Section 13.3 Audible Sound and Ultrasound

8/14/2019 Section 13.3 Audible Sound and Ultrasound

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Manhattan Press (H.K.) Ltd. 1

Section 13.3Section 13.3

Audible sound and ultrasoundAudible sound and ultrasound

Audible soundAudible sound

IntensityIntensity

Sound intensity levelSound intensity level

UltrasoundUltrasound


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13.3 Audible sound and ultrasound(SB p. 196)

Audible sound

Range of frequency we hear:

20 Hz 20 kHzaudible sound

Go to

Activity 3Activity 3

Cannot hear sound out of :

audible frequency range

Our ears most sensitive to:

500 Hz 5 kHz


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Intensity

Energy carried by sound

intensity

Go to


TimearealarPerpendicu

wavesoundofEnergy

Unit: J s1m2 or W m2

Amplitude

louder


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Intensity

Loudness is a subjective sensation

Min. intensity we can hear

1 1012 W m2

threshold of hearing intensity > 1 W m2

cause pain / deafness


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Sound intensity level

human respond to intensity of sound in log scale

Unit: decibel (dB)

Intensity level

=

0

log10I

Ih

intensity of sound

threshold of hearing

= 1 1012 W m2

Thinking 2Thinking 2


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Sound intensity level

Intensities & the corresponding intensity levels

Go to



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Example 3:Example 3:

(a) Suppose the sound intensity in a room is 1 108

W m2

,what is the corresponding sound intensity level of the

room?

(b) Find the intensity of a sound if it corresponds to 75 dB.

Solut

ion


(a)By

=

0

log10I

Ih

=

12

8

101

101log10h

= 10 log(10 4) = 40 dB

= 12101

log1075I

= 12101log5.7

I

(b)By

=

0

log10IIh

I= 3.16 105 W m2Find I in calculator:

7.5 1 101210x =


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Class Practice 2:Class Practice 2:The sound intensity level of an operating machine is 80 dB.

If it is covered by a shelter, the sound intensity produced is

halved. What is the new sound intensity level?

By

( )= ( )

The intensity without a shelter (I) = ________________

The intensity with a shelter = _________ = _____________

The new sound intensity level = _________________

= _________________

Ans

wer

80

1 104 W m2


=

0

log10I

Ih

1210log10

I

2101

4

5 105 W m2

12

5

101

105log10

76.99 dB



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Ultrasound


frequency: above 20 kHz

ultrasound / ultrasonic

out of the audible range ofhuman

transmitter

ultrasonic

waves

waves

reflected

from bottom


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Ultrasound


use ultrasound for

communication

weak eyesight

emit ultrasound & detect echo

to avoid obstacles & prey


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To section 13.4
http://e-ch13_04.ppt/http://e-ch13_04.ppt/


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Determination of human audible rangeDetermination of human audible range

Lets start:1. Connect a loudspeaker

to a signal generator.

2. Set the frequency of the signal generator to

0 Hz.

3. Ask your classmate to turn on the signalgenerator and then increase the frequency of

the sound slowly.


loudspeaker

signal generator


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4. When you hear the sound from the

loudspeaker, record the value of the

frequency.

5. Increase the frequency of the sound

continuously.

Activity 3 (Cont)Activity 3 (Cont)


About 20 Hz

Ans

wer



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6. When you cannot hear the sound, record thevalue of the frequency again.

7. What is the frequency range of the sound you

can hear (audible frequency range)?



About 20 kHz

Ans

wer


20 Hz to 20 kHzAns

wer


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8. Compare your audible frequency range with

those of your classmates. Are the ranges the

same?



Return to

TextText


AnswerNo, my classmates have different audible

ranges.


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Loudness and intensity of soundLoudness and intensity of sound

Lets start:(A) Loudness of sound

1. Connect a loudspeaker and a CRO to a signal

generator.


1 kHz and turn it to a moderate volume.


CRO

loudspeaker

signal

generator


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3. Decrease the volume of the sound slowly.

What is the change in the loudness of the

sound you hear?

4. What happens to the waveform shown on

the screen of the CRO in step 3?



The sound becomes less loud.

Ans

wer


Ans

werThe amplitude of the waveform

decreases.


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5. Can you hear the sound when the volume isturned to a very low value?



No.

Ans

wer


CRO

loudspeaker

signal

generator


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(B)Intensity of sound1. Connect a loudspeaker to a signal generator.


2 kHz.

3. Turn the signal generator to a moderatevolume and observe the loudness of the

sound.





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4. Now, connect another loudspeaker in parallelto the above loudspeaker, in such a way that

the two loudspeakers emit identical sound

intensity and the total sound intensity isdoubled.

5. Compare the loudness of this sound with the

sound in step 3. Do you feel that the loudnessof the sound is doubled?



Return to

TextText


Ans

werNo.


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Does a sound of intensity level of 0 dB meanthat it is not audible? Ans

wer

A sound of intensity level of 0 dB has an intensityof 1 1012 W m2. It is the value of the threshold

of hearing. Therefore, this sound is still audible.

Return toTextText



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Sound level meterSound level meter

Lets start:1. You are given a sound level meter, which is a

device to measure the intensity level of a

sound.

2. Use the sound level meter to find the loudest

place in your classroom. Record the value of

the intensity level.


Return to

TextText


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The following is a conversation between Louisand Sally.

Sally: Ive just checked the sound intensitylevel of my own voice. According to the

sound level meter, it is about 65 dB.



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Thinking 3(Cont)Thinking 3(Cont)

Louis: Ive also checked mine. It is about 70 dB.Sally: When we both talk at the same time, what

is the total sound intensity level of our

voices?Louis: Its simple. It is just the sum of 65 dB and

70 dB. That is 135 dB.

Sally: Really? 135 dB is a very high soundintensity level!

Is Louis correct? Why?


Ans

wer


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Thinking 3(Cont)Thinking 3(Cont)

Louis is not correct. Since intensity level is the logarithmicscale of intensity, we cannot simply add the two intensity

levels together.

By , the intensity of Sallys voice

= 3.16 106 W m2 . By , the intensity

of Louiss voice = 1 105 W m2 . When they talk at the

same time, the total intensity = 3.16 106 + 1 105

=13.16 106 W m2 .Therefore the total intensity level

= = 71.2 dB. Return to

TextText


=12

101

log1065I

=

12101log1070

I

12

6

101

1016.13log10

Section 13.3 Audible Sound and Ultrasound

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Transcript of Section 13.3 Audible Sound and Ultrasound