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Sound Waves (A) Wave nature of sound Sound shows all properties of waves. It is a kind of waves. (B) Properties of sound Some properties of sound: Sound is produced by vibrations. Sound is a longitudinal wave, which consists of a series of compressions and rarefactions. Sound can travel through solids, liquids and gases, but not in a vacuum. (Vsolid > Vliquid > Vgas) (C) Comparison of sound and light Production Sound Light Mechanical Electric and magnetic vibrations vibrations Type of Longitudinal Transverse Solids, liquids and gases Transparent vacuum 330 m s-1 (at 0 oC air) 3 x 108m s-1 in air waves Travelling medium Travelling materials and a speed (D) Ultrasonic waves Audible frequency ranges between 20 Hz and 20 kHz. Ultrasound is sound of frequency above 20 kHz. Ultrasound have wide applications in industry and medicine, such as sonar, ultrasound cleaner, ultrasound flaw detector and ultrasound cleaner. Sound Waves 1 (E) Musical notes and noise A note is a sound produced by regular vibrations of a source, while a noise is produced by irregular vibrations. (1) Musical Notes Musical notes have the following characteristics. (i) Loudness Loudness increases with the amplitude of the sound. (ii) Pitch Pitch increases with the frequency of the sound. (iii) Quality Two different musical instruments sound very differently even though we are playing notes of the same pitch and same loudness by them. They are said to have different qualities and their waveforms are different. Each sound from a musical instrument has a strong fundamental frequency, which determines the pitch of the note. Weaker frequencies are also produced. They are called overtones or harmonics, the frequencies of which are integral multiples of the fundamental frequency. Notes of the same pitch and loudness from different musical instruments have different qualities because they have different numbers and amplitudes of overtones accompanying the fundamental note. Sound Waves 2 (2) Noise Loudness or sound intensity level is measured in decibels (dB) by a sound intensity level meter. The lowest sound intensity level that can be heard is called the threshold of hearing. It should be noted that the zero point on the decibel scale is not ‘no sound’. Very loud Loud Moderate Quiet Silent Noise Level in dB 120 110 100 90 80 70 60 50 40 30 20 10 0 Sources of noise Near roar of aeroplane Violent hammering of steel plate Disco Loud motor horn Loud radio music Busy office Urban district Normal conservation Inside a running car Suburban street Quiet room Faintest audible 'sound Threshold of hearing Noise can be irritating and unbearable. It affects our work and health. Prolonged exposure to noise will damage hearing. In Hong Kong, there are noise control measures enforced by law. Sound Waves 3