Waves & Sound

Objectives Describe the measurable properties of waves and explain the relationships among them and how these properties change when the wave moves from one medium to another. Qualitatively describe the shift in frequency in sound or electromagnetic waves due to the relative motion of a source or a receiver.

Chapters 25&26 Waves are periodic disturbances that propagate through a medium or space –a medium does not travel with the wave Mechanical waves require a medium Electromagnetic waves do not require a medium

More on waves: Longitudinal waves – particles of the medium move parallel to the direction of the wave Sound is an example of this type of wave.

Transverse Waves Transverse waves – displacement of the particles of the medium are perpendicular to the direction of propagation of the wave.

Phase of a wave

Frequency

Properties of Waves 1. Rectilinear propagation – advancement of a wave is perpendicular to the wave front 2. Reflection – waves bounce off barriers and rebound in opposite direction –Law of reflection: Incident angle = reflected angle (i = r) 3. Refraction – bending (changing direction) of a wave as it travels from one medium into another

More properties… 4. Diffraction – spreading of a wave as it passes beyond the edge of a barrier 5. Interference – result of 2 or more waves passing through the same medium at the same time

Math associated with waves Frequency = 1 / Period (f = 1/T) Period = 1 / Freqency i.e., f = 60cycles per second = 60 hz = 60/s Also:v = f where velocity = frequency x wavelength

Sound Waves

Math re waves I = P/A Intensity = Power/Area Units: watt / cm 2 Intensity relates to loudness You are 1 km away from a radio tower, what is the intensity of the signal of a 100,000 kw radio station? 1 km = 10 5 cm100,000 kw = 100,000,000w =10 8 w I = P/A = 100,000 kw / 4  (10 5 ) 2 = 7.96 x w/cm 2

More math…  = 10 log I I o w/cm 2 , Intensity level I, Intensity I o, threshhold of hearing We all don’t hear the same, so this is a comparative measurement, in decibels

Flow chart for  intensity  problems If I = 4.7 x w/cm 2 : 10xlog(4.7 2 nd EE -10 / 1 2 nd EE -16) = 66.7 dB If I = 2.9 x w/cm 2 : 10xlog(2.9 2 nd EE -3 / 1 2 nd EE -16) = 135 dB

Now we are going backwards from intensity level (dB) to intensity (w/cm 2 ) If the intensity level is 83 dB, convert that to intensity in w/cm 2. B = 10 log I / Io get to a working eqtn: B /10 = log I / Io Log -1 (B/10) = Log -1 (log I/Io) Log- 1 (B/10) = I/Io

More to come… Doppler Effect is the apparent change in frequency as a result of relative motion between the source of a sound and an observer. F o = f s v v o v v s f o, frequency heard by observer f s, frequency of source v, velocity of sound in air v o, velocity of observer v s, velocity of source

More on Doppler Stationary observer: –Observer moving away: v-v s on bottom –Observer moving toward: v+v s on bottom Stationary source: –Observer moving toward: v+v o on top –Observer moving away: v-v o on top