MP Board · Class 9 · Science · Sound(a) Describe the structure and working mechanism of the human ear with reference to its three main parts: outer ear, middle ear, and inner ear. (b) What is meant by the 'Quality' (or Timbre) of a sound? Why do different musical instruments sound distinct even when playing notes of the same frequency and loudness? (c) A sound wave travels at a speed of $340\text{ m/s}$ with a frequency of $170\text{ Hz}$. Calculate its wavelength.

(a) Structure and Working of the Human Ear

\nThe human ear converts sound pressure variations in the air into electrical signals that travel to the brain via the auditory nerve. It consists of three main parts:

1. Outer Ear:

   • Pinna: The outer part that collects sound waves from the surroundings.
   • Auditory Canal: Directs the collected sound towards the eardrum.
   • Tympanic Membrane (Eardrum): A thin, stretched membrane at the end of the auditory canal. When sound compressions reach it, it is pushed inward, and during rarefactions, it moves outward, causing it to vibrate.

2. Middle Ear:

   • Contains three tiny interconnected bones (ossicles): the Hammer (Malleus), Anvil (Incus), and Stirrup (Stapes).
   • These bones amplify the mechanical vibrations received from the eardrum several times and transmit them to the inner ear.

3. Inner Ear:

   • Cochlea: A fluid-filled coiled tube that converts the amplified mechanical vibrations into electrical signals using sensory hair cells.
   • Auditory Nerve: Carries these electrical nerve impulses to the brain, which interprets them as sound.

(b) Quality (Timbre) of Sound

• Definition: Quality or timbre is that characteristic of sound which enables us to distinguish between two sounds of the same loudness and pitch.
• Explanation: A sound produced by a musical instrument is usually not a single pure frequency (tone) but a complex mixture of several frequencies called overtones or harmonics.
• Sound produced by a rich mixture of frequencies is pleasant to hear and has a distinct quality. Since different musical instruments (like a flute and a violin) produce different combinations and relative intensities of overtones, their wave forms differ, making their sounds distinct even at the same pitch and loudness.

(c) Numerical Solution

Given:

• Speed of sound wave ($v$) = $340\text{ m/s}$
• Frequency of sound wave ($\nu$) = $170\text{ Hz}$

Formula: $$\text{Speed } (v) = \text{Frequency } (\nu) \times \text{Wavelength } (\lambda)$$ $$\lambda = \frac{v}{\nu}$$

Calculation: $$\lambda = \frac{340}{170} = 2.0\text{ m}$$

Answer:\nThe wavelength of the sound wave is $2\text{ m}$.