Sound is a mechanical wave — a pattern of compression and rarefaction that propagates through a medium (air, water, metal, wood) as molecules collide with their neighbors. Unlike electromagnetic waves, sound cannot travel through a vacuum. Understanding this fundamental nature of sound as energy traveling through matter is the starting point for all acoustic work.
What Is a Sound Wave, Physically?
When a speaker cone pushes forward, it compresses the air molecules immediately in front of it, increasing their density and pressure. When it pulls backward, those molecules rarefy — density and pressure decrease. This pattern of pressure variation travels outward from the speaker at the speed of sound, carrying acoustic energy with it. The individual air molecules themselves don't travel across the room — they oscillate locally, transferring energy to their neighbors like dominos falling in a line.
This distinction matters for practical audio work. Since molecules return to their original positions after the wave passes, sound doesn't "push" air across a room in the way a wind current does. This is why opening a door doesn't significantly change how sound travels from one room to another — the energy transfer happens through the collision of molecules, not through the physical movement of air masses.
The frequency of a sound wave determines its pitch. Frequency is measured in Hertz (Hz) — one complete oscillation per second. A 440Hz wave compresses and rarefies 440 times per second, which our ears perceive as the note A above middle C. Humans can generally hear frequencies between approximately 20Hz and 20,000Hz (20kHz), though this range narrows with age and exposure to loud sounds. The frequencies between 2kHz and 4kHz are where human hearing is most sensitive — this is why consonants in speech, which occupy this frequency band, are so important for intelligibility.
Wavelength and Its Practical Implications
Wavelength is the physical length of one complete cycle of a wave — the distance between successive compression peaks. It is calculated as the speed of sound divided by frequency. At room temperature (approximately 20°C), the speed of sound in air is about 343 meters per second. This means a 100Hz wave has a wavelength of approximately 3.43 meters, while a 1kHz wave has a wavelength of 0.343 meters (about 34 centimeters).
Wavelength becomes critically important when dealing with speaker placement, room modes, and microphone technique. Low frequencies have long wavelengths — a 40Hz wave is over 8.5 meters long. This is why low frequencies are difficult to control with absorbers and diffusers that are physically small. Effective bass trapping requires treatment dimensions comparable to the wavelengths you're trying to absorb, which is why bass traps are typically thick and bulky.
At higher frequencies, wavelengths become short enough to create audible interference patterns from path length differences. When a sound reaches both ears with a time difference, our brains interpret this as directional information. Stereo recording techniques like spaced omnis exploit this phenomenon — when two microphones pick up the same source at different distances, the time arrival difference creates phase cancellations at certain frequencies, coloring the recorded sound. Understanding wavelength helps you predict and control these effects.
The Speed of Sound
The speed of sound varies depending on the medium and its properties. In air at sea level at 20°C, it's approximately 343 meters per second. In water, it's about 1,480 meters per second — more than four times faster. In steel, it reaches approximately 5,960 meters per second. The speed depends on the medium's density and elasticity — stiffer, denser materials transmit sound faster.
Temperature has a significant effect on the speed of sound in air. The formula is approximately: speed = 331 + (0.6 × temperature in °C). At 0°C, speed is 331 m/s; at 20°C, it's 343 m/s; at 35°C, it's 352 m/s. This matters for outdoor events and precision acoustic measurements where temperature variations across a venue affect how sound propagates.
Humidity also affects the speed of sound, though less significantly than temperature. At high humidity, sound actually travels slightly faster because water vapor molecules are lighter than the nitrogen and oxygen they replace. The difference between 0% and 100% relative humidity at room temperature is less than 2 m/s — usually negligible for practical purposes, but worth knowing for precision acoustic modeling.
Amplitude, Intensity, and Loudness
Amplitude describes the magnitude of the pressure variation in a sound wave. In practical terms, it's what we roughly correlate with loudness — though the relationship is not linear. A wave with twice the amplitude doesn't sound twice as loud; it takes roughly a 10dB increase (10 times the intensity) to be perceived as "twice as loud" in psychoacoustic studies.
Sound intensity (power per unit area, measured in watts per square meter) follows an inverse square law in free space: intensity decreases by a factor of four (6dB) every time you double the distance from the source. This has enormous practical implications for PA system design and live sound mixing. If your main speakers are hitting 100dB SPL at the mixing position (front-of-house), someone standing twice as far back will experience approximately 94dB SPL. The people at the back of a 100-meter-deep venue need significantly more sound to experience the same level as the audience near the stage.
Sound pressure level (SPL, measured in dB SPL) and sound intensity level (dB IL) are related but not identical. Since intensity is proportional to pressure squared, a doubling of pressure corresponds to a 6dB increase in SPL, while a doubling of intensity corresponds to a 3dB increase in IL. This matters for microphone sensitivity specifications, which may be expressed in mV/Pa (pressure) or mV/Pa (intensity) — understanding which reference is used prevents confusion when calculating signal-to-noise ratios.
Phase and Interference
When two sound waves occupy the same space, they add together. If the compression peaks of both waves align, they add constructively — the combined amplitude is greater than either wave alone. If a compression peak of one wave aligns with a rarefaction peak of another, they add destructively — the waves cancel partially or completely. This phenomenon is called phase interference.
Phase relationships matter enormously in audio. When a microphone picks up direct sound from a speaker plus a reflection from a hard surface (like a concrete wall), the two signals arrive at slightly different times. At certain frequencies where the path length difference corresponds to a half-wavelength, the reflection arrives out of phase with the direct sound and partially cancels it. This creates comb filtering — a frequency response with deep notches at regular intervals. In live sound, this is why feedback occurs at specific frequencies and why speaker placement relative to reflective surfaces matters so much.
In stereo and multi-channel recording, phase relationships between channels determine the width and stability of the stereo image. When mixing in mono, phase-cancelled content disappears. This is why checking your mix in mono is essential — if significant elements cancel when summed, your mix will sound thin and lacking in low-end when played back on mono systems, car radios, and club PAs that are often run in mono or have limited stereo imaging.
Standing Waves and Room Modes
When sound waves reflect between parallel surfaces in a room, certain frequencies can set up standing waves — waves that appear to oscillate in place rather than propagate. These standing waves occur at frequencies whose wavelengths fit neatly into the room dimensions. For example, in a room 6 meters long, a 57Hz wave (wavelength approximately 6 meters) will resonate strongly because the room length supports exactly one-quarter of the wavelength's spatial pattern.
Room modes (axial, tangential, and oblique) cause some frequencies to be reinforced while others are suppressed at specific locations in the room. At room mode peaks, the bass soundsboomy and uncontrolled — you're hearing the room's natural resonance rather than the actual bass content of the recording. At room mode nulls, bass seems weak or absent even if the recording has substantial low-frequency content. This is why listening position in a studio or listening room matters so much for bass reproduction accuracy.
Room mode calculator tools help predict which frequencies will cause problems in a given room geometry. Treatment options include bass traps (thick absorbers placed in corners where modal buildup is greatest), membrane absorbers, and tuned resonant absorbers. For live venues, subwoofer placement relative to room dimensions affects how the low-end distributes across the audience — sometimes moving subs by just a meter can significantly change bass distribution at different positions in the room.