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💡 How we hear it
Frequency and amplitude are physics; pitch and loudness are how our ears experience them.
What is pitch?
Pitch
How the frequency of a sound is perceived; high pitch means higher frequency and low pitch lower frequency.
- Pitch is how the frequency of a sound is perceived.
- Shrill sounds (whistle, siren) have high pitch (higher frequency).
- Deep sounds (thunder, a rumble) have low pitch (lower frequency).
What is loudness?
Loudness
How the amplitude of a sound is perceived; larger amplitude sounds louder. Measured in decibels.
- Loudness is how the amplitude of a sound is perceived.
- Larger-amplitude sounds are heard louder ; loudness falls with distance.
- Loudness is commonly measured in decibels (dB) and depends on the listener’s hearing.
Activity 10.8 — Let us experiment (demonstration activity)
In this Activity, we will experiment as a class demonstration to listen to how sound changes across different frequencies.
Activity 10.8 - Hearing Different
Frequencies Step 1
Play a 100 Hz tone
↓
Step 2
Increase in steps to 1000 Hz
↓
Step 3
Lower it toward 20 Hz
↓
Sound changes and fades near the hearing limits
- Open a mobile app that can generate sounds.
- Set the frequency to 100 Hz, play it, and listen carefully.
- Increase the frequency in steps of 100 Hz up to 1000 Hz and describe the change.
- Then lower the frequency from 50 Hz toward 20 Hz until you can no longer hear it.
What we observe
The sound changes with frequency and
fades near the limits
of the human hearing range.
How is sound classified by frequency?
Sound
Grouped by frequency
1. Infrasonic
Below 20 Hz
Example:
Earthquakes, whale calls
2. Audible
20 to 20000 Hz
Example:
Speech and music
3. Ultrasonic
Above 20000 Hz
Example:
Bats, SONAR, scans
What are infrasonic and ultrasonic waves?
- Humans hear roughly 20 Hz to 20 kHz (the audible range).
- Infrasonic waves are below 20 Hz (elephants can detect them).
- Ultrasonic waves are above 20 kHz (dogs, cats, bats and dolphins can detect them).
🌎 Bridging Science and Society
- Loudness is commonly measured in decibels (dB) .
- Rustling leaves are only a few dB.
- Normal conversation is about 60 dB.
- Firecrackers can exceed 100 dB.
- Unwanted or harmful sound is called noise .
- Prolonged exposure to loud sound can cause hearing loss.
- Hearing loss is tested using audiograms .
- Hearing aids use a microphone, amplifier and speaker.
👤 Meet a Scientist — Sir C. V. Raman
- Won India’s first Nobel Prize in Science for discovering the Raman Effect in light.
- Also studied the acoustics of Indian percussion instruments like the tabla and mridangam to understand their rich sound.
🧵 Threads of Curiosity
- A tone is a sound of a single frequency, like a tuning fork.
- A musical note has a lowest frequency called the fundamental.
- It also has higher frequencies called overtones .
- Together these create a rich and pleasant sound.
- Sarangi, sitar and veena use extra strings to enrich this.
- A flute, ektara and tabla play the same note, yet sound unique.
- This quality is called timbre .
- Timbre comes from shape, material and construction.
- An octave is a gap where one note has double the frequency.
- For example, 200 Hz and 400 Hz are an octave apart.
🧵 Threads of Curiosity
- Everyone’s voice sounds unique, so you recognise a friend instantly.
- Why do our voices sound different?
- Male, female and children’s voices differ in frequency .
- They also differ in how the sound is shaped .
- The throat, mouth and nasal cavities do that shaping.
- During adolescence, boys’ vocal cords lengthen and thicken.
- They then vibrate less frequently, ‘deepening’ the voice.
🧵 Threads of Curiosity
- Indian drums like the tabla and mridangam have a black patch.
- It sits at the centre of the drum head membrane.
- This patch is called the syaahi .
- It alters the vibration of the membrane.
- That lets the drum produce a rich variety of sounds.
- The syaahi gives tonal control rarely found in other drums.
📚 Ready to Go Beyond
- We often do not realise how amazing our sense of hearing is.
- Sound entering the ear makes a thin membrane vibrate.
- That membrane is the eardrum (Fig. 10.24).
- Tiny bones quickly amplify these vibrations.
- The cochlea converts them into electrical signals.
- These signals rush to the brain, which perceives them as sound.
- Having two ears lets the brain pinpoint the direction of a sound.
- It compares which ear hears the sound first.
- That time gap is often under a thousandth of a second.
- Snakes and fish sense vibrations through their bodies.
- Some insects have ear-like organs on their body parts.
✅ Test Yourself
-
What is wavelength, and what is its SI unit?
Show Answer
The distance between two consecutive crests (or troughs); the metre (m) . -
What is the SI unit of frequency?
Show Answer
Hertz (Hz) — one oscillation per second. -
Write the relation between frequency and time period.
Show Answer
ν= 1 T — they are inversely related. -
Write the relation between speed, wavelength and frequency.
Show Answer
v=λν — speed = wavelength × frequency. -
What is the human audible range?
Show Answer
20 Hz to 20,000 Hz (20 kHz).
Important Definitions
- Pitch — the way the human ear perceives the frequency of a sound.
- Loudness — the way the ear perceives the amplitude of a sound, measured in decibels.
- Audible range — the frequencies a human ear can hear, about 20 Hz to 20 kHz.
- Ultrasonic and infrasonic waves — sound above 20 kHz and below 20 Hz, outside the human audible range.
