Chapter 14 Waves and sound Dr. Haykel Abdelhamid Elabidi November/December 2013/Muh 1435

Units of Chapter 14 Types of waves Harmonic wave functions Waves on a string Sound waves Sound intensity Superposition and interference Standing waves

Types of waves A wave is a disturbance that propagates from one place to another. 1.Transvers wave: the displacement of the medium is perpendicular to the direction of motion of the wave. The wave on a string have the shape of sine or cosine; such a waves are called harmonic wave. 2.Longitudinal wave: the displacement of individual particles is parallel to the direction of propagation of the wave. The individual particle in the air oscillate back and forth about a given position. 3.Water waves: are a combination of transverse and longitudinal waves

Types of waves Transvers wave Longitudinal wave

Types of waves The period T is the time for one wavelength to pass a given point. The wavelength λ is the distance over which the wave repeats.

Types of waves Exercise 2: A tennis ball is hit back and forth between two players warming up for a match, If it takes 2.31 s for the ball to go from one player to the other. What are the period and the frequency of the ball’s motion? Solution:

The harmonic wave functions Exercise: Solution:

Waves on a string

When a wave reaches the end of a string, it will be reflecte. If the end is fixed, the reflected wave will be inverted. If the end of the string is free to move transversely, the wave will be reflected without inversion

Sound waves The human ear can hear sounds between about 20 Hz and 20 KHz f<20 Hz: Infrasonic f>20 KHz:Ultrasonic

Sound waves

Sound intensity The loudness of a sound is defined by its intensity. The intensity of a sound is the amount of energy that passes through a given area in a given time:

Sound intensity

The loudness of a sound doubles with each increase in intensity level by 10dB

Superposition and interference The superposition is a combination of a two or more waves to form a result wave Part A: Superposition

Superposition and interference Constructive interference: if two pulses combine to give a large pulse Destructive interference: if two pulses combine to give a smaller pulse Part B: Interference

Superposition and interference Two sources 1 and 2 are emitting sound to a point A: Two waves in phase Two waves have opposite phase

Superposition and interference

Calculate the path length difference

Superposition and interference Solution:

Superposition and interference

Standing waves A standing wave is the wave that is fixed in its location but oscillates with time. These waves are found on strings with both ends fixed, such as in a musical instrument or in vibrating columns of air (like a soda bottles)

Standing waves

The fundamental, or lowest, frequency on a fixed string has a wavelength twice the length of the string.

Standing waves Higher frequencies are called harmonics Points on the string which never move are called nodes (N); those which have the maximum movement are called antinodes (A).

Standing waves

Standing waves can also be excited in columns of air, such as soda bottles (closed at one end). One end (closed) is a node (N), and the other (open) is an antinode (A).

Standing waves Only odd-numbered harmonics appear: 1 st, 3 nd, 5 rd,…. Summary:

Standing waves

If the tube is open at both ends, both ends are antinodes, and the sequence of harmonics is the same as that on a string.

Thank you for your attention See you next time Inchallah