Chapter 12 – Waves and Sound NotesSection 12-1A. When an object moves periodically (repeats) back and forth, side to side, up and down, we say it vibrates, oscillates or has harmonic motion.

-When the vibration carries energy from one place to another it is called a wave

Amplitude – The maximum displacement from an objects resting position (equilibrium).

Wavelength – the distance between any 2 identical parts of successive waves.Frequency – the number of vibrations or oscillations an object makes in a given

time (usually 1 second)Units are Hertz – 1 vibration per second = 1HzAM radio waves are measured in kilohertz (kHz), FM

radio measured in Megahertz (MHz)Period of a wave – the time it takes for one complete vibration

Frequency =1/period Period = 1/frequencyAn electric toothbrush completes 90 back and forth sweeps every second.

What is its frequency? its period?

Section 12-2 – Wave SpeedA.When you speak, the air does not move, but the energy of your voices

vibrations are carried at 340 m/s.-Medium does not move – the energy carried is moved.

B. Speed of a wave-Speed = distance/time, so in a wave’s case Speed = wavelength/period

Wave speed = frequency X wavelength

Section 12-3 - Types of WavesA. Transverse Waves – Vibrates perpendicular to the direction the wave is

traveling.B. Longitudinal wave – vibrates in the same direction that the wave is going.

Section 12-4A. Sound waves are produced by vibrations that push air molecules together

(compressions) and move apart (rerefactions) forming a longitudinal wave.

B. Pitch – high pitched noises have sound waves with high frequencies and low pitched sounds have low frequencies.

-Humans can hear from 20 to 20,000 Hz-Frequencies < 20 Hz are called infrasonic and those > 20,000 Hz are called

ultrasonic waves.Demos – loudspeakers, and spoon and string

C. Speed of sound330 m/s in dry air

– faster in moist air, 340 m/s in most airand in warm air, in solids than in liquids and in liquids than air

-In water, sound travels 4X faster than air, steel is is 15 times faster.

Section 12-5 Reflection of SoundWhen sound waves hit a surface, it can be :

-Transmitted (pass through the surface), absorbed by the surface, or reflected by the surface.

The more rigid and smooth the better it reflects, the rougher and more irregular, the less it is reflected.

-The reflection is called an echo.-Angle of incidence = angle of reflection

Section 12-6 Refraction of SoundRefraction is when sound waves bend because parts of the sound waves are

moving at different speeds.-Can be caused by moving from one medium to another.-Or differences in temperature within a medium

Ex. Submarines in thermal gradients.Ultrasound sonography – outside of organs reflect

more waves than interior.-Bats and dolphins.

Section 12-7 – Forced vibration – when an object is caused by another object to vibrate at a

given frequency- Ex. - Machinery, fan, body of a musical instrument

Natural Frequency – Every object made of an elastic material vibrates at its own special frequencies.

Section 12-8 – If forced vibrations from another object are the same as an objects natural

frequency, it causes the object to vibrate at an increasingly greater amplitude. Resonance (resounding)

The second object will vibrate due to sympathetic vibrations-Soldiers marching on a bridge in Manchester, England vid eo

Tacoma Narrows Bridge CollapseGlass breaking -

Section 12-9 InterferenceWhen waves overlap each other interference occurs –

When they are in phase with each other, there is constructive interference – amplitude is greater and sound is louder

When they are out of phase, destructive interference occurs and no sound occurs.

Anti noise technology – noise cancelling headphones

-Beats - when two tones of slightly different frequencies are sounded at the same time there are fluctuations in loudness due to constructive and destructive interference.

-When waves pass through each other and go in and out of phase, they form standing waves with stationary nodes and antinodes. -produces stable regions of constructive and destructive interference-every two nodes is a wavelength

Section 12-10 – The Doppler EffectSounds moving toward you have a higher frequency and higher pitch and

moving away from you have a lower frequency and pitch.

radar, blue-shift and red shiftSection 12-11 & 12Wave barrier – When a wave source moves as fast as the waves it produces a wave barrier as the medium is compressed. When the object moves faster than the waves, the waves overlap with each other causing a (In 2-D)Bow wave

Shock wave the same effect as a bow wave in 3-D, when a supersonic jet forms a cone of overlapping waves. A high pressure cone followed by a low pressure cone. It produces a sharp crack that sound is a Sonic boom