Sound: a disturbance (longitudinal mechanical wave) that travels in a medium and produces the sensation of hearing. It is produced by a vibrating body. It cannot travel in vacuum.
Compression: region of high pressure. Rarefaction: region of low pressure.
Loudness: perception of intensity; depends on amplitude. Pitch: perception of frequency; high frequency means shrill. Quality (timbre): number and relative strength of harmonics; lets us tell two sources apart even when loudness and pitch are the same.
Intensity: energy per second through unit area, SI unit W m⁻². Intensity level: loudness measured on a scale, unit decibel (dB). Frequency: Hz.
Audible range 20 Hz to 20,000 Hz. Infrasound is below 20 Hz (used by elephants, earthquake detection). Ultrasound is above 20 kHz (cleaning, imaging, bats, SONAR, dog whistles).
Echo: reflected sound heard separately after at least 0.1 s. Reverberation: reflected sound that merges with the original (less than 0.1 s). Noise: unpleasant, irregular sound. Musical sound: pleasant and regular.
Formulas
v = f λ. Echo / SONAR distance: d = v t / 2. Minimum distance for an echo = 340 × 0.1 / 2 = 17 m. Speed in air about 330 to 350 m/s; seawater 1500 m/s; steel about 5000 m/s.
Must-know points
Speed of sound is greatest in solids, then liquids, then gases because particles are closer and pass the disturbance faster. It does not depend on frequency, and increases with temperature.
Three parts of the ear: outer ear (collects sound, ear drum vibrates), middle ear (three ossicles amplify), inner ear (cochlea changes vibrations into electrical signals sent to the brain by the auditory nerve).
Echo is not heard in a small room because the reflected sound returns in less than 0.1 s and merges with the original sound (reverberation).
To reduce echo and reverberation: use soft, porous materials such as curtains, carpets and acoustic panels; avoid bare hard walls.
Differences between pitch and loudness: pitch depends on frequency, loudness on amplitude; pitch is measured in Hz, loudness in dB; high pitch means shrill sound, loud means high energy. Loudness is related to amplitude, pitch to frequency.
Lightning is seen before thunder is heard because light is much faster than sound.
Past-paper questions with model answers
What is echo? How is it produced? (1+2): reflection of sound from a hard obstacle, heard after at least 0.1 s.
What is the minimum time for an echo? Why not heard in a small room? 0.1 s (1+2).
Nature of sound waves, compression and rarefaction (3 marks).
How is loudness related to amplitude? Bigger amplitude, more energy, louder sound.
Differentiate between pitch and loudness (any three).
SI units of intensity (W m⁻²), intensity level (dB), frequency (Hz).
Effect of changing amplitude and frequency on sound waves, with diagrams (Section C): larger amplitude gives louder sound; higher frequency gives higher pitch; draw waveforms.
Echo after 5 s, speed 344 m/s: d = 344 × 5 / 2 = 860 m. Sonar 8.5 s, 1500 m/s: depth = 6375 m.