Sound Waves

Three things to know about sound waves:1)There must be a source for a sound wave, that

source will be a vibrating object.2)The energy transferred from the source is

longitudinal.3)The sound is detected by an ear or an

instrument.

Characteristics of Sound waves• Sound can travel in different materials besides

for air. • In air, the speed of sound is 343 m/s.• Does the temperature of air effect the speed of

sound?• Which type of material do you think will have the

greatest speed of sound?• Why would someone put their ear to the ground

to determine if someone is coming towards them?

More characteristics

• Pitch: high or low sounds (like a flute compared to a tuba).

• The lower the frequency the lower the pitch.• The audible range is between 20Hz and

20,000Hz for healthy hearing.• As a person gets older, the high-frequency

limit lowers to about 10,000Hz.• Frequencies above 20,000 Hz is called

ultrasonic (different from supersonic)

Ultrasonic

• Many animals can hear ultrasonic frequencies.• Dogs can hear up to 50,000 Hz, and bats

100,000 Hz. • Autofocus cameras emit a pulse of ultrasonic

sound that travels to the object being photographed and back to the camera.

• A sensor times the reflected sound to know how far the object is.

Infrasonic

• Sound waves that are below the audible range (20Hz)

• Earthquakes, thunder, volcanoes, and waves produced by vibrating heavy machinery can produce infrasonic waves.

• Infrasonic waves, like ones that can be produced by heavy machinery can harm the human body.

Characteristics of sound

• Loudness: This is the intensity of the sound wave.

• As stated earlier, intensity varies with the inverse square of the distance.

• The human ear can detect sounds with an intensity as low as 10-12 W/m^2 and as loud as 1 W/m^2 (larger will cause pain)

• This is a huge range…

Alexander Graham Bell

• Used a logarithmic scale to measure the intensity of a sound. We call this unit of measurement a Bel (or more commonly a decibel, 10 dB = 1 bel).

Doppler Effect

Doppler effect

• As an object that emits a sound is moving towards an observer, the frequency of the sound appears to increase.

• As an object that emits sound is moving away from an observer, the frequency of the sound decreases.

• The Doppler effect has applications with sound, but will also have applications next week when we discuss light waves in more detail.

Sheldon Cooper Explains the Doppler Effect

• Big Bang Theory Clip

Sonic Booms

• When an object is moving faster than the speed of sound, it is said to have reached supersonic speed.

• An object moving faster than the speed of sound has “outrun” its sound waves.

• http://library.thinkquest.org/19537/• Doppler effect

Thought Experiment

• Can there be a phenomena similar to a “sonic boom” for E&M waves?

• Why or why not?

Interference

• When two waves pass through the same region of space at the same time.

• Principle of Superposition– The region where waves overlap, the resultant is

the algebraic sum of their separate amplitudes.– This could be constructive or destructive

interference.

Interference

Phases and interference for continuous waves

• For constructive interference to occur, waves are said to be “in phase”.

• For destructive interference to occur, waves are said to be “out of phase”

• wave interference simulation

Sound “beats” and interference• When two sounds (or more) of different

frequencies are played at the same time, there is both constructive and destructive interference.

• This causes a “beat”.• http://library.thinkquest.org/19537/java/

Beats.html• http://www.lon-capa.org/~mmp/applist/beats/

b.htmAnd the best for last…http://www.falstad.com/interference/

Standing waves

• If you have a fixed end of a cord and you can vibrate it at a certain frequency so it just looks like it is oscillating up and down without traveling down the cord, this is called a standing wave.

• Places where there is complete destructive interference are called nodes, and places where there is constructive interference are called anti-nodes.

Resonance

• Frequencies at which standing waves are produced are called “natural frequencies” or “resonant frequencies”.

• Resonance occurs because everything in nature has a natural frequency. In vibrating objects, there is only one resonant frequency.

• If this frequency is hit, then it causes the amplitude of the wave to increase… sometimes catastrophically.

• Galloping Gerdie!

Resonance in cords

• Cords are different because they have many natural resonant frequencies. Each of which is a whole-number multiple of the lowest resonant frequency.

Harmonics

• First, notice that the different resonant frequency depends on the length of the cord.

• The lowest frequency, the fundamental frequency, corresponds to one half of a wavelength, L = 1/2λ1.

• When a frequency is an integral multiple of the fundamental frequency, they are called harmonics.

• The fundamental frequency is the first harmonic

• harmonics demonstration