Class 7 Science — Chapter: Sound

Sound – Pitch, Loudness, and Vibrations

Understand how sounds are made, how they travel, and why some are high, some low, some loud, and some soft — with simple examples you meet every day.

📚 Contents

1. What is Sound?
2. How is Sound Made? (Vibrations)
3. Pitch: High vs. Low
4. Loudness & Amplitude
5. Speed of Sound & Medium
6. Human Voice & Instruments
7. How We Hear (Ear)
8. Quick Tables & Formulas
9. Try These Activities
10. FAQs
11. Mini-Quiz
12. Summary & Key Terms

1) What is Sound?

Definition: Sound is a form of energy produced by vibrating objects and travels as a longitudinal wave through a medium (air, water, or solids).

Key idea: No medium = no sound. In space (vacuum), sound cannot travel.

Everyday examples: Guitar string, your vocal cords, a tuning fork, a drum skin — all vibrate to make sound.

2) How is Sound Made? (Vibrations)

When an object vibrates, it pushes and pulls the air around it, creating regions of compression (air particles close together) and rarefaction (particles far apart). These regions move forward as a wave.

Compression = high pressure Rarefaction = low pressure One vibration = one complete to-and-fro motion

Try it: Pluck a rubber band. You can see it moving back and forth — that’s vibration. Touch it gently and the sound stops because the vibration stops.

3) Pitch: High vs. Low

Pitch tells us how “high” or “low” a sound is. It depends on the frequency of vibration (how many vibrations per second).

Frequency (f) — number of vibrations per second, measured in Hertz (Hz).

Example: 500 Hz = 500 vibrations every second (high pitch). 100 Hz = 100 vibrations every second (low pitch).

Rule: Higher frequency → higher pitch (like a whistle). Lower frequency → lower pitch (like a drum).

Everyday: A child’s voice has higher pitch than an adult male voice because the child’s vocal cords vibrate faster.

4) Loudness & Amplitude

Loudness tells us how “strong” or “soft” a sound seems. It depends on the amplitude (size) of vibration.

Amplitude (A) — maximum displacement of the vibrating object from its rest position.

Example: Strike a drum gently → small amplitude → soft sound. Strike it hard → large amplitude → loud sound.

Note: Loudness (what we feel) is measured in decibels (dB). Prolonged exposure to very loud sounds (above ~85 dB) can harm hearing.

5) Speed of Sound & Medium

Sound needs a medium and travels at different speeds in different media:

Medium	Speed (approx.)	Reason
Air (25°C)	~340 m/s	Particles far apart → slower transfer
Water	~1500 m/s	Particles closer → faster
Steel	~5000 m/s	Particles tightly packed → fastest

Observation: During a storm, you often see lightning first (light is very fast) and hear thunder later (sound is slower).

6) Human Voice & Musical Instruments

Human voice

Air from lungs makes the vocal cords vibrate in the larynx. Short, tight cords → higher pitch; long, loose cords → lower pitch.

Musical instruments

• String instruments (guitar, violin): thinner/shorter/tighter strings give higher pitch.
• Wind instruments (flute, clarinet): shorter air column → higher pitch; longer column → lower pitch.
• Membrane instruments (drum): tighter skin → higher pitch; stronger hit → louder sound.

7) How We Hear (Ear)

Sound waves enter the outer ear, make the eardrum vibrate, pass through three tiny bones in the middle ear, and then to the inner ear (cochlea), which sends signals to the brain.

Safety: Avoid loud headphones for long periods; give your ears rest.

8) Quick Tables & Formulas

Pitch ↔ Frequency (f, in Hz) Loudness ↔ Amplitude (A) Time period (T) = 1 / f Wavelength (λ) = v / f

Quantity	Symbol	Unit	Meaning
Frequency	f	Hertz (Hz)	Vibrations per second
Time Period	T	Seconds (s)	Time for one vibration (T = 1/f)
Amplitude	A	—	Size of vibration
Speed of sound	v	m/s	How fast the wave travels in a medium
Wavelength	λ	m	Distance between compressions (λ = v/f)

9) Try These Activities (Safe & Simple)

1. Rubber-band Guitar: Stretch rubber bands of different thickness over a box. Pluck each. Thinner (tighter) band → higher pitch.
2. Table Tapping: Tap softly vs. strongly. Notice change in loudness (amplitude), not pitch.
3. Ruler Vibration: Place a ruler edge on table, press one end, flick the free end. Shorter free length → faster vibrations → higher pitch.

10) FAQs

Is a loud sound always high-pitched?

No. Loudness depends on amplitude, while pitch depends on frequency. A drum can be very loud but low-pitched.

Why can’t we hear in space?

Space is a vacuum — no particles, so sound waves cannot travel.

What frequencies can humans hear?

Roughly 20 Hz to 20,000 Hz. Most adults hear a bit less at the high end.

11) Mini-Quiz

Q1 A sound of 800 Hz has a higher/lower pitch than a sound of 200 Hz.

Answer: Higher pitch (higher frequency → higher pitch).

Q2 If a wave’s speed is 340 m/s and frequency is 170 Hz, what is the wavelength λ?

Answer: λ = v/f = 340/170 = 2 m.

Q3 Increasing amplitude changes which: pitch or loudness?

Answer: Loudness.

12) Summary & Key Terms

• Vibration: To-and-fro motion that creates sound.
• Pitch: High/low quality of sound; depends on frequency (f).
• Loudness: Strength of sound; depends on amplitude (A), measured in dB.
• Speed (v): Distance covered per second by the wave; depends on medium.
• Wavelength (λ): Distance between two compressions or rarefactions; λ = v/f.

You’ve got this! Explore the sounds around you and identify pitch and loudness in daily life. 🎧