Sound

The One Thing

Sound is produced by vibrating bodies and travels through a material medium as a mechanical longitudinal wave. Its principal characteristics are determined by frequency, amplitude and waveform, while reflection and resonance explain many of its observed effects and applications.

Reactions, Processes and Experiments

Key Terms

• Sound: A form of energy that produces the sensation of hearing and is generally produced by vibrating bodies.
• Vibration: A rapid to-and-fro motion of a body about its mean position.
• Mechanical wave: A wave that requires a material medium, such as a solid, liquid or gas, for propagation.
• Longitudinal wave: A wave in which particles of the medium vibrate parallel to the direction in which the wave travels.
• Compression: A region of a sound wave where particles are crowded together, producing higher pressure and density.
• Rarefaction: A region of a sound wave where particles are spread apart, producing lower pressure and density.
• Amplitude: The maximum displacement of a vibrating particle from its mean position; it mainly determines the loudness of sound.
• Time period: The time taken by a vibrating body to complete one vibration, represented by T.
• Frequency: The number of complete vibrations made per second, measured in hertz (Hz); it determines the pitch of sound.
• Wavelength: The distance between two successive compressions or two successive rarefactions, represented by lambda.
• Wave velocity: The distance travelled by a wave per unit time, represented by v.
• Pitch: The characteristic of sound that allows it to be judged as shrill or deep; it increases with frequency.
• Loudness: The sensation that helps distinguish a loud sound from a soft sound; it depends mainly on amplitude and is also affected by the sensitivity of the ear.
• Quality or timbre: The characteristic that helps distinguish sounds having the same pitch and loudness but produced by different sources.
• Audible range: For an average human ear, the range of frequencies that can normally be heard is approximately 20 Hz to 20,000 Hz.
• Infrasonic sound: Sound with frequency below 20 Hz.
• Ultrasonic sound: Sound with frequency above 20,000 Hz.
• Reflection of sound: The return of sound into the same medium after striking a hard, smooth surface.
• Laws of reflection of sound: The angle of incidence equals the angle of reflection, and the incident sound, reflected sound and normal at the point of incidence lie in the same plane.
• Echo: A distinct repetition of sound caused by reflection from a distant surface.
• Reverberation: The persistence or prolongation of sound in an enclosure due to repeated reflections after the source has stopped.
• Resonance: The phenomenon in which a body vibrates with maximum amplitude when the frequency of an external periodic force equals its natural frequency.
• Natural frequency: The frequency at which a body vibrates freely when disturbed and then left to itself.
• Forced vibrations: Vibrations produced in a body by the repeated action of an external periodic force.
• Multiple reflection: Repeated reflection of sound between surfaces, used in devices such as megaphones, horns, stethoscopes and soundboards.
• SONAR: A system that uses reflected ultrasonic waves to detect underwater objects or measure depth.

Easily Confused

• Pitch and loudness: Pitch depends mainly on frequency, whereas loudness depends mainly on amplitude.
• Loudness and quality or timbre: Loudness distinguishes loud from soft sounds, whereas quality distinguishes sounds with the same pitch and loudness produced by different sources.
• Echo and reverberation: An echo is a distinct repetition caused by a sufficiently delayed reflection; reverberation is the persistence of sound caused by repeated reflections.
• Natural frequency and forced vibrations: Natural frequency is the frequency of free vibration, whereas forced vibrations are produced by an external periodic force.
• Frequency and time period: Frequency is the number of vibrations per second, while time period is the time taken for one vibration; f = 1/T.
• Infrasonic and ultrasonic sound: Infrasonic sound has a frequency below 20 Hz, whereas ultrasonic sound has a frequency above 20,000 Hz.
• Reflection and absorption: Hard, smooth and large surfaces reflect sound well, whereas soft, porous or irregular surfaces absorb or scatter more sound.
• Pure tone and musical note: A pure tone has one main frequency, whereas a musical note usually contains a fundamental frequency and overtones.
• Frequency and wavelength when sound changes medium: Frequency remains determined by the source, while speed and wavelength may change in another medium.
• Mechanical waves and propagation through a vacuum: Sound is a mechanical wave and therefore cannot travel through a vacuum.

What Gets Asked

• Define or explain how sound is produced and propagated. Marks are lost if sound is described without stating that vibrating bodies produce it, that it requires a material medium, or that it travels as a longitudinal wave through compressions and rarefactions.
• Use the wave equations. Questions may require v = f lambda, f = 1/T, T = 1/f or d = vt; in echo calculations, the factor of 2 must be included because sound travels to the reflector and back.
• Calculate the minimum distance for a distinct echo. The reflected sound must arrive at least 0.1 s after the original, giving approximately 17 m when the speed of sound is 340 m/s; omitting the return journey causes an incorrect result.
• Distinguish pitch, loudness and quality or timbre. The specific relationships are frequency–pitch, amplitude–loudness and waveform or overtones–quality; confusing these characteristics costs marks.
• Explain reflection, echo and reverberation. A distinct echo requires sufficient delay, whereas repeated reflections that are not separately heard produce reverberation; the type of reflecting surface should also be identified where relevant.
• Explain resonance and give examples or applications. The driving frequency must equal or be very close to the natural frequency; relevant examples include a swing being pushed at suitable intervals, air columns in pipes and one tuning fork causing another tuning fork of the same frequency to vibrate strongly.