Chapter 11: Sound

An echo is heard when the time interval between the reflected sound and the original sound is at least 0.1 seconds. As the temperature increases, the speed of sound in a medium also increases. On a hotter day, the time interval between the reflected and original sound will decrease, and an echo is audible only if the time interval between the reflected sound and the original sound is greater than 0.1 s.

(i) Reflection of sound is used to measure the speed and distance of underwater objects. This method is called SONAR.

(ii) Working of a stethoscope – The sound of a patient’s heartbeat reaches the doctor’s ear through multiple reflections of sound.

Height (s) of tower = 500 m

Velocity (v) of sound = 340 m s⁻¹

Acceleration (g) due to gravity = 10 m s⁻¹

Initial velocity (u) of the stone = 0

Time (t₁) taken by the stone to fall to the tower base:

As per the second equation of motion,

s= ut₁ + (½) g (t₁)²

500 = 0 x t₁ + (½) 10 (t₁)²

(t₁)² = 100

t₁ = 10 s

Time (t₂) taken by sound to reach the top from the tower base = 500/340 = 1.47 s

t = t₁ + t₂

t = 10 + 1.47

t = 11.47 s

Speed (v) of sound = 339 m s⁻¹

Wavelength (λ) of sound = 1.5 cm = 0.015 m

Speed of sound = Wavelength × Frequency

v = v = λ X v

v = v / λ = 339 / 0.015 = 22600 Hz

The frequency of audible sound for human beings lies between the ranges of 20 Hz to 20,000 Hz. The frequency of the given sound is more than 20,000 Hz; therefore, it is not audible.

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The continuous multiple reflections of sound in a big enclosed space are reverberation. It can be reduced by covering walls and ceilings of enclosed spaces with the help of sound-absorbing materials, such as loose woollens and fibre boards.

Loud sounds have high energy. Loudness directly depends on the amplitude of vibrations. It is proportional to the square of the amplitude of vibrations of sound.

Bats have the ability to produce high-pitched ultrasonic squeaks. These squeaks get reflected by objects, like prey, and return to their ears. This helps a bat to know how far its prey is.

Solution:

Objects that need to be cleansed are put in a cleaning solution, and ultrasonic sound waves are passed through the solution. The high frequency of ultrasound waves helps in detaching the dirt from the objects. In this way, ultrasound is used for cleaning purposes.

SONAR is an abbreviation for Sound Navigation and Ranging. It is an acoustic device used in measuring the direction, speed, and depth of underwater objects, such as shipwrecks and submarines, using ultrasound.

Also, it is used to determine the depth of oceans and seas.

A beam of ultrasonic sound is produced and travels through the seawater, which is transmitted by the transducer. When it reflects, an echo is produced, which is detected and recorded by the detector. It is then converted into electrical signals. The distance represented by ‘d’ of the under-water object is calculated from the time (represented as ‘t’) taken by the echo to return with speed (represented as ‘v’) is expressed as,

2d = v × t

This method of measuring distance is also referred to as echo-ranging.

Time (t) taken to hear the echo = 5 s

Distance (d) of an object from submarine = 3625 m

Total distance travelled by SONAR during reception and transmission in water = 2d

Velocity (v) of sound in water = 2d/t = (2 × 3625) / 5

= 1450 ms^-1