Transcript of INTRODUCTION TO SOUND Longitudinal Waves. S OUND W AVES A sound wave is a travelling disturbance of...
- Slide 1
- INTRODUCTION TO SOUND Longitudinal Waves
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- S OUND W AVES A sound wave is a travelling disturbance of
compressions regions in which air pressure rises followed by
rarefactions regions where air pressure drops compared to quiet or
still air.
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- S OUND W AVES Sound is an example of a longitudinal wave the
wave moves in a direction parallel to the direction in which the
medium moves. The wavelength of a sound wave is the distance
between successive compressions or rarefactions. The amplitude of
the sound wave is measured by how much the medium moves from its
equilibrium state..
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- S OUND S PECTRUM Sounds come to us in an entire spectrum of
frequencies. Conversation with your friends is in the range of 80
Hz to about 300 Hz. A opera singer can reach frequencies as high as
1100 Hz Musical instruments extend this range from as low as 20 Hz
to as high as 5000 Hz Bird songs can reach higher than 15 000 Hz.
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- S OUND S PECTRUM In general, your ears are able to respond to
sounds in the range of 20 Hz to 20 000 Hz, But they are most
sensitive to sounds in the range of about 2000 Hz to 5000 Hz.
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- S OUND S PECTRUM 1. Infrasonic sounds have frequencies of less
than 20 Hz. Rather than being able to hear sounds in this range,
you may feel them as a rumble that passes through your body. 2.
Audible sounds are in the range of 20 Hz to 20 kHz 3. Ultrasonic
sounds have frequencies greater than 20 kHz
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- S PEED OF S OUND The speed of sound through a medium depends on
the physical characteristics of the medium. If you compare sound
waves in a solid medium (such as steel) with those in a gaseous
medium (air), you will find that they have very different speeds.
The speed of sound in steel is almost 5800 m/s, whereas in air it
is only about 350 m/s.
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- S PEED OF S OUND There are 3 major factors that effect the
speed of sound through a medium: 1. The distance between particles
in the medium 2. The stiffness of the particles Stiffness is a
measure of how much force is required to create a compression in a
given substance 3. Temperature
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- S PEED OF S OUND
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- If an object is 6.56 m from the camera and sound travels at 344
m/s, determine the length of time it takes the emitted sound pulse
to return to the camera.
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- S PEED OF S OUND
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- What is the wavelength of a sound of frequency 225 Hz that is
produced in air at a temperature of 20.0C?
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- I NTENSITY Our ears are marvellous organs. They can respond to
the faintest of whispers or the roar of a jet engine. The ability
to distinguish variations in loudness is an important environmental
cue that humans and other animals use to navigate in their
surroundings.
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- I NTENSITY The intensity of a sound is the energy per unit area
that passes a point each second. It has units of (J/m 2 )/s or J/sm
2, which is the same as W/m 2. Our ears can respond to sounds as
faint as one-trillionth of a watt per square metre
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- D ECIBEL S CALE
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- The Greek letter beta, , is commonly used to represent sound
intensity expressed in dB. The faintest sound that humans can hear
represents the start of the decibel scale and is given a value of 0
dB. The decibel scale is a logarithmic scale that corresponds to
how our ears perceive loudness.
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- D ECIBEL S CALE
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- In general, the smallest difference in loudness that can be
detected by the human ear is 1 dB. When using the decibel scale,
every 3-dB increase in SIL is a doubling in intensity. A 10-dB
increase increases the intensity by 10 times.
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- D ECIBEL S CALE
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- 1. By what factor would the sound intensity increase if the
sound intensity level in an office increased from 68 dB to 77 dB?
2. A sound changes from an intensity of 5 x10 -6 W/m 2 to 5 x10 -7
W/m2. Has the sound intensity level increased or decreased? 3. By
how much has the sound intensity level changed in question 2?
Express your answer in decibels.
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- I NTENSITY L OUDNESS The terms loudness and intensity do not
have the same meaning. Loudness is a measure of the ears response
to sound. Two sounds can have equal intensity, but you may hear one
sound as louder than another because your ears can detect it
better.