace one another with mechanical vibrations, the disturbance is transported throughout the medium. These particle-to-particle, mechanical vibrations of sound conductance qualify sound waves as mechanical waves. Sound energy, or energy associated with the vibrations created by a vibrating source, requires a medium to travel, which makes sound energy a mechanical wave.

Pressure Sound Waves

A pressure wave, or compression wave, has a regular pattern of high- and low-pressure regions. Because sound waves consist of compressions and rarefactions, their regions fluctuate between low and high-pressure patterns. For this reason, sound waves are considered to be pressure waves. For example, as the human ear receives sound waves from the surrounding environment, it detects rarefactions as low-pressure periods and compressions as high-pressure periods.

Transverse Waves

Transverse waves move with oscillations that are perpendicular to the direction of the wave. Sound waves are not transverse waves because their oscillations are parallel to the direction of the energy transport; however sound waves can become transverse waves under very specific circumstances. Transverse waves, or shear waves, travel at slower speeds than longitudinal waves, and transverse sound waves can only be created in solids. Ocean waves are the most common example of transverse waves in nature. A more tangible example can be demonstrated by wiggling one side of a string up and down, while the other end is anchored (see standing waves video below). Still a little confused? Check out the visual comparison of transverse and longitudinal waves below.

Longitudinal Waves Transverse Waves
Visual comparison of longitudinal and transverse waves.

How to Create Standing Waves

With PASCO’s String Vibrator, Sine Wave Generator, and Strobe System, students can create, manipulate and measure standing waves in real time. The Sine Wave Generator and String Vibrator work together to propagate a sine wave through the rope, while the Strobe System can be used to “freeze” waves in time. Create clearly defined nodes, illuminate standing waves, and investigate the quantum nature of waves in real-time with this modern investigative approach. You can check out some of our favorite wave applications in the video below.

4 Properties of Sound

What makes music different from noise? A bird’s call is more melodic than a car alarm. And, we can usually tell the difference between ambulance and police sirens - but how do we do this? We use the four properties of sound: pitch, dynamics (loudness or softness), timbre (tone color), and duration.

Frequency (Pitch)

Pitch is the quality that enables us to judge sounds as being “higher” and “lower. It provides a method for organizing sounds based on a frequency-based scale. Pitch can be interpreted as the musical term for frequency, though they are not exactly the same. A high-pitched sound causes molecules to rapidly oscillate, while a low-pitched sound causes slower oscillation. Pitch can only be determined when a sound has a frequency that is clear and consistent enough to differentiate it from noise. Because pitch is primarily based on a listener’s perception, it is not an objective physical property of sound.

Amplitude (Dynamics)

The amplitude of a sound wave determines it relative loudness. In music, the loudness of a note is called its dynamic level. In physics, we measure the amplitude of sound waves in decibels (dB), which do not correspond with dynamic levels. Higher amplitudes correspond with louder sounds, while shorter amplitudes correspond with quieter sounds. Despite this, studies have shown that humans perceive sounds at very low and very high frequencies to be softer than sounds in the middle frequencies, even when they have the same amplitude.

Timbre (Tone Color)

Timbre refers to the tone color, or “feel” of the sound. Sounds with various timbres produce different wave shapes, which affect our interpretation of the sound. The sound produced by a piano has a different tone color than the sound from a guitar. In physics, we refer to this as the timbre of a sound. It’s what allows humans to quickly identify sounds (e.g. a cat’s meow, running water, the sound of a friend’s voice).

Duration (Tempo/Rhythm)

In music, duration is the amount of time that a pitch, or tone, lasts. They can be described as long, short, or as taking some amount of time. The duration of a note or tone influences the timbre and rhythm of a sound. A classical piano piece will tend to have notes with a longer duration than the notes played by a keyboardist at a pop concert. In physics, the duration of a sound or tone begins once the sound registers and ends after it cannot be detected.

Creating Music with the 4 Properties of Sound

Musicians manipulate the four properties of sound to make repeating patterns that form a song. Duration is the length of time a musical sound lasts. When you strum a guitar, the duration of the sound is stopped when you quiet the strings. Pitch is the relative highness or lowness that is heard in a sound and is determined by the frequency of sound vibrations. Faster vibrations produce a higher pitch than slower vibrations. The thicker strings of the guitar produce slower vibrations, creating a deeper pitch, while the thinner strings produce faster vibrations and a higher pitch. A sound with a definite pitch, or specific frequency, is called a tone. Tones have specific frequencies that reach the ear at equal time intervals, such as 320 cycles per second. When two tones have different pitches, they sound dissimilar, and the difference between their pitches is called an interval. Musicians frequently use an interval called an octave, which allows two tones of varying pitches to share a similar sound. Dynamics refers to a sound’s degree of loudness or softness and is related to the amplitude of the vibration that produces the sound. The harder a guitar string is plucked, the louder the sound will be. Tone color, or timbre, describes the overall feel of an instrument’s produced sound. If we were to describe a trumpet’s tone color, we may refer to it as bright or brilliant. When we consider a cello, we may say it has a rich tone color. Each instrument offers its own tone color, and new tone colors can be created by layering instruments together. Furthermore, modern music styles like EDM have introduced new tone styles, which were unavailable prior to digital music creation.

