Sound - PowerPoint Presentation

Slide1

SoundSlide2

Speed of a Wave on a String

What affects the speed of a

wave on a string?

The properties of the medium through which the wave travels determine the speed of the wave.

If the medium changes, the speed of the wave changes.Slide3

Speed of a Wave on a String

As the wave move to the right, each particle (red dot) is displaced, one after the other, from its equilibrium position.

Particle 1 pulls particle 2 upwards.

Particle 2 pulls particle 3 upwards, etc.Slide4

Speed of a Wave on a String

The speed of the wave depends on how quickly one particle of the string is accelerated by the force exerted on it by a neighboring particle.

According to Newton’s 2

nd

Law, it would make sense that a stronger net F would give a larger acceleration, and thus a faster moving wave.Slide5

Speed of a Wave on a String

The greater the

Tension

in the string, the greater the pulling force the particles exert on each other.

A smaller mass will accelerate more quickly (less inertia), so a wave will travel faster on a string whose particles have a small mass.Slide6

Speed of a Wave on a String

Where

F

is the

tension

in the string

m/L

is the

linear density

(mass per unit length) of the string

 Slide7

Example

Transverse waves travel on each string of an electric guitar after the string is plucked. The length of each string is 0.628m, and the mass is 0.208g for the highest pitched E string and 3.32 g for the lowest pitched E. Each string is under a tension of 226N. Find the speed of the waves on the two strings.

826 m/s and 207 m/sSlide8

Example

One end of each of two identical strings is attached to a wall. Each string is being pulled equally tight by someone at the other end. A transverse pulse is sent traveling along string A. A bit later an identical pulse is sent traveling along string B. What, if anything, can be done to make the pulse on string B catch up with and pass the pulse on string A?

826 m/s and 207 m/sSlide9

The Nature Of Sound

Sound is a longitudinal wave that is created by a vibrating object (ex. Guitar string, vocal cords, diaphragm of a loudspeaker)

Sound can be created or transmitted only in a medium – a solid, liquid, or gas

Demo – bell jarSlide10

The Nature Of Sound

When the diaphragm of a loudspeaker moves outward, it compresses the air directly in front of it.

It causes the air pressure to rise slightly, and this region is called a

condensationSlide11

The Nature Of Sound

The condensation moves away from the speak at the speed of sound.Slide12

The Nature Of Sound

After producing a condensation, the speaker reverses its motion and moves inward, producing a

rarefaction

The air pressure is slightly less than normalSlide13

The Nature Of Sound

As the wave passes, the air molecules are pushed and pulled about their equilibrium position (simple harmonic motion)

The wavelength of the wave is the distance between 2 successive condensations or rarefactions.Slide14

The Nature Of Sound

A sound wave spreads out in space after it is produced by the speaker

Remember, the air molecules vibrate around their equilibrium position as the wave is passing, colliding with their neighbor passing the rarefactions and condensations forward and return to their equilibrium positionSlide15

The Nature Of Sound

When the condensations and rarefactions arrive at the ear, they force the eardrum to vibrate at the same frequency as the speaker diaphragm

Each cycle of a sound wave includes one condensation and one rarefactionSlide16

The Nature Of Sound

A sound wave traveling in a tube consists of a series of alternating condensations and rarefactions.

The air pressure in the tube varies

sinusoidally

A Ruben’s Tube uses gas and a flame to “show” a pattern of different flame heights which represent the sound being played