Sound and waves

From vibration to wave#

When a guitar string, loudspeaker cone, or vocal cord vibrates, it pushes and pulls the surrounding material. Nearby particles oscillate and transfer the disturbance onward. The particles do not travel from the source all the way to the listener; energy moves through the wave.

Sound is a mechanical wave, so it needs matter such as air, water, or a solid. It cannot travel through a perfect vacuum because there are no particles to pass on the vibration.

Frequency and pitch#

Frequency counts complete vibrations per second and is measured in hertz (Hz). A 250 Hz vibration repeats 250 times each second.

Higher frequency is usually heard as higher pitch, while lower frequency is heard as lower pitch. Pitch is a perception, so the ear’s response also matters; frequency and pitch are related but are not identical words.

Amplitude, intensity, and loudness#

Amplitude describes the size of the wave’s disturbance. Greater amplitude generally carries more energy and produces greater sound intensity.

Loudness is the listener’s perception. It depends on intensity, frequency, distance, duration, and hearing sensitivity. Doubling one simple wave measurement does not necessarily mean a sound is perceived as twice as loud.

Sound level is often reported in decibels (dB), a logarithmic scale. Long or repeated exposure to high sound levels can damage hearing. Reducing the level, increasing distance, and limiting exposure time all help.

Speed and the medium#

Sound speed depends on the material and its conditions. It generally travels faster in many solids than in gases because particles interact strongly, although the exact speed depends on the material. In air, temperature also affects sound speed.

Frequency is set by the source and normally stays the same when a wave enters another medium. Speed and wavelength can change together.

Reflection and absorption#

An echo is reflected sound heard separately from the original. Hard surfaces often reflect more sound, while soft or porous materials can absorb more of its energy. Rooms use shape and materials to control reflections so speech or music remains clear.

A common mistake is saying sound travels because air moves continuously from the source to the ear. The air particles mainly vibrate around their positions while the wave transfers energy through them.