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8/14/2019 Section 13.3 Audible Sound and Ultrasound
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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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13.3 Audible sound and ultrasound(SB p. 197)
Intensity
Energy carried by sound
intensity
Go to
Activity 4Activity 4
TimearealarPerpendicu
wavesoundofEnergy
Unit: J s1m2 or W m2
Amplitude
louder
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13.3 Audible sound and ultrasound(SB p. 198)
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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13.3 Audible sound and ultrasound(SB p. 199)
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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13.3 Audible sound and ultrasound(SB p. 199)
Sound intensity level
Intensities & the corresponding intensity levels
Go to
Activity 5Activity 5
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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
13.3 Audible sound and ultrasound(SB p. 200)
(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
13.3 Audible sound and ultrasound(SB p. 201)
=
0
log10I
Ih
1210log10
I
2101
4
5 105 W m2
12
5
101
105log10
76.99 dB
Thinking 3Thinking 3
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Ultrasound
13.3 Audible sound and ultrasound(SB p. 202)
frequency: above 20 kHz
ultrasound / ultrasonic
out of the audible range ofhuman
transmitter
ultrasonic
waves
waves
reflected
from bottom
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Ultrasound
13.3 Audible sound and ultrasound(SB p. 202)
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/8/14/2019 Section 13.3 Audible Sound and Ultrasound
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Activity 3Activity 3
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.
13.3 Audible sound and ultrasound(SB p. 195)
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)
Determination of human audible rangeDetermination of human audible range
About 20 Hz
Ans
wer
13.3 Audible sound and ultrasound(SB p. 195)
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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)?
Activity 3 (Cont)Activity 3 (Cont)
Determination of human audible rangeDetermination of human audible range
About 20 kHz
Ans
wer
13.3 Audible sound and ultrasound(SB p. 195)
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?
Activity 3 (Cont)Activity 3 (Cont)
Determination of human audible rangeDetermination of human audible range
Return to
TextText
13.3 Audible sound and ultrasound(SB p. 195)
AnswerNo, my classmates have different audible
ranges.
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Activity 4Activity 4
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.
2. Set the frequency of the signal generator to
1 kHz and turn it to a moderate volume.
13.3 Audible sound and ultrasound(SB p. 196)
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?
Activity 4 (Cont)Activity 4 (Cont)
Loudness and intensity of soundLoudness and intensity of sound
The sound becomes less loud.
Ans
wer
13.3 Audible sound and ultrasound(SB p. 196)
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?
Activity 4 (Cont)Activity 4 (Cont)
Loudness and intensity of soundLoudness and intensity of sound
No.
Ans
wer
13.3 Audible sound and ultrasound(SB p. 196)
CRO
loudspeaker
signal
generator
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(B)Intensity of sound1. Connect a loudspeaker to a signal generator.
2. Set the frequency of the signal generator to
2 kHz.
3. Turn the signal generator to a moderatevolume and observe the loudness of the
sound.
Activity 4 (Cont)Activity 4 (Cont)
Loudness and intensity of soundLoudness and intensity of sound
13.3 Audible sound and ultrasound(SB p. 197)
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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?
Activity 4 (Cont)Activity 4 (Cont)
Loudness and intensity of soundLoudness and intensity of sound
Return to
TextText
13.3 Audible sound and ultrasound(SB p. 197)
Ans
werNo.
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Thinking 2Thinking 2
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
13.3 Audible sound and ultrasound(SB p. 199)
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Activity 5Activity 5
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.
13.3 Audible sound and ultrasound(SB p. 200)
Return to
TextText
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Thinking 3Thinking 3
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.
13.3 Audible sound and ultrasound(SB p. 201)
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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?
13.3 Audible sound and ultrasound(SB p. 201)
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
13.3 Audible sound and ultrasound(SB p. 201)
=12
101
log1065I
=
12101log1070
I
12
6
101
1016.13log10