Question
How is sound produced and how does it travel from source to ear? Explain the difference between loudness and pitch with examples.
(CBSE Class 8-9 — Sound)
Sound: From Source to Ear
flowchart LR
A["Vibrating Object"] --> B["Compressions & Rarefactions"]
B --> C["Sound Wave in Medium"]
C --> D["Ear Drum Vibrates"]
D --> E["Hammer, Anvil, Stirrup amplify"]
E --> F["Cochlea converts to electrical signal"]
F --> G["Brain interprets as sound"]
C --> H["Needs a medium: solid/liquid/gas"]
C --> I["Cannot travel in vacuum"]Solution — Step by Step
Sound is produced by vibrating objects. When you pluck a guitar string, it moves back and forth rapidly. This vibration pushes and pulls the surrounding air molecules, creating alternating regions of high pressure (compressions) and low pressure (rarefactions).
These compressions and rarefactions form a longitudinal wave — the particles vibrate in the same direction as the wave travels. This is different from transverse waves (like water waves) where particles move perpendicular to the wave direction.
Sound needs a medium to travel — solid, liquid, or gas. It cannot travel through vacuum (that is why there is no sound in space).
Speed of sound varies by medium:
- Solids — fastest (about 5000-6000 m/s in steel)
- Liquids — medium (about 1500 m/s in water)
- Gases — slowest (about 340 m/s in air at 20°C)
Why? Particles in solids are closest together, so vibrations transfer fastest.
| Property | Loudness | Pitch |
|---|---|---|
| Depends on | Amplitude of vibration | Frequency of vibration |
| Unit | Decibel (dB) | Hertz (Hz) |
| Example | Shouting vs whispering | Bass guitar vs flute |
| Physical meaning | How much energy the wave carries | How fast the source vibrates |
A drum hit hard produces a loud sound (large amplitude). A thin string vibrating fast produces a high-pitched sound (high frequency). These are independent — you can have a loud low-pitched sound (tabla) or a soft high-pitched sound (whistle blown gently).
Why This Works
Sound is mechanical energy transferred through vibrations. The source creates disturbances, the medium transmits them, and the ear detects them. The eardrum vibrates in sync with the incoming wave. Three tiny bones (hammer, anvil, stirrup) amplify these vibrations and pass them to the cochlea, which converts mechanical vibrations into electrical nerve signals sent to the brain.
The human ear can detect frequencies from about 20 Hz to 20,000 Hz. Below 20 Hz is infrasound (elephants communicate using it), above 20,000 Hz is ultrasound (bats and dolphins use it for navigation).
Alternative Method — Everyday Observations
We can understand sound properties through daily experiences:
- Touch your throat while speaking — you feel vibrations (production)
- Sound is muffled underwater vs in air (medium affects speed)
- Thunder is heard after lightning (light is faster than sound)
- A mosquito has high pitch (fast wing beats), a lion has low pitch (slow vocal cord vibration)
For CBSE Class 9, you need to know the formula: (speed = frequency times wavelength). This connects all three properties. If frequency increases and speed stays constant, wavelength must decrease. This relation is a favourite numerical question.
Common Mistake
Students confuse loudness with pitch. A common wrong statement: “A louder sound has higher pitch.” No — a bass drum can be very loud but has a low pitch. Loudness depends on amplitude (how far the particles move), pitch depends on frequency (how fast they vibrate). These are two independent properties of sound.