Comprehensive notes for Chapter 11 Sound. Covers Production and Propagation of Sound, Longitudinal Nature, Characteristics (Loudness, Pitch, Quality, Intensity), and Ultrasound.
Definition: Sound is a form of energy that is produced by the vibrations of bodies. Sound travels in the form of waves from one place to another.
Experiment: Striking a tuning fork against a rubber hammer makes it vibrate, producing sound. Touching a suspended ball with the vibrating prong pushes the ball, confirming vibrations.
Material Medium: Sound waves require a material medium (solid, liquid, or gas) for their propagation. They cannot travel through a vacuum.
Bell Jar Experiment: An electric bell is placed in a jar connected to a vacuum pump. As air is pumped out, the sound becomes feebler and eventually dies out, even though the hammer is still striking the gong. This proves sound needs air (medium) to travel.
Longitudinal Nature: Sound waves in air are longitudinal waves. They propagate as a series of compressions (high pressure) and rarefactions (low pressure). The particles of the medium vibrate back and forth along the direction of propagation.
1. Loudness: The characteristic of sound by which loud and faint sounds can be distinguished. It depends on:
2. Pitch: The characteristic of sound by which a shrill sound can be distinguished from a grave one. Pitch depends on frequency. Higher frequency means higher pitch (e.g., voice of women/children is shrill due to high frequency).
3. Quality (Timbre): The characteristic by which we can distinguish between two sounds of same loudness and pitch. (e.g., piano vs flute note). This is due to difference in waveform.
4. Intensity: Sound energy passing per second through a unit area held perpendicular to the direction of propagation of sound waves. Unit is Watt per square metre (Wm⁻²).
The human ear responds to intensities logarithmically. The loudness (L) is directly proportional to the logarithm of intensity (I).
L = K \log I
Weber-Fechner Law: Where K is constant. Taking faintest audible sound intensity I₀ as reference:
L - L_o = K \log \frac{I}{I_o}
This difference (L - L₀) is determining the Sound Level. In Bels:
Sound Level (SL) = \log \frac{I}{I_o} (Bels)
Since Bel is large, we use Decibels (dB) where 1 Bel = 10 dB:
Sound Level (SL) = 10 \log \frac{I}{I_o} (dB)
Speed of sound depends on the nature of the medium (elasticity and density). Sound travels faster in solids than in liquids, and faster in liquids than in gases.
v = f\lambda
Definition: When sound is incident on the surface of a medium it bounces back into the first medium. This phenomenon is called echo or reflection of sound.
Conditions for Echo: The sensation of sound persists in our brain for about 0.1 s. To hear a clear echo, the time interval between original and reflected sound must be at least 0.1 s. Since speed of sound is ~340 m/s, the total distance must be 34 m, so the reflecting surface must be at least 17 meters away.
Music vs Noise: Pleasant sounds are musical (e.g., flute), while unpleasant/jarring sounds are noise (e.g., machinery, traffic).
Effects: Hearing loss, sleep disturbance, hypertension, stress.
Safe Level: Usually 85-90 dB for an 8-hour workday. Can be reduced by using ear protection or sound barriers.
Acoustic Protection: Use of soft and porous surfaces (draperies, rugs) to absorb sound and reduce reverberation.
The range of frequencies which a human ear can hear is called the audible frequency range.
Definition: Sounds of frequency higher than 20,000 Hz comprising which are inaudible to normal human ear are called ultrasound.