Sound waves are described by a handful of core features: amplitude, frequency, wavelength, period, and timbre. Each one captures a different aspect of how a sound wave behaves physically and how we perceive it. Whether you’re answering a physics question or trying to understand what makes one sound different from another, these are the properties being described.
Amplitude and Loudness
Amplitude is the maximum displacement of an air molecule from its resting position as a sound wave passes through. Think of it as how far the air gets “pushed” in each vibration cycle. A common mistake is measuring the full top-to-bottom swing of the wave. The amplitude is only the distance from the resting (equilibrium) position to the peak, not the total height.
Amplitude directly determines how loud a sound is. Turn up the volume on a speaker and the oscillations get larger, meaning the air molecules are being displaced farther with each cycle. The energy a wave carries is proportional to the square of its amplitude, so doubling the amplitude doesn’t just double the energy; it quadruples it. This is why small increases in volume can represent large jumps in actual energy.
Loudness is measured in decibels (dB). The faintest sound a human ear can detect sits at 0 dB, while the threshold of pain falls between 120 and 140 dB. For everyday reference, sounds averaging 70 dB or lower over the course of a day are considered safe for most people, while regular exposure at or above 85 dB can cause permanent hearing loss over time.
Frequency and Pitch
Frequency describes how many times a sound wave oscillates per second. It’s measured in Hertz (Hz), where 1 Hz equals one complete oscillation per second. When a guitar string vibrates at 440 Hz, it’s completing 440 back-and-forth cycles every second, producing the note known as middle A.
Frequency is the physical measurement; pitch is what you perceive. A higher frequency sounds higher-pitched (like a whistle), and a lower frequency sounds deeper (like a bass drum). The human ear generally detects frequencies between about 20 Hz and 20,000 Hz, though that upper limit tends to drop with age.
Wavelength and Period
Wavelength is the physical distance between two identical points on consecutive waves, such as from one pressure peak to the next. It’s typically represented by the Greek letter lambda (λ). Low-frequency sounds have long wavelengths, sometimes several meters. High-frequency sounds have short wavelengths, sometimes just a few centimeters.
Period is the time it takes for one complete wave cycle to pass a given point. Frequency and period are inverses of each other: if a wave has a frequency of 100 Hz, its period is 1/100th of a second (0.01 seconds). This inverse relationship is written simply as T = 1/f.
These features tie together through the wave speed equation: speed equals frequency times wavelength (v = fλ). In air at room temperature (20°C), sound travels at about 343 meters per second. Knowing any two of these values lets you calculate the third.
Timbre: Why Instruments Sound Different
If a piano and a violin both play middle A at 440 Hz, they’re producing the same pitch at the same frequency, yet you can instantly tell them apart. The feature that explains this difference is timbre (pronounced TAM-burr).
Timbre comes from the unique mixture of overtones an instrument produces. When any instrument plays a single note, it doesn’t generate just one frequency. It creates a fundamental frequency plus a series of higher frequencies called harmonics or overtones. Some of those overtones are louder, some are quieter, some fade quickly, and some linger. The specific proportions of these overtones give each instrument its characteristic color. A flute emphasizes different overtones than a clarinet, which is why the two sound nothing alike even when playing the same note at the same volume.
Sound Travels as a Longitudinal Wave
Unlike waves on a string or ripples on water, sound is a longitudinal wave. This means the air molecules vibrate back and forth in the same direction the wave is traveling, rather than up and down perpendicular to it. As the wave moves forward, it creates alternating zones of high pressure (compressions) and low pressure (rarefactions). A single-frequency sound wave produces a smooth, repeating pressure pattern that looks like a sine wave when graphed.
This is worth understanding because diagrams often show sound as a wavy line moving up and down, which can be misleading. The “up and down” on those graphs represents pressure changes, not the physical shape of the wave in space.
How Speed Changes in Different Materials
Sound doesn’t always travel at the same speed. The medium it passes through makes a dramatic difference. In air at 0°C, sound moves at 331 meters per second. In fresh water, it jumps to about 1,480 meters per second. In steel, it reaches roughly 5,960 meters per second. Denser, stiffer materials generally transmit sound faster because their molecules are packed more tightly and transfer vibrations more efficiently.
Temperature also matters. Warming the air from 0°C to 20°C increases the speed of sound from 331 to 343 meters per second. Humidity plays a smaller role. Above about 30% relative humidity, the speed of sound increases linearly as moisture content rises, because water vapor is lighter than the nitrogen and oxygen molecules it replaces, making the air mixture slightly easier to push around.
Phase and Wave Interference
Phase describes where a wave is in its cycle at any given moment, measured in degrees from 0 to 360 (one full cycle). When two sound waves meet, their phases determine what happens next. If two waves arrive perfectly in sync, their pressure peaks line up, and you hear a louder combined sound. This is constructive interference.
If two waves arrive exactly half a cycle apart, one wave’s high-pressure zone hits the other’s low-pressure zone. They cancel each other out, reducing the volume or even producing silence at that point. This is destructive interference, and it’s the principle behind noise-canceling headphones. At most real-world listening positions, you get a mix of both effects, which is why sound in a room can be louder in some spots and quieter in others.