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W. Sautter 2007. These are also called Compressional Waves.

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Presentation on theme: "W. Sautter 2007. These are also called Compressional Waves."— Presentation transcript:

1 W. Sautter 2007

2 These are also called Compressional Waves

3 Crest Trough CompressionRarefactionCompression Rarefaction Trough Crest Rarefaction = low PressureCompression = high Pressure

4 Wavelength Frequency Velocity Wavelength Frequency Velocity v x =

5 Wave A Wave B Constructive interference Destructive interference Partially Constructive interference

6 Intensity = Power / Area Sound Source Sound radiates out from a source as concentric spheres and follows an Inverse Square function

7 Inverse Square means as distance from the source doubles, the intensity 1/4 the original. If distance triples, the intensity is 1/9 the original and so on. The surface area of a sphere is given by 4 r 2 Power is measured in watts ( 1 joule / second) Intensity = Power / Area = watts/ 4 r 2 Or Watts / meter 2

8 dB = 10 log ( I / I 0 ) I = the intensity of the sound to be evaluated I 0 = intensity of lowest sound that can be heard (1 x watts / meter 2 )

9 SINCE LOGS ARE POWERS OF 10 THEY ARE USED JUST LIKE THE POWERS OF 10 ASSOCIATED WITH SCIENTIFIC NUMBERS. WHEN LOG VALUES ARE ADDED, THE NUMBERS THEY REPRESENT ARE MULTIPLIED. WHEN LOG VALUES ARE SUBTRACTED, THE NUMBERS THEY REPRESENT ARE DIVIDED WHEN LOGS ARE MULTIPLIED, THE NUMBERS THEY REPRESENT ARE RAISED TO POWERS WHEN LOGS ARE DIVIDED, THE ROOTS OF NUMBERS THEY REPRESENT ARE TAKEN. Decibels are logarithmic functions

10 A LOGARITHM (LOG) IS A POWER OF 10. IF A NUMBER IS WRITTEN AS 10 X THEN ITS LOG IS X. FOR EXAMPLE 100 COULD BE WRITTEN AS 10 2 THEREFORE THE LOG OF 100 IS 2. IN PHYSICS CALCULATIONS OFTEN SMALL NUMBERS ARE USED LIKE.0001 OR THE LOG OF.0001 IS THEREFORE –4. FOR NUMBERS THAT ARE NOT NICE EVEN POWERS OF 10 A CALCULATOR IS USED TO FIND THE LOG VALUE. FOR EXAMPLE THE LOG OF IS –2.46 AS DETERMINED BY THE CALCULATOR. Decibels are logarithmic functions

11 Whisper 20 decibels Plane 120 decibels Conversation 60 decibels Siren 100 decibels

12 The frequency of a string depends on the Tension (N) and string Linear Density in kilograms per meter (Kg/m). Light strings under high tension yield high frequencies. Heavy strings under low tension yield low frequencies.

13 V (air) = 341 m/s at 20 o C If observer is moving towards the source, V (observer) = + If observer is moving towards the source, V (observer) = - If source is moving towards the observer, V (source) = - If source is moving towards the observer, V (source) = +

14 Slower at low temp Faster at high temp

15 0C0C

16 Moving Toward source Moving Toward observer Observed Frequency Is higher

17 Moving Away from observer Moving Away from source Observed Frequency Is lower

18 Moving Away from observer Observer At rest Observed Frequency Is lower

19 Moving Toward observer Observer At rest Observed Frequency Is higher

20 1 / / 2 Fundamental = 2 L Second Harmonic = L Third Harmonic = 2/3 L

21 fundamental d = diameter of tube L = length of tube at first resonant point If d is small compared to L (which is often true) then:

22 Since V = f If velocity is constant then as decreases, f increases In the same ratio Second Harmonic = L Fundamental = 2 L Third Harmonic = 2/3 L Third Harmonic =3 f fund Fundamental f = f fund Second Harmonic f = 2 f fund

23 1 / 4 3 / 4 5 / 4 Fundamental = 4 L Second Harmonic = 4/3 L Third Harmonic = 4/5 L

24 fundamental d = diameter of tube L = length of tube at first resonant point If d is small compared to L (which is often true) then:

25 Since V = f If velocity is constant then as decreases, f increases In the same ratio Second Harmonic = 4/3 L Fundamental = 4 L Third Harmonic = 4/5 L Third Harmonic = 5 f fund Fundamental f = f fund Second Harmonic f = 3 f fund

26 Fundamental = 2 L Second Harmonic = L Third Harmonic = 2 / 3 L Fourth Harmonic = ½ L Node VIBRATIONAL MODES

27 Since V = f If velocity is constant then as decreases, f increases In the same ratio Second Harmonic = L Fundamental = 2 L Third Harmonic = 2/3 L Third Harmonic = 3 f fund Fundamental f = f fund Second Harmonic f = 2 f fund

28 Waves from a Distant source = crest = trough Barrier with Two slits In phase waves Emerge from slits Constructive interference Destructive interference

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