“ When you sing, you begin with Do, Re, Mi..”“ Do, Re, Mi..” “ Do, Re, Mi.. The first three notes just happen to be ‘Do, Re, Mi” “ Do, Re, Mi…...Do, Re, Mi, Fa, So, La, Ti..” Fig.1. She plucks the strings with one hand while she presses the strings with the fingers of the other. We use sounds for signalling and communications. Music is produced only when the notes are arranged in a way soothing or pleasant to a listener. A note in music may mean the pitch of the wave. There are high pitch notes and low pitch notes. In key boards, such as a piano, as you move to the right, the pitch gets higher. The pitch of a note depends on the frequency of vibration. The loudness or volume depends on the amplitude. Fig.2. In a musical keyboard the frequency increases to the right. String instruments These produce musical notes with stretched strings. When a tight sering is plucked. It vibrates for a while emitting a continuous sound, This is a kind of resonance due to the forming of a standing wave. The factors that determine the frequency are the length of the string, the tension and the thickness Fig.3. Sonometer is an instrument to study vibrations in strings. When a vibrating tuning fork is kept on the box a section of spring can be made to vibrate due to resonance. The weights and the length L of the wire can be altered easily. Vibrations can easily be detected by placing bits of folded paper on the wire. When the wire vibrates they get thrown off. Longer strings give low notes. Low pitch...................Frequency is inversely proportional to length. Tighter strings give high notes. High pitch............... F Thicker strings produce low notes. Low pitch. A vibrating string has a transverse wave motion. It gives rise to a longitudinal wave in air. The frequency remains unaltered while the wavelength changes. This is due to the change in velocity. Fig.4. Formula for the frequency of a vibrating string. 2L is used as the fundamental of a string is twice the length. (Refer to the table below) Fig.5 Fundamental note and the overtones. The string column in the table shows 3 ways of vibrating a string. The first one gives the fundamental note while the others are called overtones or harmonics. A musical note produced by a string will be a blend of the fundamental and several harmonics. Fig.6. Red and blue show two harmonics (overtones) The resulting wave can be obtained by adding the amplitudes of those. This is shown as black dots. This is the reason for the difference in tone or colour in different instruments. This is described as timbre. Practice Problem.1. A stretched wire 1.00 m. Long resonates to a tuning fork of 260 Hz.
3. If the velocity in air is 340 ms-1 calculate the wavelength of the sound. Solution:-
λ= 340/60 = 5.67 m.
1. Using wires of different materials.
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