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An increase (or decrease) in the frequency of sound, light, or other waves as the source and observer move towards (or away from) each other. Example: the change in pitch of a siren as an ambulance approaches and then recedes from the listener.

The persistence of a sound after its source has stopped, caused by multiple reflections of the sound within a space. Example: the prolonged echoes heard in a large empty room or hall.

A reflection of sound that arrives at the listener with a delay after the direct sound. Example: shouting in a canyon and hearing your own voice bouncing back.

The increase in amplitude of oscillation of a system when the frequency of its oscillations matches the system's natural frequency of vibration. Example: a glass shattering when a singer hits the right pitch.

The bending of waves around obstacles and openings. Example: sound waves spreading out after passing through a narrow doorway.

The process by which a material, structure, or object takes in sound energy when sound waves are encountered, as opposed to reflecting the energy. Example: acoustical foam in recording studios.

The process of preventing sound from passing from one area to another. Example: using insulated walls to prevent noise from passing into a quiet room.

The product of the density of a medium and the speed of sound within that medium, which affects the transmission of sound waves. Example: understanding acoustic impedance is important in designing loudspeakers.

The change in direction of a wave passing from one medium to another due to a change in its speed. Example: sound bending as it passes from warm to cold air.

A vibration of a system in which some points remain fixed while others between them vibrate with the maximum amplitude. Example: the stationary pattern on a guitar string when plucked.

The difference in arrival time of a sound between two ears, helping in locating the direction of the sound source. Example: humans use this to localize where a sound is coming from.

A measure of opposition that a system presents to the flow of an acoustic wave. Example: matching impedance of speakers to an amplifier for efficient sound transfer.

A system for the detection of objects under water and for measuring the water's depth by emitting sound pulses and detecting or measuring their return after being reflected. Example: submarines use sonar to navigate.

The quality of a sound that distinguishes different types of sound production, such as voices or musical instruments. Example: telling the difference between a piano and a violin playing the same note.

The process by which the threshold of audibility for one sound is raised by the presence of another (masking) sound. Example: not hearing a conversation in a noisy cafe.

The quality of a sound governed by the rate of vibrations producing it; the degree of highness or lowness of a tone. Example: differentiating between a high note and a low note on a musical scale.

The sound associated with the shock waves created whenever an object travels through the air faster than the speed of sound. Example: the explosive noise heard when a jet breaks the sound barrier.

A frequency that is a multiple of the fundamental frequency of a sound. Example: overtones produced by a musical instrument that contributes to its timbre.

The lowest frequency of a periodic waveform. Example: the lowest tone in a musical note that is often perceived as the note's pitch.

Using the pressure of sound waves to lift objects. Example: suspending small particles or liquids in air with ultrasonic waves.

An area where direct sound waves from a source cannot reach due to obstructions or the process of absorption and scattering. Example: being unable to hear the sound of a concert hidden behind a large building.

A subjective measure of the strength or magnitude of a sound sensation as perceived by the ear. Example: the perceived volume of sound from a speaker.

A logarithmic unit used to express the ratio of two values of a physical quantity, often power or intensity. Example: sound levels are measured in decibels.

An interference pattern between two sounds of slightly different frequencies, perceived as a periodic variation in volume. Example: the throbbing sound from two tuning forks with close frequencies.