Sound

The One Thing

Sound is produced by vibrating objects and propagates through a material medium as a longitudinal mechanical wave. Its observable characteristics depend mainly on frequency, amplitude, and waveform, while its behaviour also includes transmission and reflection.

Reactions, Processes and Experiments

Key Terms

• Sound: A form of energy that produces the sensation of hearing.
• Vibration: A rapid to-and-fro motion of an object about its mean position.
• Medium: A substance such as a solid, liquid, or gas through which sound travels.
• Mechanical wave: A wave that requires a material medium for its propagation.
• Longitudinal wave: A wave in which particles of the medium vibrate parallel to the direction of wave travel.
• Compression: A region of a sound wave where particles are crowded together and pressure is high.
• Rarefaction: A region of a sound wave where particles are spread apart and pressure is low.
• Wavelength: The distance between two successive compressions or two successive rarefactions; represented by λ.
• Frequency: The number of complete vibrations made by a particle in one second; measured in hertz (Hz).
• Time period: The time taken to complete one vibration; represented by T and measured in seconds.
• Amplitude: The maximum displacement of a vibrating particle from its mean position.
• Pitch: The characteristic of sound that depends mainly on frequency; higher frequency produces a higher pitch.
• Loudness: The sensation that indicates how strong or weak a sound appears, mainly dependent on amplitude.
• Quality or timbre: The characteristic that helps distinguish sounds of the same pitch and loudness produced by different sources.
• Audible sound: Sound that can normally be heard by humans, having frequencies from about 20 Hz to 20,000 Hz.
• Infrasonic sound: Sound with a frequency below 20 Hz.
• Ultrasonic sound: Sound with a frequency above 20,000 Hz.
• Echo: A distinct repetition of sound caused by reflection from a distant surface.
• Reverberation: The persistence of sound in an enclosure due to repeated reflections.
• Noise: An unwanted or unpleasant sound, usually produced by irregular vibrations.

Easily Confused

• Compression and rarefaction: A compression is a high-pressure region in which particles are crowded together; a rarefaction is a low-pressure region in which particles are spread apart.
• Pitch and loudness: Pitch depends mainly on frequency, whereas loudness depends mainly on amplitude.
• Frequency and time period: Frequency is the number of vibrations per second, whereas time period is the time required for one vibration; f = 1/T.
• Echo and reverberation: An echo is a distinct repetition caused by reflection from a distant surface, whereas reverberation is the persistence of sound caused by repeated reflections in an enclosure.
• Audible, infrasonic, and ultrasonic sound: Audible sound has frequencies from about 20 Hz to 20,000 Hz, infrasonic sound is below 20 Hz, and ultrasonic sound is above 20,000 Hz.
• Sound production and sound propagation: A vibrating object is required to produce sound, while a material medium is required for sound to propagate.
• Quality and pitch: Quality depends on waveform and the presence and relative strengths of overtones or harmonics, whereas pitch depends mainly on frequency.
• Sound speed and sound characteristics: Sound speed depends mainly on the nature and temperature of the medium, not on the loudness or pitch of the sound.

What Gets Asked

• Explain how sound is produced and propagated, including the roles of vibrating objects, a material medium, compressions, and rarefactions. The common mark-losing error is stating that sound can travel through a vacuum.
• Use v = fλ, f = 1/T, and T = 1/f in numerical or conceptual questions. Marks are lost by confusing frequency with time period or by omitting the units Hz, s, or m/s.
• Distinguish pitch, loudness, and quality. The specific errors are linking pitch to amplitude, linking loudness to frequency, or ignoring waveform and overtones when describing quality.
• Compare sound speeds in air, water, and steel, and explain the effect of changing frequency in a fixed medium. The key distinction is that speed remains approximately constant for a given medium and temperature, while wavelength changes.
• Explain reflection, echo, and reverberation, including the 0.1 s condition and the approximate 17 m minimum distance for a distinct echo when v = 340 m/s. A frequent error is treating all repeated sound as an echo.
• State applications and effects of sound: ultrasound is used in medical imaging, detecting flaws in metals, cleaning delicate objects, and measuring underwater distances; noise pollution may cause stress, lack of sleep, reduced concentration, and hearing problems.