Sound

The One Thing

Sound is produced by vibrations and travels through a material medium as a longitudinal wave of compressions and rarefactions. Its pitch depends mainly on frequency, whereas its loudness depends mainly on amplitude; sound may also be reflected by surfaces.

Reactions, Processes and Experiments

Key Terms

• Vibration: The repeated to-and-fro motion of an object about its mean position.
• Sound: A form of energy produced by vibrating objects that creates the sensation of hearing.
• Medium: A substance through which sound travels, such as air, water or a solid.
• Longitudinal wave: A wave in which particles of the medium vibrate parallel to the direction in which the wave travels.
• Compression: A region of high pressure and high particle density in a sound wave.
• Rarefaction: A region of low pressure and low particle density in a sound wave.
• Amplitude: The maximum displacement of vibrating particles from their mean position; it is related to the loudness of sound.
• Time period: The time taken by a vibrating object to complete one vibration.
• Frequency: The number of complete vibrations made in one second, measured in hertz (Hz).
• Wavelength: The distance between two successive compressions or two successive rarefactions.
• Wave speed: The distance travelled by a sound wave per unit time.
• Pitch: The characteristic of sound that tells whether it is shrill or deep; it depends on frequency.
• Loudness: The characteristic of sound related mainly to amplitude and perceived strength of sound.
• Reflection of sound: The bouncing back of sound waves after striking a hard surface.
• Echo: A distinct repetition of sound caused by reflection from a distant surface.
• Ultrasound: Sound with frequency greater than 20,000 Hz, beyond the upper limit of human hearing.
• Infrasound: Sound with frequency below 20 Hz, below the lower limit of human hearing.

Easily Confused

• Pitch and loudness: Pitch depends mainly on frequency, whereas loudness depends mainly on amplitude.
• Frequency and amplitude: Frequency is the number of vibrations per second; amplitude is the maximum displacement from the mean position.
• Compression and rarefaction: A compression has high pressure and high particle density; a rarefaction has low pressure and low particle density.
• Echo and reverberation: An echo is a distinct repetition of sound, whereas reverberation is the persistence of sound caused by repeated reflections and reduces clarity.
• Ultrasound and infrasound: Ultrasound has a frequency greater than 20,000 Hz; infrasound has a frequency below 20 Hz.
• Reflection and absorption: Hard, smooth surfaces reflect sound well, whereas soft, porous materials such as curtains, carpets and foam absorb it.
• Sound speed and frequency: Sound speed depends on the medium and is related to frequency and wavelength by $v=f$ lambda; frequency describes the number of vibrations per second.

What Gets Asked

• Explain how sound is produced and propagated, using the tuning fork example. The key point is that the prongs vibrate and transfer energy through a medium; the medium as a whole does not move forward.
• State why sound cannot travel through a vacuum. The mark is lost if the need for particles to transfer the disturbance is omitted.
• Use $f=$ number of vibrations / time taken, $T=1/f$, or $v=f$ lambda. Units must be consistent: hertz (Hz), metre (m), and metre per second (m/s).
• Distinguish pitch from loudness. Higher frequency means higher pitch, while greater amplitude means greater loudness.
• Explain or calculate an echo. A distinct echo requires a time delay of at least 0.1 second; using 340 m/s, the reflecting surface must be about 17 m away because distance = $vt/2$.
• Describe reflection and its applications, including megaphones, horns, musical instruments, stethoscopes and sound boards. Hard, smooth surfaces reflect sound well, while soft, porous materials absorb sound.