MP Board · Class 9 · Science · Sound(a) Define the following characteristics of a sound wave and explain the factors on which they depend: Loudness Pitch Quality (Timbre) (b) Differentiate between a loud sound and a soft sound, and between a high-pitched sound and a low-pitched sound on the basis of their wave characteristics.

(a) Characteristics of Sound Waves

1. Loudness:

   • Definition: Loudness is the subjective sensation produced in the human ear that enables us to distinguish between a loud sound and a faint sound of the same frequency.
   • Dependency: It depends primarily on the amplitude of the vibrating body. Loudness is directly proportional to the square of the amplitude ($\text{Loudness} \propto \text{Amplitude}^2$). A larger amplitude carries more energy, producing a louder sound. It also depends on the surface area of the vibrating body and the distance of the listener from the source.
   • Unit: Measured in decibels (dB).

2. Pitch:

   • Definition: Pitch is the characteristic of sound by which a listener can distinguish a sharp/shrill sound from a grave/flat sound.
   • Dependency: Pitch depends strictly on the frequency of the sound wave. High frequency corresponds to a high pitch (shrill voice), whereas low frequency corresponds to a low pitch (deep/flat voice).

3. Quality or Timbre:

   • Definition: Quality (or timbre) is that characteristic of sound which enables us to distinguish between two sounds having the same pitch and loudness, produced by two different musical instruments or sources.
   • Dependency: It depends on the shape of the waveform and the number of harmonics (overtones) present in the sound. A sound of a single frequency is called a tone, while a combination of several frequencies is called a note (which is pleasant to hear).

(b) Comparative Differences

1. Loud Sound vs. Soft Sound:

• Amplitude: A loud sound has a larger amplitude wave, whereas a soft (faint) sound has a smaller amplitude wave.
• Energy: A loud sound wave carries higher energy and travels longer distances without dying out compared to a soft sound.
• Graphical Representation: On a graph plotting density/pressure vs. distance, a loud sound displays taller crests and deeper troughs, while a soft sound displays shallow crests and troughs.

2. High-Pitched Sound vs. Low-Pitched Sound:

• Frequency: High-pitched sounds have a higher number of wave cycles (compressions and rarefactions) passing per unit time, whereas low-pitched sounds have fewer cycles per unit time.
• Examples: A female voice, a bird chirping, or a whistle produces a high-pitched sound. A male voice, a roaring lion, or a beating drum produces a low-pitched sound.
• Wave Density: In a graphical representation, the waves of a high-pitched sound are packed closely together, while the waves of a low-pitched sound are spread farther apart.