Sound

Chapter 11 of the Class 9 NCERT Science textbook follows vibrations from source to ear. Learners see that sound needs a material medium, travels as longitudinal waves of compression and rarefaction, and carries pitch in frequency and loudness in amplitude. Speed across solids, liquids and gases is compared, and reflection explains echoes, reverberation and sounding boards. Audible range, infrasound, ultrasound, SONAR and medical imaging reveal uses from navigation to diagnosis, while noise control protects hearing.

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