Describes wave motion in general and specifically sound waves **More good stuff available at: www.wsautter.com and http://www.youtube.com/results?search_query=wnsautter&aq=f

1. W. Sautter 2015

2. These are also called
Compressional Waves 2

3. Crest
Trough
Compression Rarefaction Compression CompressionRarefaction Rarefaction
Trough
Crest
Rarefaction = low PressureCompression = high Pressure
3

4. Wavelength
λ
Frequency
ν
Velocity Wavelength
λ
Frequency
ν
Velocity
vx =
4

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

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

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 Π r2
Power is measured in watts ( 1 joule / second)
Intensity = Power / Area = watts/ 4 Π r2
Or
Watts / meter2
7

8. dB = 10 log ( I / I0 )
I = the intensity of the sound to be evaluated
I0 = intensity of lowest sound that can be heard
(1 x 10-12 watts / meter2)
8

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
9

10. • A LOGARITHM (LOG) IS A POWER OF 10. IF A NUMBER
IS WRITTEN AS 10X THEN ITS LOG IS X.
• FOR EXAMPLE 100 COULD BE WRITTEN AS 102
THEREFORE THE LOG OF 100 IS 2.
• IN PHYSICS CALCULATIONS OFTEN SMALL NUMBERS
ARE USED LIKE .0001 OR 10-4. 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 .00345 IS –2.46 AS
DETERMINED BY THE CALCULATOR.
Decibels are logarithmic functions
10

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

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.
12

13. V (air) = 341 m/s at 20 oC
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) = +
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