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Physics 1251 The Science and Technology of Musical SoundPowerPoint Presentation

Physics 1251 The Science and Technology of Musical Sound

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### Physics 1251The Science and Technology of Musical Sound

Unit 2

Session 16 MWF

The Wave Properties: Propagation

Physics 1251 Unit 2 Session 16 Wave Properties: Propagation

Foolscap Quiz:

What is frequency of the pitch that a person will perceive, if he hears the harmonics in the vibration recipe: 350 Hz, 525 Hz, 700 Hz and 875 Hz?

Answer:175 Hz (F3), fundamental of series:

350 Hz (F4), 525 Hz (C5), 700 Hz (F5) and

875 Hz (A5).

We hear the “difference” frequencies:

(525-350) =(700-525) =(875-350) = 175 Hz!

Physics 1251 Unit 2 Session 16 Wave Properties: Propagation

Georg Phillip Telemann

Sonata IV in E minor

Recorded by Ron DiIulio (UNT September 2001)

Physics 1251 Unit 2 Session 16 Wave Properties: Propagation

Georg Phillip Telemann

Sonata IV in E minor

Notice the “difference tones”

Physics 1251 Unit 2 Session 16 Wave Properties: Propagation

1′ Lecture:

- The intensity of sound decreases with the square of the inverse of the distance from the source in an open area.
I / I0 = (r0 /r) 2

Physics 1251 Unit 2 Session 16 Wave Properties: Propagation

1′ Lecture:

- The Sound Intensity Level (SIL) diminishes with distance thus:
SIL= SIL0 – 20 Log (r/ r0 )

Physics 1251 Unit 2 Session 16 Wave Properties: Propagation

Thought question:

What happens to a sound wave as it propagates or travels?

- Wave is absorbed.

- not much: ~ 10 dB/km

- Wave spreads out.

- depends of geometry.

Physics 1251 Unit 2 Session 16 Wave Properties: Propagation

The intensity of a spherical sound wave decreases as the distance from the source increases. [“Inverse”]

d [m] I [W/m2]

1.0 1.0

=12 /12

=12/2 2

=12 /3 2

= 12 /4 2

= 12 /5 2

As 1/r 2

2.0 0.25

3.0 0.11

4.0 0.068

5.0 0.040

Physics 1251 Unit 2 Session 16 Wave Properties: Propagation

Intensity is Power per Unit Area

Why does intensity

diminish as 1/r 2?Area = 2/3π ‧ r 2

I = Power/Area

I = I0 (A0 /A)

I= I0 (r0 / r ) 2

A = ⅔π r 2

r

Physics 1251 Unit 2 Session 16 Wave Properties: Propagation

Demonstration of Inverse Square Law:

I1 I1‧ (r1 /r2) 2 I1 ‧ (r1 /r3)2

Source

r1

r2

r3

The fraction of the sound energy that enters the ear decreases as the ratio of the inverse of the square of the distance because the fractional area decreases as 1/r 2.

Physics 1251 Unit 2 Session 16 Wave Properties: Propagation

80/20Inverse Square Law:

The intensity of sound (originating from a point source in an open environment) diminishes as the square of the inverse ratio of the distances from a source.

I/ I0= (r0 /r) 2

Physics 1251 Unit 2 Session 16 Wave Properties: Propagation

Practice:

The sound intensity is 1.0 x10 -9 W/m2 at a distance of 3.0 m from a speaker. What is the intensity 20 m away?

I2 / I1 = ( r1 /r2 ) 2 ;

I2 / (1 .0 x10-9 W/m2) =( 3.0 m / 20 m) 2

I2 = (1 .0 x10-9 W/m2) (.0225) =2.25 x10 -11 W/m2

Physics 1251 Unit 2 Session 16 Wave Properties: Propagation

80/20Inverse Square Law:

The Sound Intensity Level (SIL) decreases by 20 dB for every 10x increase in distance.

SIL= SIL0 – 20 Log (r/ r0)

10 Log(I/I Propagationthreshold) – 10 Log(I0 /Ithreshold) = 20 Log((r0 /r))

SIL – SIL0 = -20 Log(r/r0 )

Physics 1251 Unit 2 Session 16 Wave Properties: Propagation80/20Inverse Square Law:

If I/ I0= (r0 /r) 2

Then 10 Log (I/ I0 )= 10 Log((r0 /r) 2)

10 Log(I/Ithreshold) – 10 Log(I0 /Ithreshold) = 20 Log((r0 /r))

SIL – SIL0 = -20 Log(r/r0 )

SIL= SIL0 – 20 Log (r/ r0)

Physics 1251 Unit 2 Session 16 Wave Properties: Propagation

Practice:

The sound intensity level of the professor’s lecture is 80 dB on the front row of the classroom, 3 m from the his mouth. What is the SIL at the back row if it is 6 m away and there are no reflections?

SIL = SIL0 – 20 dB Log ( r/r0 )

SIL = 80 dB – 20 dB Log ( 6 m/3 m) = 74 dB

Physics 1251 Unit 2 Session 16 Wave Properties: Propagation

Summary

- The intensity of sound decreases with the square of the inverse of the distance from the source in an open area.
I / I0 = ( r /r0 ) 2

- The Sound Intensity Level diminishes with distance thus
SIL= SIL0 – 20 Log (r/ r0)

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