Refraction and reflection of sound

Sound

1. Reflection of sound Hard surfaces reflect sound.

Reflected sound is called Echo.

-Sound travels from the mouth to the wall and back to the ear.-Sound travels twice the distance between the man and the wall.-The time taken for sound to move from the man to the wall and back to the man is called echo time.

d

- Distance travelled by sound = 2d- Time taken for sound to travel to the wall and back = echo timeTherefore;

speed of sound = distance travelled = 2 x distance to walltime taken echo time

Speed of sound in different materialsMedium sound speed (m/s)

air (0C) 330

water 1400

gold 3240

brick 3650

wood 4000

concrete 5000

glass 5100

steel 5790

aluminum 6420

Example:1. A ship is 220m from a large cliff when it sounds its foghorn. The speed of sound

in air = 330m/sa. when the echo is heard on the ship, how far has the sound travelled?

2 x d = 2 x 220= 440m

b. what time delay is there before the echo is heard?echo time = 2 x d = 2 x 220 = 440

speed 330 330 = 1.33 sec

c. the ship changes its distance from the cliff. When the echo time is 0.5sec how far is the ship from the cliff?

2 x d = speed x echo time

2 x d = 330 x 0.5

2 x d = 165m

Therefore d = 1652 d = 82.5m

Exercise

1. When the horn of the ship is sounded, the passengers hear an echo from a cliff after 4.0 s. How far away is the cliff?

2. A boy is stranded on an island. He shouts for help but all he can hear in reply is the echo of his shout from the cliff 500 m away. What is the time interval between the boy shouting and hearing the echo?

Echo sounding The process of using echoes of different sounds to

measure distance and time is called echo sounding.

The principle of echo sounding is used in the following processes:

Echo-sounder: used by boats to measure the depth of water.

Electronic-tape measure: used by surveyors to measure distance between walls.

Radar: microwaves are used to detect the positions of aircrafts using the principle of echo-sounding.

Used by animals to measure distances.

2. Refraction of soundSound waves can:

undergo refraction.

travel from one medium to another. E.g.

A person under water can hear sounds made above the water.

Can bend or change direction

A person behind a hill can hear a gun shot from the other side of the hill.

Quality of sound1. Frequency and Pitch

- The sharpness of sound is called pitch.

- Pitch is related to frequency.

- The higher the frequency, the higher the pitch.

- The human ear can hear sounds of frequencies between 20Hz and 20kHz (20 000Hz)

PITCH FREQUENCY

High Upper limit of hearing 20 000Hz

Whistle 10 000Hz

High note (soprano) 1 000Hz

Low note (bass) 100Hz

Low Drum note 20Hz

exerciseUsing the wave equation, calculate the wavelengths

and complete the table below. Speed of sound in air = 330m/s

PITCH FREQUENCY WAVELENGTH

High Upper limit of hearing 20 000Hz

Whistle 10 000Hz

High note (soprano) 1 000Hz

Low note (bass) 100Hz

Low Drum note 20Hz

2. Amplitude and loudness.

- Loudness is how high or low the sound is to the ear.

- Loudness is related to amplitude.

- The higher the amplitude, the louder the sound.

A B

C D

Which sound or sounds has/have (a) the same pitch as A? (b) the same loudness as A? (c) the same pitch as B?

Ultra sound Sounds of frequencies higher than the highest

hearing limit of human beings (higher than 20kHz) is called ultra sound.

Sounds below 20Hz are called Infrasound.

Uses of Ultra-sound Metal testing: Ultrasound is used to detect leakages in metal

pipes.

Scanning the womb:

Cleaning and breaking: ultrasound is used for cleaning surfaces of delicate machinery. It is also used in the making of fine glass instruments such as pipettes and burettes.

some animals e.g. bats and dolphins, use ultrasound in echo sounding to navigate.

Thanks

.