Chapter-Sound Waves: Characteristics and Applications Science Exploration class 9 in english Medium CBSE Notes

Chapter 10. Sound Waves: Characteristics and Applications

This section explains the important characteristics of sound waves, including wavelength, frequency, time period, amplitude and speed. These quantities help describe the behaviour of sound and explain properties such as loudness and pitch. 

Characteristics of Sound Waves

Every sound wave possesses certain measurable properties that help describe its behaviour during propagation.

Wavelength (λ)

Wavelength is the distance travelled by a sound wave during one complete vibration.

Definition

The distance between the centres of two consecutive compressions or two consecutive rarefactions is called the wavelength (λ).

• Symbol – λ (Lambda)
• SI Unit – metre (m)
• Measurement – Distance between two successive compressions or rarefactions.

Frequency (ν)

Frequency tells us how many complete vibrations are produced in one second.

Definition

The number of vibrations or oscillations completed in one second is called the frequency.

• Symbol – ν (Nu) or f
• SI Unit – hertz (Hz)
• Higher Frequency – Produces a higher-pitched sound.
• Lower Frequency – Produces a lower-pitched sound.

Time Period (T)

The time period represents the time required to complete one vibration.

Definition

The time period (T) is the time taken by a vibrating particle to complete one oscillation.

• SI Unit – second (s)
• Relation – Frequency and time period are inversely proportional.

Formula: T = 1 / ν

Amplitude (A)

Amplitude is the maximum displacement of a vibrating particle from its mean position.

Definition

The maximum displacement of a vibrating particle on either side of its mean position is called the amplitude.

• Greater Amplitude – Produces louder sound.
• Smaller Amplitude – Produces softer sound.

Loudness

Loudness is the sensation produced in the ear due to sound. It mainly depends on the amplitude of vibration.

• Greater amplitude produces greater loudness.
• Measured in decibel (dB).
• Depends on the intensity of sound reaching the ear.

Pitch

Pitch is the characteristic that enables us to distinguish between high and low sounds.

• Depends on the frequency of vibration.
• Higher frequency produces higher pitch.
• Lower frequency produces lower pitch.

Speed of Sound

The speed of sound is the distance travelled by a sound wave in one second.

Formula

Speed = Frequency × Wavelength

v = νλ 

Relationship Among Wave Quantities

Quantity	Symbol	SI Unit
Wavelength	λ	metre (m)
Frequency	ν or f	hertz (Hz)
Time Period	T	second (s)
Amplitude	A	metre (m)
Speed	v	m/s

Solved Example

Question: A sound wave has a frequency of 500 Hz and a wavelength of 0.68 m. Calculate its speed.

Solution:

Given:

• Frequency (ν) = 500 Hz
• Wavelength (λ) = 0.68 m

Using the formula:

v = νλ

v = 500 × 0.68 = 340 m/s

Answer: The speed of sound is 340 m/s.

ATP Education Concept Builder

Remember the three important relationships: Amplitude controls loudness, Frequency controls pitch, and Speed depends on both frequency and wavelength. Students often confuse loudness with pitch, but they depend on different wave characteristics.

ATP Education Exam Booster

• Learn the definitions of wavelength, frequency, time period and amplitude.
• Memorise the formulas T = 1/ν and v = νλ.
• Remember that loudness depends on amplitude, while pitch depends on frequency.
• Practise numerical problems based on the speed of sound.
• CBSE competency-based questions frequently ask students to identify which wave characteristic changes the loudness or pitch of sound.

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