What Makes Sound Music or Noise?

Acousticians, or scientists who study sound acoustics, have studied how different sound types, primarily noise and music, affect humans. Randomized, unpleasant sound waves are often referred to as noise. Alternatively, constructed patterns of sound waves are known as music. Studies have shown that the human body responds differently to noise and music, which may explain why road construction on a Saturday morning makes us more tense than a pianist’s song.


Acoustics is an interdisciplinary science that studies mechanical waves, including vibration, sound, infrasound and ultrasound in various environments, such as solids, liquids and gases. Professionals in acoustics can range from acoustical engineers, who investigate new applications for sound in technology, to audio engineers, who focus on recording and manipulating sound, to acousticians, who are scientists concerned with the science of sound.

Four Properties of Sound
Sound wave diagram. A wave cycle occurs between two troughs.

Characteristics of Sound Waves

There are five main characteristics of sound waves: wavelength, amplitude, frequency, time period, and velocity. The wavelength of a sound wave indicates the distance that wave travels before it repeats itself. The wavelength itself is a longitudinal wave that shows the compressions and rarefactions of the sound wave. The amplitude of a wave defines the maximum displacement of the particles disturbed by the sound wave as it passes through a medium. A large amplitude indicates a large sound wave. The frequency of a sound wave indicates the number of sound waves produced each second. Low-frequency sounds produce sound waves less often than high-frequency sounds. The time period of a sound wave is the amount of time required to create a complete wave cycle. Each vibration from the sound source produces a wave’s worth of sound. Each complete wave cycle begins with a trough and ends at the start of the next trough. Lastly, the velocity of a sound wave tells us how fast the wave is moving and is expressed as meters per second.

Units of Sound

When we measure sound, there are four different measurement units available to us. The first unit is called the decibel (dB). The decibel is a logarithmic ratio of the sound pressure compared to a reference pressure. The next most frequently used unit is the hertz (Hz). The hertz is a measure of sound frequency. Hertz and decibels are widely used to describe and measure sounds, but phon and sone are also used. A sone is the perceived loudness of a sound and a phon is the unit of loudness for pure tones. Additionally, the phon refers to subjective loudness, while the sone is the perceived loudness.

Sound Wave Graphs Explained

Sound waves can be described by graphing either displacement or density. Displacement-time graphs represent how far the particles are from their original places and indicates which direction they’ve moved. Particles that show up on the zero line in a particle displacement graph didn’t move at all from their normal position. These seemingly motionless particles experience more compressions and rarefactions than other particles. Since pressure and density are related, a pressure versus time graph will display the same information as a density versus time graph. These graphs indicate where the particles are compressed and where they are very expanded. Unlike displacement graphs, particles along the zero line in a density graph are never squished or pulled apart. Instead, they are the particles that move back and forth the most.

Direction of Sound Waves

Sound Pressure

Sound pressure describes the local pressure deviation from the ambient atmospheric pressure as a sound wave travels. It’s important to recognize that sound pressure and air pressure are not the same concept. Overall, the speed of sound is not influenced by air pressure. As sound waves pass from the sound source through the air, they alter the pressure experienced by air nearby particles.

Sound Level

Sound level is a comparison of the sound wave’s pressure relative to the reference point. Sound level is measured in decibels, with higher decibels correlating to higher sound levels. Some sound instruments measure sound level in dBc, which is the power ratio (decibels) of a signal to its carrier signal. Other sound instruments measure the relative loudness of sounds as perceived by the human ear using a-weighted decibels, known as dBa. When dBa is used, sounds at low frequencies have their decibel values reduced and compared to unweighted decibels.

Sound Level Graph
Sound Level is a comparison of the sound wave’s pressure relative to the reference point. A dBc meter measures high and low frequencies, while a dBa meter measures mid-level frequencies.

Sound Intensity

Sound intensity is the power per unit area carried by a sound wave. The more intense the sound is, the larger the amplitude oscillations will be. As sound intensity increases, the pressure exerted by the sound waves on nearby objects also increases. Decibels are used to measure the ratio of a given intensity (I) to the threshold of hearing intensity, which typically has a value of 1000 Hz for the human ear.

Sound Intensity Graph
Sound Intensity is the power per unit area carried by a sound wave. The more intense the sound is, the larger the amplitude oscillations will be. As sound intensity increases, the pressure exerted by the sound waves on nearby objects also increases.

Sound Intensity in an Air Column

An air column is a large, hollow tube that is open on one side and closed on the other. The conditions created by an air column are especially useful for investigating sound characteristics such as intensity and resonance. Check out the video below to see how air columns can be used to investigate nodes, antinodes and resonance.