What Determines The Pitch And Loudness Of Sound

Sound Waves Pitch & Loudness Animal Hearing Music vs Noise

What Determines the Pitch and Loudness of Sound? | Class 9 Sound Chapter Explained

What Determines the Pitch and Loudness of Sound?

Sound is part of your everyday life-your alarm clock, your teacher’s voice, music from your phone, and the horn of a bus. But have you ever noticed something interesting?
Why does a baby’s cry sound sharp while a drum sounds deep?
Why does a whisper feel soft but a loudspeaker feels powerful?
These two experiences are explained using pitch and loudness of sound.
Let’s understand, step by step, what actually decides:
Why a sound is high or low (pitch)
Why a sound is soft or loud (loudness)
We’ll keep it simple, practical, and exam-focused.

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1. Basic Idea: What Is Sound?

Sound is produced when an object vibrates.

Examples:

• Guitar string --> vibrates --> sound
• School bell --> vibrates --> sound
• Speaker cone --> vibrates --> sound
• Human vocal cords --> vibrate --> sound

These vibrations create waves in air. These waves carry energy to our ears, and our brain interprets them as sound.
Two important properties of these sound waves decide how we hear them:

1. Frequency --> decides pitch
2. Amplitude --> decides loudness

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2. What Is Pitch of Sound?

Simple Definition: Pitch tells us whether a sound is high or low.

Examples:

• Mosquito sound --> high pitch
• Lion’s roar --> low pitch
• Female voice --> usually higher pitch
• Male voice --> usually lower pitch

Pitch depends on frequency.

Frequency means: Number of vibrations per second.

Unit: Hertz (Hz)

1. 100 vibrations per second = 100 Hz
2. 1000 vibrations per second = 1000 Hz

Higher frequency = more vibrations per second = higher pitch.

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3. Frequency and Pitch Relationship

Let’s see how frequency controls pitch:

So: Pitch ∝ Frequency

• If frequency increases --> pitch increases
• If frequency decreases --> pitch decreases

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4. Real-Life Example of Pitch

Example 1: Guitar String

• Tight string --> vibrates faster --> higher pitch
• Loose string --> vibrates slower --> lower pitch

Example 2: Flute vs Drum

• Flute --> thin air column --> high frequency --> high pitch
• Drum --> slow vibrations --> low frequency --> low pitch

5. What Is Loudness of Sound?

Simple Definition: Loudness tells us how strong or soft a sound is.

Examples:

• Whisper --> soft sound
• Classroom fan --> medium sound
• DJ speaker --> loud sound

Loudness depends on amplitude.

Amplitude means: Height of the sound wave (how much the particles move).

Greater vibration --> more energy --> louder sound.

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6. Amplitude and Loudness Relationship

So, Loudness ∝ Amplitude

• More amplitude = louder sound
• Less amplitude = softer sound

7. Measuring Loudness: Decibel (dB)

Loudness is measured in decibel (dB).

Safe limit for human ears: below 85 dB

Above this level for long time --> hearing damage.

8. Pitch vs Loudness (Comparison Table)

9. Why Baby Cry Sounds Different from Drum?

Let’s break it scientifically:

So,

• Different pitch = different frequency
• Different loudness = different amplitude

10. Classroom Scenario

Teacher speaks normally --> frequency ~ 200 Hz --> medium pitch
Teacher uses mic --> amplitude increases --> louder sound
But frequency stays same --> pitch stays same

So mic does NOT change pitch, only loudness.

11. Formula Summary Table

12. Real-Life Analogy Table

13. PAS for Misconception 1

“Loud sound means high pitch”

Problem: Students think louder sound is always sharper.

Agitate: This causes wrong answers in exams when identifying pitch vs loudness.

Solution: Loudness depends on amplitude, not frequency.
Pitch depends on frequency only.

A drum can be louder than a whistle but still low-pitched.

14. PAS for Misconception 2

“High frequency means louder sound”

Problem: Students mix frequency with loudness.

Agitate: This confusion leads to wrong graphs and incorrect MCQs.

Solution: High frequency = high pitch
High amplitude = loudness

15. Common Mistakes vs Correct Understanding Table

16. Case Study: Two Students in Lab

Student A hits tuning fork softly
 -> small amplitude -> soft sound
  -> frequency same

Student B hits tuning fork hard
-> large amplitude -> loud sound
-> frequency same

Pitch remains constant.
Only loudness changes.

17. Exam Tip: Graph Interpretation

18. Numerical Example

A sound wave has:

• Frequency = 500 Hz
• Amplitude = large

Interpretation:

• Pitch = medium
• Loudness = high

Another sound:

• Frequency = 2000 Hz
• Amplitude = small

Interpretation:

• Pitch = high
• Loudness = low

19. Human Ear and Perception

Ear can hear frequencies between:
20 Hz to 20,000 Hz

• Below 20 Hz --> infrasonic
• Above 20,000 Hz --> ultrasonic

Pitch is how brain interprets frequency.
Loudness is how brain interprets amplitude.

20. Short Revision Table

21. Why Understanding This Is Important for Exams

Questions come as:

• Define pitch
• Difference between pitch and loudness
• Identify graph
• MCQs
• Assertion-Reason
• Numericals on frequency

That’s why clarity is important.
You can download free notes and practice questions from our Resource Page.

22. Practical Scenario: School Bell

• Metal bell vibrates --> sound produced
• Hard hit --> large amplitude --> louder
• Bell shape --> fixed frequency --> same pitch

So hitting harder doesn’t change pitch.

23. Another Real Example: Radio Volume Knob

Volume knob controls amplitude.
Channel tuning controls frequency.

Volume ≠ pitch
Channel ≠ loudness

24. PAS for Student Doubt

“Why can two sounds be equally loud but different in pitch?”

Problem: Students expect loud sounds to sound similar.

Agitate: This causes confusion when identifying instruments.

Solution: Same amplitude = same loudness
Different frequency = different pitch

So flute and violin can be equally loud but sound different.

25. Summary Table (Key Points)

26. Final Understanding

So remember:

Pitch = frequency = sharp or deep sound
Loudness = amplitude = soft or loud sound

They are independent properties.

High pitch does not mean loud.
Loud sound does not mean high pitch.

If you want to practice this topic, you can take a quiz in Curious Corner for better practice.