What is sound?Sound is a longitudinal wave produced by a

vibrating sourceExamples of sources: tuning fork, vocal

cords, lips or reed on a musical instrumentThe vibration produces COMPRESSIONS and

RAREFACTIONS in the airMust have a medium to travel through, will

not travel through a vacuum http://dev.physicslab.org/Document.aspx?doctype=3&filename=WavesSound_IntroductionWaves.xml

Sound waves propagate in three dimensions

Compressions and Rarefactions produce wave fronts that become a spherical wave

• Energy radiates outward from sound source in a direction perpendicular to the wave front.

What determines pitch?Pitch is the audible sound our ears

perceive, and it depends upon FREQUENCYHigher frequency = higher pitch If sound source is moving, the perceived

frequency of the hearer will change, so the pitch will change

This is called the DOPPLER EFFECT

Doppler Effect Misconceptionthe frequency does not change as the

moving source approaches or recedesthe frequency only changes as the

moving source passes you ( the perceived frequency)

ResonanceResonance occurs when the frequency of a

force applied to a system matches the natural frequency of the system

Result: large amplitudes of vibrationExample: breaking glass with your voicehttp://www.youtube.com/watch?v=ofESdVdX-fY

OR a bridge collapsing

Beats

Beats

Beats

BeatsThe beat frequency (how often the

beat is heard) is determined by the difference in the source frequencies

Example: A tuning fork at 440 Hz and 448 Hz are struck at the same time. What is the beat frequency produced by the two sound waves?

As the sound source frequencies are farther and farther apart….

Beat frequency is created by superposition/interference- there is constructive & destructive interference that creates the beat

What determines volume?Energy of a wave is determined by

amplitudeVolume of sound is also determined by

amplitudeHow do we measure volume?On the Decibel (dB) scale

Measure of sound intensity

rate at which energy flows through a unit area perpendicular to the direction of wave motion

intensity = ΔE / (Δt • area) = P / areaSince the area of a sphere is equal to

4πr2 , Intensity = P / 4πr2 (units: W/m2)

E: energy (J)t: time (s)P: power (W)r: radius from source (m)

Intensity

Decibel (dB)describes the ratio of two intensities of

sound; the threshold of hearing is commonly used as the reference intensity

Ratio = Intensity of source/threshold of hearing

threshold of hearing = 1.0 x 10-12 W/m2

logarithmic scaleMeans that an increase of 10 dB is power

of ten increase (ten fold increase) in intensity

Intensity (W/m2)

Decibel level (dB)

Examples

1.0 x 10-12 0 threshold of hearing

1.0 x 10-10 20 quiet whisper

1.0 x 10-8 40 mosquito buzzing

1.0 x 10-6 60air conditioning at

6m

1.0 x 10-4 80busy traffic, alarm

clock

1.0 x 10-2 100subway, power

motor

1.0 x 100 120 threshold of pain

1.0 x 103 150 nearby jet airplane

Human hearing

Audiogram