SOUND WAVE PROPERTIES

Sound

•Sound is a longitudinal (Mechanical)wave caused by a vibrating object

•Molecules collide, producing sound

•Examples: Vocal chords, guitar or piano strings, tuning fork, etc.

Longitudinal Wave• Referred to as a PRESSURE WAVE• A sound wave has high pressure and low pressure

regions moving through a medium• The high pressure regions are called compressions,

molecules are compressed• The low pressure regions are called rarefactions,

molecules are spread out

Frequency

• The frequency of a sound wave (or any wave) is the number of complete vibrations per second.

• The frequency of sound determines its pitch

• The higher the frequency, the higher the pitch

• The lower the frequency, the lower the pitch

Wavelength

• Wavelength is the distance between two high pressures or two low pressures

• Wavelength and frequency are inversely related

• A short wavelength (high frequency) results in a high pitch

• http://phet.colorado.edu/en/simulation/sound

Frequency and the human ear• Humans can hear a range of frequencies from 20 Hz to

20,000 Hz• The older you get, the hearing range shrinks• Sound waves with frequencies below 20 Hz are called

infrasonic• Sound waves with frequencies above 20,000 Hz are

called ultrasonic

Amplitude• The AMPLITUDE of a sound wave determines it’s

loudness or softness• This means the more energy in a sound wave, the louder

the sound• Sound intensity is a measure of how much energy passes

a given point in a time period• Intensity is measured in decibels

Sound Behaviors: Reflection• Reflection of sound results in an echo• Sound waves leave a source, travel a distance, and

bounce back to the origin• Animals, like bats, uses echoes to locate prey• Other uses include determining distance between objects,

echocardiograms• The distance the sound travels to get back to the origin is

2x the distance between the sound source and boundary

Sound Behavior: Refraction• Refraction occurs when sound moves from one medium

to another• The wave bends, and the speed changes

• Even when sound moves from warmer areas to cooler areas, refraction occurs

Sound Behavior: Diffraction• Diffraction occurs when sound waves pass through an

opening or through a barrier• Low pitched sound waves travel farther than high pitched

sound waves• Animals use diffraction for communication

Velocity• Velocity of sound depends on the medium it travels

through and the phase of the medium• Sound travels faster in liquids than in air (4 times faster in

water than air)• Sound travels faster in solids than in liquids (11 times

faster in iron than in air)• Sound does not travel through a vacuum (there is no air

so sound has no medium)

Velocity and Temperature• In air at room temperature, sound travels at 343 m/s (at

20°C). This is about 766 mph.• As temperature increases, the velocity of sound increases

v= velocity of sound in air

T=temperature of air in °C

v=331 + (0.6)T

Wave Equation• The basic wave equation is also applied to sound

• V= velocity, measured in m/s• λ= wavelength, measured in meters• f= frequency, measured in hertz

Bellwork

1. What type of wave is a sound wave?

2. What is a compression? Rarefaction?

3. Based on yesterday’s class, state the relationship between wavelength and the pitch of a sound wave.

4. Explain in one sentence how blowing across a straw produces a sound.

5. How can you change the loudness of a sound you produce?

Example Problems:

1. Sound waves travel at approximately 340 m/s. What is the wavelength of a sound wave with a frequency of 20 Hz?

2. What is the speed of sound traveling in air at 20º C?

3. If the above sound wave has a frequency of 261.6 Hz, what is the wavelength of the wave?

What is the Doppler Effect?• http://molebash.com/doppler/home.htm

Doppler Effect• Sound waves move out in all directions

Definition

•The Doppler effect is a change in the apparent frequency due to the motion of the source or the receiver

•Example: As an ambulance with sirens approaches, the pitch seems high. As the ambulance moves by the pitch lowers.

Doppler Effect

• As the wave travels outward, the front of the wave bunches up, producing a shorter wavelength

• We hear a higher

frequency

•The back of the wave spreads out, producing a longer wavelength

•We hear a lower frequency•http://www.sounddogs.com/searchresults.asp?Keyword=Doppler

•Observer A hears a low pitch (lower frequency)•Observer B hears the correct pitch (no change in frequency)•Observer C hears a high pitch (high frequency)

When the source goes faster, the wave fronts in the front of the source start to bunch up closer and closer together, until...

The object actually starts to go faster than the speed of sound. A sonic boom is then created.

Uses of the Doppler Effect• Police use Doppler to measure your speed with radar

• A frequency is sent out with a radar gun• The sound wave hits your car and bounces back to the police car• Speed can be determined based on the frequency changes

received

• Radar can be used to determine the speed of baseballs• Astronomers can determine the distance to other galaxies• Bats use Doppler to locate prey

• If the bat is catching the prey, the frequency is high• If the prey is moving away from the bat, the frequency is low

Doppler Equation

• = frequency detected by observer• = frequency produced by the source• = velocity of the sound wave• = velocity of the detector (observer)• = velocity of the source

Things to remember

• The velocity detected by the observer (vd) is negative if the observer moves away from the source

• The velocity detected by the observer (vd) is positive is the observer moves toward the source

• The velocity of the source () is negative if the source moves toward the observer

• The velocity of the source () is positive if the source moves away from the observer

Example

A trumpet player plays a C note of 524 Hz while traveling in a convertible at 24.6 m/s. If the car is coming toward you, what frequency should you hear? Assume the temperature is 20°C.

Homework

p.405 # 1-5 (use table 15-1 for the speed of sound in various media)

p. 409 #6-8 Read Carefully!

Hearing Range Frequencies• http://www.movingsoundtech.com/• http://www.noiseaddicts.com/2009/03/can-you-hear-this-h

earing-test/