Class 9 Ch 11 Sound

 Class 9 Ch 11 Sound                                            

**Q: What is sound and how is it produced?**

A: Sound is a form of energy that produces a sensation of hearing, and it is produced by vibrating objects.

**Q: What is a medium in the context of sound?**

A: A medium is the matter or substance—whether solid, liquid, or gas—through which sound is transmitted from the point of generation to the listener.

**Q: What type of waves are sound waves?**

A: Sound waves are mechanical, longitudinal waves because they require a medium to travel and the particles of the medium oscillate back and forth parallel to the direction of wave propagation.

**Q: What are compressions and rarefactions?**

A: When a vibrating object moves forward, it pushes air to create a high-pressure region called a **compression (C)**, and when it moves backwards, it creates a low-pressure region called a **rarefaction (R)**.

**Q: What wave property determines the loudness of a sound?**

A: The loudness or softness of a sound is determined basically by its **amplitude**, which depends on the force with which an object is made to vibrate.

**Q: How is the pitch of a sound determined?**

A: Pitch is how the brain interprets the **frequency** of an emitted sound; a faster vibration means a higher frequency and a higher pitch.

**Q: How are the speed, wavelength, and frequency of a sound wave related?**

A: The speed of sound is equal to the product of its wavelength and frequency ($v = λ ν$).

**Q: What is the minimum distance required to hear a distinct echo?**

A: To hear a distinct echo in air at 22 ºC, the minimum distance of the obstacle from the source of sound must be at least **17.2 meters**.

**Q: What is reverberation?**

A: Reverberation is the persistence of sound in a big hall or auditorium caused by repeated reflections from the walls and roof.

**Q: What is the audible hearing range for the average human?**

A: The human audible range extends from about **20 Hz to 20,000 Hz** (20 kHz).

**Q: What are infrasound and ultrasound?**

A: Sounds with frequencies below 20 Hz are called **infrasonic sound** (or infrasound), and frequencies higher than 20 kHz are called **ultrasonic sound** (or ultrasound). 

**Q: What are some medical applications of ultrasound?**

A: Ultrasound is used in echocardiography to form images of the heart, and in ultrasonography scanners to get images of internal organs like the liver, kidneys, and uterus, as well as to examine a foetus during pregnancy.