Sound

Physics

Sound Source

• All sounds are produced by the vibrations of material objects

• The frequency of sounds is often described by the word pitch.

• The frequency range that humans can hear is limited: from about 20 to 20000 Hz– Below 20 Hz is called infrasonic– Above 20000 Hz is called ultrasonic

Loudness & Intensity• Loudness is subjective

and is related to sound intensity

• Loudness almost follows a log (10^x) pattern of intensity

• Intensity is objective and can be measured (in dB) or calc.

• Intensity = P/(4Πr2)• Units – W/m2

Sample Problem 12A

• What is the intensity of the sound waves produced by a trumpet at a distance of 3.2 m when the power output of the trumpet is 0.20 W? Assume that the sound waves are spherical.

Relative Intensity• Threshold of

Hearing• Whisper• Normal

Conversation• Lawn Mower• Car Horn at 1m• Threshold of Pain• Nearby Jet

• 0 dB 1x10-12 W/m2

• 30 1x10-9

• 50 1x10-7

• 90 1x10-3

• 110 1x10-1

• 120 1x100

• 150 1x103

Frequency• Every object has a

natural frequency• Depends on the

shape, elasticity and other factors of the object

• This frequency is the frequency at which it takes the least amount of energy to continue the vibration

• If an object is forced to vibrate at its natural frequency an increase in amplitude can occur.

• This is called resonance

• Ex. Swings, Tacoma – Narrows Bridge

Sound in Air

• Sound travels through air in longitudinal waves.

• It travels by causing molecules in any object to push close together and pull further apart, or vibrate– Compression – high pressure –

molecules move together– Rarefaction – low pressure –

molecules move apart

Sound Through Different Media

• Sound can travel through solids, liquids, and gasses.

• Since they travel by vibrating molecules, the closer the molecules are, the faster the sounds will travel.

• In general, sounds travel fastest through solids and slowest through gasses.

Speed of Sound

• You hear thunder after you see lightning This is because sound is slower than light.

• The speed of sound in air at 0C is about 330 m/s or 1200 km/h

• The speed we will use is its speed at 20C, 340 m/s

• Why does it move faster through warmer air?

Interference

• Like any waves sound waves can be made to interfere

• Constructively the amplitude, or loudness, of a sound is increased.

• Destructively the amplitude, or loudness is decreased and can even be cancelled out

• This is used in anti-noise technology

Standing Waves on a vibrating string• To create a standing

wave on a string, the string must be a length that is a multiple of ½ the wavelength

• Reorganizing the wave speed eqn v=λf

• Fundamental frequency: f = v/λ = v/2L

• This is the frequency the string will vibrate at.

• The other possible standing waves are called harmonics and are multiples of the fundamental frequency

• Harmonic Series

– fn= n(v/2L)

Standing Waves on an Air column

• Standing waves can also be created in an air column– Instruments: Sax, trumpet, organ,

trombone• The harmonics depends on

whether the pipe is open at both ends or closed at both ends

Harmonics in a pipe

• Open Pipe

– fn= n(v/2L)

• Closed Pipe

– fn= n(v/4L)

Sample Problem 13B

• What are the first three harmonics in a 2.45 m long pipe that is open at both ends? What are the first three harmonics of this pipe when one end of the pipe is closed? Assume that the speed of sound in air is 345 m/s for both of the situations.

Beats• Two sounds of different

frequencies can create beats.

• A beat is a periodic vibration in the loudness of a sound.

• When the frequencies are different, areas of alternating constructive and destructive interference are created– The ear hears this as

beats.

Doppler Effect• Change in frequency

created by objects in motion

• When an object moves towards you, the waves get closer together – frequency gets higher

• When an object moves away, the waves spread apart – frequency gets lower

Effects

Sound – Objects moving towards you have a higher pitch, objects moving away have a lower pitchLight – Objects moving towards you have a blue shift, moving away creates a red shift (explanation when we talk about light) TAYLOR IS THE BEST

Bow Wave• When the speed of

a source is equal to the speed of the waves, the waves build up

• When this happens with sound, we create the “sound barrier”

Shock Wave

• When a source travels faster than the wave, it produces a shock wave. – Boat through

water

• For sound, a sonic boom is created.