A sound wave is a mechanical wave that carries energy through an elastic medium such as air, water or a solid as a traveling pattern of pressure variations and particle motion, without transporting the medium itself over any distance. In acoustics, the sound wave is the basic physical object that every other quantity describes: frequency, amplitude, wavelength, sound pressure level, reflection and reverberation are all properties or behaviors of sound waves.
In air, a sound wave is longitudinal: molecules oscillate back and forth along the direction of travel, creating alternating regions of slightly higher pressure, called compressions, and slightly lower pressure, called rarefactions. Human hearing responds to such waves roughly between 20 Hz and 20 kHz.
How a sound wave propagates
A vibrating source, such as a loudspeaker cone or a vocal fold, pushes on neighboring air molecules. Those molecules push on their neighbors in turn, so the disturbance travels outward while each molecule only oscillates around its resting position by a tiny distance.
Two linked quantities describe the wave at any point: sound pressure, the deviation from ambient atmospheric pressure, and particle velocity, the speed of local molecular motion. Microphones usually respond to pressure or to pressure gradient, which is why pressure is the reference quantity in most audio measurements.
The pressure changes involved are extremely small compared with atmospheric pressure of about 101,325 pascals. The table below gives reference points.
| Situation | Approximate sound pressure | Sound pressure level |
|---|---|---|
| Threshold of hearing at 1 kHz | 20 micropascals | 0 dB SPL |
| Quiet room | About 0.0006 pascal | About 30 dB SPL |
| Calibrator reference tone | 1 pascal | 94 dB SPL |
| Loud concert near the stage | About 20 pascals | About 120 dB SPL |
| Threshold of pain region | Roughly 20 to 60 pascals | Roughly 120 to 130 dB SPL |
Frequency, wavelength and speed
Every periodic sound wave is characterized by its frequency, the number of cycles per second in hertz, and its wavelength, the distance between two successive compressions. They are linked by the equation speed equals frequency times wavelength, so in a given medium a higher frequency always means a shorter wavelength.
The speed of sound depends on the medium’s stiffness and density. In dry air it is about 343 m/s at 20 °C and rises by roughly 0.6 m/s per degree Celsius, while humidity has a smaller effect.
| Medium | Speed of sound | Main reason |
|---|---|---|
| Air at 0 °C | About 331 m/s | Low stiffness, low density |
| Air at 20 °C | About 343 m/s | Higher molecular speed when warmer |
| Fresh water | About 1,480 m/s | Much higher stiffness |
| Steel | About 5,000 to 6,000 m/s | Very high stiffness |
In air, the audible range spans wavelengths from about 17 meters at 20 Hz to about 1.7 centimeters at 20 kHz. That thousand-fold range explains why low and high frequencies behave so differently in rooms and around objects.
| Frequency | Wavelength | Practical consequence |
|---|---|---|
| 20 Hz | About 17 m | Larger than most rooms, hard to absorb |
| 100 Hz | About 3.4 m | Drives room modes in small studios |
| 1 kHz | About 34 cm | Comparable to furniture and heads |
| 10 kHz | About 3.4 cm | Easily blocked, beams from small sources |
How sound waves interact with surroundings
When a sound wave meets a boundary or obstacle, its behavior depends mainly on the ratio of wavelength to object size and on the material’s acoustic properties.
- Reflection occurs at hard surfaces and produces echoes and reverberation in rooms.
- Absorption converts sound energy into heat in porous or resonant materials, which acoustic panels exploit.
- Diffraction lets long wavelengths bend around obstacles, so bass is heard behind walls while treble is shadowed.
- Refraction bends waves where the speed changes, for example in temperature gradients outdoors.
- Interference adds or cancels waves that overlap, causing comb filtering and the standing waves known as room modes.
In free space, a small source radiates a roughly spherical wave, and its sound pressure level falls by about 6 dB for each doubling of distance, the inverse square law. Close to a large source or in a room, this rule no longer holds, because reflections and near-field effects dominate.
Sound waves compared with related wave phenomena
Sound waves are often confused with electromagnetic waves and with the electrical signals that represent sound. Keeping them apart avoids common errors in audio work.
| Wave type | Needs a medium | Typical speed | Example |
|---|---|---|---|
| Sound wave in air | Yes | About 343 m/s | Speech, music |
| Shear wave in solids | Yes, solids only | Slower than longitudinal waves | Structure-borne vibration |
| Electromagnetic wave | No | About 300,000 km/s | Light, radio |
| Audio signal | Conductor or storage | Not an acoustic wave | Microphone voltage, digital samples |
Sound cannot travel through a vacuum, because there are no particles to carry the disturbance. Ultrasound above 20 kHz and infrasound below 20 Hz are physically the same kind of wave, just outside the typical hearing range.
Common misconceptions about sound waves
A few misunderstandings appear repeatedly in audio and acoustic discussions.
- Air does not flow with the sound: molecules oscillate in place, and only the energy travels.
- Louder is not faster: in normal conditions, amplitude does not change the speed of sound.
- A waveform plot is not a picture of the wave in space: it shows pressure over time at one point.
- Soundproofing and absorption differ: blocking transmission needs mass and isolation, while absorption only reduces reflections.
Understanding the sound wave as a pressure disturbance with a specific wavelength is the foundation for decisions about microphone placement, room treatment and loudspeaker positioning.
Explained without jargon
Think of a long line of people standing shoulder to shoulder. If the first person gives a small push, each person bumps the next and then steps back. The push travels down the line, but nobody actually walks anywhere.
A sound wave works the same way in air. A guitar string or a voice pushes the nearest air, and that push passes from molecule to molecule until it reaches your ear. Faster pushes sound higher, and stronger pushes sound louder.
A typical use case
A podcaster records in a small spare room measuring about 3 by 4 meters and hears a boomy, uneven bass. Knowing that a 100 Hz sound wave is about 3.4 meters long, a consultant explains that the room’s walls are spaced at distances where low-frequency waves reflect and reinforce each other.
Thin foam on the walls has already tamed the short high-frequency waves but cannot affect long bass waves. The podcaster adds thick corner bass traps, moves the microphone away from the room’s center and speaks closer to it, which raises the direct sound relative to reflections. The recordings become noticeably clearer, with far less low-frequency boom.